**Environmentally Friendly Insulation Gases as Alternatives to Sulfur Hexafluoride Gas**

**Chapter 4**

F8 ),

) can satisfy the

has been

N) as the main insulat-

**Provisional chapter**

**Development Prospect of Gas Insulation Based on**

**Development Prospect of Gas Insulation Based on** 

DOI: 10.5772/intechopen.77035

The research situation of environmentally friendly gas insulation is expounded in this paper. The basic physical and chemical properties of the insulating gases are analysed, to propose several environment-friendly insulating gas of potential alternative to sulphur hexafluoride

). The insulation characteristics of different components gas mixtures with 90% of nitro-

I) and heptafluorobutyronitrile (C<sup>4</sup>

ing gas had been tested with 5–20 mm sphere-plane electrode gaps in non-uniform electric field under the power frequency voltage and positive and negative lightning impulse breakdown. The development prospects of environmentally friendly gas insulation are

> I and C4 F7

**Keywords:** electrical equipment insulation, environmentally friendly gases, alternatives

guarantees the security of its application in the gas insulating apparatus. What is more, the

insulating materials. There is little decomposing by-products after discharge or arc, which

an important industrial gas with more than 20,000 tons' produced every year all over the

guarantees the following insulating function and protects apparatus. Nowadays, SF<sup>6</sup>

F8 ,CF3

Because of its good electrical insulating properties, sulphur hexafluoride (SF<sup>6</sup>

) as buffer gas and 10% octafluorocyclobutane (c-C<sup>4</sup>

) are conducted trying to points out the further research direction.

F7

N (some friendly gases, which have the

is nontoxic and non-combustible, which

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

© 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use,

distribution, and reproduction in any medium, provided the original work is properly cited.

are stable and it can be compatible with most mental and solid

**Environmental Protection**

**Environmental Protection**

http://dx.doi.org/10.5772/intechopen.77035

Additional information is available at the end of the chapter

Additional information is available at the end of the chapter

) and carbon dioxide (CO2

insulating demands of the electrical apparatus. SF6

Trifluoroiodomethane (CF<sup>3</sup>

potential to replace SF6

forecasted. Further analysis of c-C4

Dengming Xiao

Dengming Xiao

**Abstract**

(SF6

gen (N2

gases, SF6

**1. Introduction**

chemical properties of SF6

#### **Development Prospect of Gas Insulation Based on Environmental Protection Development Prospect of Gas Insulation Based on Environmental Protection**

DOI: 10.5772/intechopen.77035

#### Dengming Xiao Dengming Xiao

Additional information is available at the end of the chapter Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/intechopen.77035

#### **Abstract**

The research situation of environmentally friendly gas insulation is expounded in this paper. The basic physical and chemical properties of the insulating gases are analysed, to propose several environment-friendly insulating gas of potential alternative to sulphur hexafluoride (SF6 ). The insulation characteristics of different components gas mixtures with 90% of nitrogen (N2 ) and carbon dioxide (CO2 ) as buffer gas and 10% octafluorocyclobutane (c-C<sup>4</sup> F8 ), Trifluoroiodomethane (CF<sup>3</sup> I) and heptafluorobutyronitrile (C<sup>4</sup> F7 N) as the main insulating gas had been tested with 5–20 mm sphere-plane electrode gaps in non-uniform electric field under the power frequency voltage and positive and negative lightning impulse breakdown. The development prospects of environmentally friendly gas insulation are forecasted. Further analysis of c-C4 F8 ,CF3 I and C4 F7 N (some friendly gases, which have the potential to replace SF6 ) are conducted trying to points out the further research direction.

**Keywords:** electrical equipment insulation, environmentally friendly gases, alternatives gases, SF6

#### **1. Introduction**

Because of its good electrical insulating properties, sulphur hexafluoride (SF<sup>6</sup> ) can satisfy the insulating demands of the electrical apparatus. SF6 is nontoxic and non-combustible, which guarantees the security of its application in the gas insulating apparatus. What is more, the chemical properties of SF6 are stable and it can be compatible with most mental and solid insulating materials. There is little decomposing by-products after discharge or arc, which guarantees the following insulating function and protects apparatus. Nowadays, SF<sup>6</sup> has been an important industrial gas with more than 20,000 tons' produced every year all over the

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

world, and 80% of that is applied as insulating gas in electrical apparatus [1]. With the continuous increase of China's electrical demand and the expansion of the electrical grid, the demand for insulating gas will continuously increase [2–4].

Although the characteristics of SF6 can satisfy the requirements as insulation gas in electrical apparatus, such as gas-insulated substations, scientists have recognised that it can influent and aggravate the greenhouse effect in recent years. SF<sup>6</sup> is a strong greenhouse gas that will cause serious harm to the environment. The Global Warming Potential GWP of SF<sup>6</sup> is 23,900 times stronger than that of CO<sup>2</sup> [5], which means that under the computing period of 100 years. Far more serious is that because of the extremely stable chemical properties, it is very hard to decompose SF6 in nature and it can exist for 3200 years in atmosphere [6], which will make the environmental influence and greenhouse effect continuously accumulated.

In the *Kyoto Protocol to the United Nations Framework Convention on Climate Change* signed in 1997 in Kyoto of Japan [2], SF6 was regarded as one of the six-kinds of greenhouse gas (CO2 ,CH4 ,N2 O,PFC,HFC and SF<sup>6</sup> ) and it demanded that developed countries should stop and reduce the total emission of greenhouse gas. With signing the *Paris Agreemen*t [3], international society are making efforts to reduce carbon emissions, which means that the application of SF6 in industry will be limited more and more [4, 5, 7]. Therefore, researching new method of gas insulating to replace SF6 becomes an urgent work.

Besides c-C4

C3

2-C4

1,3-C<sup>4</sup>

butyronitrile (C4

Fluoride (C6

CF3

c-C4

g3 (C4 F7 N/CO2

C5

(C4 F6 )

Hexafluoropropylene (C<sup>3</sup>

Fluorinated 1,3-butadiene

Fluorinated 2-butyne (C4

Fluorinated 2-butene (C4

F8

Hexafluorobutadiene (2-C<sup>4</sup>

F7

**Gas Physicochemical** 

F10O/air Nontoxic 26.9

F6

F6

F8

**Table 2.** Properties of potential alternative gas to sulphur hexafluoride (SF<sup>6</sup>

CF4 0.39

**Gas Relative breakdown** 

F8 About 1.35

F8 About 1.75

F6 About 1.50

F6

**voltage**

SF6 1 As reference of gas

power free electron c-C4

) About 2.3

ketone such as Heptafluoropropyl trifluorovinyl ether (C5

**properties**

**Toxicity Boiling** 

, organic halogenated gas, trifluoroiodomethane (CF<sup>3</sup>

**Table 1.** Relative direct current (DC) breakdown voltages of some fluorination gases [1, 8, 12].

CHF3 0.27 With weaker absorption to free electron

F12O). Properties of some potential alternative gases to SF<sup>6</sup>

**point (unit: °C)**

SF6 Nontoxic −64 1 1.00 1.00

I Low-toxicity −22.5 ≈0 1.20 0.90

F8 Nontoxic −6 0.3 1.30 —

(Pure)

) Toxic 1.2 — 1.8

) Low-toxicity 24 (Pure) 0.02 0.85–1 —

) Toxic −29.6 ≈0 1.01 —

) Toxic −25 ≈0 1.7 —

Toxic 6~7 ≈0 1.4 —

iodine (I) has been concentrated by researchers for its much lower GWP and better insulation characteristics. At the same time, ALSTOM company in France and 3M company in US produce an electrical insulation gas mixtures together, named G3, whose main ingredient is heptafluoro-

F8 0.90 With strong absorption to free electron, especially low-

**Remarks**

Relative breakdown voltage is 1

Development Prospect of Gas Insulation Based on Environmental Protection

http://dx.doi.org/10.5772/intechopen.77035

81

ABB company produces electrical insulation gas mixtures whose main ingredient is fluorinated

N), a kind of fluorinated nitrile with Novec 4710 as trade name [11]. Besides,

**Environmental characteristics**

**Relative GWP Relative insulation** 

≈0 0.75–0.85 —

) [8, 13, 14].

I), contains fluorine (F) and

F10O) and Undecafluorohexanoyl

**Electrical characteristics**

**characteristics [15]**

are shown in **Table 2**.

**Relative rising rate of recovery voltage (RRRV) characteristics**

It is important to look for environmentally insulating gas with similar insulating characteristics and physicochemical properties of SF6 to replace SF6 . SF6 belongs to inorganic fluorinated gases, and its molecular geometry is octahedron with six-fluorine (F) atoms in outer surface and one sulphur (S) atom in centre. Because of fluorine belongs to the halogens, its peripheral electronic layer is occupied by seven electrons and can become stable structure with one more electron, which allows it to strongly attract electron. Moreover, in the molecule of SF<sup>6</sup> , F atoms and S atom form more stable covalent bonds by sharing electrons. However, F atoms also have the trend to attract electrons so that the entire molecule has a trend to attract electron. Therefore, it has better insulating characteristics than other gaseous molecular without electronegativity. In addition, although the gas characteristics showed by the structure of macro element cannot show the insulation strength of gas exactly, even counterexample existing, researchers have attached importance to that and the researching emphasis of alternative gas is concentrated on the halogenated gas [8]. In 1997, the research report about the insulation characteristics and arc quenching of alternative gas of SF<sup>6</sup> written by the National Bureau of Standards of the U.S.A [9] introduced many potential alternative gases. Besides, in this work was studied the breakdown voltage under direct current (DC) uniform field of gases, such as organic fluorinated ones, compared with SF<sup>6</sup> , and this comparison is shown in **Table 1**. The result of the report shows that most fluorinated gases have good electronic adsorption, which it is related to the addition of fluorine, but not all the organic fluorinated gases have good insulation characteristics. Besides, it is not correct to evaluate the insulation characteristics just based on the elements that constitute a gas, so it is necessary to analyse different gases in detail for comparison. Because the physicochemical properties of octafluorocyclobutane (c-C4 F8 ) are close to SF6 , its cost is low and Greenhouse Warming Potential (GWP) is lower than SF6 , the report has specially indicated that c-C4 F8 and its mixture can be the study subject for long time [10], so that researchers are focused on the study of this gas.

#### Development Prospect of Gas Insulation Based on Environmental Protection http://dx.doi.org/10.5772/intechopen.77035 81


**Table 1.** Relative direct current (DC) breakdown voltages of some fluorination gases [1, 8, 12].

world, and 80% of that is applied as insulating gas in electrical apparatus [1]. With the continuous increase of China's electrical demand and the expansion of the electrical grid, the

cal apparatus, such as gas-insulated substations, scientists have recognised that it can influ-

will cause serious harm to the environment. The Global Warming Potential GWP of SF<sup>6</sup>

will make the environmental influence and greenhouse effect continuously accumulated.

100 years. Far more serious is that because of the extremely stable chemical properties, it is

In the *Kyoto Protocol to the United Nations Framework Convention on Climate Change* signed

reduce the total emission of greenhouse gas. With signing the *Paris Agreemen*t [3], international society are making efforts to reduce carbon emissions, which means that the applica-

It is important to look for environmentally insulating gas with similar insulating characteris-

gases, and its molecular geometry is octahedron with six-fluorine (F) atoms in outer surface and one sulphur (S) atom in centre. Because of fluorine belongs to the halogens, its peripheral electronic layer is occupied by seven electrons and can become stable structure with one more

and S atom form more stable covalent bonds by sharing electrons. However, F atoms also have the trend to attract electrons so that the entire molecule has a trend to attract electron. Therefore, it has better insulating characteristics than other gaseous molecular without electronegativity. In addition, although the gas characteristics showed by the structure of macro element cannot show the insulation strength of gas exactly, even counterexample existing, researchers have attached importance to that and the researching emphasis of alternative gas is concentrated on the halogenated gas [8]. In 1997, the research report about the insulation

Standards of the U.S.A [9] introduced many potential alternative gases. Besides, in this work was studied the breakdown voltage under direct current (DC) uniform field of gases, such as

result of the report shows that most fluorinated gases have good electronic adsorption, which it is related to the addition of fluorine, but not all the organic fluorinated gases have good insulation characteristics. Besides, it is not correct to evaluate the insulation characteristics just based on the elements that constitute a gas, so it is necessary to analyse different gases in detail for comparison. Because the physicochemical properties of octafluorocyclobutane

electron, which allows it to strongly attract electron. Moreover, in the molecule of SF<sup>6</sup>

to replace SF6

in industry will be limited more and more [4, 5, 7]. Therefore, researching new

becomes an urgent work.

. SF6

, its cost is low and Greenhouse Warming Potential (GWP) is lower

F8

can satisfy the requirements as insulation gas in electri-

[5], which means that under the computing period of

was regarded as one of the six-kinds of greenhouse gas

) and it demanded that developed countries should stop and

in nature and it can exist for 3200 years in atmosphere [6], which

is a strong greenhouse gas that

belongs to inorganic fluorinated

written by the National Bureau of

, and this comparison is shown in **Table 1**. The

and its mixture can be the study subject

is

, F atoms

demand for insulating gas will continuously increase [2–4].

80 Simulation and Modelling of Electrical Insulation Weaknesses in Electrical Equipment

ent and aggravate the greenhouse effect in recent years. SF<sup>6</sup>

Although the characteristics of SF6

23,900 times stronger than that of CO<sup>2</sup>

in 1997 in Kyoto of Japan [2], SF6

method of gas insulating to replace SF6

tics and physicochemical properties of SF6

characteristics and arc quenching of alternative gas of SF<sup>6</sup>

, the report has specially indicated that c-C4

for long time [10], so that researchers are focused on the study of this gas.

organic fluorinated ones, compared with SF<sup>6</sup>

) are close to SF6

O,PFC,HFC and SF<sup>6</sup>

very hard to decompose SF6

(CO2

(c-C4 F8

than SF6

,CH4 ,N2

tion of SF6

Besides c-C4 F8 , organic halogenated gas, trifluoroiodomethane (CF<sup>3</sup> I), contains fluorine (F) and iodine (I) has been concentrated by researchers for its much lower GWP and better insulation characteristics. At the same time, ALSTOM company in France and 3M company in US produce an electrical insulation gas mixtures together, named G3, whose main ingredient is heptafluorobutyronitrile (C4 F7 N), a kind of fluorinated nitrile with Novec 4710 as trade name [11]. Besides, ABB company produces electrical insulation gas mixtures whose main ingredient is fluorinated ketone such as Heptafluoropropyl trifluorovinyl ether (C5 F10O) and Undecafluorohexanoyl Fluoride (C6 F12O). Properties of some potential alternative gases to SF<sup>6</sup> are shown in **Table 2**.


**Table 2.** Properties of potential alternative gas to sulphur hexafluoride (SF<sup>6</sup> ) [8, 13, 14].

#### **2. Analysis of potential alternative gas**

#### **2.1. Octafluorocyclobutane (c-C<sup>4</sup> F8 )**

Octafluorocyclobutane, c-C<sup>4</sup> F8 is an important industrial gas. Nowadays, it is used in plasma etching technology or as refrigerant [16]. Similar to SF6 gas, the performance to absorb electron easily of fluorine in c-C<sup>4</sup> F8 is shown in the characteristics of the whole molecule, so that c-C<sup>4</sup> F8 has a stronger absorption to free electron. c-C4 F8 is colourless, odourless, nontoxic to human bodies at low concentration, non-combustible, nonexplosive and with GWP of about 8700 relative to CO2 . Though it belongs to greenhouse, but in the same conditions, its negative effects are just one third of SF6 [17]. In addition, as organic halogenated gas, c-C4 F8 does not contain chlorine or bromine, so it is not harmful to the ozone layer. The molecule of c-C4 F8 is circular with a stable chemical structure and does no harm to other solid materials in electrical apparatus, such as aluminium alloy, copper contact and epoxy supporting insulators. Recently, the price of c-C4 F8 differs with the purity of gas. The price of this gas with 99.9% purity is about 200 RMB/kg [8] (1 RMB ≈ 0.16 dollar≈0.13 euro, the same below), as the price of gas with 99.999% purity is about 500 RMB/kg, and that has obviously reduced compared with the price of about thousand RMB per kilogramme 10 years ago. This is related to more applications, such as refrigerant [18], that are using c-C4 F8 and the rise of production. Nowadays, the price of c-C<sup>4</sup> F8 is only a little bit higher than that of SF<sup>6</sup> , but if c-C4 F8 is applied widely in electrical domain, its price still can be reduced, so the cost is not the obstacle to be applied in electrical apparatus.

gas mixtures. Therefore, c-C4

**2.2. Trifluoroiodomethane (CF<sup>3</sup>**

the electrical apparatus by applying CF3

indicate, that the difference between CF<sup>3</sup>

production in China is low. Currently, CF<sup>3</sup>

Science & Technology Co., Ltd.), after CF3

price is much higher than SF6

electrical apparatus. The molecular structure of CF3

Trifluoroiodomethane (CF<sup>3</sup>

1–5 relative to CO<sup>2</sup>

Because of CF3

constant cost of CF3

that for SF6

gas mixtures.

F8

**I)**

cannot satisfy the demand of arctic alpine regions. Thus, it should be mixed with other gas in some ratios to reduce the condensing temperature of the gas mixtures and be used as

is a new industrial gas that can be used as an environmental refrigerant and alternative fireextinguishing agent. It can be used as additive or mixed composition to replace traditional refrigerant Freon and fire-extinguishing material "Halon." Because its GWP is very low, about

greenhouse is very small. At the same time, it does not contain chlorine and bromine that is commonly present in most refrigerants, so it will not damage the ozone layer, thus the United

of environmentally friendly gas, and has related basis in industrial application. As a kind of fire-extinguishing material, its efficiency is outstanding and has little negative influence on environment, and it is well compatible with normal industrial materials, so that it will not cause chemical reaction or erosion. Therefore, it has passed some related standards of the U.S.A [20]. and can be used in aerospace and other areas. In addition, it can rise the security of

switchgear (C-GIS) or compact transformer. It is especially appropriate to be used in populous regions of central city in order to reduce the conflagration or explosion caused by the bug of

halogens such as F and I, so it has strong absorption to free electron. So that it can absorb free electron at the beginning of discharge when electron avalanche forms, and then it can restrain the formation of collision ionisation, which enhances its insulation property. What is worthy to

of its structure, which makes the polarity effect of the molecule stronger. The three-F atoms in the molecule has stronger absorption to electron than I atom, so the electron cloud in the molecule trends to F atoms, and the density of the electron cloud around the carbon-iodine covalent bond formed by I atom and carbon (C) atom is reduced, and the energy barrier to absorb electron is also reduced. Therefore, the whole molecule has a strong ability to absorb electron.

I and SF6

Nations regards it as new refrigerant to replace Freon [19]. This can prove that CF3

is not suited to be applied in apparatus as pure gas, or it

Development Prospect of Gas Insulation Based on Environmental Protection

http://dx.doi.org/10.5772/intechopen.77035

I) is colourless, odourless, non-combustible and nonexplosive. CF3

I in electrical apparatus such as cubicle gas insulating

F8

I produced in China costs about 2000 RMB/kg, the

I will be used widely and will be mass-produced, the

I is shown in **Figure 1** RMB. It is affected by

, comes from the asymmetry

I in China is higher than

I (Beijing Yuji

F8 .

is much lower than most organic halogenated gases, so its influence on

, as well as c-C<sup>4</sup>

I is a new industrial gas, its application in China is not widely extended, the

I will reduce a lot with the actual cost lower than 600 RMB/kg. Moreover,

[1]. The main reason why the price of CF<sup>3</sup>

[1] is that the demand is very low. According to the producers of CF<sup>3</sup>

Since year 2000, many researchers in China and abroad begin to research this new insulating gas [21, 22]. Researchers of plasma from Mexico have calculated and measured the ionisation

by optimising and upgrading, its price will be reduced continuously like that for c-C<sup>4</sup>

I

83

I is a kind

Long before, Japanese researchers began to research the electrical properties of c-C<sup>4</sup> F8 and indicated that it had the feasibility to replace SF6 in electrical apparatus. Then, the researchers of plasma and electric-related domains from the U.S.A. and Mexico began to use Boltzmann equation, calculation of parameter of discharge particle and breakdown test to research the insulation characteristics of c-C4 F8 . Shanghai Jiao Tong University, Xi'an Jiao Tong University and other high schools in China began the researches about calculation of academic simulation and breakdown test of c-C<sup>4</sup> F8 . The results of researches have shown that the insulation characteristics of pure c-C4 F8 are better than SF<sup>6</sup> , in air pressure at 0.3 MPa and over. The breakdown voltage of the gas mixtures of c-C<sup>4</sup> F8 and N2 or CO2 is higher than the gas mixtures of SF6 with the same contents, and in low air pressure or atmospheric pressure, the breakdown voltage of the gas mixtures of c-C<sup>4</sup> F8 can approach the gas mixtures of SF6 with the same contents. In conclusion, c-C4 F8 and its gas mixtures have similar insulation characteristics with SF<sup>6</sup> , and the breakdown voltage differs a little with the composition, mixture ratio and gas pressure, so it can satisfy the demands of actual application.

The relative molecular mass of c-C4 F8 is 200, higher than that of SF<sup>6</sup> (146.06), and it means that the condensing temperature of c-C4 F8 will be high, is about −6°C, higher than −63.6°C of SF6 . The insulating gas should exist in gaseous state in the electrical apparatus, thus need to have a low enough liquefaction temperature. One way to reduce its liquefaction point is to add some buffer gas including nitrogen (N<sup>2</sup> ) or carbon dioxide (CO2 ), which may lead to a weaker insulation strength. So we need to take a balance between the low liquefaction temperature and good insulation property when considering the mixture ratio for c-C<sup>4</sup> F8

gas mixtures. Therefore, c-C4 F8 is not suited to be applied in apparatus as pure gas, or it cannot satisfy the demand of arctic alpine regions. Thus, it should be mixed with other gas in some ratios to reduce the condensing temperature of the gas mixtures and be used as gas mixtures.

#### **2.2. Trifluoroiodomethane (CF<sup>3</sup> I)**

**2. Analysis of potential alternative gas**

**F8 )**

82 Simulation and Modelling of Electrical Insulation Weaknesses in Electrical Equipment

are just one third of SF6 [17]. In addition, as organic halogenated gas, c-C4

F8

F8

F8

are better than SF<sup>6</sup>

F8

F8

F8

and gas pressure, so it can satisfy the demands of actual application.

F8

chlorine or bromine, so it is not harmful to the ozone layer. The molecule of c-C4

is an important industrial gas. Nowadays, it is used in plasma

and the rise of production. Nowadays, the price of c-C<sup>4</sup>

. Shanghai Jiao Tong University, Xi'an Jiao Tong University

. The results of researches have shown that the insulation

or CO2

) or carbon dioxide (CO2

is shown in the characteristics of the whole molecule, so that c-C<sup>4</sup>

F8

. Though it belongs to greenhouse, but in the same conditions, its negative effects

differs with the purity of gas. The price of this gas with 99.9% purity is about 200

F8

bodies at low concentration, non-combustible, nonexplosive and with GWP of about 8700 rela-

with a stable chemical structure and does no harm to other solid materials in electrical apparatus, such as aluminium alloy, copper contact and epoxy supporting insulators. Recently, the

RMB/kg [8] (1 RMB ≈ 0.16 dollar≈0.13 euro, the same below), as the price of gas with 99.999% purity is about 500 RMB/kg, and that has obviously reduced compared with the price of about thousand RMB per kilogramme 10 years ago. This is related to more applications, such as

, but if c-C4

Long before, Japanese researchers began to research the electrical properties of c-C<sup>4</sup>

price still can be reduced, so the cost is not the obstacle to be applied in electrical apparatus.

of plasma and electric-related domains from the U.S.A. and Mexico began to use Boltzmann equation, calculation of parameter of discharge particle and breakdown test to research the

and other high schools in China began the researches about calculation of academic simula-

F8

F8

. The insulating gas should exist in gaseous state in the electrical apparatus, thus need to have a low enough liquefaction temperature. One way to reduce its liquefaction point

to a weaker insulation strength. So we need to take a balance between the low liquefaction temperature and good insulation property when considering the mixture ratio for c-C<sup>4</sup>

and N2

with the same contents, and in low air pressure or atmospheric pressure, the

, and the breakdown voltage differs a little with the composition, mixture ratio

is 200, higher than that of SF<sup>6</sup>

gas, the performance to absorb electron

F8

is applied widely in electrical domain, its

in electrical apparatus. Then, the researchers

, in air pressure at 0.3 MPa and over. The

can approach the gas mixtures of SF6

will be high, is about −6°C, higher than −63.6°C

and its gas mixtures have similar insulation character-

is higher than the gas mix-

(146.06), and it means

), which may lead

is colourless, odourless, nontoxic to human

F8

F8

F8 and

with

F8

does not contain

is circular

F8

F8

etching technology or as refrigerant [16]. Similar to SF6

F8

has a stronger absorption to free electron. c-C4

**2.1. Octafluorocyclobutane (c-C<sup>4</sup>**

Octafluorocyclobutane, c-C<sup>4</sup>

easily of fluorine in c-C<sup>4</sup>

F8

refrigerant [18], that are using c-C4

insulation characteristics of c-C4

tion and breakdown test of c-C<sup>4</sup>

the same contents. In conclusion, c-C4

The relative molecular mass of c-C4

that the condensing temperature of c-C4

is to add some buffer gas including nitrogen (N<sup>2</sup>

characteristics of pure c-C4

tures of SF6

istics with SF<sup>6</sup>

of SF6

is only a little bit higher than that of SF<sup>6</sup>

indicated that it had the feasibility to replace SF6

breakdown voltage of the gas mixtures of c-C<sup>4</sup>

breakdown voltage of the gas mixtures of c-C<sup>4</sup>

tive to CO2

price of c-C4

Trifluoroiodomethane (CF<sup>3</sup> I) is colourless, odourless, non-combustible and nonexplosive. CF3 I is a new industrial gas that can be used as an environmental refrigerant and alternative fireextinguishing agent. It can be used as additive or mixed composition to replace traditional refrigerant Freon and fire-extinguishing material "Halon." Because its GWP is very low, about 1–5 relative to CO<sup>2</sup> is much lower than most organic halogenated gases, so its influence on greenhouse is very small. At the same time, it does not contain chlorine and bromine that is commonly present in most refrigerants, so it will not damage the ozone layer, thus the United Nations regards it as new refrigerant to replace Freon [19]. This can prove that CF3 I is a kind of environmentally friendly gas, and has related basis in industrial application. As a kind of fire-extinguishing material, its efficiency is outstanding and has little negative influence on environment, and it is well compatible with normal industrial materials, so that it will not cause chemical reaction or erosion. Therefore, it has passed some related standards of the U.S.A [20]. and can be used in aerospace and other areas. In addition, it can rise the security of the electrical apparatus by applying CF3 I in electrical apparatus such as cubicle gas insulating switchgear (C-GIS) or compact transformer. It is especially appropriate to be used in populous regions of central city in order to reduce the conflagration or explosion caused by the bug of electrical apparatus. The molecular structure of CF3 I is shown in **Figure 1** RMB. It is affected by halogens such as F and I, so it has strong absorption to free electron. So that it can absorb free electron at the beginning of discharge when electron avalanche forms, and then it can restrain the formation of collision ionisation, which enhances its insulation property. What is worthy to indicate, that the difference between CF<sup>3</sup> I and SF6 , as well as c-C<sup>4</sup> F8 , comes from the asymmetry of its structure, which makes the polarity effect of the molecule stronger. The three-F atoms in the molecule has stronger absorption to electron than I atom, so the electron cloud in the molecule trends to F atoms, and the density of the electron cloud around the carbon-iodine covalent bond formed by I atom and carbon (C) atom is reduced, and the energy barrier to absorb electron is also reduced. Therefore, the whole molecule has a strong ability to absorb electron.

Because of CF3 I is a new industrial gas, its application in China is not widely extended, the production in China is low. Currently, CF<sup>3</sup> I produced in China costs about 2000 RMB/kg, the price is much higher than SF6 [1]. The main reason why the price of CF<sup>3</sup> I in China is higher than that for SF6 [1] is that the demand is very low. According to the producers of CF<sup>3</sup> I (Beijing Yuji Science & Technology Co., Ltd.), after CF3 I will be used widely and will be mass-produced, the constant cost of CF3 I will reduce a lot with the actual cost lower than 600 RMB/kg. Moreover, by optimising and upgrading, its price will be reduced continuously like that for c-C<sup>4</sup> F8 .

Since year 2000, many researchers in China and abroad begin to research this new insulating gas [21, 22]. Researchers of plasma from Mexico have calculated and measured the ionisation

insulation characteristics of its gas mixtures are lower than SF<sup>6</sup>

F7

materials used in electrical assets. The relative molecular mass of C4

the development of the producers at home, the price could be reduced.

condensing temperature of −4.7°C, so that it cannot replace SF<sup>6</sup>

become gas mixtures with buffering gas such as N<sup>2</sup>

lation characteristics of its gas mixtures with CO<sup>2</sup>

**2.3. Fluorinated nitrile gas and G3 gas mixtures**

I and other gas is the key to be applied in the future.

gas. Among many organic fluorinated gases, they choose the gas, which is also

N have very good insulation performance, which can reach

Development Prospect of Gas Insulation Based on Environmental Protection

F7

. The chemical features of this gas are similar to the organic

is about 90% of the SF<sup>6</sup>

or CO2

, and this gas can also be used as arc quenching medium being applied

ALSTOM company in France and 3M company in U.S.A. have joined to research the alterna-

alternative refrigerant, and organic chemical compound that contains four-C atoms and

G3. Besides, its molecular structure is shown in **Figure 2**. The gas has replaced a fluorine atom with nitrile group (▬C☰N) on the basis of the fluorinated hydrocarbon gas, and becomes fluorinated nitrile gas. This nitrile group containing carbon-nitrogen triple bond has a special

fluorinated gas with stable chemical characteristics and can be well compatible with other

gas, related testing research is lacking. According to research result obtained by now, the insu-

in circuit-breakers [33]. Nowadays, this gas is researched and produced by 3M company and its cost is dozens of times higher than other gases [33], so the cost is one of the obstacles for its industrial application. With the accomplishment of the production technology of the gas and

seven-F atoms, with a trade name of Novec 4710 [11] and chemical formula of C4

the synergistic effect of CF<sup>3</sup>

chemical structure to make C4

about two-times of that of SF<sup>6</sup>

same amount of CO2

**Figure 2.** Molecule structure of C<sup>4</sup>

F7 N.

tive to SF6

. Therefore, the research about

http://dx.doi.org/10.5772/intechopen.77035

F7

N is 195, with a high

mixtures with the

as a single gas, it should

. Because of it is a new insulating

N, named

85

**Figure 1.** Molecule structure of CF<sup>3</sup> I.

coefficient, attachment coefficient and electron drift velocity during the process of discharge of CF3 I and its gas mixtures with N<sup>2</sup> , SF6 and other gases [23, 24]. The aforementioned work has quantified the reaction between free electron and gas molecule during the process of discharge, and has analysed the insulation strength of gas mixtures from the perspective of the parameters of discharge. Tokyo University of Japan, Tokyo Denki University and Japan Electric Power Company have researched CF3 I by testing [25, 26]. They make the breakdown test to CF<sup>3</sup> I and its gas mixtures with N<sup>2</sup> , CO2 and air by using lighting impulse. The results show that the insulation strength of pure CF3 I is better than that in SF<sup>6</sup> , about 1.2 times than SF<sup>6</sup> , and CF3 I-CO2 gas mixtures with high content also has better insulation characteristics to be able to replace SF6 . Many universities and academies in Europe also research the gas mixtures of CF<sup>3</sup> I-CO2 and CF3 I-N2 in different conditions [24]. The results show that the positive synergistic effect of the gas mixtures of CF3 I and N2 is less obvious than that of the gas mixtures of SF6 and N2 , which means that in the same mixture ratio, the insulation strength of the gas mixtures of CF3 I-CO2 cannot increase with the rising content of CF<sup>3</sup> I because of the synergistic effect [22]. In addition, the gas mixtures of CF3 I and CO2 with low content show better positive synergistic effect. Shanghai Jiao Tong University, Xi'an Jiao Tong University and Chongqing University in China has researched CF3 I and its gas mixtures by academic calculation and testing research [27–29]. Shanghai Jiao Tong University uses Boltzmann's equation to calculate and analyse the discharge parameters and insulation characteristics of the gas mixtures of CF3 I and N2 , CO2 , He and so on and get the alternating current (AC) breakdown voltage in non-uniform electric field and slightly non-uniform electric field by testing [28, 30]. Other researchers have measured partial discharge voltage and other insulation characteristics of the gas mixtures of CF3 I [31, 32]. The results show that CF<sup>3</sup> I has good electrical insulation characteristics, but the positive synergistic effect of the mixture of CF<sup>3</sup> I and normal buffering gas is not obvious, so that the insulation characteristics of its gas mixtures are lower than SF<sup>6</sup> . Therefore, the research about the synergistic effect of CF<sup>3</sup> I and other gas is the key to be applied in the future.

#### **2.3. Fluorinated nitrile gas and G3 gas mixtures**

ALSTOM company in France and 3M company in U.S.A. have joined to research the alternative to SF6 gas. Among many organic fluorinated gases, they choose the gas, which is also alternative refrigerant, and organic chemical compound that contains four-C atoms and seven-F atoms, with a trade name of Novec 4710 [11] and chemical formula of C4 F7 N, named G3. Besides, its molecular structure is shown in **Figure 2**. The gas has replaced a fluorine atom with nitrile group (▬C☰N) on the basis of the fluorinated hydrocarbon gas, and becomes fluorinated nitrile gas. This nitrile group containing carbon-nitrogen triple bond has a special chemical structure to make C4 F7 N have very good insulation performance, which can reach about two-times of that of SF<sup>6</sup> . The chemical features of this gas are similar to the organic fluorinated gas with stable chemical characteristics and can be well compatible with other materials used in electrical assets. The relative molecular mass of C4 F7 N is 195, with a high condensing temperature of −4.7°C, so that it cannot replace SF<sup>6</sup> as a single gas, it should become gas mixtures with buffering gas such as N<sup>2</sup> or CO2 . Because of it is a new insulating gas, related testing research is lacking. According to research result obtained by now, the insulation characteristics of its gas mixtures with CO<sup>2</sup> is about 90% of the SF<sup>6</sup> mixtures with the same amount of CO2 , and this gas can also be used as arc quenching medium being applied in circuit-breakers [33]. Nowadays, this gas is researched and produced by 3M company and its cost is dozens of times higher than other gases [33], so the cost is one of the obstacles for its industrial application. With the accomplishment of the production technology of the gas and the development of the producers at home, the price could be reduced.

**Figure 2.** Molecule structure of C<sup>4</sup> F7 N.

coefficient, attachment coefficient and electron drift velocity during the process of discharge of

quantified the reaction between free electron and gas molecule during the process of discharge, and has analysed the insulation strength of gas mixtures from the perspective of the parameters of discharge. Tokyo University of Japan, Tokyo Denki University and Japan Electric Power

gas mixtures with high content also has better insulation characteristics to be able to replace

which means that in the same mixture ratio, the insulation strength of the gas mixtures of

effect. Shanghai Jiao Tong University, Xi'an Jiao Tong University and Chongqing University in

[27–29]. Shanghai Jiao Tong University uses Boltzmann's equation to calculate and analyse the

and so on and get the alternating current (AC) breakdown voltage in non-uniform electric field and slightly non-uniform electric field by testing [28, 30]. Other researchers have measured partial discharge voltage and other insulation characteristics of the gas mixtures of CF3

in different conditions [24]. The results show that the positive synergistic effect

. Many universities and academies in Europe also research the gas mixtures of CF<sup>3</sup>

and other gases [23, 24]. The aforementioned work has

, about 1.2 times than SF<sup>6</sup>

I and

I-CO2

I-CO2

 and N2 ,

, CO2

, He

I [31,

, and CF3

I because of the synergistic effect [22].

I and N2

with low content show better positive synergistic

I by testing [25, 26]. They make the breakdown test to CF<sup>3</sup>

is less obvious than that of the gas mixtures of SF6

I and its gas mixtures by academic calculation and testing research

I has good electrical insulation characteristics, but the positive

I and normal buffering gas is not obvious, so that the

and air by using lighting impulse. The results show that the insu-

, SF6

I is better than that in SF<sup>6</sup>

I and CO2

discharge parameters and insulation characteristics of the gas mixtures of CF3

CF3

SF6

CF3 I-CO2

and CF3

I and its gas mixtures with N<sup>2</sup>

, CO2

I.

84 Simulation and Modelling of Electrical Insulation Weaknesses in Electrical Equipment

I and N2

cannot increase with the rising content of CF<sup>3</sup>

Company have researched CF3

its gas mixtures with N<sup>2</sup>

I-N2

lation strength of pure CF3

**Figure 1.** Molecule structure of CF<sup>3</sup>

of the gas mixtures of CF3

China has researched CF3

32]. The results show that CF<sup>3</sup>

synergistic effect of the mixture of CF<sup>3</sup>

In addition, the gas mixtures of CF3

The gas with the chemical formula of C<sup>4</sup> F7 N has two-isomeric compounds, their chemical formulas and element compositions are the same, but for the different positions of nitrile groups, their molecular structures and microcosmic natures are different. For Novec 4710 gas used in G3 gas, its nitrile group is located in the carbon atom in the middle of the organic carbon-chain, and the other isomeric compound has a nitrile group located in the carbon atom at one end of the carbon-chain, which constitute a virulent gas that cannot be used in industry. In addition, during the production of Novec 4710, by avoiding the production and the mixture of the virulent isomeric compound is key to apply this gas in a real environment. What is more, any gas will be decompounded to produce decomposed by-products in the condition of high temperature and pressure during the discharge process. Moreover, it should be continuously researched about how to guarantee that this gas will not produce toxic isomeric compounds or other gases during the process of discharge or arc interruption.

method to inflate gas mixtures to test chamber is introduced in Ref. [17]. The gases tested in

values as the gap distance gets bigger, and it shows saturation effect. The maximum electric strength of the gas mixtures gets lower values as the gap distance gets bigger, and it shows that the gas mixtures has some sensitivity to the non-uniformity of the electric field. As the non-uniformity of the electric field increases, the maximum electric field able to be tolerated

**Figure 5** shows that under different gap distances, the variety of the AC-breakdown voltage

, N2

gas mixtures increases linearly as the air pressure increases without hump effect, and

. That is to say, the breakdown voltages of gas mixtures have little difference with dif-

ferent contents, at the same time, it shows that the breakdown voltage of the gas mixtures of

is about 10% higher than that of 10%c-C<sup>4</sup>

F8

Development Prospect of Gas Insulation Based on Environmental Protection

and CO2 in Appendix **Figures A1** and **A2**.

F8 , N2 , CO2

gas mixtures as the gas pressure changes. The AC-breakdown voltage of

gas mixtures with different contents in the graphs are more concentrated

gas mixtures. When the gap distance is 20 mm, the AC-breakdown voltage

gas mixtures with different gas pressures.

gas mixtures. From **Figures 3**–**5**, we can see that the variety of the

gas mixtures with the same content as the air pressure and

F8

gas mixtures. However, the curves of breakdown

+90%N<sup>2</sup> .

is lower, this is different from the

mixtures is similar to the

87

http://dx.doi.org/10.5772/intechopen.77035

gas mixtures gets higher

the present paper are listed in **Table 3**.

SF6

of the c-C4

voltage of c-C4

and CO2

F8

properties of SF6

of 10%c-C<sup>4</sup>

than SF6

c-C4 F8

c-C4 F8 F8 , N2 , CO2

this trend is the same to SF6

breakdown voltage of the c-C<sup>4</sup>

F8

+90%CO<sup>2</sup>

**Figure 3.** AC-breakdown voltage of c-C<sup>4</sup>

F8 , N2 , CO2

From **Figures 3** and **4**, it can be observed that the behaviour of c-C4

gas mixtures, the AC-breakdown voltage of the c-C<sup>4</sup>

F8

is the highest and the gas mixtures with N<sup>2</sup>

reduces, and the trend of change is similar to SF6

the electrodes gap changes is the same to SF6

#### **2.4. Fluorinated ketone gas**

ABB company in Switzerland has supported a method for evaluating the greenhouse effect of SF6 [34, 35], and it is to take advantage of fluorinated ketone gas as the main ingredient of gas mixtures, which contains organic fluorinated gas with carbonyl group (C〓O) such as C<sup>5</sup> F10O and C6 F12O. This kind of gas is similar to fluorinated nitrile gas. It is a chemical compound, which uses the carbonyl group to replace one F atom of fluorinated hydrocarbon based on fluorinated hydrocarbon. Because of carbonyl group has carbon-oxide double bond, which is unsaturated bond as the same as the carbon-nitrogen triple bond, it has good absorption to free electron, and it shows higher insulation characteristics in macro-performance [36]. According to the existing testing data in China and abroad, the insulation characteristics of pure C<sup>5</sup> F10O and C6 F12O are about two-times higher than SF<sup>6</sup> and their GWP value approaches zero, physicochemical properties are stable and they have good compatibility with materials and industrial applicability. The fluorinated carbonyl, which ABB has applied in the gas mixtures has more than five-carbon atoms, so its relative molecular mass is bigger than other insulating gases, such as C<sup>5</sup> F10O with 266 and C6 F12O with 316. Besides,the condensing temperature of C5 F10O and C6 F12O is very high with 24 and 49°C at room condition, which means that they are liquid at normal temperature and gas pressure. Therefore, this gas cannot be used in any electrical insulating domains as single gas, and it can only be applied as gas mixtures. Limited by its high-condensing temperature, it will have low content in the gas mixtures, which causes the limitation of the insulation strength of the whole gas mixtures, so the synergistic effect of this gas and other gas mixtures is very important. Therefore, the use of this kind of gas forming gas mixtures, which allows it keep high insulation characteristics at low concentrations, is the emphasis of research in the future.

#### **3. The power frequency AC breakdown characteristics of the c-C<sup>4</sup> F8 , N2 , CO2 gas mixtures**

The breakdown voltage under AC voltage of the gas mixtures with a constant content of 10% of c-C4 F8 and different content of N<sup>2</sup> and CO2 has been measured by testing. **Figures 3** and **4** show the variety of the AC-breakdown voltage and maximum electric strength of the c-C<sup>4</sup> F8 , N2 , CO2 gas mixtures with the variety of gap distance under different air pressure. The gas discharge test chamber and other internal structure are the same with that in Ref. [37]. The method to inflate gas mixtures to test chamber is introduced in Ref. [17]. The gases tested in the present paper are listed in **Table 3**.

The gas with the chemical formula of C<sup>4</sup>

86 Simulation and Modelling of Electrical Insulation Weaknesses in Electrical Equipment

**2.4. Fluorinated ketone gas**

about two-times higher than SF<sup>6</sup>

 **gas mixtures**

and different content of N<sup>2</sup>

SF6

and C6

266 and C6

**N2**

of c-C4 F8

N2 , CO2

**, CO2**

F7

formulas and element compositions are the same, but for the different positions of nitrile groups, their molecular structures and microcosmic natures are different. For Novec 4710 gas used in G3 gas, its nitrile group is located in the carbon atom in the middle of the organic carbon-chain, and the other isomeric compound has a nitrile group located in the carbon atom at one end of the carbon-chain, which constitute a virulent gas that cannot be used in industry. In addition, during the production of Novec 4710, by avoiding the production and the mixture of the virulent isomeric compound is key to apply this gas in a real environment. What is more, any gas will be decompounded to produce decomposed by-products in the condition of high temperature and pressure during the discharge process. Moreover, it should be continuously researched about how to guarantee that this gas will not produce toxic isomeric compounds or other gases during the process of discharge or arc interruption.

ABB company in Switzerland has supported a method for evaluating the greenhouse effect of

erties are stable and they have good compatibility with materials and industrial applicability. The fluorinated carbonyl, which ABB has applied in the gas mixtures has more than five-carbon

with 24 and 49°C at room condition, which means that they are liquid at normal temperature and gas pressure. Therefore, this gas cannot be used in any electrical insulating domains as single gas, and it can only be applied as gas mixtures. Limited by its high-condensing temperature, it will have low content in the gas mixtures, which causes the limitation of the insulation strength of the whole gas mixtures, so the synergistic effect of this gas and other gas mixtures is very important. Therefore, the use of this kind of gas forming gas mixtures, which allows it keep high insulation characteristics at low concentrations, is the emphasis of research in the future.

The breakdown voltage under AC voltage of the gas mixtures with a constant content of 10%

discharge test chamber and other internal structure are the same with that in Ref. [37]. The

gas mixtures with the variety of gap distance under different air pressure. The gas

show the variety of the AC-breakdown voltage and maximum electric strength of the c-C<sup>4</sup>

atoms, so its relative molecular mass is bigger than other insulating gases, such as C<sup>5</sup>

**3. The power frequency AC breakdown characteristics of the c-C<sup>4</sup>**

and CO2

mixtures, which contains organic fluorinated gas with carbonyl group (C〓O) such as C<sup>5</sup>

testing data in China and abroad, the insulation characteristics of pure C<sup>5</sup>

F12O with 316. Besides,the condensing temperature of C5

[34, 35], and it is to take advantage of fluorinated ketone gas as the main ingredient of gas

F12O. This kind of gas is similar to fluorinated nitrile gas. It is a chemical compound, which uses the carbonyl group to replace one F atom of fluorinated hydrocarbon based on fluorinated hydrocarbon. Because of carbonyl group has carbon-oxide double bond, which is unsaturated bond as the same as the carbon-nitrogen triple bond, it has good absorption to free electron, and it shows higher insulation characteristics in macro-performance [36]. According to the existing

and their GWP value approaches zero, physicochemical prop-

N has two-isomeric compounds, their chemical

F10O

F12O are

F10O with

F12O is very high

**F8 ,** 

> F8 ,

F10O and C6

F10O and C6

has been measured by testing. **Figures 3** and **4**

From **Figures 3** and **4**, it can be observed that the behaviour of c-C4 F8 mixtures is similar to the SF6 gas mixtures, the AC-breakdown voltage of the c-C<sup>4</sup> F8 , N2 , CO2 gas mixtures gets higher values as the gap distance gets bigger, and it shows saturation effect. The maximum electric strength of the gas mixtures gets lower values as the gap distance gets bigger, and it shows that the gas mixtures has some sensitivity to the non-uniformity of the electric field. As the non-uniformity of the electric field increases, the maximum electric field able to be tolerated reduces, and the trend of change is similar to SF6 , N2 and CO2 in Appendix **Figures A1** and **A2**.

**Figure 5** shows that under different gap distances, the variety of the AC-breakdown voltage of the c-C4 F8 , N2 , CO2 gas mixtures as the gas pressure changes. The AC-breakdown voltage of c-C4 F8 gas mixtures increases linearly as the air pressure increases without hump effect, and this trend is the same to SF6 gas mixtures. From **Figures 3**–**5**, we can see that the variety of the breakdown voltage of the c-C<sup>4</sup> F8 gas mixtures with the same content as the air pressure and the electrodes gap changes is the same to SF6 gas mixtures. However, the curves of breakdown voltage of c-C4 F8 gas mixtures with different contents in the graphs are more concentrated than SF6 . That is to say, the breakdown voltages of gas mixtures have little difference with different contents, at the same time, it shows that the breakdown voltage of the gas mixtures of c-C4 F8 and CO2 is the highest and the gas mixtures with N<sup>2</sup> is lower, this is different from the properties of SF6 gas mixtures. When the gap distance is 20 mm, the AC-breakdown voltage of 10%c-C<sup>4</sup> F8 +90%CO<sup>2</sup> is about 10% higher than that of 10%c-C<sup>4</sup> F8 +90%N<sup>2</sup> .

**Figure 3.** AC-breakdown voltage of c-C<sup>4</sup> F8 , N2 , CO2 gas mixtures with different gas pressures.

**Figure 4.** Maximum electric strength of c-C<sup>4</sup> F8 , N2 , CO2 gas mixtures with different gas pressures.


**Figure 5.** AC-breakdown voltage of c-C<sup>4</sup>

F8 , N2 , CO2

**Figure 6.** Relationship between AC-breakdown voltage and mixing contents of c-C<sup>4</sup>

gas mixtures with different electrodes gap distances.

Development Prospect of Gas Insulation Based on Environmental Protection

http://dx.doi.org/10.5772/intechopen.77035

89

F8 , N2 , CO2

gas mixtures.

**Table 3.** Test gas mixtures for power frequency AC breakdown experiments.

**Figure 6** shows under different gas pressures, the variety of the AC-breakdown voltage of the c-C4 F8 , N2 , CO2 gas mixtures as the content changes. If it is make the gas mixtures of 10%c-C<sup>4</sup> F8 + 90%N<sup>2</sup> as the initial matched group, it can be seen that the breakdown voltage of the gas mixtures increases as the content of CO2 increases, and when the content of CO<sup>2</sup> exceeds 60%. In other words, with a content of N<sup>2</sup> lower than 30%, the increase of the breakdown voltage is more noticeable.

Because of during the process of discharge, N2 will make the ionisation probability of CO<sup>2</sup> increase as well, when reducing N<sup>2</sup> and increasing CO2 of the c-C4 F8 gas mixtures, the breakdown voltage of the triple gas mixtures in **Figure 6** does not has an obvious increase Development Prospect of Gas Insulation Based on Environmental Protection http://dx.doi.org/10.5772/intechopen.77035 89

**Figure 5.** AC-breakdown voltage of c-C<sup>4</sup> F8 , N2 , CO2 gas mixtures with different electrodes gap distances.

**Figure 6.** Relationship between AC-breakdown voltage and mixing contents of c-C<sup>4</sup> F8 , N2 , CO2 gas mixtures.

**Figure 6** shows under different gas pressures, the variety of the AC-breakdown voltage of

and increasing CO2

breakdown voltage of the triple gas mixtures in **Figure 6** does not has an obvious increase

gas mixtures as the content changes. If it is make the gas mixtures of

gas mixtures with different gas pressures.

 **mixing ratio (%) CO2**

as the initial matched group, it can be seen that the breakdown voltage

increases, and when the content of CO<sup>2</sup>

 **mixing ratio (%)**

lower than 30%, the increase of the break-

will make the ionisation probability of CO<sup>2</sup>

F8

gas mixtures, the

of the c-C4

the c-C4

**Number c-C<sup>4</sup>**

10%c-C<sup>4</sup>

F8 , N2 , CO2

F8 + 90%N<sup>2</sup>

down voltage is more noticeable.

**Figure 4.** Maximum electric strength of c-C<sup>4</sup>

**F8 /CF3** F8 , N2 , CO2

**I mixing ratio (%) N2**

88 Simulation and Modelling of Electrical Insulation Weaknesses in Electrical Equipment

1 10 90 0 2 10 80 10 3 10 60 30 4 10 45 45 5 10 30 60 6 10 10 80 7 10 0 90

increase as well, when reducing N<sup>2</sup>

of the gas mixtures increases as the content of CO2

**Table 3.** Test gas mixtures for power frequency AC breakdown experiments.

exceeds 60%. In other words, with a content of N<sup>2</sup>

Because of during the process of discharge, N2

immediately, and even it has a trend to reduce a little. Only after the content of N<sup>2</sup> is lower than 30% and the content of CO<sup>2</sup> is higher than 60%, the breakdown voltage can increase significantly.

#### **4. Power frequency AC-breakdown characteristics of the CF<sup>3</sup> I, N2 , CO2 gas mixtures**

To CF3 I, it has been measured the breakdown characteristics for a constant content of 10% CF3 I and with different concentrations of N<sup>2</sup> and CO2 under AC-voltage applied during the tests. The test method and experiment setup are similar to that in Section 2. The gas mixtures and mixing ratio are listed in **Table 1**. **Figures 7** and **8** show that under different air pressures, the variety of the AC-breakdown voltage applied and the maximum electric strength of the CF3 I, N2 , CO2 gas mixtures as the gap changes. From **Figure 7**, it can be seen that the breakdown voltage of CF<sup>3</sup> I gas mixtures gets higher as the electrodes gap gets bigger, but curves of different gas mixtures are more approached even closer compared with SF<sup>6</sup> and c-C4 F8 . The breakdown voltage of CF<sup>3</sup> I gas mixtures has little difference with different contents of N2 and CO2 . Moreover, N<sup>2</sup> , which has better insulation strength, does not perform better than CO<sup>2</sup> when it is mixed with CF<sup>3</sup> I. In **Figure 8**, the maximum electric strength of CF3 I gas mixtures has a trend to reduce as the electrodes gap increases, but the curves are smoother than c-C4 F8 , which shows that the sensitivity to the electric non-uniformity of CF<sup>3</sup> I is lower than c-C<sup>4</sup> F8 .

**Figure 9** shows, under different gaps of electrode, the variety of the AC-breakdown voltage for

I, N2 , CO2

gas mixtures as the gas pressure changes. Similar to the gas mixtures of SF6

gas mixtures with different gas pressures.

Development Prospect of Gas Insulation Based on Environmental Protection

http://dx.doi.org/10.5772/intechopen.77035

gas mixtures with different electrodes gap distances.

, the AC-breakdown voltage increases linearly as the air pressure increases, and without

and

91

CF3 I, N2

c-C4 F8 , CO2

**Figure 8.** Maximum electric strength of CF<sup>3</sup>

**Figure 9.** AC-breakdown voltage of CF<sup>3</sup>

I, N2 , CO2

**Figure 7.** AC-breakdown voltage of CF<sup>3</sup> I, N2 , CO2 gas mixtures with different gas pressures.

Development Prospect of Gas Insulation Based on Environmental Protection http://dx.doi.org/10.5772/intechopen.77035 91

**Figure 8.** Maximum electric strength of CF<sup>3</sup> I, N2 , CO2 gas mixtures with different gas pressures.

immediately, and even it has a trend to reduce a little. Only after the content of N<sup>2</sup>

I, it has been measured the breakdown characteristics for a constant content of 10%

gas mixtures as the gap changes. From **Figure 7**, it can be seen that the

, which shows that the sensitivity to the electric non-uniformity of CF<sup>3</sup>

gas mixtures with different gas pressures.

I gas mixtures gets higher as the electrodes gap gets bigger, but

I gas mixtures has little difference with different con-

I. In **Figure 8**, the maximum electric strength of

, which has better insulation strength, does not perform

and CO2

tests. The test method and experiment setup are similar to that in Section 2. The gas mixtures and mixing ratio are listed in **Table 1**. **Figures 7** and **8** show that under different air pressures, the variety of the AC-breakdown voltage applied and the maximum electric strength

curves of different gas mixtures are more approached even closer compared with SF<sup>6</sup>

I gas mixtures has a trend to reduce as the electrodes gap increases, but the curves are

**4. Power frequency AC-breakdown characteristics of the CF<sup>3</sup>**

90 Simulation and Modelling of Electrical Insulation Weaknesses in Electrical Equipment

is higher than 60%, the breakdown voltage can increase

than 30% and the content of CO<sup>2</sup>

I and with different concentrations of N<sup>2</sup>

. The breakdown voltage of CF<sup>3</sup>

F8

F8 .

. Moreover, N<sup>2</sup>

when it is mixed with CF<sup>3</sup>

 **gas mixtures**

I, N2

breakdown voltage of CF<sup>3</sup>

and CO2

**Figure 7.** AC-breakdown voltage of CF<sup>3</sup>

I, N2 , CO2

, CO2

significantly.

**CO2**

To CF3

of the CF3

tents of N2

better than CO<sup>2</sup>

smoother than c-C4

is lower than c-C<sup>4</sup>

CF3

c-C4 F8

CF3

is lower

and

I

**I, N2 ,** 

under AC-voltage applied during the

**Figure 9** shows, under different gaps of electrode, the variety of the AC-breakdown voltage for CF3 I, N2 , CO2 gas mixtures as the gas pressure changes. Similar to the gas mixtures of SF6 and c-C4 F8 , the AC-breakdown voltage increases linearly as the air pressure increases, and without

**Figure 9.** AC-breakdown voltage of CF<sup>3</sup> I, N2 , CO2 gas mixtures with different electrodes gap distances.

**Figure 10.** Relationship between AC-breakdown voltage and mixing contents of CF<sup>3</sup> I, N2 , CO2 gas mixtures.

hump effect or trend of saturation. Curves in **Figure 9** are similar to these in **Figure 7**, the superposition of the curves of gas mixtures with different contents is very high and the performed insulation characteristics are little different.

**6. Lightning impulse characteristics of c-C<sup>4</sup>**

with N<sup>2</sup>

**Figure 12.** Positive lightning impulse breakdown voltage of c-C<sup>4</sup>

and CO2

**Figure 11.** Relationship between power frequency breakdown voltage and mixture ratio of C<sup>3</sup>

is more appropriate to be mixed with CO<sup>2</sup>

is higher than N2

F8

mixtures of 10% c-C<sup>4</sup>

F8

breakdown voltage of CO<sup>2</sup>

eters, c-C4

**F8 , N2**

**Figures 12** and **13** show the testing curves of the positive lightning impulse voltage of gas

tures, and the breakdown voltage increases nearly linearly as the air pressure increases. From the perspective of the excitation energy and the ionisation energy of the microcosmic param-

> F8 , N2 , CO2

the electrodes gap increases without the performance of the trend to saturation in SF<sup>6</sup>

**, CO2**

Development Prospect of Gas Insulation Based on Environmental Protection

. The positive lightning impulse voltage increases as

. According with **Figures 12** and **13**, it can be

 **gas mixtures**

F7 CN/CO2 .

http://dx.doi.org/10.5772/intechopen.77035

93

and the positive lightning impulse

gas mixtures with different gas pressures.

gas mix-

**Figure 10** shows that under different gas pressures, the curves of the variety of the AC-breakdown voltage for CF<sup>3</sup> I, N2 , CO2 gas mixtures changes as the content changes. Generally, with the same mixing ratio of CF3I, the breakdown strength becomes stronger with the increasing ratio of CO<sup>2</sup> . The same as the judge of the foregoing, the change of the gas mixtures of CF3 I is not obvious as the contents of N2 and CO2 change. What is worthy to be concentrated, it is that N2 has higher insulation strength than CO2 , but it does not perform in the CF3 I gas mixtures.

#### **5. Power frequency AC-breakdown characteristics of C<sup>3</sup> F7 CN/CO2**

AC-breakdown characteristics of C<sup>4</sup> F7 CN mixed with CO<sup>2</sup> are tested for different concentrations. **Figure 11** shows that AC-breakdown voltage of C<sup>3</sup> F7 CN/CO2 gas mixtures varies as the mixture ratio changes between 0 and 10% under different air pressures. Under the same gas pressure, as the mixture ratio of C4 F7 CN *k* increases, the AC-breakdown voltage of gas mixtures shows the saturated trend to increase. The lower the gas pressure is, the smaller the growth is. It has to be said that the influence of the mixture ratio *k* on the C3 F7 CN/CO2 gas mixtures is less under low gas pressure. In addition, under high-gas pressure, increasing the mixture ratio k can increase the insulation properties of the gas mixtures. When the proportion of C3 F7 CN increases to 20%, the insulation properties of C<sup>3</sup> F7 CN/CO2 gas mixtures can approach that of pure SF6 under the same condition.

Development Prospect of Gas Insulation Based on Environmental Protection http://dx.doi.org/10.5772/intechopen.77035 93

**Figure 11.** Relationship between power frequency breakdown voltage and mixture ratio of C<sup>3</sup> F7 CN/CO2 .

#### **6. Lightning impulse characteristics of c-C<sup>4</sup> F8 , N2 , CO2 gas mixtures**

hump effect or trend of saturation. Curves in **Figure 9** are similar to these in **Figure 7**, the superposition of the curves of gas mixtures with different contents is very high and the per-

**Figure 10** shows that under different gas pressures, the curves of the variety of the

Generally, with the same mixing ratio of CF3I, the breakdown strength becomes stronger

CN mixed with CO<sup>2</sup>

the mixture ratio changes between 0 and 10% under different air pressures. Under the same

mixtures shows the saturated trend to increase. The lower the gas pressure is, the smaller the

mixtures is less under low gas pressure. In addition, under high-gas pressure, increasing the mixture ratio k can increase the insulation properties of the gas mixtures. When the propor-

gas mixtures changes as the content changes.

I, N2 , CO2

**F7**

**CN/CO2**

are tested for different concentra-

change. What is worthy

gas mixtures.

gas mixtures varies as

F7

CN/CO2

gas mixtures can

gas

, but it does not

. The same as the judge of the foregoing, the change of the

and CO2

F7

CN/CO2

F7

CN/CO2

CN *k* increases, the AC-breakdown voltage of gas

has higher insulation strength than CO2

formed insulation characteristics are little different.

I gas mixtures.

tions. **Figure 11** shows that AC-breakdown voltage of C<sup>3</sup>

I, N2

**Figure 10.** Relationship between AC-breakdown voltage and mixing contents of CF<sup>3</sup>

92 Simulation and Modelling of Electrical Insulation Weaknesses in Electrical Equipment

**5. Power frequency AC-breakdown characteristics of C<sup>3</sup>**

F7

F7

growth is. It has to be said that the influence of the mixture ratio *k* on the C3

CN increases to 20%, the insulation properties of C<sup>3</sup>

under the same condition.

, CO2

I is not obvious as the contents of N2

AC-breakdown voltage for CF<sup>3</sup>

with the increasing ratio of CO<sup>2</sup>

to be concentrated, it is that N2

AC-breakdown characteristics of C<sup>4</sup>

gas pressure, as the mixture ratio of C4

gas mixtures of CF3

perform in the CF3

tion of C3

F7

approach that of pure SF6

**Figures 12** and **13** show the testing curves of the positive lightning impulse voltage of gas mixtures of 10% c-C<sup>4</sup> F8 with N<sup>2</sup> and CO2 . The positive lightning impulse voltage increases as the electrodes gap increases without the performance of the trend to saturation in SF<sup>6</sup> gas mixtures, and the breakdown voltage increases nearly linearly as the air pressure increases. From the perspective of the excitation energy and the ionisation energy of the microcosmic parameters, c-C4 F8 is more appropriate to be mixed with CO<sup>2</sup> and the positive lightning impulse breakdown voltage of CO<sup>2</sup> is higher than N2 . According with **Figures 12** and **13**, it can be

**Figure 12.** Positive lightning impulse breakdown voltage of c-C<sup>4</sup> F8 , N2 , CO2 gas mixtures with different gas pressures.

of CO2

N2

C4

N2

F8 + 90%CO<sup>2</sup>

down voltage of 10% CF<sup>3</sup>

impulse voltage of CF3

changing trend of c-C4

the gas mixtures will have negative impact on CO<sup>2</sup>

, it is not hard to find that 10%c-C<sup>4</sup>

**7. Lightning impulse characteristics of the CF3**

different contents and ratios, it can be seen that CF<sup>3</sup>

curves in **Figure 17** have the same change with the c-C<sup>4</sup>

gas mixtures.

F8

**Figure 15.** Positive lightning impulse breakdown voltage of CF<sup>3</sup>

it is more appropriate to mix with CO<sup>2</sup>

gas mixtures consisting of 10% CF<sup>3</sup>

I with N<sup>2</sup>

trend to increase as the content of CO2

lightning impulse breakdown voltage.

in the gas mixtures increases. Because of the high resonance excitation, energy of N2

increase of breakdown voltage of the gas mixtures is not obvious, and when the content of

**Figures 15** and **16** show the curves of the positive lightning impulse (means that the impulse voltage is applied to sphere electrode, and the plane electrode is connected to ground) break-

and air pressure increase. From the difference of breakdown voltages of gas mixtures with

**Figure 17** shows the variation of the positive lightning impulse breakdown voltage of the

the breakdown voltage of the gas mixtures increases obviously and this is the same with the

as well as CO<sup>2</sup>

I, N2 , CO2

and CO2

.

I and N2

is lower than 30%, the excitation energy can weaken the ionisation of CF<sup>3</sup>

is lower than 30%, the positive lightning impulse breakdown voltage shows more obvious

increases. Comparing 10%c-C<sup>4</sup>

F8 + 90%CO<sup>2</sup>

when the content of N<sup>2</sup>

Development Prospect of Gas Insulation Based on Environmental Protection

**I, N2**

I gas mixtures increases with a little saturation as the electrodes gap

F8

**, CO2**

of different contents. The positive lightning

I has the similar properties with c-C<sup>4</sup>

in

95

exceeds 30%. The

and 10%c-

F8 and

, and

F8 + 90%N<sup>2</sup>

http://dx.doi.org/10.5772/intechopen.77035

has obviously higher positive

 **gas mixtures**

as the mixture ratio changes. The

gas mixtures, when the content of

gas mixtures with different gas pressures.

I and CO2

**Figure 13.** Positive lightning impulse breakdown voltage of c-C<sup>4</sup> F8 , N2 , CO2 gas mixtures with different electrodes gap distances.

seen that 10%c-C<sup>4</sup> F8 + 90%CO<sup>2</sup> gas mixtures have the highest breakdown voltage and 10%c-C4 F8 + 90%N<sup>2</sup> gas mixtures have the lowest breakdown voltage.

**Figure 14** shows the different curves of positive lightning impulse breakdown voltage of the gas mixtures of 10%c-C<sup>4</sup> F8 with N<sup>2</sup> and CO2 as the content of N2 and CO2 changes. Because of CO2 itself has stronger ability to tolerate positive lightning impulse and it will not have obvious ionisation with c-C<sup>4</sup> F8 compared with N<sup>2</sup> , the breakdown voltage increases as the content

**Figure 14.** Relationship between positive lightning impulse breakdown voltage and mixing contents of c-C<sup>4</sup> F8 , N2 , CO2 gas mixtures.

of CO2 in the gas mixtures increases. Because of the high resonance excitation, energy of N2 in the gas mixtures will have negative impact on CO<sup>2</sup> when the content of N<sup>2</sup> exceeds 30%. The increase of breakdown voltage of the gas mixtures is not obvious, and when the content of N2 is lower than 30%, the positive lightning impulse breakdown voltage shows more obvious trend to increase as the content of CO2 increases. Comparing 10%c-C<sup>4</sup> F8 + 90%N<sup>2</sup> and 10%c-C4 F8 + 90%CO<sup>2</sup> , it is not hard to find that 10%c-C<sup>4</sup> F8 + 90%CO<sup>2</sup> has obviously higher positive lightning impulse breakdown voltage.

#### **7. Lightning impulse characteristics of the CF3 I, N2 , CO2 gas mixtures**

**Figures 15** and **16** show the curves of the positive lightning impulse (means that the impulse voltage is applied to sphere electrode, and the plane electrode is connected to ground) breakdown voltage of 10% CF<sup>3</sup> I with N<sup>2</sup> and CO2 of different contents. The positive lightning impulse voltage of CF3 I gas mixtures increases with a little saturation as the electrodes gap and air pressure increase. From the difference of breakdown voltages of gas mixtures with different contents and ratios, it can be seen that CF<sup>3</sup> I has the similar properties with c-C<sup>4</sup> F8 and it is more appropriate to mix with CO<sup>2</sup> .

**Figure 17** shows the variation of the positive lightning impulse breakdown voltage of the gas mixtures consisting of 10% CF<sup>3</sup> I and N2 as well as CO<sup>2</sup> as the mixture ratio changes. The curves in **Figure 17** have the same change with the c-C<sup>4</sup> F8 gas mixtures, when the content of N2 is lower than 30%, the excitation energy can weaken the ionisation of CF<sup>3</sup> I and CO2 , and the breakdown voltage of the gas mixtures increases obviously and this is the same with the changing trend of c-C4 F8 gas mixtures.

seen that 10%c-C<sup>4</sup>

gas mixtures of 10%c-C<sup>4</sup>

ous ionisation with c-C<sup>4</sup>

F8 + 90%N<sup>2</sup>

C4

distances.

CO2

gas mixtures.

F8 + 90%CO<sup>2</sup>

**Figure 13.** Positive lightning impulse breakdown voltage of c-C<sup>4</sup>

F8

F8

with N<sup>2</sup>

94 Simulation and Modelling of Electrical Insulation Weaknesses in Electrical Equipment

gas mixtures have the lowest breakdown voltage.

and CO2

**Figure 14.** Relationship between positive lightning impulse breakdown voltage and mixing contents of c-C<sup>4</sup>

compared with N<sup>2</sup>

**Figure 14** shows the different curves of positive lightning impulse breakdown voltage of the

itself has stronger ability to tolerate positive lightning impulse and it will not have obvi-

as the content of N2

F8 , N2 , CO2

gas mixtures have the highest breakdown voltage and 10%c-

and CO2

, the breakdown voltage increases as the content

changes. Because of

F8 , N2 , CO2

gas mixtures with different electrodes gap

**Figure 15.** Positive lightning impulse breakdown voltage of CF<sup>3</sup> I, N2 , CO2 gas mixtures with different gas pressures.

**8. Conclusion**

/N2

/N2

ing strength of CF3

rent SF6

of c-C4 F8

and SF6

C-GIS.

feasible.

insulation.

uefaction temperature, CF3

I and N2

CF3

**1.** In the consideration of insulation strength, c-C4

gas mixtures and pure SF6

which is full with gas at 0.1–0.3MPa. Therefore, c-C<sup>4</sup>

voltage power transmission wire are promising to use c-C<sup>4</sup>

I is higher than that of SF6

mixed gas can be used as replacement of SF6

as insulation media.

Switchgear at middle and low voltage (C-GIS).

cost, especially in 30% proportion of CF<sup>3</sup>

As environmentally friendly insulation gas, CF3

**3.** Power-frequency breakdown voltage of C<sup>3</sup>

global scope for gas insulating systems. The application of CF3

Compared with compressed air or compressed N<sup>2</sup>

F8

gas tending to liquefaction and carbon decomposition. Traditional c-GIS is widely

F8

. Moreover, c-C<sup>4</sup>

used in the range of middle voltage, mainly in electric power substation and among consumers. Vacuum circuit breaker and grounded switchgear are both installed in a gas cavity shell,

switchgear of relative low voltage whose working pressure is low and function is not to break current arc, which can not only guarantee the insulation strength, but also greatly reduce the effect of insulation gas on the environment. Therefore, it has a good potential to substitute SF<sup>6</sup>

Moreover, for the areas with warm climate, electric apparatus such as transformer and high

forming gas insulation transformer (GIT), gas insulation line (GIL) and cabinet Gas Insulated

**2.** Above comprehensive of analysis, under the same pressure conditions, the insulat-

pressure, in order to reduce the sealing technology and easy to manufacture. The shortcomings of high price also can be relief after mixed with buffer gas. Therefore, using CF<sup>3</sup>

as insulating gas in C-GIS has better comprehensive performance than that of the present

sure, which has bigger advantage on the dielectric strength, liquefaction temperature and

voltage apparatus not only meets the requirements and current trends on environmental protection in the international community, but also is a new direction in the field of electrical

To sum up, taking into account environmental characteristics, insulating properties and liq-

F7

increase of mixing ratio from 0 to 10%. The relative dielectric strength of the gas mixtures showed a trend of saturated growth with the increase of mixing ratio, and power-frequency

CN/CO2

voltage system as well as GIL, GIT and other electrical devices in high-voltage system.

gas mixtures with N<sup>2</sup>

Development Prospect of Gas Insulation Based on Environmental Protection

F8

while ensuring CF<sup>3</sup>

I gas mixtures can be applied prior to C-GIS in the middle, low

insulated in C-GIS, CF<sup>3</sup>

I in mixed gases, that is the most likely to be

I and its gas mixtures is a hot-topic on the

F8

, CO2

gas mixtures can be applied to the gas

gas mixtures as insulation media

gas in the C-GIS at a low pres-

I and its gas mixtures in high-

gas mixtures increases with the

I not to be liquefied.

I can lower the

I

gas mixtures can solves the problem

http://dx.doi.org/10.5772/intechopen.77035

is prior than cur-

97

**Figure 16.** Positive lightning impulse breakdown voltage of CF<sup>3</sup> I, N2 , CO2 gas mixtures with different electrodes gap distances.

**Figure 17.** Relationship between positive lightning impulse breakdown voltage and mixing contents of CF<sup>3</sup> I, N2 , CO2 gas mixtures.

## **8. Conclusion**

**Figure 17.** Relationship between positive lightning impulse breakdown voltage and mixing contents of CF<sup>3</sup>

I, N2 , CO2

gas mixtures with different electrodes gap

**Figure 16.** Positive lightning impulse breakdown voltage of CF<sup>3</sup>

96 Simulation and Modelling of Electrical Insulation Weaknesses in Electrical Equipment

gas mixtures.

distances.

I, N2 , CO2 **1.** In the consideration of insulation strength, c-C4 F8 gas mixtures with N<sup>2</sup> , CO2 is prior than current SF6 /N2 gas mixtures and pure SF6 . Moreover, c-C<sup>4</sup> F8 gas mixtures can solves the problem of c-C4 F8 gas tending to liquefaction and carbon decomposition. Traditional c-GIS is widely used in the range of middle voltage, mainly in electric power substation and among consumers. Vacuum circuit breaker and grounded switchgear are both installed in a gas cavity shell, which is full with gas at 0.1–0.3MPa. Therefore, c-C<sup>4</sup> F8 gas mixtures can be applied to the gas switchgear of relative low voltage whose working pressure is low and function is not to break current arc, which can not only guarantee the insulation strength, but also greatly reduce the effect of insulation gas on the environment. Therefore, it has a good potential to substitute SF<sup>6</sup> and SF6 /N2 as insulation media.

Moreover, for the areas with warm climate, electric apparatus such as transformer and high voltage power transmission wire are promising to use c-C<sup>4</sup> F8 gas mixtures as insulation media forming gas insulation transformer (GIT), gas insulation line (GIL) and cabinet Gas Insulated Switchgear at middle and low voltage (C-GIS).

**2.** Above comprehensive of analysis, under the same pressure conditions, the insulating strength of CF3 I is higher than that of SF6 while ensuring CF<sup>3</sup> I not to be liquefied. Compared with compressed air or compressed N<sup>2</sup> insulated in C-GIS, CF<sup>3</sup> I can lower the pressure, in order to reduce the sealing technology and easy to manufacture. The shortcomings of high price also can be relief after mixed with buffer gas. Therefore, using CF<sup>3</sup> I as insulating gas in C-GIS has better comprehensive performance than that of the present C-GIS.

CF3 I and N2 mixed gas can be used as replacement of SF6 gas in the C-GIS at a low pressure, which has bigger advantage on the dielectric strength, liquefaction temperature and cost, especially in 30% proportion of CF<sup>3</sup> I in mixed gases, that is the most likely to be feasible.

As environmentally friendly insulation gas, CF3 I and its gas mixtures is a hot-topic on the global scope for gas insulating systems. The application of CF3 I and its gas mixtures in highvoltage apparatus not only meets the requirements and current trends on environmental protection in the international community, but also is a new direction in the field of electrical insulation.

To sum up, taking into account environmental characteristics, insulating properties and liquefaction temperature, CF3 I gas mixtures can be applied prior to C-GIS in the middle, low voltage system as well as GIL, GIT and other electrical devices in high-voltage system.

**3.** Power-frequency breakdown voltage of C<sup>3</sup> F7 CN/CO2 gas mixtures increases with the increase of mixing ratio from 0 to 10%. The relative dielectric strength of the gas mixtures showed a trend of saturated growth with the increase of mixing ratio, and power-frequency breakdown voltage of C<sup>3</sup> F7 CN/CO2 gas mixtures when C<sup>3</sup> F7 CN is 8% ratio can reach 75% of that of pure SF6 under the same condition. C3 F7 CN/CO2 gas mixtures have potential of application of substitute for SF6 in the electric power equipment, and the insulation of the other characteristics need further study. A deep insight into the partial discharge properties and corona stabilisation behaviour under strong inhomogeneous fields is needed for a full understanding.

#### **Acknowledgements**

This work is supported by the National Natural Science Foundation of China (Grant No. 51337006).

**Author details**

**Figure A2.** Maximum electric strength of SF<sup>6</sup>

Dengming Xiao

**References**

University; 2016

Address all correspondence to: dmxiao@sjtu.edu.cn

[1] Yunkun D. Basic research of the environmentally friendly insulating gas CF<sup>3</sup>

assessment of the Kyoto protocol. Nature. 1999;**401**:3466-3469

, N2 , CO2

application in electric power apparatus [PhD thesis]. Shanghai: Shanghai Jiao Tong

gas mixtures with different gas pressures.

Development Prospect of Gas Insulation Based on Environmental Protection

http://dx.doi.org/10.5772/intechopen.77035

99

[2] Reilly J, Prinn R, Harnisch J, Fitzmaurice J, Jacoby H, Kicklighter D, et al. Multi-gas

I for its

Shanghai Jiao Tong University, Shanghai, China

### **Appendix**

**Figure A1.** AC breakdown voltage of SF<sup>6</sup> , N2 , CO2 gas mixtures with different gas pressures.

Development Prospect of Gas Insulation Based on Environmental Protection http://dx.doi.org/10.5772/intechopen.77035 99

**Figure A2.** Maximum electric strength of SF<sup>6</sup> , N2 , CO2 gas mixtures with different gas pressures.

#### **Author details**

breakdown voltage of C<sup>3</sup>

**Acknowledgements**

**Figure A1.** AC breakdown voltage of SF<sup>6</sup>

, N2 , CO2

gas mixtures with different gas pressures.

application of substitute for SF6

of that of pure SF6

understanding.

51337006).

**Appendix**

F7

CN/CO2

98 Simulation and Modelling of Electrical Insulation Weaknesses in Electrical Equipment

under the same condition. C3

gas mixtures when C<sup>3</sup>

other characteristics need further study. A deep insight into the partial discharge properties and corona stabilisation behaviour under strong inhomogeneous fields is needed for a full

This work is supported by the National Natural Science Foundation of China (Grant No.

F7

F7

in the electric power equipment, and the insulation of the

CN/CO2

CN is 8% ratio can reach 75%

gas mixtures have potential of

Dengming Xiao

Address all correspondence to: dmxiao@sjtu.edu.cn

Shanghai Jiao Tong University, Shanghai, China

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[19] Macko WMJ. Toxicity review for Iodotrifluoromethane (CF<sup>3</sup>

. Journal of Chemical Physics. 1999;**110**:3811-3822

gas mixtures by the Monte Carlo method. Journal of Physics D: Applied

IRE Transactions on Aeronautical & Navigational Electronics. 2012;**2**:111-114

ernance on climate. Progress in Climate Change Research. 2016;**12**:61-67

[8] Dengming Xiao. Gas Discharge and Gas Insulation. China: Springer; 2016

Change 2017. Beijing: China Government; 2017

100 Simulation and Modelling of Electrical Insulation Weaknesses in Electrical Equipment

Washington: World Resources Institute; 1995

Awards for Science and Technology. 2016:6-6

c-C4 F8 /CO2

of CF4 , C2 F6

81-86

c-C4 F8

and N2

Physics. 2006;**39**:4204

Patents; 2013

Physics. 2008;**41**:015206

, and C3

F8

Journal of Physics D: Applied Physics. 2000;**33**:L145

Chemical Reference Data. 2001;**30**:449-473

Insulation Conference; 2016. pp. 531-534

Technical Working Conference. US. 1999


[34] Switzerland: ABB achieves breakthrough in switchgear technology with eco-efficient insulation gas. Tendersinfo News. 2014

**Chapter 5**

**Provisional chapter**

**Typical Internal Defects of Gas-Insulated Switchgear**

**Typical Internal Defects of Gas-Insulated Switchgear** 

Gas-insulated switchgear (GIS) is a common electrical equipment, which uses sulfur

and flexibility. However, GIS may have internal defects and partial discharge (PD) is then induced. PD will cause great harm to GIS and power system. Therefore, it is of great importance to study the intrinsic characteristics and detection of PD for online monitoring. In this chapter, typical internal defects of GIS and the PD characteristics are discussed. Several detection methods are also presented in this chapter including electro-

**Keywords:** GIS, internal defects, PD, intrinsic characteristics, electromagnetic detection

Gas-insulated switchgear (GIS) is an electrical equipment that conceals traditional electrical devices in a chamber. GIS has obvious advantages over traditional air-insulated switchgear (AIS). Firstly, GIS demands less area thus reducing the cost; secondly, GIS has a longer overhaul period; and finally, GIS has higher reliability. For these reasons, GIS has been widely used in the world nowadays [1–4]. However, GIS has a complex structure that internal defects may come into being during process of manufacturing, transferring, and installing [5, 6]. These defects will induce partial discharge (PD) [7–9], which causes potential internal insulation aging. The insulation aging may develop into serious fault and blackout [10, 11]. PD also reflects GIS insulation state. By monitoring PD signals, potential defects can be recognized.

) as insulating medium instead of traditional air. It has good reliability

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

© 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use,

distribution, and reproduction in any medium, provided the original work is properly cited.

DOI: 10.5772/intechopen.79090

**and Partial Discharge Characteristics**

Fuping Zeng, Ju Tang, Xiaoxing Zhang, Siyuan Zhou

magnetic method, chemical method, and optical method.

method, chemical detection method, optical detection method

**and Partial Discharge Characteristics**

Fuping Zeng, Ju Tang, Xiaoxing Zhang,

Additional information is available at the end of the chapter

Additional information is available at the end of the chapter

Siyuan Zhou and Cheng Pan

and Cheng Pan

**Abstract**

**1. Introduction**

hexafluoride (SF<sup>6</sup>

http://dx.doi.org/10.5772/intechopen.79090


#### **Typical Internal Defects of Gas-Insulated Switchgear and Partial Discharge Characteristics Typical Internal Defects of Gas-Insulated Switchgear and Partial Discharge Characteristics**

DOI: 10.5772/intechopen.79090

Fuping Zeng, Ju Tang, Xiaoxing Zhang, Siyuan Zhou and Cheng Pan Fuping Zeng, Ju Tang, Xiaoxing Zhang, Siyuan Zhou and Cheng Pan

Additional information is available at the end of the chapter Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/intechopen.79090

#### **Abstract**

[34] Switzerland: ABB achieves breakthrough in switchgear technology with eco-efficient

[35] Rabie M, Franck CM. Assessment of eco-friendly gases for electrical insulation to replace

fluoroketone for high voltage applications. IEEE Transactions on Dielectrics & Electrical

insulator under standard lightning impulse. Presented at the IEEE Electrical Insulation

. Environmental Science & Technology.

F8 /N2 and C<sup>5</sup>

with and without

F10O per-

insulation gas. Tendersinfo News. 2014

Insulation. 2017;**24**:2712-2721

2017:369-380

Conference; 2016

the most potent industrial greenhouse gas SF6

102 Simulation and Modelling of Electrical Insulation Weaknesses in Electrical Equipment

[36] Stoller PC, Doiron CB, Tehlar D, Simka P, Ranjan N. Mixtures of CO<sup>2</sup>

[37] Zhao S, Xiao D, Jiao J, Zhao X. Discharge characteristics of c-C<sup>4</sup>

Gas-insulated switchgear (GIS) is a common electrical equipment, which uses sulfur hexafluoride (SF<sup>6</sup> ) as insulating medium instead of traditional air. It has good reliability and flexibility. However, GIS may have internal defects and partial discharge (PD) is then induced. PD will cause great harm to GIS and power system. Therefore, it is of great importance to study the intrinsic characteristics and detection of PD for online monitoring. In this chapter, typical internal defects of GIS and the PD characteristics are discussed. Several detection methods are also presented in this chapter including electromagnetic method, chemical method, and optical method.

**Keywords:** GIS, internal defects, PD, intrinsic characteristics, electromagnetic detection method, chemical detection method, optical detection method

#### **1. Introduction**

Gas-insulated switchgear (GIS) is an electrical equipment that conceals traditional electrical devices in a chamber. GIS has obvious advantages over traditional air-insulated switchgear (AIS). Firstly, GIS demands less area thus reducing the cost; secondly, GIS has a longer overhaul period; and finally, GIS has higher reliability. For these reasons, GIS has been widely used in the world nowadays [1–4]. However, GIS has a complex structure that internal defects may come into being during process of manufacturing, transferring, and installing [5, 6]. These defects will induce partial discharge (PD) [7–9], which causes potential internal insulation aging. The insulation aging may develop into serious fault and blackout [10, 11]. PD also reflects GIS insulation state. By monitoring PD signals, potential defects can be recognized.

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

#### **1.1. Typical types of GIS internal defects**

There are several types of GIS internal defects, namely high-voltage (HV) conductor protrusions, free metal particles, floating potential, insulator metal pollution, and insulator gap [7]. The various defects in GIS are shown in **Figure 1**.

In recent decades, GIS has also been deployed widely in China. However, operating experience shows that although GIS equipment has high reliability, inevitable internal defects will still lead to failure and gradually major accidents. This has become a hot topic in power system [12].

According to statistics, the State Grid Corporation of China had a total of 48,498 GIS equipment in operation by the end of 2013, with a growth of 17.8% of the previous year. In the same year, 11 trips occurred in the GIS operation of the national grid system in China. CIGRE 23.10 Working Group GIS Fault Investigation Report shows that in all failures of GIS that occurred before 1985, the insulation failure accounted for 60 and 51% after 1985. According to operation analysis of the State Grid of China, at the end of June in 2008, 33 GIS accidents occurred including 24 insulation accidents, while operation failure occurred 74 times including 13 insulation accidents [4]. GIS insulation failure accidents are diverse. According to **Figure 2**, insulation faults caused by bad contact and defects of metal particles occupy a larger proportion [12].

In this chapter, we will focus on four typical types of GIS internal insulation defect, that is, free-metal particles, conductor protrusions, insulator gap, and insulator metal pollution.

*1.1.2. Metal protrusions defect (denoted as N-type defect)*

**Figure 2.** GIS equipment-defect type statistics.

under some overvoltage, it may cause breakdown and GIS fault.

for a long-time and damage the operation insulation of GIS [12].

*1.1.3. Insulator gap defect (denoted as G-type defect)*

and serious insulation breakdown may follow;

the insulating properties of the basin-type insulator [13].

Metal protrusion defects refer to the defects that form on the protruding parts such as HV conductor inside GIS. Just like free-metal particles defect, these protrusions are usually formed during process of assembling, installation, or operation. Due to the sharp tip of the protrusions, the electric field will be distorted and strong electric field will then come into being. Under the rated working voltage, the strong electric field will induce a stable PD, but

Typical Internal Defects of Gas-Insulated Switchgear and Partial Discharge Characteristics

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105

The discharge characteristics of protrusion on HV conductor and that on inner wall of shell are different. Protrusions on the HV conductor usually discharge in the negative half-cycle of the power frequency, while protrusions on the inner wall of the shell usually discharge in the positive half-cycle of the power frequency. Some tiny protrusions will be ablated in long-term discharge and will not threaten the insulation of GIS. However, larger protrusions will persist

Insulator gap defects in GIS mainly happen on the basin-type insulator, which can be classified into two types. One type is due to internal bubbles of epoxy resin resulting during process of manufacturing. Then during operation, PD will take place in these bubbles under strong electric field, resulting in gradual insulation deterioration of the basin-type insulator,

The other type is due to electric force in the long-term operation. Mechanical vibration process may result in connection loosening of basin-type insulator and HV conductive rod connection loosening. Then an insulator gap defect forms and induces PD, resulting in deterioration of

#### *1.1.1. Free metal particles defect (denoted as P-type defect)*

Free-metal particles defects in GIS are one of the main causes of insulation failure. During the GIS assembling, installation or operation process, its metal parts may rub against each other, thus creating free metal particles. Due to their small size, these metal particles will move and beat under the electric field forces. If the range of particle movement is large enough, it is possible to form conductive paths or arc passages between the HV conductor and the shell, causing serious damage to the GIS. The path forming depends on many factors including applied voltage, shape and size of particles, and the position of the particles [12].

**Figure 1.** GIS internal insulation-defect type diagram.

**Figure 2.** GIS equipment-defect type statistics.

**1.1. Typical types of GIS internal defects**

system [12].

The various defects in GIS are shown in **Figure 1**.

104 Simulation and Modelling of Electrical Insulation Weaknesses in Electrical Equipment

*1.1.1. Free metal particles defect (denoted as P-type defect)*

**Figure 1.** GIS internal insulation-defect type diagram.

There are several types of GIS internal defects, namely high-voltage (HV) conductor protrusions, free metal particles, floating potential, insulator metal pollution, and insulator gap [7].

In recent decades, GIS has also been deployed widely in China. However, operating experience shows that although GIS equipment has high reliability, inevitable internal defects will still lead to failure and gradually major accidents. This has become a hot topic in power

According to statistics, the State Grid Corporation of China had a total of 48,498 GIS equipment in operation by the end of 2013, with a growth of 17.8% of the previous year. In the same year, 11 trips occurred in the GIS operation of the national grid system in China. CIGRE 23.10 Working Group GIS Fault Investigation Report shows that in all failures of GIS that occurred before 1985, the insulation failure accounted for 60 and 51% after 1985. According to operation analysis of the State Grid of China, at the end of June in 2008, 33 GIS accidents occurred including 24 insulation accidents, while operation failure occurred 74 times including 13 insulation accidents [4]. GIS insulation failure accidents are diverse. According to **Figure 2**, insulation faults caused by bad contact and defects of metal particles occupy a larger proportion [12].

In this chapter, we will focus on four typical types of GIS internal insulation defect, that is, free-metal particles, conductor protrusions, insulator gap, and insulator metal pollution.

Free-metal particles defects in GIS are one of the main causes of insulation failure. During the GIS assembling, installation or operation process, its metal parts may rub against each other, thus creating free metal particles. Due to their small size, these metal particles will move and beat under the electric field forces. If the range of particle movement is large enough, it is possible to form conductive paths or arc passages between the HV conductor and the shell, causing serious damage to the GIS. The path forming depends on many factors including

applied voltage, shape and size of particles, and the position of the particles [12].

#### *1.1.2. Metal protrusions defect (denoted as N-type defect)*

Metal protrusion defects refer to the defects that form on the protruding parts such as HV conductor inside GIS. Just like free-metal particles defect, these protrusions are usually formed during process of assembling, installation, or operation. Due to the sharp tip of the protrusions, the electric field will be distorted and strong electric field will then come into being. Under the rated working voltage, the strong electric field will induce a stable PD, but under some overvoltage, it may cause breakdown and GIS fault.

The discharge characteristics of protrusion on HV conductor and that on inner wall of shell are different. Protrusions on the HV conductor usually discharge in the negative half-cycle of the power frequency, while protrusions on the inner wall of the shell usually discharge in the positive half-cycle of the power frequency. Some tiny protrusions will be ablated in long-term discharge and will not threaten the insulation of GIS. However, larger protrusions will persist for a long-time and damage the operation insulation of GIS [12].

#### *1.1.3. Insulator gap defect (denoted as G-type defect)*

Insulator gap defects in GIS mainly happen on the basin-type insulator, which can be classified into two types. One type is due to internal bubbles of epoxy resin resulting during process of manufacturing. Then during operation, PD will take place in these bubbles under strong electric field, resulting in gradual insulation deterioration of the basin-type insulator, and serious insulation breakdown may follow;

The other type is due to electric force in the long-term operation. Mechanical vibration process may result in connection loosening of basin-type insulator and HV conductive rod connection loosening. Then an insulator gap defect forms and induces PD, resulting in deterioration of the insulating properties of the basin-type insulator [13].

#### *1.1.4. Insulator metal pollution defect (denoted as M-type defect)*

The surface of the insulator sometimes adsorbs some metal particles which move under the electric field force. Some of the particles may not be dangerous at first, but due to the mechanical vibration under electrostatic force, their movement facilitates the discharge and then induces the PD.

series of chemical substances including SO<sup>2</sup>

sible to determine whether there is a PD source [23, 24].

decomposition components.

let light (UV), infrared ray (IR), and visible light have been developed.

The spectral range of optical signal generated by PD in SF<sup>6</sup>

signals through the optical converter [17, 18].

optical sensors must be installed inside the GIS.

insulation material is epoxy resin [25].

electrodes simulate the metal shell of GIS.

*2.1.1. N-type defect*

CH<sup>4</sup>

SF<sup>6</sup>

, and SF<sup>4</sup>

by detecting SF<sup>6</sup>

ment of PD in GIS.

*1.2.3. Optical detection method*

F2 , SOF<sup>2</sup>

position components, their ratio, and gas generating rate. So one can also identify the PD type

In the process of PD, molecular ionization, ion recombination, and atomic energy level transition will excite and radiate optical signals. Optical detection methods of PD based on ultravio-

is mainly visible light. The basic principle of optical detection method is to use optical sensors to receive optical signals generated by the PD source and convert optical signals into electrical

Optical detection is not affected by strong EM interference on site, its anti-interference ability is more outstanding than the other two methods, and real-time monitoring of GIS PD phenomenon can be achieved. However, due to poor-optical signal transmission and GIS is a closed structure of equipment, the optical method cannot be used for outer GIS detection;

**2. Physical model of typical defects and the electrical field simulation**

**2.1. Typical detection methods of PD construction of insulation defect physical model**

In order to simulate GIS insulation defects and PD, what we choose for the physical model of insulation defect designed in this chapter is stainless steel, aluminum, and brass, and the solid

Under steady-state AC voltage, the prominent parts are distributed in the electric field and form the local high field strength zone. This corona sometimes appears to be relatively stable as the discharge only occurs in a local area instead of throughout the entire electrode. In this chapter, pin-plate electrodes are used to simulate N-type defects. As shown in **Figure 3**, the pin electrodes are used to simulate abnormal protrusions on HV conductors and the plate

 decomposition component method is able to locate the fault to find fault gas chamber, response accurately and timely to sudden failure, and judge the type of defect. It is also free from the scene of EM and noise interference, and regular detection can reflect the develop-

Studies have shown that PD sources caused by different types of defects differ in SF<sup>6</sup>

, CF<sup>4</sup>

. By detecting these decomposed components in the GIS gas chamber, it is pos-

, SO<sup>2</sup>

Typical Internal Defects of Gas-Insulated Switchgear and Partial Discharge Characteristics

, SOF<sup>4</sup> , S2

F10, SiF<sup>4</sup>

http://dx.doi.org/10.5772/intechopen.79090

gas is roughly 460–550 nm, which

, HF, CO, CO<sup>2</sup>

decom-

,

107

Due to strong adsorption, some of the particles will not move. Particles fixed on the insulator surface forms insulator surface pollution defects. These fixed metal particles have the following characteristics: on their surface, charges will accumulate, and these surface charges sometimes aggravate the distortion of the electric field, causing PD. Particle-induced discharge will cause insulator surface damage, resulting in surface tree marks. Eventually, it may cause serious insulation breakdown and flashover [14].

#### **1.2. Typical detection methods of PD**

Under the operating voltage, the insulation defect will cause the local electric field distortion in the insulation medium. When the local electric field reaches the critical breakdown field strength, PD will be induced and a large amount of charged particles will be generated. Charged particles under electric field will migrate, recombine, and adhere, resulting in pulse current, and accompanied by optical, electrical, thermal, and acoustic effects. By effective detection of these signals, PD can be measured in the GIS. At present, there are five commonly used PD signal detection methods, that is, pulse current method [15], ultra-high frequency (UHF) method [16], ultrasonic method, chemical detection method, and optical detection method [17, 18]. In this chapter, we will focus on the following three PD signal detection methods:

#### *1.2.1. UHF method*

When a PD occurs, a non-periodically changing current pulse excites a changing magnetic field and radiates a high-frequency electromagnetic (EM) wave through the insulator. Due to the short-duration of the PD current pulse and the steep rising edge, the excitation frequency of the EM wave ranges from several MHz to several GHz [19, 20].

Because GIS is a good coaxial waveguide structure, high-frequency EM waves can be effectively transmitted within the GIS. Through the high-frequency sensors installed inside or outside GIS, the detection of these EM signals and PD signals can be achieved. This method is called UHF method.

The UHF method has many advantages. Firstly, it uses the UHF signal to avoid EM interference due to low frequency in the power grid and has strong anti-interference ability. Secondly, it can pinpoint the location of the PD [21]. Finally, this method has a large detection range and requires fewer sensors to be installed [11, 22].

#### *1.2.2. Chemical detection method*

Many studies show that the SF<sup>6</sup> gas will decompose under PD and the decomposed components will further react with moisture and oxygen in the gas chamber of GIS to generate a series of chemical substances including SO<sup>2</sup> F2 , SOF<sup>2</sup> , CF<sup>4</sup> , SO<sup>2</sup> , SOF<sup>4</sup> , S2 F10, SiF<sup>4</sup> , HF, CO, CO<sup>2</sup> , CH<sup>4</sup> , and SF<sup>4</sup> . By detecting these decomposed components in the GIS gas chamber, it is possible to determine whether there is a PD source [23, 24].

Studies have shown that PD sources caused by different types of defects differ in SF<sup>6</sup> decomposition components, their ratio, and gas generating rate. So one can also identify the PD type by detecting SF<sup>6</sup> decomposition components.

SF<sup>6</sup> decomposition component method is able to locate the fault to find fault gas chamber, response accurately and timely to sudden failure, and judge the type of defect. It is also free from the scene of EM and noise interference, and regular detection can reflect the development of PD in GIS.

#### *1.2.3. Optical detection method*

*1.1.4. Insulator metal pollution defect (denoted as M-type defect)*

106 Simulation and Modelling of Electrical Insulation Weaknesses in Electrical Equipment

serious insulation breakdown and flashover [14].

**1.2. Typical detection methods of PD**

*1.2.1. UHF method*

is called UHF method.

*1.2.2. Chemical detection method*

Many studies show that the SF<sup>6</sup>

requires fewer sensors to be installed [11, 22].

The surface of the insulator sometimes adsorbs some metal particles which move under the electric field force. Some of the particles may not be dangerous at first, but due to the mechanical vibration under electrostatic force, their movement facilitates the discharge and then induces the PD.

Due to strong adsorption, some of the particles will not move. Particles fixed on the insulator surface forms insulator surface pollution defects. These fixed metal particles have the following characteristics: on their surface, charges will accumulate, and these surface charges sometimes aggravate the distortion of the electric field, causing PD. Particle-induced discharge will cause insulator surface damage, resulting in surface tree marks. Eventually, it may cause

Under the operating voltage, the insulation defect will cause the local electric field distortion in the insulation medium. When the local electric field reaches the critical breakdown field strength, PD will be induced and a large amount of charged particles will be generated. Charged particles under electric field will migrate, recombine, and adhere, resulting in pulse current, and accompanied by optical, electrical, thermal, and acoustic effects. By effective detection of these signals, PD can be measured in the GIS. At present, there are five commonly used PD signal detection methods, that is, pulse current method [15], ultra-high frequency (UHF) method [16], ultrasonic method, chemical detection method, and optical detection method [17,

18]. In this chapter, we will focus on the following three PD signal detection methods:

of the EM wave ranges from several MHz to several GHz [19, 20].

When a PD occurs, a non-periodically changing current pulse excites a changing magnetic field and radiates a high-frequency electromagnetic (EM) wave through the insulator. Due to the short-duration of the PD current pulse and the steep rising edge, the excitation frequency

Because GIS is a good coaxial waveguide structure, high-frequency EM waves can be effectively transmitted within the GIS. Through the high-frequency sensors installed inside or outside GIS, the detection of these EM signals and PD signals can be achieved. This method

The UHF method has many advantages. Firstly, it uses the UHF signal to avoid EM interference due to low frequency in the power grid and has strong anti-interference ability. Secondly, it can pinpoint the location of the PD [21]. Finally, this method has a large detection range and

nents will further react with moisture and oxygen in the gas chamber of GIS to generate a

gas will decompose under PD and the decomposed compo-

In the process of PD, molecular ionization, ion recombination, and atomic energy level transition will excite and radiate optical signals. Optical detection methods of PD based on ultraviolet light (UV), infrared ray (IR), and visible light have been developed.

The spectral range of optical signal generated by PD in SF<sup>6</sup> gas is roughly 460–550 nm, which is mainly visible light. The basic principle of optical detection method is to use optical sensors to receive optical signals generated by the PD source and convert optical signals into electrical signals through the optical converter [17, 18].

Optical detection is not affected by strong EM interference on site, its anti-interference ability is more outstanding than the other two methods, and real-time monitoring of GIS PD phenomenon can be achieved. However, due to poor-optical signal transmission and GIS is a closed structure of equipment, the optical method cannot be used for outer GIS detection; optical sensors must be installed inside the GIS.

## **2. Physical model of typical defects and the electrical field simulation**

In order to simulate GIS insulation defects and PD, what we choose for the physical model of insulation defect designed in this chapter is stainless steel, aluminum, and brass, and the solid insulation material is epoxy resin [25].

#### **2.1. Typical detection methods of PD construction of insulation defect physical model**

#### *2.1.1. N-type defect*

Under steady-state AC voltage, the prominent parts are distributed in the electric field and form the local high field strength zone. This corona sometimes appears to be relatively stable as the discharge only occurs in a local area instead of throughout the entire electrode. In this chapter, pin-plate electrodes are used to simulate N-type defects. As shown in **Figure 3**, the pin electrodes are used to simulate abnormal protrusions on HV conductors and the plate electrodes simulate the metal shell of GIS.

**Figure 3.** N-type insulation defect model. (a) Model diagram and (b) physical diagram.

In order to obtain a stable PD, an electrode is adopted with a tip radius of curvature of about 0.3 mm as well as a cone angle of 30°, and a ground plate electrode diameter of 120 mm as well as 10 mm thickness. Aluminum needle electrodes are designed and manufactured, with ground electrode material stainless steel, and electrode surface all were well polished.

damage to the surface of the insulator, causing surface tree marks in the power frequency field.

Typical Internal Defects of Gas-Insulated Switchgear and Partial Discharge Characteristics

contact surface of electrodes and cylindrical insulator is polished to avoid potential air gap discharge. The model structure is shown in **Figure 5**. The plate diameter is 120 mm, the epoxy resin cylindrical insulator diameter is 60 mm, and the thickness is 25 mm. The HV electrode

G-type defects are often formed in the manufacturing process such as epoxy curing shrinkage and internal voids [17]. The mechanism of air gap discharge is complicated, and it is generally believed that there are three ways of air gap discharge, that is, the throughout discharge, the discharge along the surface of the upper and lower electrode and the discharge along the air gap wall. In this chapter, G defects model is shown in **Figure 6**. The cylindrical insulator and the grounding electrode are closely adhered with epoxy glue to ensure that there are no gaps or bubbles between them. The air gap size at the interface between the high voltage plate electrode, and the insulator is about 1–3 mm. In order to reflect the real air gap situation, the insulator is slightly concave at the center of the upper surface, and the edge of the air gap is arc shaped.

In this chapter, the finite-element analysis software COMSOL is used to simulate the electric field distribution of four insulation defect models. The simulation results are used to evaluate the feasibility of the model and provide the preliminary data reference for the following PD test.

eters of the simulation model and the relative dielectric constants are shown in **Tables 1** and **2**.

and the boundary conditions are grounded. The specific technical param-

) is used to stimulate M-type defect. The

http://dx.doi.org/10.5772/intechopen.79090

109

discharge gas chamber. The solu-

Once the discharge channel is formed, a serious insulation accident will be caused.

**Figure 4.** P-type insulation defect model. (a) Model diagram and (b) physical diagram.

material is aluminum, and the ground plate electrode material is stainless steel.

In this chapter, rectangular copper cutting (5 × 18 mm<sup>2</sup>

**2.2. Insulation defect electric field simulation**

In the simulation, a cylindrical cavity is used to simulate the SF<sup>6</sup>

*2.1.4. G-type defect*

tion domain is set as SF<sup>6</sup>

#### *2.1.2. P-type defect*

Conductive particles have the shape of powder, flake or large solid particles, etc.; they get the charge in the electric field and will move or beat under electrostatic force. If the electric field is strong enough and the energy obtained by the conductive particles is large enough, particles are possible to cross the gap between the shell and the HV conductor or move to a point where the insulation is damaged.

The motion intensity of the conductive particles depends on the material, the shape, and the applied voltage, as well as the strength and duration of the external electric field strength and the location of the particles in GIS cavity. When the metal particles come close without touching the HV conductor, then PD arises as the electrical characteristics. Half of the actual GIS equipment uses a structure with a coaxial cylinder between the HV conductor and the shell, that is, a slightly uneven electric field structure [26]. In order to effectively simulate the slightly heterogeneous electric field structure of the coaxial cylinder inside the real GIS, the ball-bowl electrode shown in **Figure 4** is selected in this chapter. The bowl electrode is cut by a half of a stainless steel hollow sphere. In order to ensure the steady PD experiment, it is necessary to limit the beating range of the copper scrap. HV terminal ball electrode diameter is designed to be 44 mm, the ground bowl diameter is designed to be 120 mm, and particle maximum beating range up to 40 mm.

#### *2.1.3. M-type defect*

Due to electric force, some metal particles are absorbed on the insulator, thus distorting the insulator surface electric field and causing PD. Some metal particles on the insulator may not be dangerous at first, but under mechanical vibration and electric force, there will be slight movement and potential danger. Metal particles on the surface of the insulator will form surface charge aggregation, thereby increasing the possibility of failure. Particle discharge can cause

**Figure 4.** P-type insulation defect model. (a) Model diagram and (b) physical diagram.

damage to the surface of the insulator, causing surface tree marks in the power frequency field. Once the discharge channel is formed, a serious insulation accident will be caused.

In this chapter, rectangular copper cutting (5 × 18 mm<sup>2</sup> ) is used to stimulate M-type defect. The contact surface of electrodes and cylindrical insulator is polished to avoid potential air gap discharge. The model structure is shown in **Figure 5**. The plate diameter is 120 mm, the epoxy resin cylindrical insulator diameter is 60 mm, and the thickness is 25 mm. The HV electrode material is aluminum, and the ground plate electrode material is stainless steel.

#### *2.1.4. G-type defect*

In order to obtain a stable PD, an electrode is adopted with a tip radius of curvature of about 0.3 mm as well as a cone angle of 30°, and a ground plate electrode diameter of 120 mm as well as 10 mm thickness. Aluminum needle electrodes are designed and manufactured, with

Conductive particles have the shape of powder, flake or large solid particles, etc.; they get the charge in the electric field and will move or beat under electrostatic force. If the electric field is strong enough and the energy obtained by the conductive particles is large enough, particles are possible to cross the gap between the shell and the HV conductor or move to a point where

The motion intensity of the conductive particles depends on the material, the shape, and the applied voltage, as well as the strength and duration of the external electric field strength and the location of the particles in GIS cavity. When the metal particles come close without touching the HV conductor, then PD arises as the electrical characteristics. Half of the actual GIS equipment uses a structure with a coaxial cylinder between the HV conductor and the shell, that is, a slightly uneven electric field structure [26]. In order to effectively simulate the slightly heterogeneous electric field structure of the coaxial cylinder inside the real GIS, the ball-bowl electrode shown in **Figure 4** is selected in this chapter. The bowl electrode is cut by a half of a stainless steel hollow sphere. In order to ensure the steady PD experiment, it is necessary to limit the beating range of the copper scrap. HV terminal ball electrode diameter is designed to be 44 mm, the ground bowl diameter is designed to

Due to electric force, some metal particles are absorbed on the insulator, thus distorting the insulator surface electric field and causing PD. Some metal particles on the insulator may not be dangerous at first, but under mechanical vibration and electric force, there will be slight movement and potential danger. Metal particles on the surface of the insulator will form surface charge aggregation, thereby increasing the possibility of failure. Particle discharge can cause

be 120 mm, and particle maximum beating range up to 40 mm.

ground electrode material stainless steel, and electrode surface all were well polished.

**Figure 3.** N-type insulation defect model. (a) Model diagram and (b) physical diagram.

108 Simulation and Modelling of Electrical Insulation Weaknesses in Electrical Equipment

*2.1.2. P-type defect*

*2.1.3. M-type defect*

the insulation is damaged.

G-type defects are often formed in the manufacturing process such as epoxy curing shrinkage and internal voids [17]. The mechanism of air gap discharge is complicated, and it is generally believed that there are three ways of air gap discharge, that is, the throughout discharge, the discharge along the surface of the upper and lower electrode and the discharge along the air gap wall. In this chapter, G defects model is shown in **Figure 6**. The cylindrical insulator and the grounding electrode are closely adhered with epoxy glue to ensure that there are no gaps or bubbles between them. The air gap size at the interface between the high voltage plate electrode, and the insulator is about 1–3 mm. In order to reflect the real air gap situation, the insulator is slightly concave at the center of the upper surface, and the edge of the air gap is arc shaped.

#### **2.2. Insulation defect electric field simulation**

In this chapter, the finite-element analysis software COMSOL is used to simulate the electric field distribution of four insulation defect models. The simulation results are used to evaluate the feasibility of the model and provide the preliminary data reference for the following PD test.

In the simulation, a cylindrical cavity is used to simulate the SF<sup>6</sup> discharge gas chamber. The solution domain is set as SF<sup>6</sup> and the boundary conditions are grounded. The specific technical parameters of the simulation model and the relative dielectric constants are shown in **Tables 1** and **2**.

**Cavity height/mm** **Cavity diameter/mm**

**Table 1.** Model technical parameters.

**Plate electrode diameter/mm**

**Table 2.** Relative dielectric constant of each part of the material.

**Figure 7.** N-type insulation defect space electric field simulation output (mm).

**Figure 8.** P-type insulation defect space electric field simulation output.

**Plate electrode thickness/mm**

350 180 120 10 0.3 30 60 25/20

**Material SF6 Aluminum Stainless steel Aerosols (Copper) Epoxy resin**

Relative permittivity 1.002 1.0 1.0 8000 3.8

**Needle curvature radius/mm** **Cone sharp corners/°**

Typical Internal Defects of Gas-Insulated Switchgear and Partial Discharge Characteristics

**Insulator diameter/mm**

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**Insulator thickness/mm**

111

**Figure 5.** M-type insulation defect model. (a) Model diagram and (b) physical diagram.

**Figure 6.** G-type insulation defect model. (a) Model diagram and (b) physical diagram.

#### *2.2.1. N-type insulation defect*

Due to the axial symmetry of N-type insulation defect, a two-dimensional axisymmetric model is adopted in this chapter. The HV-terminal needle electrode potential is set to 25 kV, the plate electrode with the cavity shell boundary is set to ground, and the needle-plate spacing is set to 10 mm.

The results of the electric field simulation of N-type insulation defect are shown in **Figure 7**. It can be seen from the figure that the distribution of the electric field between the needle and plate is extremely uneven. The electric field strength value at the tip of the needle electrode is high, and the electric field distortion at the tip of the needle electrode reaches up to 351 kV/cm.


**Table 1.** Model technical parameters.

*2.2.1. N-type insulation defect*

ing is set to 10 mm.

Due to the axial symmetry of N-type insulation defect, a two-dimensional axisymmetric model is adopted in this chapter. The HV-terminal needle electrode potential is set to 25 kV, the plate electrode with the cavity shell boundary is set to ground, and the needle-plate spac-

**Figure 6.** G-type insulation defect model. (a) Model diagram and (b) physical diagram.

**Figure 5.** M-type insulation defect model. (a) Model diagram and (b) physical diagram.

110 Simulation and Modelling of Electrical Insulation Weaknesses in Electrical Equipment

The results of the electric field simulation of N-type insulation defect are shown in **Figure 7**. It can be seen from the figure that the distribution of the electric field between the needle and plate is extremely uneven. The electric field strength value at the tip of the needle electrode is high, and the electric field distortion at the tip of the needle electrode reaches up to 351 kV/cm.


**Table 2.** Relative dielectric constant of each part of the material.

**Figure 7.** N-type insulation defect space electric field simulation output (mm).

**Figure 8.** P-type insulation defect space electric field simulation output.

#### *2.2.2. P-type insulation defects*

Like N-type insulation defect, P-type insulation defect is also axisymmetric, so a two-dimensional axisymmetric model is adopted again for P-type insulation defect. In the simulation, the potential of the HV terminal ball electrode is set to be 30 kV, and the potential of the bowl electrode is set to be grounded. The distance between the ball and the bowl is set to 30 mm, and metal particles with a diameter of about 2 mm are placed in the bowl electrode. The floating particles are treated with the virtual large dielectric constant method.

The results of the electric field simulation of the P-type insulation defects are shown in **Figure 8**. The metal particles cause a distortion of the electric field between the electrodes. When the electric force is greater than the gravity of the metal particles, the particles will move or beat under the force. It can be seen from the figure that the electric field on the surface of metal particles close to the high voltage end is seriously distorted, and the maximum field strength reaches 155 kV/cm.

> As shown in **Figure 10**, the simulation results of the electric field are mainly concentrated in the air gap between the high-voltage conductor and the insulator. The maximum field strength is 71.3 kV/cm. From the simulation results, G-type insulation defect has higher initial

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113

As mentioned in Section 1, PD can be detected by UHF method. In this section, UHF charac-

The detecting and measuring platform for PD is shown in **Figure 11**. The regulator (T1) input voltage is 220 V, the output voltage is adjustable from 0 to 250 V, the regulator output voltage through non-halo test transformer (T2: 10 kVA/50 kV) is boosted as the test voltage and is applied

the amplitude of the short circuit current which may appear after the breakdown of the test object. To measure the test voltage, a capacitor divider in parallel on both ends of the test object is used [27]. The experimental voltage is acquired by outer UHF antenna developed by the authors (ultrahigh frequency microstrip antenna, with 340–440 MHz bandwidth) and displayed on the digital storage oscilloscope (DSO: Lecroy WavePro 7100). The DSO has the largest sample rate of 20 GS/s.

Experiments can be done on the platform in **Figure 15**. Large numbers of data can then be acquired and processed. All sampling data are unified and normalized, so that the resulting mathematical models are more universal [28]. Unification means that each PD signal consists of 10,000 sampling points at a sampling rate of 20 GS/s (i.e., the sampling time is 500 ns, the sampling step is 0.05 ns, and the trigger point is set at the 4000th point). Normalization means

that each value of the sample points is divided by the maximum absolute value.

). The protection resistor is used to limit

teristic of PD will be discussed. Experiments and analysis will be shown as follows.

discharge voltage.

**3.1. Experimental setup**

**3.2. Data acquiring and processing**

**3. UHF characteristics of typical defects PD**

**Figure 10.** G-type insulation defect space electric field simulation output.

to the test object through a 10 kΩ protection resistor (Rr

#### *2.2.3. M-type insulation defects*

In the simulation, the electrode potential of the HV terminal plate is set to be 30 kV, the boundary of the lower plate electrode and the cavity are set to be grounded, the thickness of the cylindrical insulator is set to 25 mm, and the surface is pasted with a metal copper cuttings of about 5 × 18 mm<sup>2</sup> .

As shown in **Figure 9**, the simulation results show that the electric field at the surface of the insulator where the metal pollutants are located has been distorted, forming a very uneven field with the maximum field strength of 192 kV/cm. Based on the simulation results, insulator surface metal contamination will lead to PD before the insulator flashover.

#### *2.2.4. G-type insulation defects*

In the simulation, the potential of HV board is set to 60 kV. The boundary between lower board electrode and cavity is grounded. The upper surface of insulator is slightly concave with an average thickness of 20 mm. Between the HV board is arc-shaped air gap, with maximum gap 2 mm.

**Figure 9.** M-type insulation defect space electric field simulation output.

Typical Internal Defects of Gas-Insulated Switchgear and Partial Discharge Characteristics http://dx.doi.org/10.5772/intechopen.79090 113

**Figure 10.** G-type insulation defect space electric field simulation output.

As shown in **Figure 10**, the simulation results of the electric field are mainly concentrated in the air gap between the high-voltage conductor and the insulator. The maximum field strength is 71.3 kV/cm. From the simulation results, G-type insulation defect has higher initial discharge voltage.

## **3. UHF characteristics of typical defects PD**

As mentioned in Section 1, PD can be detected by UHF method. In this section, UHF characteristic of PD will be discussed. Experiments and analysis will be shown as follows.

#### **3.1. Experimental setup**

*2.2.2. P-type insulation defects*

strength reaches 155 kV/cm.

*2.2.3. M-type insulation defects*

*2.2.4. G-type insulation defects*

.

**Figure 9.** M-type insulation defect space electric field simulation output.

of about 5 × 18 mm<sup>2</sup>

mum gap 2 mm.

Like N-type insulation defect, P-type insulation defect is also axisymmetric, so a two-dimensional axisymmetric model is adopted again for P-type insulation defect. In the simulation, the potential of the HV terminal ball electrode is set to be 30 kV, and the potential of the bowl electrode is set to be grounded. The distance between the ball and the bowl is set to 30 mm, and metal particles with a diameter of about 2 mm are placed in the bowl electrode. The float-

The results of the electric field simulation of the P-type insulation defects are shown in **Figure 8**. The metal particles cause a distortion of the electric field between the electrodes. When the electric force is greater than the gravity of the metal particles, the particles will move or beat under the force. It can be seen from the figure that the electric field on the surface of metal particles close to the high voltage end is seriously distorted, and the maximum field

In the simulation, the electrode potential of the HV terminal plate is set to be 30 kV, the boundary of the lower plate electrode and the cavity are set to be grounded, the thickness of the cylindrical insulator is set to 25 mm, and the surface is pasted with a metal copper cuttings

As shown in **Figure 9**, the simulation results show that the electric field at the surface of the insulator where the metal pollutants are located has been distorted, forming a very uneven field with the maximum field strength of 192 kV/cm. Based on the simulation results, insula-

In the simulation, the potential of HV board is set to 60 kV. The boundary between lower board electrode and cavity is grounded. The upper surface of insulator is slightly concave with an average thickness of 20 mm. Between the HV board is arc-shaped air gap, with maxi-

tor surface metal contamination will lead to PD before the insulator flashover.

ing particles are treated with the virtual large dielectric constant method.

112 Simulation and Modelling of Electrical Insulation Weaknesses in Electrical Equipment

The detecting and measuring platform for PD is shown in **Figure 11**. The regulator (T1) input voltage is 220 V, the output voltage is adjustable from 0 to 250 V, the regulator output voltage through non-halo test transformer (T2: 10 kVA/50 kV) is boosted as the test voltage and is applied to the test object through a 10 kΩ protection resistor (Rr ). The protection resistor is used to limit the amplitude of the short circuit current which may appear after the breakdown of the test object. To measure the test voltage, a capacitor divider in parallel on both ends of the test object is used [27]. The experimental voltage is acquired by outer UHF antenna developed by the authors (ultrahigh frequency microstrip antenna, with 340–440 MHz bandwidth) and displayed on the digital storage oscilloscope (DSO: Lecroy WavePro 7100). The DSO has the largest sample rate of 20 GS/s.

#### **3.2. Data acquiring and processing**

Experiments can be done on the platform in **Figure 15**. Large numbers of data can then be acquired and processed. All sampling data are unified and normalized, so that the resulting mathematical models are more universal [28]. Unification means that each PD signal consists of 10,000 sampling points at a sampling rate of 20 GS/s (i.e., the sampling time is 500 ns, the sampling step is 0.05 ns, and the trigger point is set at the 4000th point). Normalization means that each value of the sample points is divided by the maximum absolute value.

Mathematical models of PD for VHF measurement are established by fitting to Gaussian plots

**Figures 12**–**15** show time domain and frequency domain UHF PD characteristics, and they, respectively, denote G-type defect, M-type defect, N-type defect, and P-type defect. In time domain, the unit of x-axis is nanosecond, while in frequency domain, the unit of x-axis is gigahertz. Notice that the y-axis in time domain and frequency domain has no unit because it

massive experiment data, these parameters can be calculated as shown in **Table 3**.

represents normalized data, that is, the U\* and A\* both stand for per unit.

**Figure 12.** G-type defect UHF PD characteristics. (a) Time domain and (b) frequency domain.

**Figure 13.** M-type defect UHF PD characteristics. (a) Time domain and (b) frequency domain.

are parameters and different of defects have different parameters. Based on

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(1)

115

function, the selected mathematical model is [28]:

where *ai*

, *bi ,* and *ci*

**Figure 11.** Detecting and measuring platform.


**Table 3.** Parameters of different types of defect.

Mathematical models of PD for VHF measurement are established by fitting to Gaussian plots function, the selected mathematical model is [28]:

$$f(\mathbf{x}) = \sum\_{i=1}^{s} a\_i \bullet e^{-\frac{(\mathbf{x} \cdot \mathbf{b}\_i)^2}{c\_i^2}} \tag{1}$$

where *ai* , *bi ,* and *ci* are parameters and different of defects have different parameters. Based on massive experiment data, these parameters can be calculated as shown in **Table 3**.

**Figures 12**–**15** show time domain and frequency domain UHF PD characteristics, and they, respectively, denote G-type defect, M-type defect, N-type defect, and P-type defect. In time domain, the unit of x-axis is nanosecond, while in frequency domain, the unit of x-axis is gigahertz. Notice that the y-axis in time domain and frequency domain has no unit because it represents normalized data, that is, the U\* and A\* both stand for per unit.

**Figure 12.** G-type defect UHF PD characteristics. (a) Time domain and (b) frequency domain.

**Figure 11.** Detecting and measuring platform.

114 Simulation and Modelling of Electrical Insulation Weaknesses in Electrical Equipment

**Table 3.** Parameters of different types of defect.

**Types of defects Coefficients Value Coefficients Value Coefficients Value** N-type a1 0.2002 b1 4852 c1 435.3

P-type a1 0.2858 b1 4305 c1 39.6

M-type a1 0.9757 b1 4000 c1 23.2

G-type a1 1.0100 b1 3998 c1 30.3

a2 0.1863 b2 4444 c2 215.2 a3 −0.9328 b3 3998 c3 19.2 a4 0.3613 b4 6097 c4 769.7 a5 −0.3475 b5 5767 c5 119

a2 0.2332 b2 4531 c2 97.7 a3 0.1117 b3 4762 c3 104.1 a4 −0.9565 b4 3998 c4 19.1 a5 0.0454 b5 6181 c5 211.8

a2 0.7679 b2 5298 c2 521.2 a3 0.7750 b3 6661 c3 606.1 a4 1.1040 b4 5951 c4 591.9 a5 −1.5420 b5 5981 c5 1006.0

a2 0.0588 b2 5156 c2 358.2 a3 0.8981 b3 4368 c3 177.9 a4 −1.0730 b4 4358 c4 201.4 a5 0.0232 b5 7225 c5 152.5

**Figure 13.** M-type defect UHF PD characteristics. (a) Time domain and (b) frequency domain.

**4. Chemical characteristics of typical defects PD**

mass spectrometry (GCMS), with its type Shimadzu QP-2010 Ultra.

The experiment is carried out in the gas chamber, which is closed filled of SF<sup>6</sup>

every 12 h, and the concentration of characteristic decomposition components CF<sup>4</sup>

evacuated. Repeat the process until the chamber is filled with pure 0.2 MPa SF<sup>6</sup>

chromatograph is used to analyze the concentration of gas components.

shown in **Table 4**. The experimental process is as follows:

**1.** The insulation defect model is installed in SF<sup>6</sup>

tinue to raise at the experimental voltage.

 **decomposition characteristics**

lead to different decomposition components. Based on this idea, chemical methods can be

The detecting and measuring platform for PD is shown in **Figure 16**. The measuring platform is similar to that in Section 3.1, but the UHF antenna will be replaced by gas chromatography

specific pressure. Certain type of insulation defects for PD is also placed in the chamber. The coupling capacitor (Ck: 500 pF/100 kV) provides a high frequency and low-impedance path to the pulsed current and is converted to a voltage signal via a sense-less impedance (Zm: 50 Ω), and it is displayed by digital storage oscilloscope. The decomposed components generated

carried out. Under each type, the experiments last for 96 h. The decomposition gas is collected

is measured. The initial discharge voltage and the test voltage of various insulation defects are

ber, the vacuum chamber is first evacuated and then filled with fresh SF6 gas, and then

**2.** Connect the test circuit according to **Figure 20**, and then adjust the regulator to slowly increase the test voltage until the oscilloscope can detect PD on the defect model. Record

**4.** After a 96-h continuous experiment on a defect model, another model of the defect will replace it and continue the experiment according to the aforementioned steps until all four

The decomposition components under the four types of defects are shown in **Figure 17(a)**–**(d)**, that is, N-type, M-type, P-type, and G-type defect, respectively. Four characteristic

decomposition experiments under four types of insulation defects are

decomposition. In addition, different defects will

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Typical Internal Defects of Gas-Insulated Switchgear and Partial Discharge Characteristics

gas with a

117

, CO<sup>2</sup>

.

partial discharge decomposition gas cham-

at this time, that is, the initial discharge voltage. Then, con-

gas is collected. The single collection gas volume is about 100 mL. Gas

, SO<sup>2</sup> F2

In GIS, PD takes place accompanied by SF<sup>6</sup>

used to detect PD [23].

**4.1. Experimental setup**

under PD are detected by GCMS.

the experimental voltage U<sup>0</sup>

types of defects are all done.

**4.2. Experimental steps**

In this chapter, SF<sup>6</sup>

**3.** Every 12 h, SF<sup>6</sup>

**4.3. SF6**

**Figure 14.** N-type defect UHF PD characteristics. (a) Time domain and (b) frequency domain.

**Figure 15.** P-type defect UHF PD characteristics. (a) Time domain and (b) frequency domain.

For G-type defect characteristics curve, in time domain, there is a sharp jump at the 4000th point, and the curve is smooth after the jump; while in frequency domain, there is a jump after the original point, and the curve is smooth after the jump except for some protuberant points. For M-type defect characteristics curve, in time domain, there is a sharp jump at the 4000th point, and the curve is smooth after the jump with two peaks; while in frequency domain, there is a jump after the original point with a smaller jump afterwards, and the curve is smooth except for some protuberant points. For N-type defect characteristics curve, in time domain, there is a sharp jump at the 4000th point toward the negative direction, and the curve is smooth after the jump with a flat segment and then two peaks; while in frequency domain, there is oscillation on the whole frequency axis. For P-type defect characteristics curve, in time domain, there is a sharp jump at the 4000th point toward the negative direction, and several peaks follow afterwards; while in frequency domain, there is a jump after the original point with several oscillations afterwards.

## **4. Chemical characteristics of typical defects PD**

In GIS, PD takes place accompanied by SF<sup>6</sup> decomposition. In addition, different defects will lead to different decomposition components. Based on this idea, chemical methods can be used to detect PD [23].

#### **4.1. Experimental setup**

The detecting and measuring platform for PD is shown in **Figure 16**. The measuring platform is similar to that in Section 3.1, but the UHF antenna will be replaced by gas chromatography mass spectrometry (GCMS), with its type Shimadzu QP-2010 Ultra.

The experiment is carried out in the gas chamber, which is closed filled of SF<sup>6</sup> gas with a specific pressure. Certain type of insulation defects for PD is also placed in the chamber. The coupling capacitor (Ck: 500 pF/100 kV) provides a high frequency and low-impedance path to the pulsed current and is converted to a voltage signal via a sense-less impedance (Zm: 50 Ω), and it is displayed by digital storage oscilloscope. The decomposed components generated under PD are detected by GCMS.

#### **4.2. Experimental steps**

For G-type defect characteristics curve, in time domain, there is a sharp jump at the 4000th point, and the curve is smooth after the jump; while in frequency domain, there is a jump after the original point, and the curve is smooth after the jump except for some protuberant points. For M-type defect characteristics curve, in time domain, there is a sharp jump at the 4000th point, and the curve is smooth after the jump with two peaks; while in frequency domain, there is a jump after the original point with a smaller jump afterwards, and the curve is smooth except for some protuberant points. For N-type defect characteristics curve, in time domain, there is a sharp jump at the 4000th point toward the negative direction, and the curve is smooth after the jump with a flat segment and then two peaks; while in frequency domain, there is oscillation on the whole frequency axis. For P-type defect characteristics curve, in time domain, there is a sharp jump at the 4000th point toward the negative direction, and several peaks follow afterwards; while in frequency domain, there is a jump after the original point

**Figure 14.** N-type defect UHF PD characteristics. (a) Time domain and (b) frequency domain.

116 Simulation and Modelling of Electrical Insulation Weaknesses in Electrical Equipment

**Figure 15.** P-type defect UHF PD characteristics. (a) Time domain and (b) frequency domain.

with several oscillations afterwards.

In this chapter, SF<sup>6</sup> decomposition experiments under four types of insulation defects are carried out. Under each type, the experiments last for 96 h. The decomposition gas is collected every 12 h, and the concentration of characteristic decomposition components CF<sup>4</sup> , CO<sup>2</sup> , SO<sup>2</sup> F2 is measured. The initial discharge voltage and the test voltage of various insulation defects are shown in **Table 4**. The experimental process is as follows:


#### **4.3. SF6 decomposition characteristics**

The decomposition components under the four types of defects are shown in **Figure 17(a)**–**(d)**, that is, N-type, M-type, P-type, and G-type defect, respectively. Four characteristic

**Figure 16.** Detecting and measuring platform.


**Table 4.** Test voltage under different insulation defects.

decomposition components are generated, but the amounts of different characteristic components are quite different.

Under the N-type defect, at the end of the experiment, that is, at 96 h, concentration of SOF<sup>2</sup> was as high as 1114.5 μL/L, SO<sup>2</sup> F2 was 471.2 μL/L, CO<sup>2</sup> was 124.8 μL/L, and CF<sup>4</sup> was only a few μL/L. It was detected in the experiment that the concentration of components in ascending order is SOF<sup>2</sup> > SO<sup>2</sup> F<sup>2</sup> > CO<sup>2</sup> > CF<sup>4</sup> . Concentrations of SOF<sup>2</sup> , SO<sup>2</sup> F2 , and CO<sup>2</sup> all have an almost linear increase, indicating that PD is stable. The gas production rate dropped within a few tens of hours before the end of the experiment. It is preliminarily inferred that the moisture and oxygen in the gas chamber decreased after being consumed in experiment, resulting in a corresponding slowdown of various chemical reaction rates. Although the concentration of CF<sup>4</sup> generally increases, it does not increase simply linearly, and even decreases sometimes. The reason for this is that concentration of CF<sup>4</sup> is too low. Although the gas chromatograph detector sensitivity is very high, the final calculation of the test results needs to be integrated on the resulting chromatographic peak, when the concentration result is low, the impact of integral error will be greater.

surface contamination is gradually ablated by discharge; its effect on the electric field distortion becomes weaker and weaker, resulting in gradual decrease in discharge intensity. Because of sufficient fluorine atoms generated by the discharge and carbon atoms provided

**Figure 17.** Decomposition concentration under four types of defects. (a) N-type defects, (b) M-type defects, (c) P-type

linearly, and sometimes even decreases. The main reason for this is the concentration of

The characteristic decomposition components under the P-type defect are shown in **Figure 17(c)**. Under this defect, four characteristic decomposition components were also gener-

was 16.63 μL/L, and the concentration of CF<sup>4</sup>

the four characteristic components did not increase linearly. In the first 24 h of the experiment, the components concentration increased linearly, and the increasing rate was larger; from 24

is low under M-type defect, and the integral error of the gas chromatograph has a

tion increases substantially linearly with time. The increase of CO<sup>2</sup>

centration of components in ascending order is SOF<sup>2</sup> > CF<sup>4</sup> > CO<sup>2</sup> > SO<sup>2</sup>

is relatively less affected by the discharge intensity and its concentra-

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119

was 238.9 μL/L, the concentration of SO<sup>2</sup>

does not simply grow

was 15.82 μL/L,

F2

F2

was 32.68 μL/L. The con-

. The concentration of

by the insulator, CF<sup>4</sup>

defects, and (d) G-type defects.

greater impact on it.

the concentration of CO<sup>2</sup>

ated. At 96 h, the concentration of SOF<sup>2</sup>

CO<sup>2</sup>

The decomposition components under the M-type defect are shown in **Figure 17(b)**. The amounts of different characteristic components are also different. However, compared with the N-type defect, the difference is much smaller. At 96 h, concentration of SOF<sup>2</sup> was 42.78 μL/L, concentration of SO<sup>2</sup> F2 was 14.95 μL/L, concentration of CO<sup>2</sup> was 2.18 μL/L, and concentration of CF<sup>4</sup> was 6.18 μL/L. In the experiment, the concentration of components in ascending order is SOF<sup>2</sup> > SO<sup>2</sup> F<sup>2</sup> > CF<sup>4</sup> > CO<sup>2</sup> . Concentration of SOF<sup>2</sup> and SO<sup>2</sup> F2 gradually increased, but their increasing rate gradually decreased, especially SO<sup>2</sup> F2 . Its concentration almost stopped increasing at the end of the experiment. That is because the insulator

Typical Internal Defects of Gas-Insulated Switchgear and Partial Discharge Characteristics http://dx.doi.org/10.5772/intechopen.79090 119

**Figure 17.** Decomposition concentration under four types of defects. (a) N-type defects, (b) M-type defects, (c) P-type defects, and (d) G-type defects.

decomposition components are generated, but the amounts of different characteristic compo-

Under the N-type defect, at the end of the experiment, that is, at 96 h, concentration of SOF<sup>2</sup>

μL/L. It was detected in the experiment that the concentration of components in ascending

linear increase, indicating that PD is stable. The gas production rate dropped within a few tens of hours before the end of the experiment. It is preliminarily inferred that the moisture and oxygen in the gas chamber decreased after being consumed in experiment, resulting in a corresponding slowdown of various chemical reaction rates. Although the concentration of

generally increases, it does not increase simply linearly, and even decreases sometimes.

detector sensitivity is very high, the final calculation of the test results needs to be integrated on the resulting chromatographic peak, when the concentration result is low, the impact of

The decomposition components under the M-type defect are shown in **Figure 17(b)**. The amounts of different characteristic components are also different. However, compared with the N-type defect, the difference is much smaller. At 96 h, concentration of SOF<sup>2</sup>

tion almost stopped increasing at the end of the experiment. That is because the insulator

was 14.95 μL/L, concentration of CO<sup>2</sup>

was 6.18 μL/L. In the experiment, the concentration of components in

. Concentration of SOF<sup>2</sup>

. Concentrations of SOF<sup>2</sup>

was 124.8 μL/L, and CF<sup>4</sup>

, and CO<sup>2</sup>

is too low. Although the gas chromatograph

, SO<sup>2</sup> F2

**N-type (kV) M-type (kV) P-type (kV) G-type (kV)**

was only a few

was

gradually

. Its concentra-

was 2.18 μL/L, and

F2

and SO<sup>2</sup>

F2

all have an almost

was 471.2 μL/L, CO<sup>2</sup>

Starting discharge voltage 16.2 21.6 17.5 25.7 Experimental voltage 19.4 25.9 21.0 30.8

F2

118 Simulation and Modelling of Electrical Insulation Weaknesses in Electrical Equipment

F2

F<sup>2</sup> > CF<sup>4</sup> > CO<sup>2</sup>

increased, but their increasing rate gradually decreased, especially SO<sup>2</sup>

F<sup>2</sup> > CO<sup>2</sup> > CF<sup>4</sup>

**Voltage Defect type**

**Figure 16.** Detecting and measuring platform.

**Table 4.** Test voltage under different insulation defects.

The reason for this is that concentration of CF<sup>4</sup>

nents are quite different.

order is SOF<sup>2</sup> > SO<sup>2</sup>

CF<sup>4</sup>

was as high as 1114.5 μL/L, SO<sup>2</sup>

integral error will be greater.

42.78 μL/L, concentration of SO<sup>2</sup>

ascending order is SOF<sup>2</sup> > SO<sup>2</sup>

concentration of CF<sup>4</sup>

surface contamination is gradually ablated by discharge; its effect on the electric field distortion becomes weaker and weaker, resulting in gradual decrease in discharge intensity. Because of sufficient fluorine atoms generated by the discharge and carbon atoms provided by the insulator, CF<sup>4</sup> is relatively less affected by the discharge intensity and its concentration increases substantially linearly with time. The increase of CO<sup>2</sup> does not simply grow linearly, and sometimes even decreases. The main reason for this is the concentration of CO<sup>2</sup> is low under M-type defect, and the integral error of the gas chromatograph has a greater impact on it.

The characteristic decomposition components under the P-type defect are shown in **Figure 17(c)**. Under this defect, four characteristic decomposition components were also generated. At 96 h, the concentration of SOF<sup>2</sup> was 238.9 μL/L, the concentration of SO<sup>2</sup> F2 was 15.82 μL/L, the concentration of CO<sup>2</sup> was 16.63 μL/L, and the concentration of CF<sup>4</sup> was 32.68 μL/L. The concentration of components in ascending order is SOF<sup>2</sup> > CF<sup>4</sup> > CO<sup>2</sup> > SO<sup>2</sup> F2 . The concentration of the four characteristic components did not increase linearly. In the first 24 h of the experiment, the components concentration increased linearly, and the increasing rate was larger; from 24 to 36 h, the concentration of components also increased, whereas the increasing rate dropped sharply; from 36 to 48 h, the increasing rate rapidly increased. After 48 h, the increasing rate decreased slowly. At the end of the experiment, all characteristics components concentration almost stopped increasing. The main reason for this is that the discharge formed by the defects of P-type defect is unstable; the free particles are moved under strong electric field force due to their small mass. Only when moved to the position conducive to discharge will the particles lead to discharge. These particles may move randomly, which will lead to unstable discharge.

A conclusion of SF<sup>6</sup>

a small amount of CO<sup>2</sup>

the concentration of CO<sup>2</sup>

In GIS, PD will ionize SF<sup>6</sup>

**5.1. Experimental setup**

SO<sup>2</sup> F2

SOF<sup>2</sup>

more CO<sup>2</sup>

decomposition component under four types of insulation defects can be

Typical Internal Defects of Gas-Insulated Switchgear and Partial Discharge Characteristics

F2

molecules, and electrons will release and gain energy during the

is higher than

121

is detected.

are detected, but

in G-type defect is higher than

http://dx.doi.org/10.5772/intechopen.79090

. Under M-type defect, only

also vary with different types of defects,

and CO<sup>2</sup>

. Under N-type and M-type defects, the

drawn that, amounts and ratio of decomposition components are different under different defects. Under each defect, the decomposition components under four types of defects com-

and CO<sup>2</sup>

decomposition components increasing rate is stable, which is due to the reason that PD is stable under the two defects. In contrast, under P-type and G-type defects, PD is unstable. The reason is that particles and gap is not conducive to stable PD. The repetition rate varies with time, as well as the discharge amplitude. Especially under P-type defect, due to movement of

ionization process. When the electrons release energy, they will release photons at the same time, which are called luminescence; the positive and negative ions after ionization also recombine to release photons and become a composite light. Optical measurement uses photoelectric sensors to detect PD in the light intensity it generated to determine its strength. For optical measurement of signal generated by PD in GIS, detection system is less affected by outside interference and has higher sensitivity of measurement. It can detect PD in real time and identify the position of PD. Therefore, it can be used for on-line monitoring of PD in GIS. At present, there are mainly two ways to detect the optical signal generated by PD in the GIS by optical measurement: one is to directly use the photoelectric sensor to detect the optical signal generated by the PD; the other is to insert the optical fiber sensor into the GIS to detect the optical signal generated by the PD. The former is more flexible to install, but the detection range is smaller, whereas the latter installation is more fixed, but the detection range is larger. Different types of insulation defects lead to different optical signals released by the PD, so the

The schematic diagram of the fluorescence optical fiber sensing system used to study the optical characteristics of the typical defects is shown in **Figure 19**. The optical fiber sensor system mainly comprises an optical sensor unit, optical transmission unit, photoelectric conversion unit, power supply module, and electrical signal transmission and acquisition unit [12].

Four types of single-defect models in this paper are respectively put into the device. After the preparation, the experimental device is applied with the experimental voltage. Slowly raise the test voltage and record the initial discharge voltage of the four single-defect models. Continue to

is larger than that of CO<sup>2</sup>

is detected under N-type defect, and only a smaller amount of CF<sup>4</sup>

is detected; under G-type defect, both CF<sup>4</sup>

pared with each other is as shown in **Figure 18**. For example, the amount of SOF<sup>2</sup>

is higher than that of CF<sup>4</sup>

PD can be identified and diagnosed by using the optical measurement [12].

metal particles, concentrations of decomposition components vary most intensively.

under the N-type defect, whereas the amount of SO<sup>2</sup>

**5. Optical characteristics of typical defects PD**

. The decomposition amounts of CF<sup>4</sup>

Under P-type defect, the amount of CF<sup>4</sup>

The characteristic decomposition components under G-type defect are shown in **Figure 17(d)**. The concentration of the four characteristic decomposition components under the insulation defect is low. At the end of the experiment, the concentration of SOF<sup>2</sup> was 3.71 μL/L, the concentration of SO<sup>2</sup> F2 was 7.57 μL/L, the concentration of CO<sup>2</sup> was 6.37 μL/L, and the concentration of CF<sup>4</sup> was 1.01 μL/L. The concentration of components in ascending order is SO<sup>2</sup> F<sup>2</sup> > CO<sup>2</sup> > SOF<sup>2</sup> > CF<sup>4</sup> . There is no obvious regularity in the increasing of concentration of the four characteristic components. The time-varying increasing rate is mainly due to the unstable PD, sometimes the discharge is very intense, and sometimes discharge stops. In addition, the overall discharge repetition rate is not high, resulting in the overall concentration of decomposition products not high and growth not regular.

**Figure 18.** Decomposition components amount under four types of defects. (a) SOF<sup>2</sup> amount, (b) SO<sup>2</sup> F2 amount, (c) CO<sup>2</sup> amount, and (d) CF<sup>4</sup> amount.

A conclusion of SF<sup>6</sup> decomposition component under four types of insulation defects can be drawn that, amounts and ratio of decomposition components are different under different defects. Under each defect, the decomposition components under four types of defects compared with each other is as shown in **Figure 18**. For example, the amount of SOF<sup>2</sup> is higher than SO<sup>2</sup> F2 under the N-type defect, whereas the amount of SO<sup>2</sup> F2 in G-type defect is higher than SOF<sup>2</sup> . The decomposition amounts of CF<sup>4</sup> and CO<sup>2</sup> also vary with different types of defects, more CO<sup>2</sup> is detected under N-type defect, and only a smaller amount of CF<sup>4</sup> is detected. Under P-type defect, the amount of CF<sup>4</sup> is larger than that of CO<sup>2</sup> . Under M-type defect, only a small amount of CO<sup>2</sup> is detected; under G-type defect, both CF<sup>4</sup> and CO<sup>2</sup> are detected, but the concentration of CO<sup>2</sup> is higher than that of CF<sup>4</sup> . Under N-type and M-type defects, the decomposition components increasing rate is stable, which is due to the reason that PD is stable under the two defects. In contrast, under P-type and G-type defects, PD is unstable. The reason is that particles and gap is not conducive to stable PD. The repetition rate varies with time, as well as the discharge amplitude. Especially under P-type defect, due to movement of metal particles, concentrations of decomposition components vary most intensively.

## **5. Optical characteristics of typical defects PD**

In GIS, PD will ionize SF<sup>6</sup> molecules, and electrons will release and gain energy during the ionization process. When the electrons release energy, they will release photons at the same time, which are called luminescence; the positive and negative ions after ionization also recombine to release photons and become a composite light. Optical measurement uses photoelectric sensors to detect PD in the light intensity it generated to determine its strength. For optical measurement of signal generated by PD in GIS, detection system is less affected by outside interference and has higher sensitivity of measurement. It can detect PD in real time and identify the position of PD. Therefore, it can be used for on-line monitoring of PD in GIS.

At present, there are mainly two ways to detect the optical signal generated by PD in the GIS by optical measurement: one is to directly use the photoelectric sensor to detect the optical signal generated by the PD; the other is to insert the optical fiber sensor into the GIS to detect the optical signal generated by the PD. The former is more flexible to install, but the detection range is smaller, whereas the latter installation is more fixed, but the detection range is larger. Different types of insulation defects lead to different optical signals released by the PD, so the PD can be identified and diagnosed by using the optical measurement [12].

#### **5.1. Experimental setup**

to 36 h, the concentration of components also increased, whereas the increasing rate dropped sharply; from 36 to 48 h, the increasing rate rapidly increased. After 48 h, the increasing rate decreased slowly. At the end of the experiment, all characteristics components concentration almost stopped increasing. The main reason for this is that the discharge formed by the defects of P-type defect is unstable; the free particles are moved under strong electric field force due to their small mass. Only when moved to the position conducive to discharge will the particles lead to discharge. These particles may move randomly, which will lead to unstable discharge. The characteristic decomposition components under G-type defect are shown in **Figure 17(d)**. The concentration of the four characteristic decomposition components under the insula-

was 7.57 μL/L, the concentration of CO<sup>2</sup>

of the four characteristic components. The time-varying increasing rate is mainly due to the unstable PD, sometimes the discharge is very intense, and sometimes discharge stops. In addition, the overall discharge repetition rate is not high, resulting in the overall concentra-

was 1.01 μL/L. The concentration of components in ascending order is

. There is no obvious regularity in the increasing of concentration

was 3.71 μL/L,

was 6.37 μL/L, and the

amount, (b) SO<sup>2</sup>

F2

amount, (c) CO<sup>2</sup>

tion defect is low. At the end of the experiment, the concentration of SOF<sup>2</sup>

tion of decomposition products not high and growth not regular.

**Figure 18.** Decomposition components amount under four types of defects. (a) SOF<sup>2</sup>

F2

120 Simulation and Modelling of Electrical Insulation Weaknesses in Electrical Equipment

the concentration of SO<sup>2</sup>

F<sup>2</sup> > CO<sup>2</sup> > SOF<sup>2</sup> > CF<sup>4</sup>

concentration of CF<sup>4</sup>

amount, and (d) CF<sup>4</sup>

amount.

SO<sup>2</sup>

The schematic diagram of the fluorescence optical fiber sensing system used to study the optical characteristics of the typical defects is shown in **Figure 19**. The optical fiber sensor system mainly comprises an optical sensor unit, optical transmission unit, photoelectric conversion unit, power supply module, and electrical signal transmission and acquisition unit [12].

Four types of single-defect models in this paper are respectively put into the device. After the preparation, the experimental device is applied with the experimental voltage. Slowly raise the test voltage and record the initial discharge voltage of the four single-defect models. Continue to

**Figure 19.** Figure of the fluorescent fiber sensor system.

slowly increase the experimental voltage and collect PD signal of different discharge intensity. Because PD signal of every power frequency cycle needs to be collected, a reference voltage signal should be introduced before the PD signal is collected to correct the phase of PD. Oscilloscope sampling frequency is set to 50 Ms/s, the total acquisition signal length 20 ms, and sampling points 1 M. In experiments, the fluorescent fiber sensing system stores signals in a time domain waveform. Therefore, PD light pulses must be extracted from the time-domain waveform that record the PD signal for each cycle. The method comprises the following steps: set a threshold firstly according to noise amplitude and extract a PD light pulse whose amplitude is greater than the threshold and record and store the amplitude and corresponding phase of the PD light pulse.

#### **5.2.** *φ-u-n* **distribution characteristics of photodetector PDs with different defects**

In this chapter, the *φ-u-n* spectral is used to analyze PD. In *φ-u-n* space, *φ* represents the phase of PD power frequency, *u* represents the amplitude of PD light pulse signal, characterizing the PD discharge level, and *n* represents the number of discharges. The space surface is constructed by dividing the power-frequency phase *φ*-axis into 256 intervals from 0° to 360° and dividing the amplitude of the optical pulse signal from 0 to 0.1 V into 128 small sections so that the *φ-u* plane is divided into 128 × 256 cells; count *φ-u* plane discharge times within each cell, and one can get the space surface. The *φ-u-n* space surface constructed in this paper is based on 200 power-frequency signals. The three-dimensional map of the different defects obtained from the collected PD data is shown in **Figure 20**. There is a significant difference between the three-dimensional spectra of the *φ-u-n* obtained by detecting different internal defects in the GIS using the optical method.

space charge generated by discharge diffuses rapidly in the gas. The influence of space charge on the external electric field is very small. The initial discharge voltage of corona discharge is almost equal to the extinction voltage. Therefore, the light pulses are distributed around the 270° phase of the negative half cycle of the power frequency and symmetrical about 270°.

**Figure 20.** The *φ-u-n* chart of PD induced by the four types of defects in GIS. (a) N-type, (b) G-type, (c) M-type, and (d) P-type.

Typical Internal Defects of Gas-Insulated Switchgear and Partial Discharge Characteristics

http://dx.doi.org/10.5772/intechopen.79090

123

For G-type defect, the pulse repetition rate is high, the average amplitude is small, the amplitude range is wide, and the light pulse distribution has obvious phase characteristics, which are all distributed at the phases of 90 and 270°. Phase width distributed in the 90–180° is greater than 0–90°, and 180–270° phase width greater than 270–360°. The reason is that the PD caused by the G-type defect between the insulator and the metal conductor is not very stable. The light intensity generated by a single discharge is not uniform, but the light intensity generated by the discharge is relatively small. Therefore, the average light pulse detected has small amplitude and large amplitude range. As the insulator hinders the spread of the space charge, the space charge will cause the distortion of the external electric field so that the initial discharge voltage caused by the air gap defect between the insulator and the metal conductor is higher than the extinction voltage. Therefore, the light pulse phase width distributed between 90 and

180° is greater than 0–90° and between 180–270° phase width is greater than 270–360°.

For M-type defect, the pulse repetition rate is low, the average pulse amplitude is small, the amplitude range is large, and the light pulse distribution has obvious phase characteristics

For N-type defect, the repetition rate of light pulse is high, the average amplitude is large, the range of amplitude variation is small, and the light pulse distribution has obvious phase characteristics and symmetrical about 270° in phase. The reason is that the PD under N-type defect is a typical corona discharge which is relatively stable, the intensity of the single discharge is small, the light signal generated by the discharge is relatively stable, and the light intensity generated by the single discharge is relatively large. Therefore, the average amplitude of the detected light pulse is large, and the range of the amplitude is small. The initial discharge voltage of positive half cycle of corona discharge is higher than the negative half cycle of power frequency, and the Typical Internal Defects of Gas-Insulated Switchgear and Partial Discharge Characteristics http://dx.doi.org/10.5772/intechopen.79090 123

slowly increase the experimental voltage and collect PD signal of different discharge intensity. Because PD signal of every power frequency cycle needs to be collected, a reference voltage signal should be introduced before the PD signal is collected to correct the phase of PD. Oscilloscope sampling frequency is set to 50 Ms/s, the total acquisition signal length 20 ms, and sampling points 1 M. In experiments, the fluorescent fiber sensing system stores signals in a time domain waveform. Therefore, PD light pulses must be extracted from the time-domain waveform that record the PD signal for each cycle. The method comprises the following steps: set a threshold firstly according to noise amplitude and extract a PD light pulse whose amplitude is greater than the threshold and record and store the amplitude and corresponding phase of the PD light pulse.

**5.2.** *φ-u-n* **distribution characteristics of photodetector PDs with different defects**

defects in the GIS using the optical method.

**Figure 19.** Figure of the fluorescent fiber sensor system.

122 Simulation and Modelling of Electrical Insulation Weaknesses in Electrical Equipment

In this chapter, the *φ-u-n* spectral is used to analyze PD. In *φ-u-n* space, *φ* represents the phase of PD power frequency, *u* represents the amplitude of PD light pulse signal, characterizing the PD discharge level, and *n* represents the number of discharges. The space surface is constructed by dividing the power-frequency phase *φ*-axis into 256 intervals from 0° to 360° and dividing the amplitude of the optical pulse signal from 0 to 0.1 V into 128 small sections so that the *φ-u* plane is divided into 128 × 256 cells; count *φ-u* plane discharge times within each cell, and one can get the space surface. The *φ-u-n* space surface constructed in this paper is based on 200 power-frequency signals. The three-dimensional map of the different defects obtained from the collected PD data is shown in **Figure 20**. There is a significant difference between the three-dimensional spectra of the *φ-u-n* obtained by detecting different internal

For N-type defect, the repetition rate of light pulse is high, the average amplitude is large, the range of amplitude variation is small, and the light pulse distribution has obvious phase characteristics and symmetrical about 270° in phase. The reason is that the PD under N-type defect is a typical corona discharge which is relatively stable, the intensity of the single discharge is small, the light signal generated by the discharge is relatively stable, and the light intensity generated by the single discharge is relatively large. Therefore, the average amplitude of the detected light pulse is large, and the range of the amplitude is small. The initial discharge voltage of positive half cycle of corona discharge is higher than the negative half cycle of power frequency, and the

**Figure 20.** The *φ-u-n* chart of PD induced by the four types of defects in GIS. (a) N-type, (b) G-type, (c) M-type, and (d) P-type.

space charge generated by discharge diffuses rapidly in the gas. The influence of space charge on the external electric field is very small. The initial discharge voltage of corona discharge is almost equal to the extinction voltage. Therefore, the light pulses are distributed around the 270° phase of the negative half cycle of the power frequency and symmetrical about 270°.

For G-type defect, the pulse repetition rate is high, the average amplitude is small, the amplitude range is wide, and the light pulse distribution has obvious phase characteristics, which are all distributed at the phases of 90 and 270°. Phase width distributed in the 90–180° is greater than 0–90°, and 180–270° phase width greater than 270–360°. The reason is that the PD caused by the G-type defect between the insulator and the metal conductor is not very stable. The light intensity generated by a single discharge is not uniform, but the light intensity generated by the discharge is relatively small. Therefore, the average light pulse detected has small amplitude and large amplitude range. As the insulator hinders the spread of the space charge, the space charge will cause the distortion of the external electric field so that the initial discharge voltage caused by the air gap defect between the insulator and the metal conductor is higher than the extinction voltage. Therefore, the light pulse phase width distributed between 90 and 180° is greater than 0–90° and between 180–270° phase width is greater than 270–360°.

For M-type defect, the pulse repetition rate is low, the average pulse amplitude is small, the amplitude range is large, and the light pulse distribution has obvious phase characteristics and is distributed around the phase 90 and 270°. Phase width distributed in the 90–180° is greater than 0–90° and in the 270–360° phase width is greater than 180–270°. The reason is that the PD produced by M-type defect will generate electrical branches on the surface of the insulator, which will affect the insulation of the insulator surface. As a result, the PD is not very stable, and the light intensity produced by a single discharge is different. However, the overall light intensity is relatively small, so the optical measurement method detecting average amplitude of the light pulse is small, with a wide range of amplitude.

**Author details**

**References**

2003;**19**(2):23-30

**17**(1):247-255

Fuping Zeng\*, Ju Tang, Xiaoxing Zhang, Siyuan Zhou and Cheng Pan

School of Electrical Engineering, Wuhan University, Wuhan, China

Dielectrics & Electrical Insulation. 2013;**20**(4):1049-1055

and diagnostic techniques. Electra. 1998;**176**(2):67-95

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[2] Dai D et al. Feature extraction of GIS partial discharge signal based on S-transform and singular value decomposition. Iet Science Measurement & Technology. 2017;**11**(2):186-193

[3] Okabe S, Ueta G, Hama H, et al. New aspects of UHF PD diagnostics on gas-insulated systems. IEEE Transactions on Dielectrics and Electrical Insulation. 2014;**21**(5):2245-2258

[4] Cigre WG. 33/23-12 Insulation coordination of GIS: Return of experience, on site tests

[5] Pearson JS, Hampton BE, Sellars AG. A continuous UHF monitor for gas-insulated sub-

[6] Cavallini A, Montanari GC, Contin A, Pulletti F. A new approach to the diagnosis of solid insulation systems based on PD signal inference. IEEE Electrical Insulation Magazine.

[7] Tang J, Zhou R, Wang DB, et al. Application of SA-SVM incremental algorithm in GIS PD pattern recognition. Journal of Electrical Engineering and Technology. 2016;**11**(1):192-199

[8] Koo JY et al. Identification of insulation defects in gas-insulated switchgear by chaotic analysis of partial discharge. Iet Science Measurement & Technology. 2010;**4**(3):115-124

[9] Gao W, Ding D, Liu W. Research on the typical partial discharge using the UHF detection method for GIS. IEEE Transactions on Power Delivery. 2011;**26**(4):2621-2629 [10] DEA M et al. Charge accumulation effects on time transition of partial discharge activity at GIS spacer defects. IEEE Transactions on Dielectrics & Electrical Insulation. 2010;

[11] Zhang X et al. GIS partial discharge pattern recognition based on the chaos theory. IEEE

[12] Zhuo R. Feature optimization and fault diagnosis of GIS based on combined detection

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[PhD thesis]. Chongqing University; 2014 (in Chinese)

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\*Address all correspondence to: fuping.zeng@whu.edu.cn

For P-type defect, the pulse repetition rate is low, the average amplitude is large, the range of the amplitude changes is large, and the phase distribution of the light pulse is not characterized. The reason is that metal particles in the external electric field obtain the induced charge and will move under electric force. The movement intensity of the metal particles depends on the induced charges, the shape of the particles, the direction of movement of the particles, and whether the particles collide with other objects during the movement. PD generated by P-type defects is caused by the movement of the metal particles. Therefore, it is very unstable, and the phase of PD is also irregular.

## **6. Conclusion**

In this chapter, typical defects in GIS are discussed and physical model is established, then different resulting PD is studied. Four typical defects and their respective PD UHF characteristics, chemical characteristics, and optical characteristics are then obtained by experiments. Different figures and data owing to different types of PD are compared with each other so that unique features could be further extracted.

As for UHF characteristics, it can be seen visually that waveforms of different defects have obvious difference. Then some parameters can be designed to measure the essential difference, which can be presented as fingerprints. In time domain, statistics parameters are selected as features. For example, these parameters include mean, variance, skewness, kurtosis, etc. While in frequency domain, these parameters also works. In addition, Shannon entropy, wavelet sub-band energy, and absolute value of peaks can also be included. Based on these features, methods such as support vector machine can be applied to classify the defects.

For chemical characteristics, it can be concluded that the SF<sup>6</sup> PD decomposition components amount under the four types of insulation defects are obviously different, and so is their ratio. So the insulation defect can be identified by detecting PD decomposition component of SF<sup>6</sup> . Methods such as artificial neural network can then be set up to classify the defects. The concentration and ratio of each decomposition components are the input variables and during training process the defect is finally classified.

For optical characteristics, just as UHF characteristics, the spectrals of different defects have obvious difference. So some statistics parameters are introduced. Because the optical spectral has three dimensions, projection on two-dimension plane is firstly needed, and then parameters are extracted. The classification step is like that of UHF or chemical characteristics.

## **Author details**

and is distributed around the phase 90 and 270°. Phase width distributed in the 90–180° is greater than 0–90° and in the 270–360° phase width is greater than 180–270°. The reason is that the PD produced by M-type defect will generate electrical branches on the surface of the insulator, which will affect the insulation of the insulator surface. As a result, the PD is not very stable, and the light intensity produced by a single discharge is different. However, the overall light intensity is relatively small, so the optical measurement method detecting aver-

For P-type defect, the pulse repetition rate is low, the average amplitude is large, the range of the amplitude changes is large, and the phase distribution of the light pulse is not characterized. The reason is that metal particles in the external electric field obtain the induced charge and will move under electric force. The movement intensity of the metal particles depends on the induced charges, the shape of the particles, the direction of movement of the particles, and whether the particles collide with other objects during the movement. PD generated by P-type defects is caused by the movement of the metal particles. Therefore, it is very unstable,

In this chapter, typical defects in GIS are discussed and physical model is established, then different resulting PD is studied. Four typical defects and their respective PD UHF characteristics, chemical characteristics, and optical characteristics are then obtained by experiments. Different figures and data owing to different types of PD are compared with each other so that

As for UHF characteristics, it can be seen visually that waveforms of different defects have obvious difference. Then some parameters can be designed to measure the essential difference, which can be presented as fingerprints. In time domain, statistics parameters are selected as features. For example, these parameters include mean, variance, skewness, kurtosis, etc. While in frequency domain, these parameters also works. In addition, Shannon entropy, wavelet sub-band energy, and absolute value of peaks can also be included. Based on these features, methods such as support vector machine can be applied to classify the defects.

amount under the four types of insulation defects are obviously different, and so is their ratio. So the insulation defect can be identified by detecting PD decomposition component of

For optical characteristics, just as UHF characteristics, the spectrals of different defects have obvious difference. So some statistics parameters are introduced. Because the optical spectral has three dimensions, projection on two-dimension plane is firstly needed, and then parameters are extracted. The classification step is like that of UHF or chemical characteristics.

. Methods such as artificial neural network can then be set up to classify the defects. The concentration and ratio of each decomposition components are the input variables and during

PD decomposition components

age amplitude of the light pulse is small, with a wide range of amplitude.

124 Simulation and Modelling of Electrical Insulation Weaknesses in Electrical Equipment

and the phase of PD is also irregular.

unique features could be further extracted.

training process the defect is finally classified.

For chemical characteristics, it can be concluded that the SF<sup>6</sup>

**6. Conclusion**

SF<sup>6</sup>

Fuping Zeng\*, Ju Tang, Xiaoxing Zhang, Siyuan Zhou and Cheng Pan

\*Address all correspondence to: fuping.zeng@whu.edu.cn

School of Electrical Engineering, Wuhan University, Wuhan, China

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**Section 3**

**Insulation Weaknessess Modelling for Electrical**

**Devices Used in Smart Grids**


**Insulation Weaknessess Modelling for Electrical Devices Used in Smart Grids**

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partial discharge recognition under negative DC conditions. Energies. 2017;**10**:556 [24] Dong M, Zhang C, Ren M, Albarracín R, Ye R. Electrochemical and infrared absorption

products part 2: Feature extraction and decision tree-based pattern recognition. IEEE

[26] Zhuo R et al. Study on relationship between optical signals and charge quantity of partial discharge under four typical insulation defects. Electrical Insulation and Dielectric

[27] Dong Y. Research on feature extraction and severity assessment of partial discharge in

[28] Tang J et al. Study on mathematical model for VHF partial discharge of typical insulated defects in GIS. IEEE Transactions on Dielectrics & Electrical Insulation. 2007;**14**(1):30-38

decomposition products. Sensors. 2017;**17**:2627

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charge measuring methods and sensors. Sensors. 2018;**18**:720

source localization algorithms. Sensors. 2017;**17**:2666

spectroscopy detection of SF<sup>6</sup>

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and calibration of impact factors [PhD thesis]. Chongqing University; 2013 (in Chinese)

under PD & recognition of PD category and

under PD & recognition of PD category

and

decomposition

**Chapter 6**

**Provisional chapter**

**Electrical Insulation Weaknesses in Wide Bandgap**

**Electrical Insulation Weaknesses in Wide Bandgap** 

DOI: 10.5772/intechopen.77657

The power electronics research community is balancing on the edge of a game-changing technological innovation: as traditionally silicon (Si) based power semiconductors approach their material limitations, next-generation wide bandgap (WBG) power semiconductors are poised to overtake them. Promising WBG materials are silicon carbide

(AlN). They can operate at higher voltages, temperatures, and switching frequencies with greater efficiencies compared to existing Si, in power electronics. These characteristics can reduce energy consumption, which is critical for national economic, health, and security interests. However, increased voltage blocking capability and trend toward more compact packaging technology for high-power density WBG devices can enhance the local electric field that may become large enough to raise partial discharges (PDs) within the module. High activity of PDs damages the insulating silicone gel, lead to electrical insulation failure and reduce the reliability of the module. Among WBG devices, electrical insulation weaknesses in WBG-based Insulated Gate Bipolar Transistor (IGBT) have been more investigated. The chapter deals with (a) current standards for evaluation of the insulation systems of power electronics modules, (b) simulation and modeling of the electric field stress inside modules, (c) diagnostic tests on modules, and (d) PD control methods in modules.

The growing integration of distributed generation resources, envisagement of direct current (DC) microgrids and high-voltage direct current (HVDC) networks, the continued electrification and grid-level power flow controls call for advanced power electronics with improved

O3

) and aluminum nitride

(SiC), gallium nitride (GaN), diamond (C), gallium oxide (Ga2

**Keywords:** wide bandgap devices, partial discharge, electric field stress

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

© 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use,

distribution, and reproduction in any medium, provided the original work is properly cited.

**Devices**

**Devices**

Mona Ghassemi

**Abstract**

**1. Introduction**

Additional information is available at the end of the chapter

Mona GhassemiAdditional information is available at the end of the chapter

http://dx.doi.org/10.5772/intechopen.77657

#### **Electrical Insulation Weaknesses in Wide Bandgap Devices Electrical Insulation Weaknesses in Wide Bandgap Devices**

DOI: 10.5772/intechopen.77657

#### Mona Ghassemi

Additional information is available at the end of the chapter Mona GhassemiAdditional information is available at the end of the chapter

http://dx.doi.org/10.5772/intechopen.77657

#### **Abstract**

The power electronics research community is balancing on the edge of a game-changing technological innovation: as traditionally silicon (Si) based power semiconductors approach their material limitations, next-generation wide bandgap (WBG) power semiconductors are poised to overtake them. Promising WBG materials are silicon carbide (SiC), gallium nitride (GaN), diamond (C), gallium oxide (Ga2 O3 ) and aluminum nitride (AlN). They can operate at higher voltages, temperatures, and switching frequencies with greater efficiencies compared to existing Si, in power electronics. These characteristics can reduce energy consumption, which is critical for national economic, health, and security interests. However, increased voltage blocking capability and trend toward more compact packaging technology for high-power density WBG devices can enhance the local electric field that may become large enough to raise partial discharges (PDs) within the module. High activity of PDs damages the insulating silicone gel, lead to electrical insulation failure and reduce the reliability of the module. Among WBG devices, electrical insulation weaknesses in WBG-based Insulated Gate Bipolar Transistor (IGBT) have been more investigated. The chapter deals with (a) current standards for evaluation of the insulation systems of power electronics modules, (b) simulation and modeling of the electric field stress inside modules, (c) diagnostic tests on modules, and (d) PD control methods in modules.

**Keywords:** wide bandgap devices, partial discharge, electric field stress

#### **1. Introduction**

The growing integration of distributed generation resources, envisagement of direct current (DC) microgrids and high-voltage direct current (HVDC) networks, the continued electrification and grid-level power flow controls call for advanced power electronics with improved

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

efficiency, reliability and power density [1]. Undergoing dynamic evolution in power electronics is mainly due to the development of power semiconductor devices seeking simultaneous operation at a higher voltage, power, and switching frequency. Having higher blocking voltage capability, higher temperature tolerance, and higher switching frequency than Si technology, wide-bandgap (WBG) semiconductor materials such as silicon carbide (SiC) and gallium nitride (GaN) are expected to be a response to the mentioned challenge [2]. Si has a bandgap of 1.1 eV, whereas the bandgap of SiC and GaN is, respectively, 3.3 and 3.4 eV. The bandgap is the energy required to transfer an electron from the valence to the conduction band. Insulators, semiconductors, and conductors have large, small and very small bandgaps, respectively. While the highest commercial Si IGBT breakdown voltage capability is 6.5 kV, a record high blocking voltage of 15 kV was reported for the SiC IGBT produced in [3] and higher voltage capability up to 20–30 kV is expected shortly [4].

bonded copper (DBC) or active metal brazing (AMB). **Figure 1a** shows active metal brazing method [7]. No braze layers are needed for DBC as shown in **Figure 2**. Then a soft dielectric such as silicone gel is used to encapsulate the whole module to prevent electrical discharges in air as well as to protect semiconductors, substrates, and connections against humidity, dirt, and vibration. As a commercial example, "SYLGARD™ 527 Silicone Dielectric Gel" manufactured by the Dow Chemical Company has a dielectric constant of 2.85 and dielectric strength

**Figure 2.** A schematic of an IGBT module with DBC, (a) connection to bus bars, (b) bond wire, (c) diode, (d) IGBT, (e)

Electrical Insulation Weaknesses in Wide Bandgap Devices

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131

plastic case, (f) baseplate, (g) silicone gel, (h) AlN ceramic, and (i) copper metallization.

Silicone gel also prevents thermal induced movements of bond wires attached to the semiconductor. The final encapsulation is achieved using polymer housing. The schematic layout of an IGBT with AMB is similar to that shown in **Figure 2** where brazes are also added to the structure. As a consequence of higher blocking voltage, new packaging solutions to provide electrical insulation between the grounded heat sink and the HV terminals of the module are required. Due to the high electric fields, especially at the edges of the copper metallization, PDs can be initiated from these regions. The situation gets worse at protrusions shown in **Figure 1b** with extremely sharp edges of some braze below the metallization. High activity of PDs damages the insulating silicone gel and leads to electrical insulation failure and reduces the reliability of the module. Moreover, high-frequency PD pulses can lead to disturbance of the power electronics and cause severe shortcomings in high-power applications. The PD issue is one of the most crucial challenges to the development of HV high power density WBG power semiconductor devices. A description of current standards on PD tests on power electronics modules and relevant technical gaps is presented in Section 2. Section 3 deals with simulation and modeling of electric field stress inside power electronics modules. The various PD detection techniques employed for modules and correlation between measurements and electric field calculation is discussed in Section 4, and finally, PD control methods to relieve high field

**2. International standards on PD tests on power electronics devices**

IEC 61287-1: "Railway applications-power converters installed on board rolling stock-part 1: characteristics and test methods" is the current standard commonly used for IGBT working at 1.5 kV or more [10]. The test voltage is a 50 Hz or 60 Hz alternating current (AC) root mean

of 17 kV/mm [9].

regions is explained in Section 5.

The metalized ceramic substrate shown in **Figure 1a** is well-known and established insulation technology for blocking voltages up to 3.3 kV, but it exhibits some weaknesses due to partial discharges (PDs) in silicone gel at higher voltages. In a sufficient electric stress condition localized gaseous breakdowns known as PDs can occur within an insulation system. Various measuring techniques and sensors have been developed for PD detection to perform an accurate condition monitoring and assessment of the insulation status of power equipment [5, 6]. We will discuss this topic in Section 4. The blocking voltage places across substrate solid insulating material, which is aluminum nitride (AlN) or alumina (Al2 O3 ) ceramics where HV electrode is IGBT or diode and the ground electrode is copper or aluminum silicon carbide (AlSiC) base plate connected to the heat sink. However, both sides of the insulating ceramic are metalized by copper to evacuate better and transfer the heat generated by IGBTs or diodes to the base plate [7]. IGBTs, diodes and base plate are soldered onto the metalized ceramic substrate [7]. In this regard, solid dielectric substrates should also have appropriate thermal properties such as resistance to high temperatures and good thermal conductivity in addition to their desirable electrical insulation and mechanical properties. This is the case for AlN and Al2 O3 with a thermal conductivity of typically, respectively, 180 and 27 W/mK [8]. Note that, however, the thermal resistance of the AlN substrates assembled with IGBTs is around a factor of only three less than Al2 O3 substrates [8] (not 180/27 = 6.7 times for AlN and Al2 O3 materials itself). Attaching the copper metallization to the ceramic substrate can be done by direct

**Figure 1.** (a) A schematic of an IGBT substrate with active metal brazing (AMB) of the metallization and (b) protrusions with extremely sharp edges of some braze below the metallization.

efficiency, reliability and power density [1]. Undergoing dynamic evolution in power electronics is mainly due to the development of power semiconductor devices seeking simultaneous operation at a higher voltage, power, and switching frequency. Having higher blocking voltage capability, higher temperature tolerance, and higher switching frequency than Si technology, wide-bandgap (WBG) semiconductor materials such as silicon carbide (SiC) and gallium nitride (GaN) are expected to be a response to the mentioned challenge [2]. Si has a bandgap of 1.1 eV, whereas the bandgap of SiC and GaN is, respectively, 3.3 and 3.4 eV. The bandgap is the energy required to transfer an electron from the valence to the conduction band. Insulators, semiconductors, and conductors have large, small and very small bandgaps, respectively. While the highest commercial Si IGBT breakdown voltage capability is 6.5 kV, a record high blocking voltage of 15 kV was reported for the SiC IGBT produced in [3] and

The metalized ceramic substrate shown in **Figure 1a** is well-known and established insulation technology for blocking voltages up to 3.3 kV, but it exhibits some weaknesses due to partial discharges (PDs) in silicone gel at higher voltages. In a sufficient electric stress condition localized gaseous breakdowns known as PDs can occur within an insulation system. Various measuring techniques and sensors have been developed for PD detection to perform an accurate condition monitoring and assessment of the insulation status of power equipment [5, 6]. We will discuss this topic in Section 4. The blocking voltage places across substrate solid

electrode is IGBT or diode and the ground electrode is copper or aluminum silicon carbide (AlSiC) base plate connected to the heat sink. However, both sides of the insulating ceramic are metalized by copper to evacuate better and transfer the heat generated by IGBTs or diodes to the base plate [7]. IGBTs, diodes and base plate are soldered onto the metalized ceramic substrate [7]. In this regard, solid dielectric substrates should also have appropriate thermal properties such as resistance to high temperatures and good thermal conductivity in addition to their desirable electrical insulation and mechanical properties. This is the case for AlN and

 with a thermal conductivity of typically, respectively, 180 and 27 W/mK [8]. Note that, however, the thermal resistance of the AlN substrates assembled with IGBTs is around a fac-

als itself). Attaching the copper metallization to the ceramic substrate can be done by direct

**Figure 1.** (a) A schematic of an IGBT substrate with active metal brazing (AMB) of the metallization and (b) protrusions

substrates [8] (not 180/27 = 6.7 times for AlN and Al2

O3

) ceramics where HV

O3

materi-

higher voltage capability up to 20–30 kV is expected shortly [4].

130 Simulation and Modelling of Electrical Insulation Weaknesses in Electrical Equipment

insulating material, which is aluminum nitride (AlN) or alumina (Al2

O3

with extremely sharp edges of some braze below the metallization.

Al2 O3

tor of only three less than Al2

**Figure 2.** A schematic of an IGBT module with DBC, (a) connection to bus bars, (b) bond wire, (c) diode, (d) IGBT, (e) plastic case, (f) baseplate, (g) silicone gel, (h) AlN ceramic, and (i) copper metallization.

bonded copper (DBC) or active metal brazing (AMB). **Figure 1a** shows active metal brazing method [7]. No braze layers are needed for DBC as shown in **Figure 2**. Then a soft dielectric such as silicone gel is used to encapsulate the whole module to prevent electrical discharges in air as well as to protect semiconductors, substrates, and connections against humidity, dirt, and vibration. As a commercial example, "SYLGARD™ 527 Silicone Dielectric Gel" manufactured by the Dow Chemical Company has a dielectric constant of 2.85 and dielectric strength of 17 kV/mm [9].

Silicone gel also prevents thermal induced movements of bond wires attached to the semiconductor. The final encapsulation is achieved using polymer housing. The schematic layout of an IGBT with AMB is similar to that shown in **Figure 2** where brazes are also added to the structure.

As a consequence of higher blocking voltage, new packaging solutions to provide electrical insulation between the grounded heat sink and the HV terminals of the module are required. Due to the high electric fields, especially at the edges of the copper metallization, PDs can be initiated from these regions. The situation gets worse at protrusions shown in **Figure 1b** with extremely sharp edges of some braze below the metallization. High activity of PDs damages the insulating silicone gel and leads to electrical insulation failure and reduces the reliability of the module. Moreover, high-frequency PD pulses can lead to disturbance of the power electronics and cause severe shortcomings in high-power applications. The PD issue is one of the most crucial challenges to the development of HV high power density WBG power semiconductor devices. A description of current standards on PD tests on power electronics modules and relevant technical gaps is presented in Section 2. Section 3 deals with simulation and modeling of electric field stress inside power electronics modules. The various PD detection techniques employed for modules and correlation between measurements and electric field calculation is discussed in Section 4, and finally, PD control methods to relieve high field regions is explained in Section 5.

#### **2. International standards on PD tests on power electronics devices**

IEC 61287-1: "Railway applications-power converters installed on board rolling stock-part 1: characteristics and test methods" is the current standard commonly used for IGBT working at 1.5 kV or more [10]. The test voltage is a 50 Hz or 60 Hz alternating current (AC) root mean square (RMS) voltage equal to 1.5 *Um*/<sup>√</sup> \_\_ 2 or higher where *Um* is the maximum blocking voltage of the module in (V). For a 6.5 kV IGBT, it is 1.5 × 6.5/√ \_\_ 2 ≈ 6.9kV. The voltage is ramped up to 1.5 *Um*/√ \_\_ 2 in 10 s and is maintained for *t* 1 = 1 min as shown in **Figure 3**. The rate of the ramp as shown in **Figure 3** is (1.5 *Um*/<sup>√</sup> \_ 2)/10kV/s. During this time *t* 1 , some PDs may be observed. After *t* 1 , the voltage is decreased to 1.1 *Um*/<sup>√</sup> \_\_ 2 in 10 s. For a 6.5 kV IGBT, it is 1.1 × 6.5/√ \_\_ 2 ≈ 5.1kV. The voltage 1.1 *Um*/√ \_\_ 2 is applied for *t* 2 = 30 s. During the last 5 s of *t* 2 , the peak magnitude of partial discharge in pC is measured. A typical value to pass the test for a component and a subassembly is, respectively, 10 and 50 pC.

However, IGBT modules are subjected to pulse width modulator (PWM) stress-type instead of power frequency AC voltages. To elucidate this stress-type, consider a single-phase fullbridge inverter as shown in **Figure 4a**. An inverter changes a DC input voltage to a symmetrical ac output voltage of desired magnitude and frequency. When switches (which can be IGBTs) *Q*<sup>1</sup> and *Q*<sup>2</sup> are turned on at the same time, the input voltage *Us* appears across the load. In this situation, the voltage on *Q*<sup>3</sup> and *Q*<sup>4</sup> which are off will be *Us* . If switches *Q*<sup>3</sup> and *Q*<sup>4</sup> are turned on simultaneously, the voltage across the load is −*Us* . In this situation, the voltage on *Q*<sup>1</sup> and *Q*<sup>2</sup> which are off will be −*Us* . **Figure 4b** shows the waveform for the output voltage. Thus a unipolar square wave voltage with a magnitude of *Us* for (0 − *T*<sup>0</sup> /2) and almost zero for (*T*0 /2 − *T*<sup>0</sup> ) places on *Q*<sup>3</sup> and *Q*<sup>4</sup> and a unipolar square wave voltage with an amagnitude of nearly zero for (0 − *T*<sup>0</sup> /2) and −*Us* for (*T*<sup>0</sup> /2 − *T*<sup>0</sup> ) places on *Q*<sup>1</sup> and *Q*<sup>2</sup> .

In many industrial applications, it is often required to control the output voltage of inverters (1) to cope with the variations of dc input voltage, (2) for voltage regulation of inverters, and (3) for the constant volts/frequency control requirement [11]. The most efficient method of controlling the gain is to incorporate pulse-width-modulation (PWM) control with the inverters. In this regard, the commonly used techniques are:

rising time of about 400 μs, fast rise positive unipolar square and fast rise negative unipolar square both with rise time of about 100 ns it is 12, 9 and 7 kV, respectively. Moreover, the rate of increase in the PD magnitude concerning voltage is higher for steeper voltage rise [14].

**Figure 3.** IEC 61287-1: "Railway applications-power converters installed on board rolling stock-part 1: characteristics

Electrical Insulation Weaknesses in Wide Bandgap Devices

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133

Therefore new standards are needed to take into account actual voltages for power electronic modules. It was shown in [16] that although IGBTs could pass 50 Hz sinusoidal test under IEC 61287-1, insulation failure occurs when applying PWM input voltage with 50 Hz modulating frequency with 1 kHz carrier frequency and a rise time of 10 μs that is the stress condition

According to IEC 61287-1, the collector, emitter, and gate of a power electronics module should be connected, and PDs are measured when an alternating voltage is applied between the interconnected terminal and the metal base plate. The drawback is that it tests only the insulation

more similar to the real operating conditions.

and test methods" for partial discharge test.

**Figure 4.** Single-phase full-bridge inverter.


Here we describe only sinusoidal-pulse-width modulation (SPWM) technique which is widely used. For other methods see [11]. In SPWM, the width of each gating signal can be varied in proportion to the amplitude of a sine wave. As shown in **Figure 5**, the gating signals are generated by comparing a sinusoidal reference signal with a triangular carrier wave of frequency, *f c* , and peak, *Ac* . In this case, *Q*<sup>3</sup> and *Q*<sup>4</sup> should withstand *uab* which is a fast-rising and fall square waveform known as PWM-stress in (0 − *π*) and *Q*<sup>1</sup> and *Q*<sup>2</sup> should withstand *uab* in (*π* − 2 *π*).

It has been known that repetitive voltage impulses generated as PWM-stress can lead to insulation premature failure of stator winding due to partial discharges in inverter-fed motors [12, 13]. About IGBTs the studies in [14, 15] show that PD behavior under 50 Hz or 60 Hz AC sinusoidal voltage is different from that for fast rise bipolar high-frequency square wave voltages. In this regard, for example for the test sample in [14] while partial discharge inception voltage (PDIV) under 50 Hz sinusoidal test voltage is 13 kV, for a bipolar square voltage with

**Figure 3.** IEC 61287-1: "Railway applications-power converters installed on board rolling stock-part 1: characteristics and test methods" for partial discharge test.

rising time of about 400 μs, fast rise positive unipolar square and fast rise negative unipolar square both with rise time of about 100 ns it is 12, 9 and 7 kV, respectively. Moreover, the rate of increase in the PD magnitude concerning voltage is higher for steeper voltage rise [14].

Therefore new standards are needed to take into account actual voltages for power electronic modules. It was shown in [16] that although IGBTs could pass 50 Hz sinusoidal test under IEC 61287-1, insulation failure occurs when applying PWM input voltage with 50 Hz modulating frequency with 1 kHz carrier frequency and a rise time of 10 μs that is the stress condition more similar to the real operating conditions.

According to IEC 61287-1, the collector, emitter, and gate of a power electronics module should be connected, and PDs are measured when an alternating voltage is applied between the interconnected terminal and the metal base plate. The drawback is that it tests only the insulation

**Figure 4.** Single-phase full-bridge inverter.

square (RMS) voltage equal to 1.5 *Um*/<sup>√</sup>

shown in **Figure 3** is (1.5 *Um*/<sup>√</sup>

2 is applied for *t*

respectively, 10 and 50 pC.

the voltage is decreased to 1.1 *Um*/<sup>√</sup>

and *Q*<sup>2</sup>

) places on *Q*<sup>3</sup>

**1.** Single-pulse-width modulation

**5.** Phase-displacement control

. In this case, *Q*<sup>3</sup>

known as PWM-stress in (0 − *π*) and *Q*<sup>1</sup>

**2.** Multiple-pulse-width modulation **3.** Sinusoidal-pulse-width modulation

zero for (0 − *T*<sup>0</sup> /2) and −*Us*

load. In this situation, the voltage on *Q*<sup>3</sup>

which are off will be −*Us*

and *Q*<sup>4</sup>

2 in 10 s and is maintained for *t*

2

1.5 *Um*/√ \_\_

1.1 *Um*/√ \_\_

be IGBTs) *Q*<sup>1</sup>

and *Q*<sup>2</sup>

on *Q*<sup>1</sup>

peak, *Ac*

(*T*0 /2 − *T*<sup>0</sup>

of the module in (V). For a 6.5 kV IGBT, it is 1.5 × 6.5/√

\_

132 Simulation and Modelling of Electrical Insulation Weaknesses in Electrical Equipment

\_\_

1

= 30 s. During the last 5 s of *t*

\_\_

are turned on simultaneously, the voltage across the load is −*Us*

for (*T*<sup>0</sup> /2 − *T*<sup>0</sup>

Thus a unipolar square wave voltage with a magnitude of *Us*

ers. In this regard, the commonly used techniques are:

**4.** Modified sinusoidal pulse-width modulation

and *Q*<sup>4</sup>

2)/10kV/s. During this time *t*

2 or higher where *Um* is the maximum blocking voltage

= 1 min as shown in **Figure 3**. The rate of the ramp as

2 ≈ 6.9kV. The voltage is ramped up to

, some PDs may be observed. After *t*

2 ≈ 5.1kV. The voltage

appears across the

and *Q*<sup>4</sup>

*c* , and

. If switches *Q*<sup>3</sup>

. In this situation, the voltage

for (0 − *T*<sup>0</sup> /2) and almost zero for

\_\_

, the peak magnitude of partial discharge

1 ,

\_\_

1

which are off will be *Us*

and a unipolar square wave voltage with an amagnitude of nearly

should withstand *uab* which is a fast-rising and fall square waveform

should withstand *uab* in (*π* − 2 *π*).

 and *Q*<sup>2</sup> .

. **Figure 4b** shows the waveform for the output voltage.

2 in 10 s. For a 6.5 kV IGBT, it is 1.1 × 6.5/√

2

in pC is measured. A typical value to pass the test for a component and a subassembly is,

However, IGBT modules are subjected to pulse width modulator (PWM) stress-type instead of power frequency AC voltages. To elucidate this stress-type, consider a single-phase fullbridge inverter as shown in **Figure 4a**. An inverter changes a DC input voltage to a symmetrical ac output voltage of desired magnitude and frequency. When switches (which can

are turned on at the same time, the input voltage *Us*

and *Q*<sup>4</sup>

) places on *Q*<sup>1</sup>

In many industrial applications, it is often required to control the output voltage of inverters (1) to cope with the variations of dc input voltage, (2) for voltage regulation of inverters, and (3) for the constant volts/frequency control requirement [11]. The most efficient method of controlling the gain is to incorporate pulse-width-modulation (PWM) control with the invert-

Here we describe only sinusoidal-pulse-width modulation (SPWM) technique which is widely used. For other methods see [11]. In SPWM, the width of each gating signal can be varied in proportion to the amplitude of a sine wave. As shown in **Figure 5**, the gating signals are generated by comparing a sinusoidal reference signal with a triangular carrier wave of frequency, *f*

It has been known that repetitive voltage impulses generated as PWM-stress can lead to insulation premature failure of stator winding due to partial discharges in inverter-fed motors [12, 13]. About IGBTs the studies in [14, 15] show that PD behavior under 50 Hz or 60 Hz AC sinusoidal voltage is different from that for fast rise bipolar high-frequency square wave voltages. In this regard, for example for the test sample in [14] while partial discharge inception voltage (PDIV) under 50 Hz sinusoidal test voltage is 13 kV, for a bipolar square voltage with

and *Q*<sup>2</sup>

**3. Simulation and modeling of electric stress inside the module**

measuring points at a distance of 20 μm from edges were benefited.

are studied in [20, 22] on reducing the electric field stress values.

Defining an offset of the two metallization layers as *r*

<sup>∇</sup><sup>2</sup> *<sup>U</sup>* <sup>=</sup> <sup>−</sup> *<sup>ρ</sup>*\_\_\_\_*<sup>v</sup>*

AlN ceramic edge to the edge of the upper Cu metallization) and *r*

**1.** The thickness of the metallization layer,

**2.** The thickness of the substrate,

**4.** Metal/conductive layer offset.

**3.** The shape of the edge,

*r*

Poisson's equation.

the effect of one contributing factor.

Since a combination of material defects in gel and the high electric stress due to sharp edges leads to partial discharge, PDs do not occur all along the sharp edges. However, identifying the critical spots with the maximum electric field magnitude due to only sharp edges can be useful to develop geometrical strategies to reduce the electric field magnitude peaks due to

Note that the maximum electric field magnitude at perfectly sharp edges is theoretically infinite. Thus, the smaller mesh size, the higher electric stress and mathematically there is no convergence point. Assuming a rounded edge converges to a finite maximum electric field intensity with increasing resolution of the mesh grid. However, the value depends on the assumed edge radius. The smaller assumed edge radius, the higher amount of maximum electric field magnitude. To overcome this difficulty, it was shown in [20, 21] that when the distance to sharp edges becomes larger than 20 μm for the assumed geometry and dimensions, the differences between the electric field magnitudes for different meshing sizes are less than 1%. To be on the safe side, measuring points were considered at a distance of 50 μm to sharp edges in [20, 21]. In [22], both strategies containing rounded edges and considering

Assuming the measuring points defined above, the influence of following geometrical options

Among four parameters above, the thickness of the substrate and metal/conductive layer offset have a strong influence on the electric field magnitude. By varying the thickness of the ceramic, the electric field stress does not follow the equation of a plate capacitor: a doubling of the thickness (1–2 mm) reduces the electric field stress only by about 30% and not by 50%. However, an increased substrate thickness decreases cooling efficiency of the semiconductors, and this technique may not meet the miniaturization needs of power electronics as well.

AlN ceramic edge to the edge of the lower Cu metallization) shown in **Figure 7a**. **Figure 8** shows the electric field stress values at measuring point located on L1 for different values of

*off* for a d = 630 μm ceramic layer [22]. For that (**Figure 7a**), a finite-element method (FEM) model was developed in the *Electrostatics* (*es*) module of COMSOL Multiphysics solving

> ϵ0 ϵ*r*

*off* = *r <sup>u</sup>* − *r l* for *r u*

*l*

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135

(the distance from the

(the distance from the

(1)

**Figure 5.** Sinusoidal pulse-width modulation.

of the substrate and the bulk of the gel is not tested. To address this issue, the test voltage is proposed as an AC voltage superimposed on a direct current (DC) one directly applied to the component turned off using a negative gate polarization [17–19]. The inverse DC offset of magnitude higher than the AC peak value as shown for an example in **Figure 6a** used as the test voltage avoids diode conduction [18]. The discharge inception voltage (*UDIV*) is then defined as the peak value of the applied voltage (*UDC* <sup>+</sup> *UAC*) [17]. **Figure 6b** shows an experimental set-up generating such test voltage [17].

This method leads to detect PD for voltages lower than the one necessary to trig them during IEC 61287-1 test [17–19]. Although neither the test proposed in [17–19] nor IEC 61287-1 test can represent thoroughly the stresses endured by the power modules in inverters, the testing method proposed in [17–19] can provide more useful information on PDs during normal operation by stressing all the components involved in the packaging.

**Figure 6.** (a) New test voltage waveform and (b) set-up generating new test voltage for PD detection of an IGBT [18], License No. 4383271241884.

## **3. Simulation and modeling of electric stress inside the module**

Since a combination of material defects in gel and the high electric stress due to sharp edges leads to partial discharge, PDs do not occur all along the sharp edges. However, identifying the critical spots with the maximum electric field magnitude due to only sharp edges can be useful to develop geometrical strategies to reduce the electric field magnitude peaks due to the effect of one contributing factor.

Note that the maximum electric field magnitude at perfectly sharp edges is theoretically infinite. Thus, the smaller mesh size, the higher electric stress and mathematically there is no convergence point. Assuming a rounded edge converges to a finite maximum electric field intensity with increasing resolution of the mesh grid. However, the value depends on the assumed edge radius. The smaller assumed edge radius, the higher amount of maximum electric field magnitude. To overcome this difficulty, it was shown in [20, 21] that when the distance to sharp edges becomes larger than 20 μm for the assumed geometry and dimensions, the differences between the electric field magnitudes for different meshing sizes are less than 1%. To be on the safe side, measuring points were considered at a distance of 50 μm to sharp edges in [20, 21]. In [22], both strategies containing rounded edges and considering measuring points at a distance of 20 μm from edges were benefited.

Assuming the measuring points defined above, the influence of following geometrical options are studied in [20, 22] on reducing the electric field stress values.


of the substrate and the bulk of the gel is not tested. To address this issue, the test voltage is proposed as an AC voltage superimposed on a direct current (DC) one directly applied to the component turned off using a negative gate polarization [17–19]. The inverse DC offset of magnitude higher than the AC peak value as shown for an example in **Figure 6a** used as the test voltage avoids diode conduction [18]. The discharge inception voltage (*UDIV*) is then defined as the peak value of the applied voltage (*UDC* <sup>+</sup> *UAC*) [17]. **Figure 6b** shows an experimental set-up

This method leads to detect PD for voltages lower than the one necessary to trig them during IEC 61287-1 test [17–19]. Although neither the test proposed in [17–19] nor IEC 61287-1 test can represent thoroughly the stresses endured by the power modules in inverters, the testing method proposed in [17–19] can provide more useful information on PDs during normal

**Figure 6.** (a) New test voltage waveform and (b) set-up generating new test voltage for PD detection of an IGBT [18],

operation by stressing all the components involved in the packaging.

134 Simulation and Modelling of Electrical Insulation Weaknesses in Electrical Equipment

generating such test voltage [17].

**Figure 5.** Sinusoidal pulse-width modulation.

License No. 4383271241884.

**4.** Metal/conductive layer offset.

Among four parameters above, the thickness of the substrate and metal/conductive layer offset have a strong influence on the electric field magnitude. By varying the thickness of the ceramic, the electric field stress does not follow the equation of a plate capacitor: a doubling of the thickness (1–2 mm) reduces the electric field stress only by about 30% and not by 50%. However, an increased substrate thickness decreases cooling efficiency of the semiconductors, and this technique may not meet the miniaturization needs of power electronics as well.

Defining an offset of the two metallization layers as *r off* = *r <sup>u</sup>* − *r l* for *r u* (the distance from the AlN ceramic edge to the edge of the upper Cu metallization) and *r l* (the distance from the AlN ceramic edge to the edge of the lower Cu metallization) shown in **Figure 7a**. **Figure 8** shows the electric field stress values at measuring point located on L1 for different values of *r off* for a d = 630 μm ceramic layer [22]. For that (**Figure 7a**), a finite-element method (FEM) model was developed in the *Electrostatics* (*es*) module of COMSOL Multiphysics solving Poisson's equation.

$$
\nabla^2 \mathcal{U} = -\frac{\rho\_v}{\mathbf{e}\_0 \mathbf{e}\_r} \tag{1}
$$

$$E = -\nabla \mathcal{U} \tag{2}$$

**4. Partial discharge measurements**

rise sharply with increasing voltage.

also reported for **Figure 11b** [21].

in the gel.

Research carried out on PD detection, and localization inside an IGBT can mainly be divided into electrical and optical PD measurements. For electrical PD measurement, measured phaseresolved partial discharge (PRPD) patterns were analyzed to identify the type and location of PD. As shown in **Figure 9** [30, 31], it was observed that the PD of a metalized ceramic in an isolating liquid occurs at the maximum voltage at 90° and 270° and the amount of PD does not

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However, as shown in **Figure 10** [21, 30, 31] for the same metalized ceramic embedded in silicone gel, PD was found at a phase between zero and the maximum voltage, between 0–90° and 180–270°. Since the number and magnitude of the PDs strongly increase with rising voltage, it was argued that the origin of this discharge phenomenon is due to discharges at the interface between the silicone gel and the substrate and not due to locally restricted cavities

In [21] the calculated electric field intensity and the measured PDIV were correlated. Combining the calculated electric field intensity in four measuring points ML1-ML4 shown in **Figure 11a**, the PDIV was plotted as shown in **Figure 11b** as a function of the geometric mean of *E* values at ML1-ML4. A fitted equation as "PDIV (kV) = 20.4–0.25E (kV/mm)" was

However, through an artificial spherical void embedded in silicone gel, it was shown in [32] that voids inside the silicone gel significantly accelerate the aging of the materials even at a normal operating electric stress. It was also found that an extremely non-uniform electric field resulted by a needle-sphere electrode with no artificial void inside the material can also lead to rapid aging at a normal operating electric stress [32]. Thus, it was concluded that the electrical treeing in front of the needle tip produces gas-filled voids inside the silicone and these week points besides conductive channels of trees lead to shortening the lifetime of the insulation [32]. In [33, 34] an optical PD localization setup benefitting from compact charge-coupled device (CCD) camera modules was used to record the small light intensities emitted by electroluminescence effects as well as the light caused by PD. It should be noted that before partial discharge inception, insulating polymers subjected to high electrical fields usually display electroluminescence as a result of the radiative relaxation of excited molecular states within the gel excited by high electrical field [34]. The measurement of electroluminescence allows the critical regions of high electric fields to be identified in the translucent silicone gel insulation

**Figure 9.** PD spectroscopy of AlN substrates in an insulating liquid [31], License No. 4383271013906.

where *ρ<sup>v</sup>* is volume charge density which is *ρ<sup>v</sup>* = 0 in the model considered in [22], ϵ*<sup>r</sup>* is relative permittivity which for AlN and gel were considered, respectively, 8.9 and 2.7 in [22], *E* is electric field intensity, and *U* is electric potential.

As shown in **Figure 7b**, an extremely custom fine meshing with a maximum element size of 0.001 mm was used for Area 1 shown in **Figure 7a** to obtain precise results for electric field

**Figure 7.** (a) The geometries considered for simulations in COMSOL Multiphysics and (b) meshing strategy.

intensity along L1 [22]. Such meshing strategy, using several levels of extremely custom fine meshing for the study area having sharp edges and a normal meshing for other areas to increase the computational efficiency was used in [23–29] as well. From **Figure 8**, it can be seen that with decreasing offset the electric field magnitude reduces. In other words, an increase in the length of the upper metal layer relieves the worst high field region. It is due to the influence of the grounded based plate, since the more extended top metal layer, the less nonuniform electric field. Changing *roff* from 0.35 to −0.5 mm reduces the electric field intensity up to 57% that presents the method as an efficient electric field control technique [22].

**Figure 8.** Influence of *r off* on electric field intensity.

#### **4. Partial discharge measurements**

*E* = −∇*U* (2)

is volume charge density which is *ρ<sup>v</sup>* = 0 in the model considered in [22], ϵ*<sup>r</sup>*

tive permittivity which for AlN and gel were considered, respectively, 8.9 and 2.7 in [22], *E* is

As shown in **Figure 7b**, an extremely custom fine meshing with a maximum element size of 0.001 mm was used for Area 1 shown in **Figure 7a** to obtain precise results for electric field

intensity along L1 [22]. Such meshing strategy, using several levels of extremely custom fine meshing for the study area having sharp edges and a normal meshing for other areas to increase the computational efficiency was used in [23–29] as well. From **Figure 8**, it can be seen that with decreasing offset the electric field magnitude reduces. In other words, an increase in the length of the upper metal layer relieves the worst high field region. It is due to the influence of the grounded based plate, since the more extended top metal layer, the less nonuniform electric field. Changing *roff* from 0.35 to −0.5 mm reduces the electric field intensity up to 57%

**Figure 7.** (a) The geometries considered for simulations in COMSOL Multiphysics and (b) meshing strategy.

that presents the method as an efficient electric field control technique [22].

**Figure 8.** Influence of *r off* on electric field intensity.

is rela-

where *ρ<sup>v</sup>*

electric field intensity, and *U* is electric potential.

136 Simulation and Modelling of Electrical Insulation Weaknesses in Electrical Equipment

Research carried out on PD detection, and localization inside an IGBT can mainly be divided into electrical and optical PD measurements. For electrical PD measurement, measured phaseresolved partial discharge (PRPD) patterns were analyzed to identify the type and location of PD. As shown in **Figure 9** [30, 31], it was observed that the PD of a metalized ceramic in an isolating liquid occurs at the maximum voltage at 90° and 270° and the amount of PD does not rise sharply with increasing voltage.

However, as shown in **Figure 10** [21, 30, 31] for the same metalized ceramic embedded in silicone gel, PD was found at a phase between zero and the maximum voltage, between 0–90° and 180–270°. Since the number and magnitude of the PDs strongly increase with rising voltage, it was argued that the origin of this discharge phenomenon is due to discharges at the interface between the silicone gel and the substrate and not due to locally restricted cavities in the gel.

In [21] the calculated electric field intensity and the measured PDIV were correlated. Combining the calculated electric field intensity in four measuring points ML1-ML4 shown in **Figure 11a**, the PDIV was plotted as shown in **Figure 11b** as a function of the geometric mean of *E* values at ML1-ML4. A fitted equation as "PDIV (kV) = 20.4–0.25E (kV/mm)" was also reported for **Figure 11b** [21].

However, through an artificial spherical void embedded in silicone gel, it was shown in [32] that voids inside the silicone gel significantly accelerate the aging of the materials even at a normal operating electric stress. It was also found that an extremely non-uniform electric field resulted by a needle-sphere electrode with no artificial void inside the material can also lead to rapid aging at a normal operating electric stress [32]. Thus, it was concluded that the electrical treeing in front of the needle tip produces gas-filled voids inside the silicone and these week points besides conductive channels of trees lead to shortening the lifetime of the insulation [32].

In [33, 34] an optical PD localization setup benefitting from compact charge-coupled device (CCD) camera modules was used to record the small light intensities emitted by electroluminescence effects as well as the light caused by PD. It should be noted that before partial discharge inception, insulating polymers subjected to high electrical fields usually display electroluminescence as a result of the radiative relaxation of excited molecular states within the gel excited by high electrical field [34]. The measurement of electroluminescence allows the critical regions of high electric fields to be identified in the translucent silicone gel insulation

**Figure 9.** PD spectroscopy of AlN substrates in an insulating liquid [31], License No. 4383271013906.

**Figure 10.** PD spectroscopy of AlN substrates in silicone gel (a) from [31], License No. 4383271013906, (b) from [21], License No. 4383270757365.

even before electrical aging begins. Increasing the voltage, PD starts at distinct locations. Bright shining spots in the image as seen in **Figure 12** show the higher possibility for PD inception.

In [35] the results concerning both electrical and optical detection of PDs occurring in the silicone gel were presented. That work showed that optical measurements could be used to study PDs in transparent gels, with any voltage shape and with very high sensitivity (<1 pC). In recent years, micro silicon photomultipliers (SiPM) were also examined and compared to conventional photomultiplier tubes (PMT) for optical PD detection [36].

Converters are often located in cubicles under atmospheric pressure, and the most widely used material for encapsulation of power electronic circuits is silicone gel [8, 15–22, 30–38, 42–45, 48, 51–54]. However, for variable-frequency drive (VFD) fed motors used in the subsea factory for oil and gas production at depths more than 3000 m, the development of pressure tolerant power electronics is envisaged where an incompressible insulating material is needed for power electronic modules. Thus, liquid embedded power electronics are investigated. In [14, 39] PDs in liquid embedded power electronics under three different waveforms as sinusoidal (50 Hz) voltage, a slow rise bipolar square voltage with a rise time of 400 μs, and a fast unipolar positive and negative rise square voltage with a rise time of 100 ns were investigated. Both electrical and optical techniques were used to study PD behavior of IGBT insulation. Regarding a good correlation found in [14, 39] between the measured electrical and optical PDs, optical PDs can also be considered for the characterization of PD phenomena. Another significant result obtained in [14, 39] is that the fast rise square voltage has the lowest PDIV while the sinusoidal voltage has the highest one. Moreover, it was reported in [14, 39] that the number and magnitude of PDs decrease when the pressure of the liquid in the test cell increases. In other words, pressure can collapse the propagation of the streamers, and that is

**Figure 12.** Optical localization of PD for an AIN substrate embedded in silicone gel. The discharges are located at the

the great merit of liquid embedded power electronics used for the subsea application.

connected to ground. Sharp edges were rounded to ensure the set-up is PD free.

**Figure 13a** with a dimension of 50 × 24 × 1 mm<sup>3</sup>

outer edges of the copper metallization [34], License No. 4383270494717.

Various liquid dielectrics such as Nytro 10XN, Midel 7131 and Galden HT230 were examined in [40, 41] for pressure tolerant liquid embedded power electronics modules for deep, and ultra-deepwater. The test object used in [40] is a printed circuit board (PCB) card shown in

thickness of copper metallization at both sides is 420 μm. The trench located at the upper metallization layer has a width of 2 mm. The left end of the board was connected to a high voltage source and the other end of the board and the base plate (the lower metallization layer) was

and a schematic shown in **Figure 13b**. The

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In [37, 38], besides PRPD measurements, other diagnostic and quality control test methods to discriminate the dielectric condition between new and aged IGBT samples and reveal the influence of moisture on dielectric state of IGBT modules were used. They are time-dependent dielectric response measurements such as insulation resistance and polarization index, and frequency-dependent dielectric response measurements such as loss factor and frequency response analysis (FRA). Humidity as a result of the condensation caused by the difference in the interior and exterior temperatures may impact on the dielectric integrity of IGBT modules.

**Figure 11.** PDIV as a function of the geometric mean of MP1-4 [21], License No. 4383270757365.

**Figure 12.** Optical localization of PD for an AIN substrate embedded in silicone gel. The discharges are located at the outer edges of the copper metallization [34], License No. 4383270494717.

even before electrical aging begins. Increasing the voltage, PD starts at distinct locations. Bright shining spots in the image as seen in **Figure 12** show the higher possibility for PD inception.

**Figure 10.** PD spectroscopy of AlN substrates in silicone gel (a) from [31], License No. 4383271013906, (b) from [21],

In [35] the results concerning both electrical and optical detection of PDs occurring in the silicone gel were presented. That work showed that optical measurements could be used to study PDs in transparent gels, with any voltage shape and with very high sensitivity (<1 pC). In recent years, micro silicon photomultipliers (SiPM) were also examined and compared to

In [37, 38], besides PRPD measurements, other diagnostic and quality control test methods to discriminate the dielectric condition between new and aged IGBT samples and reveal the influence of moisture on dielectric state of IGBT modules were used. They are time-dependent dielectric response measurements such as insulation resistance and polarization index, and frequency-dependent dielectric response measurements such as loss factor and frequency response analysis (FRA). Humidity as a result of the condensation caused by the difference in the interior and exterior temperatures may impact on the dielectric integrity of IGBT modules.

conventional photomultiplier tubes (PMT) for optical PD detection [36].

138 Simulation and Modelling of Electrical Insulation Weaknesses in Electrical Equipment

**Figure 11.** PDIV as a function of the geometric mean of MP1-4 [21], License No. 4383270757365.

License No. 4383270757365.

Converters are often located in cubicles under atmospheric pressure, and the most widely used material for encapsulation of power electronic circuits is silicone gel [8, 15–22, 30–38, 42–45, 48, 51–54]. However, for variable-frequency drive (VFD) fed motors used in the subsea factory for oil and gas production at depths more than 3000 m, the development of pressure tolerant power electronics is envisaged where an incompressible insulating material is needed for power electronic modules. Thus, liquid embedded power electronics are investigated. In [14, 39] PDs in liquid embedded power electronics under three different waveforms as sinusoidal (50 Hz) voltage, a slow rise bipolar square voltage with a rise time of 400 μs, and a fast unipolar positive and negative rise square voltage with a rise time of 100 ns were investigated. Both electrical and optical techniques were used to study PD behavior of IGBT insulation. Regarding a good correlation found in [14, 39] between the measured electrical and optical PDs, optical PDs can also be considered for the characterization of PD phenomena. Another significant result obtained in [14, 39] is that the fast rise square voltage has the lowest PDIV while the sinusoidal voltage has the highest one. Moreover, it was reported in [14, 39] that the number and magnitude of PDs decrease when the pressure of the liquid in the test cell increases. In other words, pressure can collapse the propagation of the streamers, and that is the great merit of liquid embedded power electronics used for the subsea application.

Various liquid dielectrics such as Nytro 10XN, Midel 7131 and Galden HT230 were examined in [40, 41] for pressure tolerant liquid embedded power electronics modules for deep, and ultra-deepwater. The test object used in [40] is a printed circuit board (PCB) card shown in **Figure 13a** with a dimension of 50 × 24 × 1 mm<sup>3</sup> and a schematic shown in **Figure 13b**. The thickness of copper metallization at both sides is 420 μm. The trench located at the upper metallization layer has a width of 2 mm. The left end of the board was connected to a high voltage source and the other end of the board and the base plate (the lower metallization layer) was connected to ground. Sharp edges were rounded to ensure the set-up is PD free.

**Figure 13.** (a) PCB card test object and (b) the schematic of PCB card [40], License No. 4383270012789.

**Table 1** shows the U1%, U50%, and U63% breakdown probability. For U63%, the cumulative Weibull function was considered given by

$$f(\nu) = 1 - e^{\|\cdot - \nu(a)\|}\tag{3}$$

**5. Partial discharge control**

No. 4383261112180.

4383261112180.

**5.1. Linear resistive electric field control**

Applying functional materials on the highly stressed region can reduce the electric field. Two types of stress relieving composite dielectrics are as follows. (1) The conductivity of the material varies with the electric field, field-dependent conductivity (FDC) [43, 48], (2) the permittivity of the material changes with the electric field, field dependent permittivity (FDP) [50]. In FDC stress relieving control, also called resistive field control, a conductive layer is applied at the metallization edge. The field distribution is modified by flowing the conduction current

**Figure 15.** Influence of temperature on average PD current versus voltage (ac 50 Hz, r<sup>0</sup> = 1.4 μm, d = 4 mm) [42], License

**Figure 14.** PRPD patterns at two temperatures (a: 20°C, b: 100°C) with a point-plane sample embedded in silicone gel (ac 50 Hz, the tip radius of curvature for the point of r0 = 1.4 μm, gap distance of d = 4 mm, V = 8 kV) [42], License No.

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where *ν* is voltage, *f*(*ν*) is the probability of failure, *α* is the characteristic breakdown voltage and *β* is the range of failure voltages within the distribution. The higher the *β*, the lower the scatter is. Besides sinusoidal voltage (38.5 Hz), tests were carried out under a fast-rising positive square wave voltage. From **Table 1** it can be seen that Galden has a superior breakdown characteristic.

The influence of temperature on PD characteristics in silicone gel was experimentally investigated in [42]. It was found that with increasing the temperature, the PDIV and the overall shape of PD patterns remain unchanged, but both discharge amplitudes and number increase as shown in **Figure 14** for PRPD patterns in the same sample at a fixed applied voltage at 20 and 100°C [42].

**Figure 15** shows the influence of temperature on the variation of the average PD current (*I av*), which is the sum of all recorded discharges per unit time, versus voltage in the same experiments [42]. It can be seen that with increasing the temperature, the PDIV remains unchanged. However, due to the increase of PD amplitudes and number the increase of *I av* is faster at high temperature [42].


**Table 1.** U1%, U50%, and U63% breakdown probability for PCB card test object [40].

**Figure 14.** PRPD patterns at two temperatures (a: 20°C, b: 100°C) with a point-plane sample embedded in silicone gel (ac 50 Hz, the tip radius of curvature for the point of r0 = 1.4 μm, gap distance of d = 4 mm, V = 8 kV) [42], License No. 4383261112180.

**Figure 15.** Influence of temperature on average PD current versus voltage (ac 50 Hz, r<sup>0</sup> = 1.4 μm, d = 4 mm) [42], License No. 4383261112180.

#### **5. Partial discharge control**

**Voltage type Samples U1% (kV) U50% (kV)** *α* **(kV)** *β* Sinusoidal voltage Nytro 20.01 40.89 43.89 5.92

However, due to the increase of PD amplitudes and number the increase of *I*

**Table 1** shows the U1%, U50%, and U63% breakdown probability. For U63%, the cumulative Weibull

**Figure 13.** (a) PCB card test object and (b) the schematic of PCB card [40], License No. 4383270012789.

where *ν* is voltage, *f*(*ν*) is the probability of failure, *α* is the characteristic breakdown voltage and *β* is the range of failure voltages within the distribution. The higher the *β*, the lower the scatter is. Besides sinusoidal voltage (38.5 Hz), tests were carried out under a fast-rising positive square wave voltage. From **Table 1** it can be seen that Galden has a superior breakdown characteristic. The influence of temperature on PD characteristics in silicone gel was experimentally investigated in [42]. It was found that with increasing the temperature, the PDIV and the overall shape of PD patterns remain unchanged, but both discharge amplitudes and number increase as shown in **Figure 14** for PRPD patterns in the same sample at a fixed applied voltage at 20

**Figure 15** shows the influence of temperature on the variation of the average PD current (*I*

which is the sum of all recorded discharges per unit time, versus voltage in the same experiments [42]. It can be seen that with increasing the temperature, the PDIV remains unchanged.

(3)

*av*),

*av* is faster at high

function was considered given by

and 100°C [42].

temperature [42].

*f*(*ν*) = 1 − *e*[−*ν*/*α*]*<sup>β</sup>*

140 Simulation and Modelling of Electrical Insulation Weaknesses in Electrical Equipment

Positive square voltage Nytro 19.19 22.5 22.86 26.29

**Table 1.** U1%, U50%, and U63% breakdown probability for PCB card test object [40].

Midel 25.94 39.76 41.26 9.92 Galden 27.56 41.98 43.54 10.06

Midel 15.28 22.98 23.75 10.43 Galden 20.80 32.17 33.41 9.7

#### **5.1. Linear resistive electric field control**

Applying functional materials on the highly stressed region can reduce the electric field. Two types of stress relieving composite dielectrics are as follows. (1) The conductivity of the material varies with the electric field, field-dependent conductivity (FDC) [43, 48], (2) the permittivity of the material changes with the electric field, field dependent permittivity (FDP) [50].

In FDC stress relieving control, also called resistive field control, a conductive layer is applied at the metallization edge. The field distribution is modified by flowing the conduction current through the layer. Materials used for resistive field control can be linear or nonlinear. The conductivity of linear resistive field control materials is not field dependent. Therefore, the conductivity of the layer made of linear materials must be carefully selected. For too low conductivity, the layer has no role in electric stress control [48]. On the other hand, if the conductivity of the layer is too high and for the case of a non-bridging layer, the layer behaves as a prolongation of the metallization and the high field problem is merely transferred to the end of the layer [48]. For the case of a layer bridging HV and ground potential, the layer leads to massive leakage current [48].

In [43], a 300-nm high impedance layer having an electrical conductivity of 105 Ω cm made of semiconducting amorphous silicon, a-Si: H, was applied by plasma-enhanced chemical vapor deposition (PECVD) process to the edge of the substrate connecting the top copper metallization with the bottom. The mentioned conductivity was adjusted to homogenize the electric field by having the magnitude of the conduction current higher than the capacitive current. By electric field simulations, the same value of the electric conductivity of the layer was reported in [44, 45]. Two sample modules with and without a-Si:H coating built under manufacturing conditions were tested in [43]. While the partial discharge increases sharply at low voltages of 3–4 kV without an a-Si:H coating, it does not exceed 10 pC up to a voltage of 10 kV with an a-Si:H coating layer satisfying the partial discharge requirements based on IEC 61287-1. Note that the linear resistive field control depends on the frequency and its advantage reduces with increasing frequency.

**5.2. FDP stress relieving control**

4383260855923.

titanate [51] this advantage will disappear [50].

**5.3. The quality and type of substrates**

composite were also examined.

As mentioned in Section 5.1 although a high permittivity coating layer relieves high electric field stress adjacent to the copper metallization, it leads to higher electric field stress in the gel and in particular the weak interface between the layer and the gel encapsulation. This means a high permittivity material as a coating layer may not be efficient. Thus in [50] employing it as a filler was examined. The filler studied in [50] was a ferroelectric filler, barium titanate, in the base

**Figure 16.** The influence of nonlinear FDC coating layer on reducing the electric field in the module [48], License No.

By enhancing polarization mechanisms, the ferroelectric filler particles can reduce high electrical stresses. However, it should be noted that this electric field control method works only under ac fields and at the temperatures higher than Curie temperature which is 130°C for pure barium

It is shown in [50] that applying a high permittivity non-dependent field filler can reduce electric stress by around 10% while with a dependent-field one a reduction of 29% can be achieved.

Despite all publications, which have concluded that PDs occur in the silicone gel or at the interface between the substrate and the gel, a different conclusion about the origin of PDs was reported in [52]. To explore the actual origin of PDs, six insulating liquids including Silicon oil #1 (Sil20), Silicon oil #1 (Sil350), Transformer oil (Toil), Synthetic capacitor liquid (Scl), Synthetic transformer liquid (Stl) and Ester liquid (Est), which have different PD properties

A rather large variation in PDIV was observed for six mentioned liquids used in a point-plane electrode geometry under 50 Hz AC voltage at room temperature (20°C). However, a substrate test geometry similar to an IGBT shows almost no changes in PDIV for the mentioned different liquids. Moreover, for the IGBT test geometry, PDs appear in both polarities and provides somewhat symmetrical patterns with good stability. However, asymmetrical PRPD

(*E*) as ϵ*<sup>r</sup>*

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O3

, and glass/epoxy

(*E*) = 6.4 + 1.3*E*.

silicone gel to form an FDP stress relieving dielectric material having a ϵ*<sup>r</sup>*

were used instead of gel, and three substrate materials including AlN, Al2

The intrinsic semi-conductive nature of the particles and their connectivity lead to non-linear behavior of nonlinear resistive electric field control composites. In this regard, the particle to particle contact is possible if the filler concentration is above a prescribed limit. The electrical field magnitude must also be high enough to allow conduction through the semi-conductive particles and barriers between particles.

A theory-based evaluation of the behavior of field grading materials with strongly fielddependent conductivities is presented in [46] with a survey of ZnO microvaristors in various applications in [47]. ZnO microvaristor layer was studied to relieve high field regions in an IGBT [48]. An advantage of nonlinear materials compared to linear materials is that losses are not permanent. They occur only when the electrical field magnitude passes a threshold known as switching filed where the material switches to a conductive behavior.

An electrostatic FEM model developed in ACE TripleC was used for electric field calculations in [48]. **Figure 16a** shows electric field distribution for without a coating layer on the protrusion considered in the model. In this case, the maximum electric stress, *Emax*, obtained 2.6 × 108 V/m at the gel adjacent to the protrusion [48]. To relieve this high field stress region, a layer for coating the metallization edges was considered in three cases with polyimide layer (*ε<sup>r</sup>* = 3.5), a high permittivity (*ε<sup>r</sup>* = 40) layer of a polymer/ceramic composite and ZnO microvaristor layer described above. A comprehensive study of the general structure of polymers, their properties and applications can be found in [49]. For polyimide layer, *Emax* in the layer (adjacent to the protrusion) and gel will be 2.3 × 108 and 0.18 × 108 V/m, respectively [48]. In this regard, although polymer/ ceramic composite with *ε<sup>r</sup>* = 40 can reduce the maximum electric field in the layer adjacent to the protrusion to 0.3 × 108 V/m, the electric field in the gel reaches higher values (0.2 × 108 V/m) than with polyimide coating. Employing a ZnO microvaristor layer, *Emax* in both the layer (0.066 × 108 V/m as shown in **Figure 16b**) and gel (0.06 × 108 V/m) dramatically decreases.

**Figure 16.** The influence of nonlinear FDC coating layer on reducing the electric field in the module [48], License No. 4383260855923.

#### **5.2. FDP stress relieving control**

through the layer. Materials used for resistive field control can be linear or nonlinear. The conductivity of linear resistive field control materials is not field dependent. Therefore, the conductivity of the layer made of linear materials must be carefully selected. For too low conductivity, the layer has no role in electric stress control [48]. On the other hand, if the conductivity of the layer is too high and for the case of a non-bridging layer, the layer behaves as a prolongation of the metallization and the high field problem is merely transferred to the end of the layer [48]. For the case of a layer bridging HV and ground potential, the layer leads to massive leakage current [48]. In [43], a 300-nm high impedance layer having an electrical conductivity of 105 Ω cm made of semiconducting amorphous silicon, a-Si: H, was applied by plasma-enhanced chemical vapor deposition (PECVD) process to the edge of the substrate connecting the top copper metallization with the bottom. The mentioned conductivity was adjusted to homogenize the electric field by having the magnitude of the conduction current higher than the capacitive current. By electric field simulations, the same value of the electric conductivity of the layer was reported in [44, 45]. Two sample modules with and without a-Si:H coating built under manufacturing conditions were tested in [43]. While the partial discharge increases sharply at low voltages of 3–4 kV without an a-Si:H coating, it does not exceed 10 pC up to a voltage of 10 kV with an a-Si:H coating layer satisfying the partial discharge requirements based on IEC 61287-1. Note that the linear resistive field control depends on the frequency and its

142 Simulation and Modelling of Electrical Insulation Weaknesses in Electrical Equipment

The intrinsic semi-conductive nature of the particles and their connectivity lead to non-linear behavior of nonlinear resistive electric field control composites. In this regard, the particle to particle contact is possible if the filler concentration is above a prescribed limit. The electrical field magnitude must also be high enough to allow conduction through the semi-conductive

A theory-based evaluation of the behavior of field grading materials with strongly fielddependent conductivities is presented in [46] with a survey of ZnO microvaristors in various applications in [47]. ZnO microvaristor layer was studied to relieve high field regions in an IGBT [48]. An advantage of nonlinear materials compared to linear materials is that losses are not permanent. They occur only when the electrical field magnitude passes a threshold

An electrostatic FEM model developed in ACE TripleC was used for electric field calculations in [48]. **Figure 16a** shows electric field distribution for without a coating layer on the protrusion considered in the model. In this case, the maximum electric stress, *Emax*, obtained 2.6 × 108 V/m at the gel adjacent to the protrusion [48]. To relieve this high field stress region, a layer for coating

above. A comprehensive study of the general structure of polymers, their properties and applications can be found in [49]. For polyimide layer, *Emax* in the layer (adjacent to the protrusion) and gel will be 2.3 × 108 and 0.18 × 108 V/m, respectively [48]. In this regard, although polymer/

the protrusion to 0.3 × 108 V/m, the electric field in the gel reaches higher values (0.2 × 108 V/m) than with polyimide coating. Employing a ZnO microvaristor layer, *Emax* in both the layer (0.066 × 108 V/m as shown in **Figure 16b**) and gel (0.06 × 108 V/m) dramatically decreases.

= 40) layer of a polymer/ceramic composite and ZnO microvaristor layer described

= 40 can reduce the maximum electric field in the layer adjacent to

= 3.5), a high per-

known as switching filed where the material switches to a conductive behavior.

the metallization edges was considered in three cases with polyimide layer (*ε<sup>r</sup>*

advantage reduces with increasing frequency.

particles and barriers between particles.

mittivity (*ε<sup>r</sup>*

ceramic composite with *ε<sup>r</sup>*

As mentioned in Section 5.1 although a high permittivity coating layer relieves high electric field stress adjacent to the copper metallization, it leads to higher electric field stress in the gel and in particular the weak interface between the layer and the gel encapsulation. This means a high permittivity material as a coating layer may not be efficient. Thus in [50] employing it as a filler was examined. The filler studied in [50] was a ferroelectric filler, barium titanate, in the base silicone gel to form an FDP stress relieving dielectric material having a ϵ*<sup>r</sup>* (*E*) as ϵ*<sup>r</sup>* (*E*) = 6.4 + 1.3*E*. By enhancing polarization mechanisms, the ferroelectric filler particles can reduce high electrical stresses. However, it should be noted that this electric field control method works only under ac fields and at the temperatures higher than Curie temperature which is 130°C for pure barium titanate [51] this advantage will disappear [50].

It is shown in [50] that applying a high permittivity non-dependent field filler can reduce electric stress by around 10% while with a dependent-field one a reduction of 29% can be achieved.

#### **5.3. The quality and type of substrates**

Despite all publications, which have concluded that PDs occur in the silicone gel or at the interface between the substrate and the gel, a different conclusion about the origin of PDs was reported in [52]. To explore the actual origin of PDs, six insulating liquids including Silicon oil #1 (Sil20), Silicon oil #1 (Sil350), Transformer oil (Toil), Synthetic capacitor liquid (Scl), Synthetic transformer liquid (Stl) and Ester liquid (Est), which have different PD properties were used instead of gel, and three substrate materials including AlN, Al2 O3 , and glass/epoxy composite were also examined.

A rather large variation in PDIV was observed for six mentioned liquids used in a point-plane electrode geometry under 50 Hz AC voltage at room temperature (20°C). However, a substrate test geometry similar to an IGBT shows almost no changes in PDIV for the mentioned different liquids. Moreover, for the IGBT test geometry, PDs appear in both polarities and provides somewhat symmetrical patterns with good stability. However, asymmetrical PRPD patterns for the point-plane electrode geometry were obtained. Using the gel in the mentioned experiments produces no change. Thus, it was concluded that PDs recorded with the substrate indeed do not occur within the liquid or the gel. The only remaining possibility is that PDs originate from the porous nature of the AlN or Al<sup>2</sup> O3 substrates. This is a hypothesis opposed to the ideas commonly accepted. The experiments carried out with another sintered porous material, and with a non-porous material (epoxy resin) confirm this hypothesis where with epoxy, no stable PD regime can be achieved.

**Author details**

Mona Ghassemi

**References**

p. 257-260

Butterworths; 1989

Florida, USA: CRC Press; 2005

Address all correspondence to: monag@vt.edu

10.1109/JESTPE.2013.2271111

2155-2163. DOI: 10.1109/TPEL.2013.2268900

Lab.; 2006. NREL/SR-500-38515(in press)

31/11-3136-sylgard-527-dielectric-gel.pdf

part 1: Characteristics and test methods. 3rd ed. 2014

Virginia Polytechnic Institute and State University, ECE Department, USA

[1] Kassakian JG, Jahns TM. Evolving and emerging applications of power electronics in systems. IEEE Emerging and Selected Topics in Power Electronics. 2013;**1**(2):47-58. DOI:

Electrical Insulation Weaknesses in Wide Bandgap Devices

http://dx.doi.org/10.5772/intechopen.77657

145

[2] Millan J, Godignon P, Perpina X, Perez-Tomas A, Rebollo J. A survey of wide bandgap power semiconductor devices. IEEE Transactions on Power Electronics. 2014;**29**(5):

[3] Ryu SH, Capell C, Jonas C, Cheng L, O'Loughlin M, Burk A, Agarwal A, Palmour J, Hefner A. Ultra high voltage (*>*12 kV), high-performance 4H-SiC IGBTs. In: Proceedings of the International Symposium Power Semiconductor Devices and ICs; 3-7 June 2012; Belgium.

[4] Marckx DA. Breakthrough in Power Electronics from SiC. USA: Nat. Renewable Energy

[5] Kreuger F. Partial Discharge Detection in High-Voltage Equipment. London, England:

[6] Álvarez-Gómez F, Albarracín-Sánchez R, Garnacho-Vecino F, Granizo-Arrabé R. Diagnosis of insulation condition of MV switchgears by application of different partial discharge measuring methods and sensors. Sensors. 2018;**18**:720. DOI: 10.3390/s18030720 [7] Sheng WW, Colino RP. Power Electronic Modules: Design and Manufacture. Boca Raton,

[8] Mitic G, Sommer KH, Dieci D, Lefranc G. The thermal impedance of new power semiconductor modules using AlN substrates. In: Proceedings of the IEEE 33th Industry

[9] Dow. SYLGARD™ 527 Silicone Dielectric Gel Technical Data Sheet. Available from: https://consumer.dow.com/en-us/document-viewer.html?ramdomVar=3447338133 693297098&docPath=/content/dam/dcc/documents/en-us/productdatasheet/11/11-

[10] IEC 61287-1: Railway applications—Power converters installed on board rolling stock-

Applications Society (IAS) Meeting; 12-15 Oct. 1998; USA. p. 1026-1030

In the almost same direction, it was experimentally shown in [53, 54] that surface discharges initiated at the triple junction and propagated at the gel-AlN substrate interface creates cavities composed of tree-like structure and spherical sub-cavities leading to the degradation of AlN substrate [53] as well as give rise to the growth of cavities in the gel [54]. Regarding the first issue, other substrates such as Al<sup>2</sup> O3 and glass were compared with AlN. The cavities usually start from the triple junction with high voltage and being pushed away from the high voltage conductor through a conductive channel on the power module substrate. Focusing on the conductive channel, it was found that during repetitive surface discharges, desorption of nitrogen from AlN substrate results in the formation of Al and this leads to a decrease in the resistance of cavity path that was measured around 5 kΩ/100 μm for AlN compared to above 1 MΩ/100 μm for glass and Al<sup>2</sup> O3 . Thus, it was justified that the high electric field at the tip of surface conductive paths is the reason for elongation the cavity stopping length for AlN to more than twice than that on other substrates. To address the second issue, the dynamic potential distribution of surface discharges in gel was measured by a two-dimensional sensing technique with a Pockels crystal [54].

Another survey of the topics discussed in this book chapter can be found in [55] where other papers, as well as other aspects of the documents reviewed in this book chapter, are evaluated. These two publications, [55] and this book chapter, cover together almost all electrical insulation issues in power electronics modules.

#### **6. Conclusion**

The book chapter reviews some technical issues raised for electrical insulation weaknesses in high power IGBTs. FEM modeling of electric stress inside modules, which have perfectly sharp edges, is a challenge where using rounded edges or assuming measuring points at a distance from edges are used to address this issue. Although PRPD patterns can be used to identify the origin and location of PDs, the hypotheses proposed based on the measured patterns have not reached an agreement. Further investigation is also needed to determine permissible levels for time and frequency dependent diagnostic methods for modules. The optical technique is a promising technique to localize PDs in a power electronics module. Using linear and nonlinear resistive electric field control as a coating layer or using field dependent permittivity materials as a filler in the silicone gel can be used to control PD in modules. However, these mitigation solutions are not mature and need further research.

## **Author details**

patterns for the point-plane electrode geometry were obtained. Using the gel in the mentioned experiments produces no change. Thus, it was concluded that PDs recorded with the substrate indeed do not occur within the liquid or the gel. The only remaining possibility is

opposed to the ideas commonly accepted. The experiments carried out with another sintered porous material, and with a non-porous material (epoxy resin) confirm this hypothesis where

In the almost same direction, it was experimentally shown in [53, 54] that surface discharges initiated at the triple junction and propagated at the gel-AlN substrate interface creates cavities composed of tree-like structure and spherical sub-cavities leading to the degradation of AlN substrate [53] as well as give rise to the growth of cavities in the gel [54]. Regarding the

usually start from the triple junction with high voltage and being pushed away from the high voltage conductor through a conductive channel on the power module substrate. Focusing on the conductive channel, it was found that during repetitive surface discharges, desorption of nitrogen from AlN substrate results in the formation of Al and this leads to a decrease in the resistance of cavity path that was measured around 5 kΩ/100 μm for AlN compared to above

of surface conductive paths is the reason for elongation the cavity stopping length for AlN to more than twice than that on other substrates. To address the second issue, the dynamic potential distribution of surface discharges in gel was measured by a two-dimensional sens-

Another survey of the topics discussed in this book chapter can be found in [55] where other papers, as well as other aspects of the documents reviewed in this book chapter, are evaluated. These two publications, [55] and this book chapter, cover together almost all electrical

The book chapter reviews some technical issues raised for electrical insulation weaknesses in high power IGBTs. FEM modeling of electric stress inside modules, which have perfectly sharp edges, is a challenge where using rounded edges or assuming measuring points at a distance from edges are used to address this issue. Although PRPD patterns can be used to identify the origin and location of PDs, the hypotheses proposed based on the measured patterns have not reached an agreement. Further investigation is also needed to determine permissible levels for time and frequency dependent diagnostic methods for modules. The optical technique is a promising technique to localize PDs in a power electronics module. Using linear and nonlinear resistive electric field control as a coating layer or using field dependent permittivity materials as a filler in the silicone gel can be used to control PD in modules. However, these

O3

O3

mitigation solutions are not mature and need further research.

O3

and glass were compared with AlN. The cavities

. Thus, it was justified that the high electric field at the tip

substrates. This is a hypothesis

that PDs originate from the porous nature of the AlN or Al<sup>2</sup>

144 Simulation and Modelling of Electrical Insulation Weaknesses in Electrical Equipment

with epoxy, no stable PD regime can be achieved.

first issue, other substrates such as Al<sup>2</sup>

ing technique with a Pockels crystal [54].

insulation issues in power electronics modules.

1 MΩ/100 μm for glass and Al<sup>2</sup>

**6. Conclusion**

Mona Ghassemi

Address all correspondence to: monag@vt.edu

Virginia Polytechnic Institute and State University, ECE Department, USA

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s17112595


**Chapter 7**

Provisional chapter

**Simulation and Optimization of Electrical Insulation in**

DOI: 10.5772/intechopen.77187

Thanks to the Smart Grid initiative, the focus for medium-voltage MV (13.8–34 kV) smart meters leveraged the development of sensors for distribution application. In order to be useful at power quality monitoring, the sensors needs to attend, at least, the International Electrotechnical Commission (IEC) 61000–4-30 and IEC 61000–4-7 standards with highaccuracy in terms of voltage (less than 0.1%), current (less than 1.0%) and measuring the waveform distortion data up to the 50th harmonic of 50 or 60 Hz alternating frequency. This kind of sensor is built with two capacitors connected in series. The first capacitor is a commercial electronic low-voltage device. One terminal of this capacitor is connected to the medium-voltage (MV) conductor. The second one, is connected to the other capacitor that is constructed using the own sensor packaging. This second capacitor has an electrode, that is connected with the first capacitor and the other terminal is connected to the ground. The voltage is measured between the terminals of the low voltage capacitor. The performance of this capacitor depends on the geometry and the materials used in the electrical insulation. This chapter describes the simulations and modeling of the capacitor electrodes using a finite-elements software, COMSOL Multiphysics, for modeling in order

to optimize the performance of sensor in terms of electric field distribution.

Keywords: simulation, electrical insulation, sensors, power quality monitoring,

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and eproduction in any medium, provided the original work is properly cited.

© 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use,

distribution, and reproduction in any medium, provided the original work is properly cited.

Simulation and Optimization of Electrical Insulation in

**Power Quality Monitoring Sensors Applied in the**

Power Quality Monitoring Sensors Applied in the

**Medium-Voltage**

Medium-Voltage

Joao Batista Rosolem

Joao Batista Rosolem

Abstract

Sender Rocha dos Santos, Rodrigo Peres, Wagner Francisco Rezende Cano and

Sender Rocha dos Santos, Rodrigo Peres, Wagner Francisco Rezende Cano and

Additional information is available at the end of the chapter

Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/intechopen.77187

medium-voltage, capacitive divider

#### **Simulation and Optimization of Electrical Insulation in Power Quality Monitoring Sensors Applied in the Medium-Voltage** Simulation and Optimization of Electrical Insulation in Power Quality Monitoring Sensors Applied in the Medium-Voltage

DOI: 10.5772/intechopen.77187

Sender Rocha dos Santos, Rodrigo Peres, Wagner Francisco Rezende Cano and Joao Batista Rosolem Sender Rocha dos Santos, Rodrigo Peres, Wagner Francisco Rezende Cano and Joao Batista Rosolem

Additional information is available at the end of the chapter Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/intechopen.77187

#### Abstract

Thanks to the Smart Grid initiative, the focus for medium-voltage MV (13.8–34 kV) smart meters leveraged the development of sensors for distribution application. In order to be useful at power quality monitoring, the sensors needs to attend, at least, the International Electrotechnical Commission (IEC) 61000–4-30 and IEC 61000–4-7 standards with highaccuracy in terms of voltage (less than 0.1%), current (less than 1.0%) and measuring the waveform distortion data up to the 50th harmonic of 50 or 60 Hz alternating frequency. This kind of sensor is built with two capacitors connected in series. The first capacitor is a commercial electronic low-voltage device. One terminal of this capacitor is connected to the medium-voltage (MV) conductor. The second one, is connected to the other capacitor that is constructed using the own sensor packaging. This second capacitor has an electrode, that is connected with the first capacitor and the other terminal is connected to the ground. The voltage is measured between the terminals of the low voltage capacitor. The performance of this capacitor depends on the geometry and the materials used in the electrical insulation. This chapter describes the simulations and modeling of the capacitor electrodes using a finite-elements software, COMSOL Multiphysics, for modeling in order to optimize the performance of sensor in terms of electric field distribution.

Keywords: simulation, electrical insulation, sensors, power quality monitoring, medium-voltage, capacitive divider

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and eproduction in any medium, provided the original work is properly cited. © 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

## 1. Introduction

One of the fundaments of the Smart Grid concept is that user safety should be ensured while monitoring, updating and continuously reliably distributing electricity grid by adding smart meters and monitoring systems to the power grid is obtained. This is necessary in order to ensure electronic communication between suppliers and consumers [1]. Smart grid monitoring systems require various types of sensors and transducers to monitor the grid conditions. After the Smart Grid initiative, the focus for medium-voltage (MV) smart meters leveraged the development of energy quality sensors for distribution application [2]. In order to be useful at power quality monitoring, the sensors needs to attend at least the International Electrotechnical Commission (IEC) 61000–4-30 and IEC 61000–4-7 standards [3, 4] with highaccuracy in terms of voltage (less than 0.1%), current (less than 1.0%) and measure waveform distortion data up to the 50th harmonic. In addition, this sensor must be easy to install and remove without disconnect the distribution network and it must monitor the grid for a period that may last longer than 1 week.

This chapter is organized as follows. Section 2 describes the physical structure of the sensor studied in this work. Section 3 describes the analytical modeling of the sensor. Section 4 describes the finite-element method (FEM) review mainly focused in simulation of an electrostatic field and the sensor. In addition in this section it is presented the sensor simulation

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The proposed case study was the modeling of a current and voltage sensor for MV applications

According to Figure 1, a capacitive divider represented by C1 and C2 (F) provides the voltage measurement. The capacitor C1 is a commercial capacitor used in electronic applications and is placed in an electronic board of the sensor. C2 is a capacitor formed by an electrode and a grounded pipe isolated by an insulation material (such as polymer, ceramic, glass or oilimpregnated paper) where a high-intensity electric field remains concentrated. To meet accuracy of less than 0.5% in the voltage measurements, it is necessary to connect the LV terminal of the sensor to the ground. Thus, it is not possible to use the parasitic capacitance as the C2

Figure 1. The voltage sensor for power quality applications. In the right is shown the elements that create the capacitance

C2.

validation. Finally, Section 5 presents the conclusions.

2. Voltage sensor for power quality applications

to be applied in live lines for the evaluation of energy quality.

In this context, two power quality-monitoring technologies are prominent: wireless sensors [5] or optical fiber sensors [6, 7]. Wireless sensors have the advantage of not needing any physical medium to transmit the data to a remote measuring unit, but need to use batteries in order to keep the electronic circuits working. On the other hand, optical fiber sensors have the advantage of no need for electrical powering, but they need a physical link to the remote measuring unit.

Independently of the technologic choice, current and voltage waves have to be measured in the medium-voltage (MV) in order to obtain the power quality parameters. Particularly, for voltage measurements, a capacitive or a resistive circuit divider [8–10] can be used to obtain a voltage sample of the MV conductor. In this work, it is analyzed the capacitive case. The low capacitance of this circuit accumulates more than 99.9% of the total voltage (e.g. 13.8 or 34 kV) and is totally constructed using the own sensor packaging. In this capacitive circuit the first capacitor is a commercial electronic low-voltage (LV) device. One terminal of this capacitor is connected to the medium-voltage conductor. The second one, is connected to the other capacitor that is constructed using the own sensor packaging. This second capacitor has an electrode, that is connected with the first capacitor and the other terminal is connected to the ground.

A rigorous design is necessary for these sensors considering the safety aspects regarding to the technician activities and to some environmental effects that can influence their performance, such as, temperature, pressure and wind. Besides, it should be taken into account that the external elements in the proximities of the sensors can alter the electrical and magnetic field acting inside of them.

The current sensor for power quality measurements was not evaluated in this work since its operation is different from the voltage sensor and in general it does not affect the insulation properties of the power quality sensor. Traditional devices used as current sensor are current transformer or Rogowski coil [11], and they are connected direct to the MV conductor without ground connection.

This chapter is organized as follows. Section 2 describes the physical structure of the sensor studied in this work. Section 3 describes the analytical modeling of the sensor. Section 4 describes the finite-element method (FEM) review mainly focused in simulation of an electrostatic field and the sensor. In addition in this section it is presented the sensor simulation validation. Finally, Section 5 presents the conclusions.

#### 2. Voltage sensor for power quality applications

1. Introduction

that may last longer than 1 week.

unit.

acting inside of them.

ground connection.

One of the fundaments of the Smart Grid concept is that user safety should be ensured while monitoring, updating and continuously reliably distributing electricity grid by adding smart meters and monitoring systems to the power grid is obtained. This is necessary in order to ensure electronic communication between suppliers and consumers [1]. Smart grid monitoring systems require various types of sensors and transducers to monitor the grid conditions. After the Smart Grid initiative, the focus for medium-voltage (MV) smart meters leveraged the development of energy quality sensors for distribution application [2]. In order to be useful at power quality monitoring, the sensors needs to attend at least the International Electrotechnical Commission (IEC) 61000–4-30 and IEC 61000–4-7 standards [3, 4] with highaccuracy in terms of voltage (less than 0.1%), current (less than 1.0%) and measure waveform distortion data up to the 50th harmonic. In addition, this sensor must be easy to install and remove without disconnect the distribution network and it must monitor the grid for a period

152 Simulation and Modelling of Electrical Insulation Weaknesses in Electrical Equipment

In this context, two power quality-monitoring technologies are prominent: wireless sensors [5] or optical fiber sensors [6, 7]. Wireless sensors have the advantage of not needing any physical medium to transmit the data to a remote measuring unit, but need to use batteries in order to keep the electronic circuits working. On the other hand, optical fiber sensors have the advantage of no need for electrical powering, but they need a physical link to the remote measuring

Independently of the technologic choice, current and voltage waves have to be measured in the medium-voltage (MV) in order to obtain the power quality parameters. Particularly, for voltage measurements, a capacitive or a resistive circuit divider [8–10] can be used to obtain a voltage sample of the MV conductor. In this work, it is analyzed the capacitive case. The low capacitance of this circuit accumulates more than 99.9% of the total voltage (e.g. 13.8 or 34 kV) and is totally constructed using the own sensor packaging. In this capacitive circuit the first capacitor is a commercial electronic low-voltage (LV) device. One terminal of this capacitor is connected to the medium-voltage conductor. The second one, is connected to the other capacitor that is constructed using the own sensor packaging. This second capacitor has an electrode, that is

connected with the first capacitor and the other terminal is connected to the ground.

A rigorous design is necessary for these sensors considering the safety aspects regarding to the technician activities and to some environmental effects that can influence their performance, such as, temperature, pressure and wind. Besides, it should be taken into account that the external elements in the proximities of the sensors can alter the electrical and magnetic field

The current sensor for power quality measurements was not evaluated in this work since its operation is different from the voltage sensor and in general it does not affect the insulation properties of the power quality sensor. Traditional devices used as current sensor are current transformer or Rogowski coil [11], and they are connected direct to the MV conductor without The proposed case study was the modeling of a current and voltage sensor for MV applications to be applied in live lines for the evaluation of energy quality.

According to Figure 1, a capacitive divider represented by C1 and C2 (F) provides the voltage measurement. The capacitor C1 is a commercial capacitor used in electronic applications and is placed in an electronic board of the sensor. C2 is a capacitor formed by an electrode and a grounded pipe isolated by an insulation material (such as polymer, ceramic, glass or oilimpregnated paper) where a high-intensity electric field remains concentrated. To meet accuracy of less than 0.5% in the voltage measurements, it is necessary to connect the LV terminal of the sensor to the ground. Thus, it is not possible to use the parasitic capacitance as the C2

Figure 1. The voltage sensor for power quality applications. In the right is shown the elements that create the capacitance C2.

capacitor. The capacitive divider is designed in such way that C2 retains practically the totally line voltage. The voltage on C1 measured by the sensor is defined by:

$$\mathcal{U}L\_{\mathbb{C}\_1} = \mathcal{U}\_T \frac{\mathbb{C}\_2}{(\mathbb{C}\_1 + \mathbb{C}\_2)} \approx \mathcal{U}\_T \frac{\mathbb{C}\_2}{\mathbb{C}\_1} \tag{1}$$

where UT (V) is the total voltage. The approximation in Eq. (1) is valid when C<sup>1</sup> ≫ C2.

The maximum voltage concentrated in C1 is around 5 V. The line voltage in MV can be 1 or 35 kV according [12]. Examples of voltage classifications between these values are: 4.16, 12.47, 13.2, 13.8, 24.94 and 34.5 kV.

The C2 design must meet some important requirements, such as, adequate dielectric strength to support pulse voltage up to 100 kV [13], homogeneous field electric around the electrode and absence of air bubbles near the electrode interface in order to reduce the growing of partial discharges in the sensor [14]. The growing of partial discharges in the insulation due to electric field concentration in a specific place and over time causes premature aging and breakdown of insulation systems [15–19].

The dimensions of the sensor and characteristics of the insulation material will be described in Section 3.

#### 3. Capacitive analytical modeling for power quality sensor

The basic reference structure adopted for the voltage sensor is composed of two coaxial cylinders terminated in hemispheres, as shown in Figure 2. This type of geometry simplifies the practical construction of C2 and allows the creation of an analytical model. The internal electrode is at line potential (U) and the external electrode is in the earth potential (0 V). The internal and external electrode radii are, respectively, a (mm) and b (mm), the electrical permittivity of the medium is ε and the cylinder length is L (mm). It should be noted that there is no region of electric flux dispersion through the upper horizontal line due to the presence of a guard's electrode not shown in the figure. This figure also shows an equipotential line defined by the distance x whose origin is the longitudinal axis of the cylinders. The equipotential line has a value exact in the region between the cylinders because the electric field in this region is constant and uniform while the equipotential in the hemispheres region has a value approximated because the electric field is non-uniform. As shown in [20], the equipotential surface tends toward a hemisphere and by heuristic approach this is a good approximation.

The analytical study demonstrates the optimum relation between the radii of the cylinders. The electric field is radial and is given by [21]:

$$E(\mathbf{x}) = \frac{q}{2\pi Lx\varepsilon} \tag{2}$$

U ¼ ðb a

Substituting q from Eq. (3) into Eq. (2) leads to:

E xð Þdx <sup>¼</sup> <sup>q</sup>

Figure 2. Structure of voltage sensor and equipotential surface. Dimension of geometric parameters in millimeters.

E xð Þ¼ <sup>U</sup> ln <sup>b</sup> a � � 1

where, substituting x to a, being a ≪ b, it is evident that the maximum value of the electric field occurs on the surface of the internal electrode. This maximum field value is given by [22]:

> Emax <sup>¼</sup> <sup>U</sup> ln <sup>b</sup> a � � 1

It is emphasized that the maximum value of the electric field is of interest in the design of the sensor, since it determines the beginning of the insulation rupture process through the dielectric. An analysis of Eq. (5) shows that this maximum field tends to infinity for the boundary conditions a ! 0 and a ! b. The first case refers to an extremely fine internal electrode, where the electric field on its surface is extremely high. The second case tends to an infinitesimal distance between the electrodes, which has to withstand voltage, . Therefore, it is clear that there is at least one minimum region between these two conditions, which is the optimal condition for

2πLε

ln <sup>b</sup> a

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� � (3)

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<sup>x</sup> (4)

<sup>a</sup> (5)

where q (C) is the charge on the cylindrical part of the central electrode. The integral of the electric field between the electrodes provides the voltage U between them, i.e.:

Simulation and Optimization of Electrical Insulation in Power Quality Monitoring Sensors Applied… http://dx.doi.org/10.5772/intechopen.77187 155

Figure 2. Structure of voltage sensor and equipotential surface. Dimension of geometric parameters in millimeters.

$$
\Delta U = \int\_{a}^{b} E(\mathbf{x})d\mathbf{x} = \frac{q}{2\pi L\varepsilon} \ln\left(\frac{b}{a}\right) \tag{3}
$$

Substituting q from Eq. (3) into Eq. (2) leads to:

capacitor. The capacitive divider is designed in such way that C2 retains practically the totally

C2 ð Þ C<sup>1</sup> þ C<sup>2</sup>

The maximum voltage concentrated in C1 is around 5 V. The line voltage in MV can be 1 or 35 kV according [12]. Examples of voltage classifications between these values are: 4.16, 12.47,

The C2 design must meet some important requirements, such as, adequate dielectric strength to support pulse voltage up to 100 kV [13], homogeneous field electric around the electrode and absence of air bubbles near the electrode interface in order to reduce the growing of partial discharges in the sensor [14]. The growing of partial discharges in the insulation due to electric field concentration in a specific place and over time causes premature aging and breakdown of

The dimensions of the sensor and characteristics of the insulation material will be described in

The basic reference structure adopted for the voltage sensor is composed of two coaxial cylinders terminated in hemispheres, as shown in Figure 2. This type of geometry simplifies the practical construction of C2 and allows the creation of an analytical model. The internal electrode is at line potential (U) and the external electrode is in the earth potential (0 V). The internal and external electrode radii are, respectively, a (mm) and b (mm), the electrical permittivity of the medium is ε and the cylinder length is L (mm). It should be noted that there is no region of electric flux dispersion through the upper horizontal line due to the presence of a guard's electrode not shown in the figure. This figure also shows an equipotential line defined by the distance x whose origin is the longitudinal axis of the cylinders. The equipotential line has a value exact in the region between the cylinders because the electric field in this region is constant and uniform while the equipotential in the hemispheres region has a value approximated because the electric field is non-uniform. As shown in [20], the equipotential surface

tends toward a hemisphere and by heuristic approach this is a good approximation.

electric field between the electrodes provides the voltage U between them, i.e.:

The electric field is radial and is given by [21]:

The analytical study demonstrates the optimum relation between the radii of the cylinders.

E xð Þ¼ <sup>q</sup>

where q (C) is the charge on the cylindrical part of the central electrode. The integral of the

<sup>2</sup>πLx<sup>ε</sup> (2)

≈ UT C2 C1

(1)

line voltage. The voltage on C1 measured by the sensor is defined by:

154 Simulation and Modelling of Electrical Insulation Weaknesses in Electrical Equipment

13.2, 13.8, 24.94 and 34.5 kV.

insulation systems [15–19].

Section 3.

UC<sup>1</sup> ¼ UT

3. Capacitive analytical modeling for power quality sensor

where UT (V) is the total voltage. The approximation in Eq. (1) is valid when C<sup>1</sup> ≫ C2.

$$E(\mathbf{x}) = \frac{U}{\ln\left(\frac{b}{a}\right)} \frac{1}{\mathbf{x}}\tag{4}$$

where, substituting x to a, being a ≪ b, it is evident that the maximum value of the electric field occurs on the surface of the internal electrode. This maximum field value is given by [22]:

$$E\_{\text{max}} = \frac{U}{\ln\left(\frac{b}{a}\right)} \frac{1}{a} \tag{5}$$

It is emphasized that the maximum value of the electric field is of interest in the design of the sensor, since it determines the beginning of the insulation rupture process through the dielectric. An analysis of Eq. (5) shows that this maximum field tends to infinity for the boundary conditions a ! 0 and a ! b. The first case refers to an extremely fine internal electrode, where the electric field on its surface is extremely high. The second case tends to an infinitesimal distance between the electrodes, which has to withstand voltage, . Therefore, it is clear that there is at least one minimum region between these two conditions, which is the optimal condition for the design of the sensor. Given b, the minimum value of Emaxð Þa can be obtained by differentiating the maximum field with respect to a and making this result equal to zero:

$$\frac{dE\_{\text{max}}(a)}{da} = 0 \tag{6}$$

elongated, increase the radius of the center electrode, tending to a revolution ellipsoid in an attempt to reduce the electric field at the electrode surface. These variations could be evaluated

Simulation and Optimization of Electrical Insulation in Power Quality Monitoring Sensors Applied…

As discussed earlier, the hemispheric region may be modified in order to attenuate the electric field at the tip surface of the central electrode. However, this makes analytical modeling difficult, and therefore the numerical calculations become necessary. For the purpose of analytical calculations, the hemispheric configuration for the electrodes (internal and external) is assumed, according to Figure 2. In the region between the hemispheres, the electric field is

E xð Þ¼ qh

where qh (C) is the charge on the hemispheric part of the central electrode. The integral of the

E xð Þdx <sup>¼</sup> qh

E xð Þ¼ Uab

In a similar way to the case of the cylinder, the maximum field occurs on the surface of the

Emax <sup>¼</sup> Ub

Differentiating Eq. (6) with respect to a and making this result equal to zero, the minimum of

Figure 4 shows the electric field on the surfaces of the cylinder and the hemisphere, for different values of the a=b ratio. It is observed that the minimum field of the hemisphere occurs for a=b ¼ 0:5 as showed by Eq. (13). The modification of the cylinder termination, passing from a flat half ellipsoid of revolution to a hemisphere and to a stretched half ellipsoid of revolution, aims to optimize the electric field on this region considering the cylindrical region as a reference. This can be done taken into account Equation 7. The dimensions of the ellipsoid could be

The capacitance between the electrodes can be easily calculated by integrating the electric flux

obtained through FEM developed in the COMSOL Multiphysics software.

along the spatial surface defined in Figure 3 resulting in:

2πε

ð Þ b � a

1 a � 1 b

1

electric field between the electrodes provides the voltage between them:

U ¼ ðb a

<sup>2</sup>πx<sup>2</sup><sup>ε</sup> (9)

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� � (10)

<sup>x</sup><sup>2</sup> (11)

a bð Þ � <sup>a</sup> (12)

b ¼ 2a (13)

through COMSOL Multiphysics software.

radial and could be described as follows:

Substituting qh from Eq. (10) into Eq. (9) gives:

this function occurs for the condition:

electrode:

what provides the optimal condition:

$$\frac{b}{a} = e\tag{7}$$

where e ¼ 2:718 is the basis of natural logarithms. Therefore, the dimensioning of the electrodes should be such that the ratio of the radii of the internal and external electrode obeys the Eq. (7). Taking the condition of Eq. (7) into Eq. (5) it is obtained:

$$E\_{\text{max}} = \frac{U}{a} \tag{8}$$

Figure 3 shows the behavior of the maximum electric field for an external electrode of b ¼ 50 mm of radius and different radii for the internal electrode when a voltage of 100 kV is applied between the electrodes. It is observed that the minimum region is obtained by Eq. (7), where a ¼ 18:4 mm. It is also noted in Figure 3 that the electric field variation around the minimum value is very small, such as small variations around the optimum relation expressed in Eq. (7) do not compromise the design. It should be noted that the critical region for the rupture of the dielectric occurs in the hemispheric region, however this region can be

Figure 3. Electrical field on the surface of the internal electrode (0 < a < b), for b ¼ 50 mm and UT ¼ 100 kV.

elongated, increase the radius of the center electrode, tending to a revolution ellipsoid in an attempt to reduce the electric field at the electrode surface. These variations could be evaluated through COMSOL Multiphysics software.

As discussed earlier, the hemispheric region may be modified in order to attenuate the electric field at the tip surface of the central electrode. However, this makes analytical modeling difficult, and therefore the numerical calculations become necessary. For the purpose of analytical calculations, the hemispheric configuration for the electrodes (internal and external) is assumed, according to Figure 2. In the region between the hemispheres, the electric field is radial and could be described as follows:

$$E(\mathbf{x}) = \frac{q\_h}{2\pi\mathbf{x}^2\varepsilon} \tag{9}$$

where qh (C) is the charge on the hemispheric part of the central electrode. The integral of the electric field between the electrodes provides the voltage between them:

$$\mathcal{U}\,\mathcal{U} = \int\_{a}^{b} E(\mathbf{x})d\mathbf{x} = \frac{q\_h}{2\pi\varepsilon} \left(\frac{1}{a} - \frac{1}{b}\right) \tag{10}$$

Substituting qh from Eq. (10) into Eq. (9) gives:

the design of the sensor. Given b, the minimum value of Emaxð Þa can be obtained by differentiat-

da <sup>¼</sup> <sup>0</sup> (6)

<sup>a</sup> <sup>¼</sup> <sup>e</sup> (7)

<sup>a</sup> (8)

dEmaxð Þa

b

where e ¼ 2:718 is the basis of natural logarithms. Therefore, the dimensioning of the electrodes should be such that the ratio of the radii of the internal and external electrode obeys the

Emax <sup>¼</sup> <sup>U</sup>

Figure 3 shows the behavior of the maximum electric field for an external electrode of b ¼ 50 mm of radius and different radii for the internal electrode when a voltage of 100 kV is applied between the electrodes. It is observed that the minimum region is obtained by Eq. (7), where a ¼ 18:4 mm. It is also noted in Figure 3 that the electric field variation around the minimum value is very small, such as small variations around the optimum relation expressed in Eq. (7) do not compromise the design. It should be noted that the critical region for the rupture of the dielectric occurs in the hemispheric region, however this region can be

Figure 3. Electrical field on the surface of the internal electrode (0 < a < b), for b ¼ 50 mm and UT ¼ 100 kV.

ing the maximum field with respect to a and making this result equal to zero:

156 Simulation and Modelling of Electrical Insulation Weaknesses in Electrical Equipment

Eq. (7). Taking the condition of Eq. (7) into Eq. (5) it is obtained:

what provides the optimal condition:

$$E(\mathbf{x}) = \frac{\mathsf{U}lab}{\left(b - a\right)\mathbf{x}^2} \frac{\mathbf{1}}{\mathbf{x}^2} \tag{11}$$

In a similar way to the case of the cylinder, the maximum field occurs on the surface of the electrode:

$$E\_{\text{max}} = \frac{\text{U}b}{a(b-a)}\tag{12}$$

Differentiating Eq. (6) with respect to a and making this result equal to zero, the minimum of this function occurs for the condition:

$$b = 2a \tag{13}$$

Figure 4 shows the electric field on the surfaces of the cylinder and the hemisphere, for different values of the a=b ratio. It is observed that the minimum field of the hemisphere occurs for a=b ¼ 0:5 as showed by Eq. (13). The modification of the cylinder termination, passing from a flat half ellipsoid of revolution to a hemisphere and to a stretched half ellipsoid of revolution, aims to optimize the electric field on this region considering the cylindrical region as a reference. This can be done taken into account Equation 7. The dimensions of the ellipsoid could be obtained through FEM developed in the COMSOL Multiphysics software.

The capacitance between the electrodes can be easily calculated by integrating the electric flux along the spatial surface defined in Figure 3 resulting in:

Figure 4. Electric field on the surface of the internal electrode, for b ¼ 50 mm and U ¼ 100 kV. The marks indicate minimum values on the surfaces of the cylinder and the hemisphere, for different values of the a=b ratio.

$$\mathbf{C} = 2\pi\varepsilon \left[ \frac{L}{\ln\left(\frac{b}{a}\right)} + \frac{ab}{(b-a)} \right] \tag{14}$$

unknown variables, which are only solved for the nodes (element corners). Thus, instead of solving an analytical equation, these unknowns are determined by a set of algebraic equations and the results for regions other than the nodes can be obtained by interpolation. Proper domain discretization is crucial to ensure the accuracy of results since the mesh format must adequately reproduce the original geometry of the structure. An example of domain discretization can be

Simulation and Optimization of Electrical Insulation in Power Quality Monitoring Sensors Applied…

This procedure reduces the generality of the mathematical framework, but enables the study of components of complex geometry. Many real world study cases involve the analysis of such

A succinct overview of FEM for the simulation of an electrostatic field is presented next considering a stationary solving method since the voltage boundary condition chosen has a constant value. A more detailed explanation can be found in [24–28]. The voltage distribution

where <sup>ε</sup><sup>0</sup> <sup>¼</sup> <sup>8</sup>:<sup>85419</sup> � <sup>10</sup>�<sup>12</sup> <sup>F</sup>=m, <sup>ε</sup><sup>r</sup> is the relative permittivity, <sup>E</sup> (kV/mm) is the electric field

Substituting Eq. (18) into Eq. (17) and considering a homogenous dielectric with r<sup>v</sup> ¼ 0 results

Eq. (19) has to be transformed into an energy functional form that relates directly to the energy of the system in order to be used in FEM. This function can be written for an element α as:

> F<sup>α</sup> ¼ ð α 1 2 ε0εrE<sup>2</sup>

/m<sup>2</sup> for a one-dimensional domain, for example.

Figure 5. Example of one-dimensional domain discretization.

) is the free charge density. The relation between electric field and the voltage U is

∇∙ð Þ¼ ε0εrE r<sup>v</sup> (17)

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159

�∇U ¼ E (18)

∇∙∇U ¼ 0 (19)

dα (20)

in a dielectric of arbitrary geometry is described by the following differential equation:

problems, which are virtually impossible to be done by analytical methods.

seen in Figure 5.

and <sup>r</sup><sup>v</sup> (C/m<sup>3</sup>

in the Laplace's equation:

with units of V<sup>2</sup>

defined as:

By inserting the condition of Eq. (7) into Eq. (14) one obtains:

$$\mathcal{L} = 2\pi\varepsilon \left[ L + \frac{ae}{(e-1)} \right] \tag{15}$$

This can also be expressed as:

$$C = 0.0556 \varepsilon\_r [L + 1.58a] \tag{16}$$

where ε<sup>r</sup> is the relative permittivity of the medium and the capacitance C is obtained in pF for L and a in millimeters. For example, considering ε<sup>r</sup> ¼ 3:8 (acetal, dielectric strength >50 kV/mm, DIN 53481 [23]), L ¼ 50 mm, and a ¼ 18:4 mm (b ¼ 50 mm), C ¼ 16:7 pF is obtained.

It is observed that the minimum field of the hemisphere occurs for a=b ¼ 0:5:

#### 4. FEM review and simulation validation

FEM is based on the solution of a boundary value problem composed of a governing equation and boundary conditions. The main idea behind this method is the division of the domain of interest in subdomains known as elements and the adoption of shape functions for the unknown variables, which are only solved for the nodes (element corners). Thus, instead of solving an analytical equation, these unknowns are determined by a set of algebraic equations and the results for regions other than the nodes can be obtained by interpolation. Proper domain discretization is crucial to ensure the accuracy of results since the mesh format must adequately reproduce the original geometry of the structure. An example of domain discretization can be seen in Figure 5.

This procedure reduces the generality of the mathematical framework, but enables the study of components of complex geometry. Many real world study cases involve the analysis of such problems, which are virtually impossible to be done by analytical methods.

A succinct overview of FEM for the simulation of an electrostatic field is presented next considering a stationary solving method since the voltage boundary condition chosen has a constant value. A more detailed explanation can be found in [24–28]. The voltage distribution in a dielectric of arbitrary geometry is described by the following differential equation:

$$\nabla \cdot (\varepsilon\_0 \varepsilon\_r \mathbf{E}) = \rho\_v \tag{17}$$

where <sup>ε</sup><sup>0</sup> <sup>¼</sup> <sup>8</sup>:<sup>85419</sup> � <sup>10</sup>�<sup>12</sup> <sup>F</sup>=m, <sup>ε</sup><sup>r</sup> is the relative permittivity, <sup>E</sup> (kV/mm) is the electric field and <sup>r</sup><sup>v</sup> (C/m<sup>3</sup> ) is the free charge density. The relation between electric field and the voltage U is defined as:

$$-\nabla \mathcal{U} I = \mathcal{E} \tag{18}$$

Substituting Eq. (18) into Eq. (17) and considering a homogenous dielectric with r<sup>v</sup> ¼ 0 results in the Laplace's equation:

$$
\nabla \cdot \nabla U = 0 \tag{19}
$$

Eq. (19) has to be transformed into an energy functional form that relates directly to the energy of the system in order to be used in FEM. This function can be written for an element α as:

$$F\_a = \int\_{\alpha} \frac{1}{2} \varepsilon\_0 \varepsilon\_r E^2 d\alpha \tag{20}$$

with units of V<sup>2</sup> /m<sup>2</sup> for a one-dimensional domain, for example.

Figure 5. Example of one-dimensional domain discretization.

C ¼ 2πε

minimum values on the surfaces of the cylinder and the hemisphere, for different values of the a=b ratio.

By inserting the condition of Eq. (7) into Eq. (14) one obtains:

158 Simulation and Modelling of Electrical Insulation Weaknesses in Electrical Equipment

4. FEM review and simulation validation

This can also be expressed as:

L ln <sup>b</sup> a � � þ

Figure 4. Electric field on the surface of the internal electrode, for b ¼ 50 mm and U ¼ 100 kV. The marks indicate

where ε<sup>r</sup> is the relative permittivity of the medium and the capacitance C is obtained in pF for L and a in millimeters. For example, considering ε<sup>r</sup> ¼ 3:8 (acetal, dielectric strength >50 kV/mm,

FEM is based on the solution of a boundary value problem composed of a governing equation and boundary conditions. The main idea behind this method is the division of the domain of interest in subdomains known as elements and the adoption of shape functions for the

C ¼ 2πε L þ

DIN 53481 [23]), L ¼ 50 mm, and a ¼ 18:4 mm (b ¼ 50 mm), C ¼ 16:7 pF is obtained.

It is observed that the minimum field of the hemisphere occurs for a=b ¼ 0:5:

ab ð Þ b � a

ae ð Þ e � 1 � �

C ¼ 0:0556εr½ � L þ 1:58a (16)

(14)

(15)

" #

The function gives the voltage distribution that satisfies the governing partial equation when differentiated with respected to U and equaled to zero. Thus, the change in the global function due to the change in the voltage of node i is:

$$\frac{\partial F}{\partial \mathcal{U}\_i} = \sum\_a \frac{\partial F\_a}{\partial \mathcal{U}\_i} \tag{21}$$

where the summation represents the contribution from all elements associated with Ui or all the elements connected to node i. These derivatives are equaled to zero resulting in a group of simultaneous equations arranged in matrix form as:

$$[S\_t] \{ \mathcal{U}\_i \} = 0 \tag{22}$$

Figure 6. Voltage sensor modeled considering its axis of symmetry, voltage boundary conditions and paths of interest for

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Figure 8. Cylindrical region—comparison of analytical and numerical electric fields for mesh selection.

the validation (dimensions in millimeters).

Figure 7. Meshes used for the mesh convergence study.

where St ½ �<sup>n</sup>�<sup>n</sup> is the stiffness matrix whose terms are defined by geometric parameters of the nodes and material properties. f g Ui <sup>n</sup>�<sup>1</sup> is a column matrix of voltages, which are the unknowns to be solved. Additionally, the boundary values of voltage associated to some of the nodes are applied to this matrix. After the determination of the voltage distribution, the electric field can be obtained by numerically solving Eq. (18).

As it can be noticed, the assumptions made in the original analytical problem for the FEM strongly affect the results. Thereby, the results provided by the simulation have to be validated to ensure their accuracy. To do so, these results are compared to the ones of an analytical model derived for specific conditions as previously presented. Analytical results of electric field from Eqs. (4) and (11) are taken for, respectively, the cylindrical and hemispheric regions as references for the simulation whose parameters of interest are the order of the shape functions and the mesh refinement.

It is worth to mention that the dielectric geometry has a longitudinal axis-symmetry, which implies that the unknown values do not change along the azimuthal axis and thus only a transversal plan needs to be modeled in FEM and the rest of the solution can be extrapolated. Figure 6 shows the structure modeled around its axis of symmetry for a ¼ 18:4 mm (inner radius) and b ¼ 50 mm (outer radius) in both cylindrical and hemispheric regions. Additionally, voltage boundary conditions were the same as the ones used for the analytical model. Electric field values were extracted along paths P1-P2 and P3-P4, respectively, in the cylindrical and hemispheric regions.

As a default, COMSOL uses a second order shape function in order to improve the results' accuracy. This choice seems to be adequate as the analytical results showed that the radial variation of the electric field fallows a quadratic pattern. However, a mesh convergence study is still needed in order to minimize domain discretization effects on the results. In this procedure, the mesh is successively refined and the values of interest are compared to a reference. Figure 7 shows the meshes evaluated.

Figures 8 and 9 show the comparison of analytical results given by Eqs. (4) and (11) with numerical results for three mesh refinements using quadratic shape functions, respectively, for the cylindrical and hemispheric regions. The electric field results obtained by the simulations presented good agreement with the analytical results indicating that quadratic shape functions Simulation and Optimization of Electrical Insulation in Power Quality Monitoring Sensors Applied… http://dx.doi.org/10.5772/intechopen.77187 161

Figure 6. Voltage sensor modeled considering its axis of symmetry, voltage boundary conditions and paths of interest for the validation (dimensions in millimeters).

Figure 7. Meshes used for the mesh convergence study.

The function gives the voltage distribution that satisfies the governing partial equation when differentiated with respected to U and equaled to zero. Thus, the change in the global function

> <sup>¼</sup> <sup>X</sup> α

where the summation represents the contribution from all elements associated with Ui or all the elements connected to node i. These derivatives are equaled to zero resulting in a group of

where St ½ �<sup>n</sup>�<sup>n</sup> is the stiffness matrix whose terms are defined by geometric parameters of the nodes and material properties. f g Ui <sup>n</sup>�<sup>1</sup> is a column matrix of voltages, which are the unknowns to be solved. Additionally, the boundary values of voltage associated to some of the nodes are applied to this matrix. After the determination of the voltage distribution, the

As it can be noticed, the assumptions made in the original analytical problem for the FEM strongly affect the results. Thereby, the results provided by the simulation have to be validated to ensure their accuracy. To do so, these results are compared to the ones of an analytical model derived for specific conditions as previously presented. Analytical results of electric field from Eqs. (4) and (11) are taken for, respectively, the cylindrical and hemispheric regions as references for the simulation whose parameters of interest are the order of the shape functions and

It is worth to mention that the dielectric geometry has a longitudinal axis-symmetry, which implies that the unknown values do not change along the azimuthal axis and thus only a transversal plan needs to be modeled in FEM and the rest of the solution can be extrapolated. Figure 6 shows the structure modeled around its axis of symmetry for a ¼ 18:4 mm (inner radius) and b ¼ 50 mm (outer radius) in both cylindrical and hemispheric regions. Additionally, voltage boundary conditions were the same as the ones used for the analytical model. Electric field values were extracted along paths P1-P2 and P3-P4, respectively, in the cylindrical

As a default, COMSOL uses a second order shape function in order to improve the results' accuracy. This choice seems to be adequate as the analytical results showed that the radial variation of the electric field fallows a quadratic pattern. However, a mesh convergence study is still needed in order to minimize domain discretization effects on the results. In this procedure, the mesh is successively refined and the values of interest are compared to a reference.

Figures 8 and 9 show the comparison of analytical results given by Eqs. (4) and (11) with numerical results for three mesh refinements using quadratic shape functions, respectively, for the cylindrical and hemispheric regions. The electric field results obtained by the simulations presented good agreement with the analytical results indicating that quadratic shape functions

∂F<sup>α</sup> ∂Ui

St ½ �f g¼ Ui 0 (22)

(21)

∂F ∂Ui

due to the change in the voltage of node i is:

160 Simulation and Modelling of Electrical Insulation Weaknesses in Electrical Equipment

simultaneous equations arranged in matrix form as:

electric field can be obtained by numerically solving Eq. (18).

the mesh refinement.

and hemispheric regions.

Figure 7 shows the meshes evaluated.

Figure 8. Cylindrical region—comparison of analytical and numerical electric fields for mesh selection.

Figure 9. Hemispheric region—comparison of analytical and numerical electric fields for mesh selection.


Table 1. Analytical and numerical capacitance values.

are indeed a good choice. In addition, it can be seen that mesh refinement led to a better solution field as the relative error computed using the analytical values as reference decreased. Besides, the duration of the simulations for the three cases did not increase significantly with the last case taking about 1 s. The computer used was a workstation with an Intel(R) Core™ i7– 4790 3.60 GHz CPU and 16 GB of RAM.

termination shall change from a flat geometry up to a stretched geometry between points P5 (region next to transition from cylindrical region to the elliptic region) and P6 (region at the tip

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Figure 10. Illustration of parametric study based on the geometric modification of the hemispheric region.

From the simulation of the electric field for different values of semi-axis v, it is verified that for small values of semi-axis v, the electric field is more concentrated on the edges and assumes large values in the transition region from cylinder region to elliptic region presenting irregular distribution, as shown in Figure 11. Similarly, an irregular distribution is observed for large values of semi-axis v, but with more concentration of electric field at the tip of the central electrode. On the other hand, for values of semi-axis v near the hemispheric geometry of radius a, a better electric field distribution for the entire surface is observed with a minimum concentration. This result shows that the investigation for the optimum condition should be concen-

visualization for the distribution of electric field norm for different geometries of the central electrode, as shown in Figure 10. As an illustration of this procedure, considering two distinct geometries having different lengths and shapes of original paths P5-P6, such as the flat and stretched cases of Figure 11, their electric filed can be directly compared thought the path normalization proposed. This is performed in Figure 12 where the flat geometry shows an

<sup>0</sup> and F<sup>0</sup> is used in order to provide a better

<sup>0</sup> while for the stretched geometry the concentration

<sup>0</sup> and P6 0 . This

), as expected. Additionally, the hemispheric geometry produces

of centre electrode).

trated around the hemispheric geometry.

electric field concentration close to point P5

0

a small electric field norm variation along the trajectory between points P5

is observed at the tip (point P6

To do so, a normalized path between points P5

Finally, another way to validate the simulations is to compute capacitance values from the simulations by numerical integration of the electrical flux along the inner surface in which 100 kV was applied. These values are compared with the analytical one described by Eq. (14) as shown in Table 1.

Since the model related to the fine mesh provided electric field distributions that best agreed with the analytical results in both cylindrical and hemispheric regions of the voltage sensor, and get a capacitance value that closest matched with Eq. (14), this model was successfully validated and will be used in the case study presented below.

#### 5. A case study using COMSOL

As explained in Section 3, the modification of the cylinder termination of the central electrode, passing from a flat half ellipsoid of revolution to a stretched half ellipsoid of revolution, aims to optimize the electric field on this region. Therefore, the optimum geometry of the ellipsoid could be obtained through a parametric study, as shown in Figure 10. Varying only the vertical semi-axis (v) and setting the horizontal semi-axis (h) constant, the geometry of the cylinder Simulation and Optimization of Electrical Insulation in Power Quality Monitoring Sensors Applied… http://dx.doi.org/10.5772/intechopen.77187 163

Figure 10. Illustration of parametric study based on the geometric modification of the hemispheric region.

are indeed a good choice. In addition, it can be seen that mesh refinement led to a better solution field as the relative error computed using the analytical values as reference decreased. Besides, the duration of the simulations for the three cases did not increase significantly with the last case taking about 1 s. The computer used was a workstation with an Intel(R) Core™ i7–

Numerical (pF) Analytical (pF) Difference (%)

Coarse mesh 16.812 0.500 Normal mesh 16.810 16.728 0.488 Fine mesh 16.809 0.482

Figure 9. Hemispheric region—comparison of analytical and numerical electric fields for mesh selection.

162 Simulation and Modelling of Electrical Insulation Weaknesses in Electrical Equipment

Finally, another way to validate the simulations is to compute capacitance values from the simulations by numerical integration of the electrical flux along the inner surface in which 100 kV was applied. These values are compared with the analytical one described by Eq. (14)

Since the model related to the fine mesh provided electric field distributions that best agreed with the analytical results in both cylindrical and hemispheric regions of the voltage sensor, and get a capacitance value that closest matched with Eq. (14), this model was successfully

As explained in Section 3, the modification of the cylinder termination of the central electrode, passing from a flat half ellipsoid of revolution to a stretched half ellipsoid of revolution, aims to optimize the electric field on this region. Therefore, the optimum geometry of the ellipsoid could be obtained through a parametric study, as shown in Figure 10. Varying only the vertical semi-axis (v) and setting the horizontal semi-axis (h) constant, the geometry of the cylinder

4790 3.60 GHz CPU and 16 GB of RAM.

Table 1. Analytical and numerical capacitance values.

5. A case study using COMSOL

validated and will be used in the case study presented below.

as shown in Table 1.

termination shall change from a flat geometry up to a stretched geometry between points P5 (region next to transition from cylindrical region to the elliptic region) and P6 (region at the tip of centre electrode).

From the simulation of the electric field for different values of semi-axis v, it is verified that for small values of semi-axis v, the electric field is more concentrated on the edges and assumes large values in the transition region from cylinder region to elliptic region presenting irregular distribution, as shown in Figure 11. Similarly, an irregular distribution is observed for large values of semi-axis v, but with more concentration of electric field at the tip of the central electrode. On the other hand, for values of semi-axis v near the hemispheric geometry of radius a, a better electric field distribution for the entire surface is observed with a minimum concentration. This result shows that the investigation for the optimum condition should be concentrated around the hemispheric geometry.

To do so, a normalized path between points P5 <sup>0</sup> and F<sup>0</sup> is used in order to provide a better visualization for the distribution of electric field norm for different geometries of the central electrode, as shown in Figure 10. As an illustration of this procedure, considering two distinct geometries having different lengths and shapes of original paths P5-P6, such as the flat and stretched cases of Figure 11, their electric filed can be directly compared thought the path normalization proposed. This is performed in Figure 12 where the flat geometry shows an electric field concentration close to point P5 <sup>0</sup> while for the stretched geometry the concentration is observed at the tip (point P6 0 ), as expected. Additionally, the hemispheric geometry produces a small electric field norm variation along the trajectory between points P5 <sup>0</sup> and P6 0 . This

Figure 11. Electric field distribution for three geometries.

emphasizes that the best geometry for the central electrode in terms of electric field distribution must be close to the hemispheric geometry.

Therefore, this work has been focused on parametric studies around the hemispheric geometry with variations in the value of the semi-axis v of the central electrode. Eleven cases were simulated with a step of 0.2 mm being five cases below 18.5 mm (tip flattening) and five cases above 18.5 mm (tip stretching), as shown in Figure 13. It is important to note the effect of geometric modification on electric field norm along normalized path P5 0 -P6 0 . As an example, the response curve for the flattening geometry has bigger values of electric field close to the point P5 <sup>0</sup> and lower values close to the point P6 0 . Also, these curves indicate the existence of a case whose maximum electric field value along E<sup>0</sup> -F<sup>0</sup> path is the smaller one among the other curves, making it a candidate for the optimum condition.

Figure 14 presents the maximum electric field norm along the normalized path P5 0 -P6 <sup>0</sup> for each case presented in Figure 13. The existence of an optimum condition is evidenced by the trend in the results provided by the simulated cases. The exact value of the semi-axis v for this optimum condition is obtained through interpolation, leading to an optimum value of 17.5 mm.

A new parametric study was performed in order to confirm the best condition found. Three cases around the optimum semi-axis v of 17.5 mm were simulated with a smaller step of 0.1 mm. Figure 15 shows these results indicating that the solution for v ¼ 17:5 mm is indeed the optimum solution since it ensures the lowest electric field norm when the whole normalized path P5 0 -P6 <sup>0</sup> is considered. Another form of analysis is to compute a relative percent difference taking the electric filed norm from the cylindrical region, which is constant for any modifications in the geometry of the hemispheric region, as a reference (5.447 kV/mm). As it

can be seen, again the optimum condition curve presents smaller values than the other curves

0 -P6 0 .

0 -P6

<sup>0</sup> for three geometries.

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<sup>0</sup> is considered. This conclusion is supported by an extension of the last parametric study, as shown in the Figure 16, where 21 different values around 17.5 mm with a step of 0.1 mm for the semi-axis v were computed. The relative difference metric was calculated for the maximum value of each electric field norm distribution evidencing the optimum condition of 17.5 mm, as expected.

0 -P6

Figure 13. Effect of tip deformation on electric field norm along normalized path P5

when the whole normalized path P5

Figure 12. Electric field norm along normalized path P5

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Figure 12. Electric field norm along normalized path P5 0 -P6 <sup>0</sup> for three geometries.

emphasizes that the best geometry for the central electrode in terms of electric field distribu-

Therefore, this work has been focused on parametric studies around the hemispheric geometry with variations in the value of the semi-axis v of the central electrode. Eleven cases were simulated with a step of 0.2 mm being five cases below 18.5 mm (tip flattening) and five cases above 18.5 mm (tip stretching), as shown in Figure 13. It is important to note the effect of

the response curve for the flattening geometry has bigger values of electric field close to the

case presented in Figure 13. The existence of an optimum condition is evidenced by the trend in the results provided by the simulated cases. The exact value of the semi-axis v for this optimum condition is obtained through interpolation, leading to an optimum value of

A new parametric study was performed in order to confirm the best condition found. Three cases around the optimum semi-axis v of 17.5 mm were simulated with a smaller step of 0.1 mm. Figure 15 shows these results indicating that the solution for v ¼ 17:5 mm is indeed the optimum solution since it ensures the lowest electric field norm when the whole normal-

difference taking the electric filed norm from the cylindrical region, which is constant for any modifications in the geometry of the hemispheric region, as a reference (5.447 kV/mm). As it

<sup>0</sup> is considered. Another form of analysis is to compute a relative percent

0

0 -P6 0

. Also, these curves indicate the existence of a


. As an example,

<sup>0</sup> for each

0 -P6

geometric modification on electric field norm along normalized path P5

Figure 14 presents the maximum electric field norm along the normalized path P5

tion must be close to the hemispheric geometry.

Figure 11. Electric field distribution for three geometries.

164 Simulation and Modelling of Electrical Insulation Weaknesses in Electrical Equipment

<sup>0</sup> and lower values close to the point P6

curves, making it a candidate for the optimum condition.

case whose maximum electric field value along E<sup>0</sup>

point P5

17.5 mm.

ized path P5

0 -P6

Figure 13. Effect of tip deformation on electric field norm along normalized path P5 0 -P6 0 .

can be seen, again the optimum condition curve presents smaller values than the other curves when the whole normalized path P5 0 -P6 <sup>0</sup> is considered.

This conclusion is supported by an extension of the last parametric study, as shown in the Figure 16, where 21 different values around 17.5 mm with a step of 0.1 mm for the semi-axis v were computed. The relative difference metric was calculated for the maximum value of each electric field norm distribution evidencing the optimum condition of 17.5 mm, as expected.

Figure 14. Interpolation of results for optimum condition.

Another conclusion is that, for this optimum geometry, a reduction of approximately 1.5% in the maximum electric field is achieved when compared to the hemispheric geometry. This improvement is strategic to ensure the safe operation of the sensor since partial discharge is a localized phenomenon influenced by electric field concentration. In a real scenario, because of imperfections in the dielectric due to the manufacturing process, such as air bubbles, a lightning impulse of 100 kV can lead to local discharges in points of high-electric field concentra-

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Figure 16. Relative difference of maximum electric field norm considering cylindrical region as reference.

Finally, Figure 17 shows the C2 capacitance variation for a wide-range of values of semi-axis v covering the geometries presented in Figure 11. It is possible to see that the value of C2 for the optimum condition is very close to the ones shown in Table 1 since the geometry of the optimum condition is very close to a hemisphere. In addition, the range of capacitance values

tion, which will cause the failure of the sensor.

obtained is compatible to the design of the capacitive divider.

Figure 15. Electric field norm and relative difference metric along normalized path P5 0 -P6 <sup>0</sup> for optimum condition verification.

Simulation and Optimization of Electrical Insulation in Power Quality Monitoring Sensors Applied… http://dx.doi.org/10.5772/intechopen.77187 167

Figure 16. Relative difference of maximum electric field norm considering cylindrical region as reference.

Figure 14. Interpolation of results for optimum condition.

166 Simulation and Modelling of Electrical Insulation Weaknesses in Electrical Equipment

Figure 15. Electric field norm and relative difference metric along normalized path P5

verification.

0 -P6

<sup>0</sup> for optimum condition

Another conclusion is that, for this optimum geometry, a reduction of approximately 1.5% in the maximum electric field is achieved when compared to the hemispheric geometry. This improvement is strategic to ensure the safe operation of the sensor since partial discharge is a localized phenomenon influenced by electric field concentration. In a real scenario, because of imperfections in the dielectric due to the manufacturing process, such as air bubbles, a lightning impulse of 100 kV can lead to local discharges in points of high-electric field concentration, which will cause the failure of the sensor.

Finally, Figure 17 shows the C2 capacitance variation for a wide-range of values of semi-axis v covering the geometries presented in Figure 11. It is possible to see that the value of C2 for the optimum condition is very close to the ones shown in Table 1 since the geometry of the optimum condition is very close to a hemisphere. In addition, the range of capacitance values obtained is compatible to the design of the capacitive divider.

imperfections in the dielectric due to manufacturing process imprecision, such as air bubbles, a lightning impulse of 100 kV can lead to local discharges in points of high electric field concen-

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This work was funded by FINEP (Brazilian Innovation Agency) grant number 0115002800

CNPq (National Counsel of Technological and Scientific Development) sponsors the author

Sender Rocha dos Santos, Rodrigo Peres, Wagner Francisco Rezende Cano and

CPqD – Research and Development Center in Telecommunications, Campinas, SP, Brazil

[1] Colak I. Introduction to smart grid. In: Proceedings of the International Smart Grid Workshop and Certificate Program (ISGWCP); Istanbul; 2016. pp. 1-5. DOI: 10.1109/

[2] Parker DM, McCollough ND. Medium-voltage sensors for the smart grid: Lessons learned. In: Proceedings of the IEEE Power and Energy Society General Meeting; San

[3] International Electrotechnical Commission. IEC 61000-4-30:2015 - Electromagnetic Compatibility (EMC)—Part 4-30: Testing and AQ01 25 Measurement Techniques - Power

tration, which will cause the failure of the sensor.

The authors wish to thank their colleague Celio Fonseca Barbosa.

Acknowledgements

Conflict of interest

Author details

Joao Batista Rosolem\*

References

ISGWCP.2016.7548265

Joao B. Rosolem under scholarship DT.

The authors declare no conflict of interest.

\*Address all correspondence to: rosolem@cpqd.com.br

Diego; 2011. pp. 1-7. DOI: 10.1109/PES.2011.6039775

Quality Measurement Methods. Geneva, Switzerland; 2015

0407/14.

Figure 17. Capacitance values C2 for different values of semi-axis v. The mark indicates optimized electric field condition.

#### 6. Conclusion

A rigorous capacitor design is necessary for MV sensors when capacitive divider is used to obtain a voltage sample. This is due to safety aspects regarding to the technician activities and some environmental effects that can influence their performance, such as temperature, pressure and wind. In addition, external elements in the vicinity of the sensor can alter the electric field acting inside of it.

The basic structure adopted for the voltage sensor is composed of two-coaxial cylinders terminated in a hemisphere, which simplifies the practical construction of the capacitor. Although, this work demonstrated that the geometry of the electrode termination should be different of a hemisphere in order to minimize the electric field distribution in this region.

This conclusion was based in finite-element studies developed in COMSOL software. The first one, considered a hemispheric electrode termination of 18.4 mm of radius and was compared to an analytical model for validation purposes. Next, a parametric study was developed, in which the termination was changed from a flat to a stretched geometry, to obtain the optimum condition. The electric field distribution along the termination for this condition was compared to a reference value extracted from the cylindrical region (5.447 kV/mm). The result is that the optimum geometry is slightly flatter than a hemisphere having a semi-axis v equals 17.5 mm.

Additionally, the electric field distributions of the optimum and the hemispheric geometries were compared evidencing a magnitude reduction of approximately 1.5%. This improvement is strategic to ensure the safe operation of the sensor since partial discharge is a localized phenomenon influenced by electric field concentration. In a real scenario, because of imperfections in the dielectric due to manufacturing process imprecision, such as air bubbles, a lightning impulse of 100 kV can lead to local discharges in points of high electric field concentration, which will cause the failure of the sensor.

#### Acknowledgements

The authors wish to thank their colleague Celio Fonseca Barbosa.

This work was funded by FINEP (Brazilian Innovation Agency) grant number 0115002800 0407/14.

CNPq (National Counsel of Technological and Scientific Development) sponsors the author Joao B. Rosolem under scholarship DT.

## Conflict of interest

The authors declare no conflict of interest.

### Author details

6. Conclusion

field acting inside of it.

A rigorous capacitor design is necessary for MV sensors when capacitive divider is used to obtain a voltage sample. This is due to safety aspects regarding to the technician activities and some environmental effects that can influence their performance, such as temperature, pressure and wind. In addition, external elements in the vicinity of the sensor can alter the electric

Figure 17. Capacitance values C2 for different values of semi-axis v. The mark indicates optimized electric field condition.

168 Simulation and Modelling of Electrical Insulation Weaknesses in Electrical Equipment

The basic structure adopted for the voltage sensor is composed of two-coaxial cylinders terminated in a hemisphere, which simplifies the practical construction of the capacitor. Although, this work demonstrated that the geometry of the electrode termination should be different of a hemisphere in order to minimize the electric field distribution in this region.

This conclusion was based in finite-element studies developed in COMSOL software. The first one, considered a hemispheric electrode termination of 18.4 mm of radius and was compared to an analytical model for validation purposes. Next, a parametric study was developed, in which the termination was changed from a flat to a stretched geometry, to obtain the optimum condition. The electric field distribution along the termination for this condition was compared to a reference value extracted from the cylindrical region (5.447 kV/mm). The result is that the optimum geometry is slightly flatter than a hemisphere having a semi-axis v equals 17.5 mm. Additionally, the electric field distributions of the optimum and the hemispheric geometries were compared evidencing a magnitude reduction of approximately 1.5%. This improvement is strategic to ensure the safe operation of the sensor since partial discharge is a localized phenomenon influenced by electric field concentration. In a real scenario, because of Sender Rocha dos Santos, Rodrigo Peres, Wagner Francisco Rezende Cano and Joao Batista Rosolem\*

\*Address all correspondence to: rosolem@cpqd.com.br

CPqD – Research and Development Center in Telecommunications, Campinas, SP, Brazil

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[4] International Electrotechnical Commission. IEC 61000-4-7:2002+AMD1:2008 CSV-Electromagnetic Compatibility (EMC)—Part 4-7: Testing and Measurement Techniques—General Guide on Harmonics and Interharmonics Measurements and Instrumentation, for Power Supply Systems and Equipment Connected Thereto. Geneva, Switzerland; 2008

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[8] Štefanka M. Application of sensors and digitalization based on IEC 61850 in medium voltage networks and switchgears [thesis]. Brno: Brno University of Technology, Faculty

[9] Aurilio G, Crotti G, Gallo D, Giordano D, Landi C, Luiso M. MV divider with fiber optic insulation. In: Proceedings of the IEEE International Workshop on Applied Measurements for Power Systems (AMPS); Aachen; 2013. pp. 1-6. DOI: 10.1109/AMPS.2013.6656216

[10] Sevlian R, Rajagopal R. Actively calibrated line mountable capacitive voltage transducer for power systems applications. IEEE Transactions on Smart Grid. 2016;99:1-1. DOI:

[11] Moreno MVR, Robles G, Albarracín R, Rey JA, Tarifa JMM. Study on the self-integration of a Rogowski coil used in the measurement of partial discharges pulses. Electrical Engi-

[12] International Electrotechnical Commission. IEC 60038:2009 – IEC Standard Voltages.

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[14] Siada AA. Power Transformer Condition Monitoring and Diagnosis. Chapter 2: Power Transformer Condition Monitoring and Diagnosis: Concepts and Challenges. Albarracín R, Robles G, Ardila-Rey JA, Cavallini A, Passaglia R. Pagination: c; 2018. 300pp. ISBN:

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978-1-78561-254-1

Butterworth-Heinemann Ltd; 1989


**Section 4**

**Insulation Aging Modelling of Power**

**Generators**

**Insulation Aging Modelling of Power Generators**

**Chapter 8**

**Provisional chapter**

**Generator Insulation-Aging On-Line Monitoring**

**Generator Insulation-Aging On-Line Monitoring** 

Additional information is available at the end of the chapter

Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/intechopen.78065

Peter Kung

Peter Kung

**Abstract**

vibration, Fiber optic

**1. Introduction**

**Technique Based on Fiber Optic Detecting Technology**

The relationship between insulation aging and generator lifespan using fiber optic sensors (FOSs) is explored to ultimately improve asset lifespan through smart choices in running conditions and maintenance. Insulation aging is a major factor that causes generator failure. FOS provides the rare opportunity of being installed up close to the insulation, monitoring degradations that are otherwise difficult to detect. FOSs, unlike purely electrical transducers, are immune to high voltage (HV) and strong electromagnetic (EM) fields. They are small and have a proven long life by their deployment in the Telecom industry. The proposed FOS is a Fabry-Perot cavity made up of two identical fiber Bragg gratings (FBGs) using light wave interference as the working principle. Such architecture delivers simultaneous vibration (10 Hz–1 kHz) and temperature (0.1°C resolution) monitoring, both helping to spot irregular vibration patterns (signatures) and hot-spots inside the generator stator slots. The signal processing unit equipped with a gateway device can help to connect the large volume of sensor data, allowing correlation with the supervisory control and data acquisition (SCADA) system data of the plant. This chapter also elaborates on the field test jointly conducted with Calpine Corporation and Oz Optics, Ltd. (Ottawa, Ontario, Canada).

**Keywords:** hot-spot, fiber Bragg grating (FBG), Brillouin scattering, generator,

Insulation aging phenomenon in air-cooled gas-fired generators is a problem confronting both original equipment manufacturers (OEMs) and owners of these competitive assets [1]. The expected life of these generators largely depends on design, manufacturing workmanship and choice of material. It also depends on the way they are used. They are used to

**Technique Based on Fiber Optic Detecting Technology**

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

© 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use,

distribution, and reproduction in any medium, provided the original work is properly cited.

DOI: 10.5772/intechopen.78065

#### **Generator Insulation-Aging On-Line Monitoring Technique Based on Fiber Optic Detecting Technology Generator Insulation-Aging On-Line Monitoring Technique Based on Fiber Optic Detecting Technology**

DOI: 10.5772/intechopen.78065

#### Peter Kung Peter Kung

Additional information is available at the end of the chapter Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/intechopen.78065

#### **Abstract**

The relationship between insulation aging and generator lifespan using fiber optic sensors (FOSs) is explored to ultimately improve asset lifespan through smart choices in running conditions and maintenance. Insulation aging is a major factor that causes generator failure. FOS provides the rare opportunity of being installed up close to the insulation, monitoring degradations that are otherwise difficult to detect. FOSs, unlike purely electrical transducers, are immune to high voltage (HV) and strong electromagnetic (EM) fields. They are small and have a proven long life by their deployment in the Telecom industry. The proposed FOS is a Fabry-Perot cavity made up of two identical fiber Bragg gratings (FBGs) using light wave interference as the working principle. Such architecture delivers simultaneous vibration (10 Hz–1 kHz) and temperature (0.1°C resolution) monitoring, both helping to spot irregular vibration patterns (signatures) and hot-spots inside the generator stator slots. The signal processing unit equipped with a gateway device can help to connect the large volume of sensor data, allowing correlation with the supervisory control and data acquisition (SCADA) system data of the plant. This chapter also elaborates on the field test jointly conducted with Calpine Corporation and Oz Optics, Ltd. (Ottawa, Ontario, Canada).

**Keywords:** hot-spot, fiber Bragg grating (FBG), Brillouin scattering, generator, vibration, Fiber optic

#### **1. Introduction**

Insulation aging phenomenon in air-cooled gas-fired generators is a problem confronting both original equipment manufacturers (OEMs) and owners of these competitive assets [1]. The expected life of these generators largely depends on design, manufacturing workmanship and choice of material. It also depends on the way they are used. They are used to

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

**2.1. Insulation aging reasons**

vibration aging combined together [6].

caused by thermal expansion of the polymer [7].

*2.1.1. Heat-aging effect*

*2.1.2. Mechanical stress effect*

*2.1.3. Sparking effect*

For the gas-fired generator, the stator winding, which is the direct carrier that generates electricity, would sustain different kinds of combined effects in the process of operation simultaneously, such as electric, heat, mechanical and any other actions [4]. The investigation and researching results gained by many researchers, show that a series of physical and chemical changes will occur during the insulation materials operating for a long time, such as insulation medium softening, pinhole, cracking, ionization, etc. [5]. It is generally assumed that insulation aging is affected mainly by thermal cycling, heat aging, electric aging, mechanical

Generator Insulation-Aging On-Line Monitoring Technique Based on Fiber Optic Detecting…

http://dx.doi.org/10.5772/intechopen.78065

177

Many generator insulation materials consist of mica and epoxides. Mica and epoxides both have excellent heat resistant characteristics. The occurring rate of the aging phenomenon is slow while the generator is working under normal temperatures, and the higher the temperature of the insulating material is, the faster the heat aging. The insulation materials performance decreases with the increase of temperature. For the generator, the main reasons for the increase of insulation temperature are the resistance heat of the conductor, partial discharge, leakage current of the insulation and the heat caused by the dielectric loss. For the epoxidemica insulation medium, there are two ways to affect the insulation performance: one is the epoxy-mica temper embrittlement and thermal degradation and another is the local defects

The generator stator windings are nested in the mechanical supporting structure. The stator winding is generally running under mechanical stress. At the same time, there always exists vibration, which corresponds with stress. The electromagnetic force generated on the stator windings changes the rotor turning speed variation. Due to the effect of long time fluctuating mechanical force, there would produce some joint loosening and vibration. Under the action of alternating stress and vibration, the conductive carbon coating attached to insulation material would loosen and shed because of the vibration fatigue. We call it mechanical insulation aging which is caused by mechanical stress. The alternating mechanical force comes from static mechanical force, the start-stop electromagnetic force, and mechanical vibration force at running time [8]. Mechanical vibration force at running time produces as illustrated in **Figure 2**.

The winding insulation of generators are made of epoxy & mica combined together. Due to the difference of expansion coefficient and manufacturing process, there exists some microgaps between materials and carbon coating, different insulation layers, under the effect of electric field, the sparking caused by partial discharge would be showing up at the micro-gap location. Due to the sparking, there are three damage types. One type is that the main insulation thickness becomes thinner because the adhesive between the coating and the insulation medium is carbonized by the high temperature in the micro-gap caused by the partial discharge. The second type is that the edges of the insulation medium and the air gap wall

**Figure 1.** At the left, local loose core found using simple knife check. At the middle, defects start to appear as pinholes. At the right, partial discharge removes the conducting paint without affecting the underlying mica.

adapt to the intermittent nature of renewable energies, subjecting them to many start-stop cycles. Such cycles give rise to stresses and creeps from material expansion and contraction. Under constant cost reduction, pressure generators are getting less expensive and their quality also suffers. Their characteristic strong vibrations that shake the structure loose further aggravate this.

**Figure 1** (left) shows how vibration causes delamination in the core, allowing the varnish to wear out and then eddy currents to introduce hot-spots affecting the performance of the winding in the slot located close by. It becomes a downward spiral of mechanical degradation. Some of the cost reducing innovations that were previously introduced worked well in a base load operation. However, in a constant start-stop mode of operation, generators are susceptible to outage early in their life because they are air-cooled. The constant thermal cycling combined with the variable characteristics of the air contribute to early wear. They can suffer from a new failure mode called vibration sparking. This is the first time that this failure mechanism has been observed as failure in progress as shown in **Figure 1** (middle). They usually become uncovered upon a complete breakdown. A simple model that combines thermal aging and a mechanical vibration-assisted degradation process is introduced in the following sections.

Thermal aging of the insulation is often related to temperature as generators have been around for more than 100 years. The insulation material has gone through many innovative improvements [2]. A greater focus is given on the mica material and winding design that has a top layer of conductive carbon paint or conductive tape, which makes connection to the grounded stator. This is the working principle of the generator and other rotating machines such as the large industrial motors.

### **2. Insulation aging**

The investigation result of generator fault event shows that the failure probability of the single generator will be increasing with the generator's capacity and applying time increasing. The investigation results show that more than 50% electrical equipment failures are caused by the insulation system [3]. How to detect generator insulation aging and degradation is of great economic and social significance. Improved reliability in power distribution affects every level of society.

#### **2.1. Insulation aging reasons**

For the gas-fired generator, the stator winding, which is the direct carrier that generates electricity, would sustain different kinds of combined effects in the process of operation simultaneously, such as electric, heat, mechanical and any other actions [4]. The investigation and researching results gained by many researchers, show that a series of physical and chemical changes will occur during the insulation materials operating for a long time, such as insulation medium softening, pinhole, cracking, ionization, etc. [5]. It is generally assumed that insulation aging is affected mainly by thermal cycling, heat aging, electric aging, mechanical vibration aging combined together [6].

#### *2.1.1. Heat-aging effect*

adapt to the intermittent nature of renewable energies, subjecting them to many start-stop cycles. Such cycles give rise to stresses and creeps from material expansion and contraction. Under constant cost reduction, pressure generators are getting less expensive and their quality also suffers. Their characteristic strong vibrations that shake the structure loose

**Figure 1.** At the left, local loose core found using simple knife check. At the middle, defects start to appear as pinholes. At

the right, partial discharge removes the conducting paint without affecting the underlying mica.

176 Simulation and Modelling of Electrical Insulation Weaknesses in Electrical Equipment

**Figure 1** (left) shows how vibration causes delamination in the core, allowing the varnish to wear out and then eddy currents to introduce hot-spots affecting the performance of the winding in the slot located close by. It becomes a downward spiral of mechanical degradation. Some of the cost reducing innovations that were previously introduced worked well in a base load operation. However, in a constant start-stop mode of operation, generators are susceptible to outage early in their life because they are air-cooled. The constant thermal cycling combined with the variable characteristics of the air contribute to early wear. They can suffer from a new failure mode called vibration sparking. This is the first time that this failure mechanism has been observed as failure in progress as shown in **Figure 1** (middle). They usually become uncovered upon a complete breakdown. A simple model that combines thermal aging and a mechanical vibration-assisted degradation process is introduced in the

Thermal aging of the insulation is often related to temperature as generators have been around for more than 100 years. The insulation material has gone through many innovative improvements [2]. A greater focus is given on the mica material and winding design that has a top layer of conductive carbon paint or conductive tape, which makes connection to the grounded stator. This is the working principle of the generator and other rotating machines

The investigation result of generator fault event shows that the failure probability of the single generator will be increasing with the generator's capacity and applying time increasing. The investigation results show that more than 50% electrical equipment failures are caused by the insulation system [3]. How to detect generator insulation aging and degradation is of great economic and social significance. Improved reliability in power distribution affects

further aggravate this.

following sections.

such as the large industrial motors.

**2. Insulation aging**

every level of society.

Many generator insulation materials consist of mica and epoxides. Mica and epoxides both have excellent heat resistant characteristics. The occurring rate of the aging phenomenon is slow while the generator is working under normal temperatures, and the higher the temperature of the insulating material is, the faster the heat aging. The insulation materials performance decreases with the increase of temperature. For the generator, the main reasons for the increase of insulation temperature are the resistance heat of the conductor, partial discharge, leakage current of the insulation and the heat caused by the dielectric loss. For the epoxidemica insulation medium, there are two ways to affect the insulation performance: one is the epoxy-mica temper embrittlement and thermal degradation and another is the local defects caused by thermal expansion of the polymer [7].

#### *2.1.2. Mechanical stress effect*

The generator stator windings are nested in the mechanical supporting structure. The stator winding is generally running under mechanical stress. At the same time, there always exists vibration, which corresponds with stress. The electromagnetic force generated on the stator windings changes the rotor turning speed variation. Due to the effect of long time fluctuating mechanical force, there would produce some joint loosening and vibration. Under the action of alternating stress and vibration, the conductive carbon coating attached to insulation material would loosen and shed because of the vibration fatigue. We call it mechanical insulation aging which is caused by mechanical stress. The alternating mechanical force comes from static mechanical force, the start-stop electromagnetic force, and mechanical vibration force at running time [8]. Mechanical vibration force at running time produces as illustrated in **Figure 2**.

#### *2.1.3. Sparking effect*

The winding insulation of generators are made of epoxy & mica combined together. Due to the difference of expansion coefficient and manufacturing process, there exists some microgaps between materials and carbon coating, different insulation layers, under the effect of electric field, the sparking caused by partial discharge would be showing up at the micro-gap location. Due to the sparking, there are three damage types. One type is that the main insulation thickness becomes thinner because the adhesive between the coating and the insulation medium is carbonized by the high temperature in the micro-gap caused by the partial discharge. The second type is that the edges of the insulation medium and the air gap wall

**Figure 2.** Mechanical vibration force at running time produces illustrated. Courtesy of QPS Photronics.

would appear to erode into pit and pinhole defects because of the striking by a large number of charged particles at high speed, which lead to a decline in insulation. The third type is that the discharge can produce ozone, which damages the insulation and copper conductor by combining with water, NO and NO<sup>2</sup> [9–15].

*2.2.2. Strong vibration aging*

*2.2.3. Hot-spot vibration sparking aging*

tial directions.

Photronics.

Vibration is an equally powerful aging mechanism. Vibration is inherent in the design structure of any rotating machine. Most generators are two-pole machines where uneven air gaps can be introduced due to misalignment, giving rise to a two-time line frequency (2xLF) nominal signature [19]. Then there is the effect of unbalancing, which gives a strong line frequency (1xLF) component. Vibration can become much stronger when the material and structure become close to a resonance mode. Excessive vibration can start rubbing the insulation, triggering shortened turns or shorts to ground. **Figure 5** shows the interactions inside a complex winding. Vibration in a generator can occur in both the radial and tangen-

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179

**Figure 4.** Depiction of grounded conductive outermost layer. Courtesy of QPS Photronics.

Vibration sparking is a special process in gas-fired generators combining insulation thermal aging together with strong vibration effects [20–22]. Hot-spot measurement was performed in a random wound electric machine coil [23]. Vibration measurement was also performed

**Figure 5.** Eight strands within a stator slot interacting with each other under the effect of vibration. Courtesy of QPS

#### **2.2. Gas-fired generator aging**

#### *2.2.1. Insulation thermal aging*

The reliability of the insulation is dependent on the thickness of the mica layer being applied in different parts of the windings [16–18] as illustrated in **Figure 3**.

Then, a top coating of conductive layer is overlaid to complete the structure. Therefore, the full voltage will now be exercised across the insulation as depicted in **Figure 4**.

The layers are quite thick, normally free of pinholes, defects, and other imperfections. However, the material ages with time, which results in reduction of their insulation properties. Defects start to form from partial discharge (PD) occurring where the windings show traces of contamination left behind during manufacturing and handling. This phenomenon is one among other degradation processes at work.

**Figure 3.** Conceptual depiction of the various insulation subdivisions. Courtesy of QPS Photronics.

Generator Insulation-Aging On-Line Monitoring Technique Based on Fiber Optic Detecting… http://dx.doi.org/10.5772/intechopen.78065 179

**Figure 4.** Depiction of grounded conductive outermost layer. Courtesy of QPS Photronics.

#### *2.2.2. Strong vibration aging*

would appear to erode into pit and pinhole defects because of the striking by a large number of charged particles at high speed, which lead to a decline in insulation. The third type is that the discharge can produce ozone, which damages the insulation and copper conductor by

The reliability of the insulation is dependent on the thickness of the mica layer being applied

Then, a top coating of conductive layer is overlaid to complete the structure. Therefore, the

The layers are quite thick, normally free of pinholes, defects, and other imperfections. However, the material ages with time, which results in reduction of their insulation properties. Defects start to form from partial discharge (PD) occurring where the windings show traces of contamination left behind during manufacturing and handling. This phenomenon is

[9–15].

**Figure 2.** Mechanical vibration force at running time produces illustrated. Courtesy of QPS Photronics.

178 Simulation and Modelling of Electrical Insulation Weaknesses in Electrical Equipment

full voltage will now be exercised across the insulation as depicted in **Figure 4**.

**Figure 3.** Conceptual depiction of the various insulation subdivisions. Courtesy of QPS Photronics.

in different parts of the windings [16–18] as illustrated in **Figure 3**.

one among other degradation processes at work.

combining with water, NO and NO<sup>2</sup>

**2.2. Gas-fired generator aging**

*2.2.1. Insulation thermal aging*

Vibration is an equally powerful aging mechanism. Vibration is inherent in the design structure of any rotating machine. Most generators are two-pole machines where uneven air gaps can be introduced due to misalignment, giving rise to a two-time line frequency (2xLF) nominal signature [19]. Then there is the effect of unbalancing, which gives a strong line frequency (1xLF) component. Vibration can become much stronger when the material and structure become close to a resonance mode. Excessive vibration can start rubbing the insulation, triggering shortened turns or shorts to ground. **Figure 5** shows the interactions inside a complex winding. Vibration in a generator can occur in both the radial and tangential directions.

#### *2.2.3. Hot-spot vibration sparking aging*

Vibration sparking is a special process in gas-fired generators combining insulation thermal aging together with strong vibration effects [20–22]. Hot-spot measurement was performed in a random wound electric machine coil [23]. Vibration measurement was also performed

**Figure 5.** Eight strands within a stator slot interacting with each other under the effect of vibration. Courtesy of QPS Photronics.

using a wideband fiber optic vibration sensor (see [21]). For confirming the effect of vibration sparking on insulation performance, some methods were actually developed to monitor the end winding of a large power generator. For example, a simple fiber Bragg grating (FBG) have been used to measure hot-spots and they found the measured value affected by vibration of the motor (see [23]). A thin vibration sensor was developed for monitoring winding vibration inside the transformer [24] by making use of the long gauge effect, namely a length of singlemode fiber spliced onto the cavity rendered the whole fiber a distributed vibration sensor. Meanwhile, the field test was performed in cooperation with Calpine Corporation. When Calpine Corporation found signs of disturbance in the winding insulation, they realized that it was time to perform a major maintenance, leading to a rewind. Some samples of the affected windings were examined (**Figure 6**). There seems to be various stages of degradation.

appeared and were associated with different deliberately introduced faults like open-circuits, short-circuits, and bearing digs. Note that they are recoverable after the experiment. It is observed that they are very distinct from those obtained when the motor was restored to its

Generator Insulation-Aging On-Line Monitoring Technique Based on Fiber Optic Detecting…

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181

It is hypothesized these were related hot-spots developed in the stator, caused by eddy current loops, formed when damage occurs between the insulation and the neighboring laminated steel plate. The hot-spots reduced the performance of the insulation and PD subsequently occurred, eroding further the carbon paint. Furthermore, mica insulation also suffered from damage. PD activities would not have sufficient energy to puncture the mica and another failure mechanism might be at work, vibration sparking. Such process is defined by excessive vibration occurring in the slot so that the carbon paint, normally maintaining ground contact to the stator, failed and HV appears at some of those disturbed locations and the air breaks down to form a plasma. **Figure 7** (left) illustrates the concept. For the other side of the vibrating part, the plasma lost contact with its current source. This is a powerful source of electro-etching, an industrial process used to etch hard material like ceramics. If this process continues undetected, it destroys the mica insulation, leading to an unplanned outage as shown in **Figure 7** (right).

In order to master the law of aging and reduce the losses due to aging, some aging models of insulation have been established based on practical experience and theoretical analysis.

The Power Reciprocal model in low electric field and the Index model in high electric field are proposed based on a lot of electric stress affected researches. The Power Reciprocal model is

*L* = *kE*<sup>−</sup>*<sup>n</sup>* (1)

where: *L* represents failure time, *E* represents external applied voltage, *k* and *n* are empirical

*L* = *a* exp(−*bE*) (2)

where: *L* represents failure time, *E* represents external applied voltage, *a* and *b* are empirical

original healthy condition (see [19]).

**3. Insulation aging detection theory**

**3.1. Insulation aging model**

*3.1.1. The aging model of single factor*

The Index model is as follows [26]:

(1) Electric stress aging model

as follows [25]:

constants.

constants.

(2) Thermal aging model

All the measurement research above shows that as the revolutions per minute (RPM) are increased, various local resonances started to appear. Then, unique frequency signatures

**Figure 6.** Patches of insulation damage on winding. Close-up highlights pinholes formed in the conductive carbon paint. Courtesy of QPS Photronics and Calpine Corporation.

**Figure 7.** At left, conceptualization of vibration sparking. At right, complete winding insulation became destroyed at another power plant suspected of having similar problem.

appeared and were associated with different deliberately introduced faults like open-circuits, short-circuits, and bearing digs. Note that they are recoverable after the experiment. It is observed that they are very distinct from those obtained when the motor was restored to its original healthy condition (see [19]).

It is hypothesized these were related hot-spots developed in the stator, caused by eddy current loops, formed when damage occurs between the insulation and the neighboring laminated steel plate. The hot-spots reduced the performance of the insulation and PD subsequently occurred, eroding further the carbon paint. Furthermore, mica insulation also suffered from damage. PD activities would not have sufficient energy to puncture the mica and another failure mechanism might be at work, vibration sparking. Such process is defined by excessive vibration occurring in the slot so that the carbon paint, normally maintaining ground contact to the stator, failed and HV appears at some of those disturbed locations and the air breaks down to form a plasma. **Figure 7** (left) illustrates the concept. For the other side of the vibrating part, the plasma lost contact with its current source. This is a powerful source of electro-etching, an industrial process used to etch hard material like ceramics. If this process continues undetected, it destroys the mica insulation, leading to an unplanned outage as shown in **Figure 7** (right).

## **3. Insulation aging detection theory**

#### **3.1. Insulation aging model**

using a wideband fiber optic vibration sensor (see [21]). For confirming the effect of vibration sparking on insulation performance, some methods were actually developed to monitor the end winding of a large power generator. For example, a simple fiber Bragg grating (FBG) have been used to measure hot-spots and they found the measured value affected by vibration of the motor (see [23]). A thin vibration sensor was developed for monitoring winding vibration inside the transformer [24] by making use of the long gauge effect, namely a length of singlemode fiber spliced onto the cavity rendered the whole fiber a distributed vibration sensor. Meanwhile, the field test was performed in cooperation with Calpine Corporation. When Calpine Corporation found signs of disturbance in the winding insulation, they realized that it was time to perform a major maintenance, leading to a rewind. Some samples of the affected

180 Simulation and Modelling of Electrical Insulation Weaknesses in Electrical Equipment

windings were examined (**Figure 6**). There seems to be various stages of degradation.

All the measurement research above shows that as the revolutions per minute (RPM) are increased, various local resonances started to appear. Then, unique frequency signatures

**Figure 6.** Patches of insulation damage on winding. Close-up highlights pinholes formed in the conductive carbon paint.

**Figure 7.** At left, conceptualization of vibration sparking. At right, complete winding insulation became destroyed at

Courtesy of QPS Photronics and Calpine Corporation.

another power plant suspected of having similar problem.

In order to master the law of aging and reduce the losses due to aging, some aging models of insulation have been established based on practical experience and theoretical analysis.

#### *3.1.1. The aging model of single factor*

#### (1) Electric stress aging model

The Power Reciprocal model in low electric field and the Index model in high electric field are proposed based on a lot of electric stress affected researches. The Power Reciprocal model is as follows [25]:

$$L = kE^{-n} \tag{1}$$

where: *L* represents failure time, *E* represents external applied voltage, *k* and *n* are empirical constants.

The Index model is as follows [26]:

$$L = a \exp(-bE) \tag{2}$$

where: *L* represents failure time, *E* represents external applied voltage, *a* and *b* are empirical constants.

(2) Thermal aging model

The thermal aging model based on the equation that describing the relationship between the rate constant of the chemical reaction and the temperature as follows:

$$\text{1nt} = \text{1n}A + \frac{E\_0}{RT} \tag{3}$$

(3) Crine model

ing abilities.

depicted in **Figure 8**.

*3.2.2. Fiber Bragg grating sensor model*

solving the light equation as follows [29]:

*L* = \_\_\_*<sup>h</sup>*

particle charge involved in the aging process.

**3.2. Fiber grating detection technology**

*3.2.1. Principle of optical Fiber sensing technology*

Crine proposed that the process of aging could be characterized by the energy barrier, and considered that the age of the insulation medium equals the time of the charged particles crossing the barrier. Based on the hypothesis that the average time of collective carriers through the potential barrier are equal to the time of a single carrier passing through the barrier, the aging model described by the relation of thermal-dynamic is obtained as follows:

*kT* exp[

where: L represents the failure time, *h* represents the Plank constant, *k* represents Pohl Seidman constant, Δ*G* represents free energy, *λ* represents the width of barrier, and *e* represents the

The mathematical models of insulation aging above show how to theoretically predict the temperature, PD and stress in a timely fashion. It is the foundation of predicting the life of insulation materials and reducing safety accidents. In the context of the demand for electric power equipment that goes increasingly up, researchers nowadays have their full attention on predictive methods to trend the aging of insulation and therefore predict the generator's service life.

Due to the complex environment caused by electricity, heat, machinery and chemistry, there are few sensors able to detect the insulation's aging in generators. With the increasing sophistication of fiber optic based technology, FBG based sensor has become the research focus in the field of sensors because of its inherent advantages, such as compact structure, corrosion resistance, intrinsic passivity, indifference to electromagnetic interference and its multiplex-

Optical fiber sensing technology senses and transmits external environment parameter variations based on the optical fiber medium. The optical fiber has the characteristic and ability to sense and transmit the information to the optical-electric field in itself directly or indirectly. When a beam of light illuminates through optical fiber, the change of the external environment parameters (e.g. vibration) could be sensed. The optical-electric fields characteristic value in the optical fiber, such as amplitude, phase, wavelength and polarization, would be affected by the signals sensed while the sensed signal propagates in the fiber. Furthermore, by using a demodulation device, the changes of the external environment physical parameters quantity could be obtained by using the signal reversing method [29]. The principle of optical fiber sensing is

Based on the light coupling-mode theory at the scale of micro-disturbances, the effective refractive index of grating region and the central wavelength of FBG could be obtained by

\_\_\_\_\_\_\_ Δ*G* − *eE*

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where: *t* represents the set testing time, *A* is constant, *E*<sup>0</sup> represents the energy loss during the process of aging (unit: kJ/mol), R equals to the gas constant (8.314 J/mol·K), T represents temperature (K).

(3) Mechanical stress aging model

The mechanical stress aging model of a large motor is generally expressed by the empirical formula as follows:

$$L = K\_n S^{-n} \tag{4}$$

where: *L* represents the failure time, *S* represents the mechanical stress, *m* and *K* are empirical constants related to vibration frequency [27].

#### *3.1.2. Electric-thermal two-factor aging model*

With the in-depth study of single factor electric and thermal aging, it is found that the reasons for the insulation aging are not isolated. The electric-thermal two-factor aging model that is widely accepted is as follows:

#### (1) Simoni model

Based on the function of hypothetical electric field *F*(*E*) <sup>=</sup> ln(*E*/*E*<sup>0</sup> ), Simoni proposed the twofactor aging model as follows:

$$L(T, E) = L\_0 \left(\frac{E}{E\_0}\right)^A \exp\left[-B\Delta\left(\frac{1}{T}\right)\right] \tag{5}$$

where: *<sup>A</sup>* <sup>=</sup> *<sup>n</sup>* <sup>−</sup> *<sup>b</sup>*Δ(1/*T*), *n* represents the index of the power reciprocal, *L*<sup>0</sup> represents the breakdown time at *<sup>E</sup>* <sup>=</sup> *<sup>E</sup>*<sup>0</sup> , *E* represents applied electric-field, *E*<sup>0</sup> represents a reference electric-field value, *b* represents an empirical constant determined by the insulation materials, *B* represents the constant of the single factor thermal aging model, *T* represents the temperature (K).

#### (2) Ramu model

Based on the rate of Eyring physical chemistry reaction and considering the temperature function as constant, the Ramu the model is established from the Power Reciprocal aging model as follows [28]:

$$L\{T, E\} = c(T) \to^{-s(\gamma)} \exp\left|-B\Delta\left(\frac{1}{T}\right)\right|\tag{6}$$

where: parameter definitions are identical as the ones found in the Simoni model, *c* and *n* are empirical constants.

(3) Crine model

The thermal aging model based on the equation that describing the relationship between the

the process of aging (unit: kJ/mol), R equals to the gas constant (8.314 J/mol·K), T represents

The mechanical stress aging model of a large motor is generally expressed by the empirical

*L* = *K<sup>m</sup> S*<sup>−</sup>*<sup>m</sup>* (4)

where: *L* represents the failure time, *S* represents the mechanical stress, *m* and *K* are empirical

With the in-depth study of single factor electric and thermal aging, it is found that the reasons for the insulation aging are not isolated. The electric-thermal two-factor aging model that is

> \_\_*E E*0) −*A*

value, *b* represents an empirical constant determined by the insulation materials, *B* represents the constant of the single factor thermal aging model, *T* represents the temperature (K).

Based on the rate of Eyring physical chemistry reaction and considering the temperature function as constant, the Ramu the model is established from the Power Reciprocal aging

where: parameter definitions are identical as the ones found in the Simoni model, *c* and *n* are

exp[−*B*Δ(

exp[−*B*Δ(

\_\_1

\_\_1

*E*\_\_\_0

*RT* (3)

represents the energy loss during

), Simoni proposed the two-

represents the break-

*<sup>T</sup>*)] (5)

represents a reference electric-field

*<sup>T</sup>*)] (6)

rate constant of the chemical reaction and the temperature as follows:

182 Simulation and Modelling of Electrical Insulation Weaknesses in Electrical Equipment

ln*t* = ln*A* +

temperature (K).

formula as follows:

(3) Mechanical stress aging model

constants related to vibration frequency [27].

Based on the function of hypothetical electric field *F*(*E*) <sup>=</sup> ln(*E*/*E*<sup>0</sup>

where: *<sup>A</sup>* <sup>=</sup> *<sup>n</sup>* <sup>−</sup> *<sup>b</sup>*Δ(1/*T*), *n* represents the index of the power reciprocal, *L*<sup>0</sup>

, *E* represents applied electric-field, *E*<sup>0</sup>

*3.1.2. Electric-thermal two-factor aging model*

*<sup>L</sup>*(*T*, *<sup>E</sup>*) <sup>=</sup> *<sup>L</sup>*<sup>0</sup> (

*L*(*T*, *E*) = *c*(*T*) *E*<sup>−</sup>*n*(*T*)

widely accepted is as follows:

factor aging model as follows:

(1) Simoni model

down time at *<sup>E</sup>* <sup>=</sup> *<sup>E</sup>*<sup>0</sup>

(2) Ramu model

model as follows [28]:

empirical constants.

where: *t* represents the set testing time, *A* is constant, *E*<sup>0</sup>

Crine proposed that the process of aging could be characterized by the energy barrier, and considered that the age of the insulation medium equals the time of the charged particles crossing the barrier. Based on the hypothesis that the average time of collective carriers through the potential barrier are equal to the time of a single carrier passing through the barrier, the aging model described by the relation of thermal-dynamic is obtained as follows:

$$L = \frac{h}{kT} \exp\left[\frac{\Delta G - e\lambda E}{kT}\right] \tag{7}$$

where: L represents the failure time, *h* represents the Plank constant, *k* represents Pohl Seidman constant, Δ*G* represents free energy, *λ* represents the width of barrier, and *e* represents the particle charge involved in the aging process.

The mathematical models of insulation aging above show how to theoretically predict the temperature, PD and stress in a timely fashion. It is the foundation of predicting the life of insulation materials and reducing safety accidents. In the context of the demand for electric power equipment that goes increasingly up, researchers nowadays have their full attention on predictive methods to trend the aging of insulation and therefore predict the generator's service life.

#### **3.2. Fiber grating detection technology**

Due to the complex environment caused by electricity, heat, machinery and chemistry, there are few sensors able to detect the insulation's aging in generators. With the increasing sophistication of fiber optic based technology, FBG based sensor has become the research focus in the field of sensors because of its inherent advantages, such as compact structure, corrosion resistance, intrinsic passivity, indifference to electromagnetic interference and its multiplexing abilities.

#### *3.2.1. Principle of optical Fiber sensing technology*

Optical fiber sensing technology senses and transmits external environment parameter variations based on the optical fiber medium. The optical fiber has the characteristic and ability to sense and transmit the information to the optical-electric field in itself directly or indirectly. When a beam of light illuminates through optical fiber, the change of the external environment parameters (e.g. vibration) could be sensed. The optical-electric fields characteristic value in the optical fiber, such as amplitude, phase, wavelength and polarization, would be affected by the signals sensed while the sensed signal propagates in the fiber. Furthermore, by using a demodulation device, the changes of the external environment physical parameters quantity could be obtained by using the signal reversing method [29]. The principle of optical fiber sensing is depicted in **Figure 8**.

#### *3.2.2. Fiber Bragg grating sensor model*

Based on the light coupling-mode theory at the scale of micro-disturbances, the effective refractive index of grating region and the central wavelength of FBG could be obtained by solving the light equation as follows [29]:

$$
\delta n\_{\rm eff} = \begin{array}{c}
\overline{\delta n\_{\rm eff}} \left[1 + \cos\left(\frac{2\pi}{\Lambda}z\right)\right] \\
\end{array} \tag{8}
$$

From the FBG central wavelength formula, we can conclude that: (A) when a beam of incident light transmits through the grating region, there always exists a fraction of the light that reflects back, and the wavelength of the reflected light must satisfy the Bragg wavelength Eq. (9); (B) The Bragg wavelength of the FBG only depends on the grating period Λ and the

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A proper FBG will have its central wavelength solely determined by the corrugation period and effective refractive index. The FBG central wavelength, period, and effective refractive index are all fixed constants, meaning that the reflection spectrum of the FBG central wavelength is always fixed at reflecting peak on the condition that the FBG sensor is under the same environmental conditions [30, 31]. The frame of the FBG and its reflecting schematic

Due to the central wavelength shifted with the change of temperature, based on the formula (9) and the assumption that the fiber grating is only affected by the temperature, the central

where: *αs* <sup>=</sup> (1/Λ)(*Δ*Λ/*T*) represents the fiber's thermal expansion coefficient, and is used to describe the grating pitch variation with the temperature. *ζs* <sup>=</sup> (1/*neff*)(Δ*neff* /*T*) represents the FBG thermo-optical coefficient, and is used to describe the variation of the material refractive

Based on the Eq. (10), the variation of the ambient temperature could be reversed by detecting

Due to principle of the fiber grating pitch and refractive index that vary with the fiber undergoing strain along the axial direction, based on the central wavelength formula (9) and the assumption that the fiber grating is only affected by it, the central wavelength shifting value

> = \_\_\_ ΔΛ <sup>Λ</sup> + <sup>Δ</sup>*<sup>n</sup>* \_\_\_\_*eff neff*

When strain is applied along the axial direction in the fiber, the refractive index variation of

, ΔΛ and Δ*neff* represent the changing value of FBG central wavelength, grating period

[(1 − *μ*) *P*<sup>12</sup> − *μP*11]*ε* = −*Pe*

.

*λB*

= (*αs* + *ζs*)*T* (10)

(11)

*ε* (12)

refractive index *neff*.

diagram are shown in **Figure 9**.

*3.2.3. Fiber Bragg grating sensor model for temperature*

<sup>Δ</sup>*λ*\_\_\_\_*<sup>B</sup>*

index changing with the temperature.

the variation of the central wavelength Δ*λ<sup>B</sup>*

<sup>Δ</sup>*λ*\_\_\_\_*<sup>B</sup>*

the FBG satisfies the following formula:

and refractive index separately.

<sup>Δ</sup>*<sup>n</sup>* \_\_\_\_*eff*

where: Δ*λ<sup>B</sup>*

*3.2.4. Fiber Bragg grating sensor model for vibration*

of the Bragg fiber grating could be obtained as follows:

*neff*

= \_\_1 <sup>2</sup> *neff* 2

wavelength shifting value of the FBG could be obtained as follows:

*λB*

where: *neff* represents the effect refractive index, Λ represents grating period, *δneff* −−−−− represents the change of the average effective refractive index.

The characteristic wavelength that interacts with the optical fiber corrugation is represented by:

$$
\lambda\_{\mathfrak{g}} = \mathcal{D} \mathfrak{n}\_{\mathfrak{g}'} \Lambda \tag{9}
$$

where: *λ<sup>B</sup>* represents the Bragg (central) wavelength.

**Figure 8.** The principle of optical fiber sensing.

**Figure 9.** The frame of the Bragg fiber grating and its reflecting schematic diagram. Courtesy of QPS Photronics.

From the FBG central wavelength formula, we can conclude that: (A) when a beam of incident light transmits through the grating region, there always exists a fraction of the light that reflects back, and the wavelength of the reflected light must satisfy the Bragg wavelength Eq. (9); (B) The Bragg wavelength of the FBG only depends on the grating period Λ and the refractive index *neff*.

A proper FBG will have its central wavelength solely determined by the corrugation period and effective refractive index. The FBG central wavelength, period, and effective refractive index are all fixed constants, meaning that the reflection spectrum of the FBG central wavelength is always fixed at reflecting peak on the condition that the FBG sensor is under the same environmental conditions [30, 31]. The frame of the FBG and its reflecting schematic diagram are shown in **Figure 9**.

#### *3.2.3. Fiber Bragg grating sensor model for temperature*

*δneff* = *δneff*

**Figure 8.** The principle of optical fiber sensing.

by:

where: *λ<sup>B</sup>*

the change of the average effective refractive index.

represents the Bragg (central) wavelength.

184 Simulation and Modelling of Electrical Insulation Weaknesses in Electrical Equipment

−−−−−

where: *neff* represents the effect refractive index, Λ represents grating period, *δneff*

[1 + cos(

The characteristic wavelength that interacts with the optical fiber corrugation is represented

*λ<sup>B</sup>* = 2 *neff* Λ (9)

**Figure 9.** The frame of the Bragg fiber grating and its reflecting schematic diagram. Courtesy of QPS Photronics.

\_\_\_ 2*π*

<sup>Λ</sup> *z*)] (8)

−−−−− represents

Due to the central wavelength shifted with the change of temperature, based on the formula (9) and the assumption that the fiber grating is only affected by the temperature, the central wavelength shifting value of the FBG could be obtained as follows:

$$\frac{\Lambda \lambda\_s}{\lambda\_s} = \langle \alpha\_s + \zeta\_s \rangle \Delta T \tag{10}$$

where: *αs* <sup>=</sup> (1/Λ)(*Δ*Λ/*T*) represents the fiber's thermal expansion coefficient, and is used to describe the grating pitch variation with the temperature. *ζs* <sup>=</sup> (1/*neff*)(Δ*neff* /*T*) represents the FBG thermo-optical coefficient, and is used to describe the variation of the material refractive index changing with the temperature.

Based on the Eq. (10), the variation of the ambient temperature could be reversed by detecting the variation of the central wavelength Δ*λ<sup>B</sup>* .

#### *3.2.4. Fiber Bragg grating sensor model for vibration*

Due to principle of the fiber grating pitch and refractive index that vary with the fiber undergoing strain along the axial direction, based on the central wavelength formula (9) and the assumption that the fiber grating is only affected by it, the central wavelength shifting value of the Bragg fiber grating could be obtained as follows:

$$\frac{\Delta\lambda\_{\rm g}}{\lambda\_{\rm g}} = \frac{\Delta\Lambda}{\Lambda} + \frac{\Delta n\_{\rm rf}}{n\_{\rm rf}} \tag{11}$$

where: Δ*λ<sup>B</sup>* , ΔΛ and Δ*neff* represent the changing value of FBG central wavelength, grating period and refractive index separately.

When strain is applied along the axial direction in the fiber, the refractive index variation of the FBG satisfies the following formula:

$$\frac{\Delta n\_{\eta\overline{\ell}}}{n\_{\eta\overline{\ell}}} = \frac{1}{2} n\_{\eta\overline{\ell}}^2 \left[ (1 - \mu) P\_{12} - \mu P\_{11} \right] \varepsilon \ = -P\_i \varepsilon \tag{12}$$

where: *Pe* represents the elasto-optical coefficient, and defined as *Pe* <sup>=</sup> *<sup>n</sup>eff* 2 [(1 <sup>−</sup> *<sup>μ</sup>*) *<sup>P</sup>*<sup>12</sup> <sup>−</sup> *μP*11]/2, *ε* represents strain of the fiber grating along the axial direction, *μ* represents the Poisson's ratio of the fiber material. *P*11 and *<sup>P</sup>*12 represent the elastic tensor components of the fiber grating.

Based on the definition of axial strain and Eq. (11), we could obtain the strain detecting formula as follows:

$$\frac{\Delta\lambda\_{\rm g}}{\lambda\_{\rm g}} = \left(1 - P\_e\right)\varepsilon^{-1}$$

Based on the relationship between strain and wavelength and the detecting value of the center wavelength, the vibration could be reversed by using the relationship between stress and strain.

#### **3.3. Long-gauge vibration/temperature sensor**

#### *3.3.1. Introduction to QPS Photronics*

QPS is an innovator in the field of fiber optic sensors and specifically FBG's. They bring gratings from research into marketable products.

#### *3.3.2. Introduction of long-gauge vibration/temperature sensor*

Because fiber optics are made of glass with no conducting materials, fitting well in environments undergoing high-voltages and strong electromagnetic fields, based on the FBG vibration/temperature sensor model, this section introduces how the sensors were designed to satisfy the power industry requirements using FBGs.

both the laser current (LC) and the thermoelectric cooler (TEC) analog controls of the laser. When vibration occurs, the fringe pattern will be moving right and left, it forces the operating point to ride up and down the slope, translated into linear intensity changes. The changes accurately reflect the actual vibration that is occurring. Since the cavity is also affected by temperature, a self-calibration algorithm was introduced to re-establish the operating point and such compensation delivers an indirect method to measure temperature (see **Figure 11**).

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**Figure 11.** Correspondence between fringe oscillation and resulting vibration. Courtesy of QPS Photronics.

Another invention linked to the cavity is the long gauge technology. By splicing a length of single-mode optical fiber on to the cavity, a new cavity sprouts between the two matching FBGs cavity and the interrogation system connector. Said connector triggers a Fresnel broadband reflection, enabling a thin in-slot vibration distributed sensor to be used in a field test inside a gas-fired generator [32]. The FBG based architecture is housed within a 2 mm thin package that allows easy insertion into tight spots, whereas another supplier of the same field requires two separate large sensors that limit locations where they could be installed [33].

Based on the system for temperature and vibration monitoring shown in **Figure 12**, the generator insulation's temperature and vibration becomes possible by using different sensing units that are placed at different locations. By control and analyzing different signals received from different sensors, not only the insulation temperature and vibration can be obtained, but insulation hot-spots can be pinpointed by using the relation between wavelength and vibration.

The detecting system consists of three parts: the signal control & processing unit, signal transmission unit and sensing unit. After the output of the optical signal is converted into an electrical equivalent by the photoelectric converter, and acquired & collected by the data collecting

**4. Sensing temperature and vibration**

The ability to measure both vibration and temperature is based on interference: two identical FBGs are printed on the same fiber at a small distance, which forms a cavity. When a laser beam with matching center wavelength is emitted into the cavity, it is reflected and goes through a 180° phase shift, giving rise to two interfering beams and a dense fringe spectrum as shown in **Figure 10**.

The larger the cavity length, the denser will be the fringe pack with steeper slope, playing on sensitivity. The vibration function is realized by programming an operating point at the midpoint of the rising slope of a selected fringe. This operating point stays locked using

**Figure 10.** Correlation between fiber optic structure and resulting spectrum where a selected fringe will be monitored for changes due to both static/dynamic strain and temperature. Courtesy of QPS Photronics.

**Figure 11.** Correspondence between fringe oscillation and resulting vibration. Courtesy of QPS Photronics.

both the laser current (LC) and the thermoelectric cooler (TEC) analog controls of the laser. When vibration occurs, the fringe pattern will be moving right and left, it forces the operating point to ride up and down the slope, translated into linear intensity changes. The changes accurately reflect the actual vibration that is occurring. Since the cavity is also affected by temperature, a self-calibration algorithm was introduced to re-establish the operating point and such compensation delivers an indirect method to measure temperature (see **Figure 11**).

Another invention linked to the cavity is the long gauge technology. By splicing a length of single-mode optical fiber on to the cavity, a new cavity sprouts between the two matching FBGs cavity and the interrogation system connector. Said connector triggers a Fresnel broadband reflection, enabling a thin in-slot vibration distributed sensor to be used in a field test inside a gas-fired generator [32]. The FBG based architecture is housed within a 2 mm thin package that allows easy insertion into tight spots, whereas another supplier of the same field requires two separate large sensors that limit locations where they could be installed [33].

## **4. Sensing temperature and vibration**

where: *Pe*

mula as follows:

*Δλ*\_\_\_\_*<sup>B</sup>*

**3.3. Long-gauge vibration/temperature sensor**

ings from research into marketable products.

*3.3.2. Introduction of long-gauge vibration/temperature sensor*

satisfy the power industry requirements using FBGs.

*3.3.1. Introduction to QPS Photronics*

as shown in **Figure 10**.

represents the elasto-optical coefficient, and defined as *Pe* <sup>=</sup> *<sup>n</sup>eff*

186 Simulation and Modelling of Electrical Insulation Weaknesses in Electrical Equipment

*λB*

resents strain of the fiber grating along the axial direction, *μ* represents the Poisson's ratio of the fiber material. *P*11 and *P*12 represent the elastic tensor components of the fiber grating.

Based on the definition of axial strain and Eq. (11), we could obtain the strain detecting for-

= (1 − *Pe*)*ε*

Based on the relationship between strain and wavelength and the detecting value of the center wavelength, the vibration could be reversed by using the relationship between stress and strain.

QPS is an innovator in the field of fiber optic sensors and specifically FBG's. They bring grat-

Because fiber optics are made of glass with no conducting materials, fitting well in environments undergoing high-voltages and strong electromagnetic fields, based on the FBG vibration/temperature sensor model, this section introduces how the sensors were designed to

The ability to measure both vibration and temperature is based on interference: two identical FBGs are printed on the same fiber at a small distance, which forms a cavity. When a laser beam with matching center wavelength is emitted into the cavity, it is reflected and goes through a 180° phase shift, giving rise to two interfering beams and a dense fringe spectrum

The larger the cavity length, the denser will be the fringe pack with steeper slope, playing on sensitivity. The vibration function is realized by programming an operating point at the midpoint of the rising slope of a selected fringe. This operating point stays locked using

**Figure 10.** Correlation between fiber optic structure and resulting spectrum where a selected fringe will be monitored for

changes due to both static/dynamic strain and temperature. Courtesy of QPS Photronics.

2

[(1 <sup>−</sup> *<sup>μ</sup>*) *<sup>P</sup>*<sup>12</sup> <sup>−</sup> *μP*11]/2, *ε* rep-

Based on the system for temperature and vibration monitoring shown in **Figure 12**, the generator insulation's temperature and vibration becomes possible by using different sensing units that are placed at different locations. By control and analyzing different signals received from different sensors, not only the insulation temperature and vibration can be obtained, but insulation hot-spots can be pinpointed by using the relation between wavelength and vibration.

The detecting system consists of three parts: the signal control & processing unit, signal transmission unit and sensing unit. After the output of the optical signal is converted into an electrical equivalent by the photoelectric converter, and acquired & collected by the data collecting system, the signal can be transmitted to the computer and processed by the specific software. Then the hot-spot/temperature and vibration can be determined. The related parameters of the vibration of the measured object can be obtained after the analysis and processing of the data on the computer through the vibration monitoring software. The signal control & processing unit consists of an incident optical source, interrogator, display and computer. Incident optical source is used as the stimulating source of the incident light, computer that installed the control software was considered as the head of the detecting system and could send the information and analyze the feedback signal that come from the interrogator complete.

are protected by Teflon tubes where the sensor tip is housed in a molded PEEK package as shown in **Figure 15** (left). It is an open face design. The same sensor was also used to measure vibration inside the transformer. The long gauge vibration sensor is a wideband sensor, able to detect frequencies ranging from 10 Hz to 1 kHz for standard version and 5 Hz to 2 kHz for

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Gas-fired generators are known to produce strong vibration and noise [34]. The in-slot vibration sensor, which has been installed onto the wedge (see **Figure 14**), gave a very clear vibra-

We observed a strong line frequency (1xLF) vibration peak together with a strong two times line frequency (2xLF) component. The 1xLF component measures 15-mil peak-to-peak and the 2xLF component measured 10-mil peak-to-peak. This was very high compared with

**Figure 14.** The in-slot vibration sensor installed between the Brillouin temperature sensing fiber loop (U-turn not

extended sensing range as depicted in **Figure 15** (right).

**Figure 13.** The sensing transducer array. Courtesy of QPS Photronics.

shown). Courtesy of Calpine Corporation and QPS Photronics.

**4.2. Vibration sensing**

*4.2.1. In-slot vibration*

tion response as illustrated in **Figure 16**.

In order to get more information of different locations in the insulation, several fiber gratings having different center wavelength are set in different locations of the optical fiber. Many optical fibers are connected to the interrogator and form the sensing transducer array. The system of the temperature and vibration detection is shown as **Figure 12** and the sensing transducer array is shown as **Figure 13**.

#### **4.1. Details of the field test**

There are two types of sensors. One of them makes use of the Brillouin technology where two laser beams are fed into the same fiber optic loop and both lasers have very close center wavelengths that are made to beat against each other, generating a beat frequency. This beat frequency is sensitive to any index changes in the fiber as hot spots. Hence, it is able to report their location along the sensing fiber. The temperature sensor forms a loop inside the slot. In the middle of it a single point temperature (yet distributed for vibration) sensor is installed (see **Figure 14**). The top slightly bluish fiber together with the bottom yellow fiber are the Brillouin temperature sensing loop; the dark hexagon houses the temperature sensing cavity with vibration sensitivity for the whole length of the fiber extension. The whole length of the slot, which measures five meters long, has its vibration captured. The single point temperature measurement provides a temperature reference for the Brillouin hot spot temperature sensing measurement.

Besides the Brillouin hot spots sensor and the in-slot vibration sensor, two other cavity vibration sensors were installed. One is coupled with the end winding lead and another one with the end winding bus, all are done on the neutral phase of the generator. All sensing fibers

**Figure 12.** System diagram for temperature and vibration detection. Courtesy of QPS Photronics.

are protected by Teflon tubes where the sensor tip is housed in a molded PEEK package as shown in **Figure 15** (left). It is an open face design. The same sensor was also used to measure vibration inside the transformer. The long gauge vibration sensor is a wideband sensor, able to detect frequencies ranging from 10 Hz to 1 kHz for standard version and 5 Hz to 2 kHz for extended sensing range as depicted in **Figure 15** (right).

#### **4.2. Vibration sensing**

system, the signal can be transmitted to the computer and processed by the specific software. Then the hot-spot/temperature and vibration can be determined. The related parameters of the vibration of the measured object can be obtained after the analysis and processing of the data on the computer through the vibration monitoring software. The signal control & processing unit consists of an incident optical source, interrogator, display and computer. Incident optical source is used as the stimulating source of the incident light, computer that installed the control software was considered as the head of the detecting system and could send the information and analyze the feedback signal that come from the interrogator complete.

188 Simulation and Modelling of Electrical Insulation Weaknesses in Electrical Equipment

In order to get more information of different locations in the insulation, several fiber gratings having different center wavelength are set in different locations of the optical fiber. Many optical fibers are connected to the interrogator and form the sensing transducer array. The system of the temperature and vibration detection is shown as **Figure 12** and the sensing

There are two types of sensors. One of them makes use of the Brillouin technology where two laser beams are fed into the same fiber optic loop and both lasers have very close center wavelengths that are made to beat against each other, generating a beat frequency. This beat frequency is sensitive to any index changes in the fiber as hot spots. Hence, it is able to report their location along the sensing fiber. The temperature sensor forms a loop inside the slot. In the middle of it a single point temperature (yet distributed for vibration) sensor is installed (see **Figure 14**). The top slightly bluish fiber together with the bottom yellow fiber are the Brillouin temperature sensing loop; the dark hexagon houses the temperature sensing cavity with vibration sensitivity for the whole length of the fiber extension. The whole length of the slot, which measures five meters long, has its vibration captured. The single point temperature measurement provides a

temperature reference for the Brillouin hot spot temperature sensing measurement.

**Figure 12.** System diagram for temperature and vibration detection. Courtesy of QPS Photronics.

Besides the Brillouin hot spots sensor and the in-slot vibration sensor, two other cavity vibration sensors were installed. One is coupled with the end winding lead and another one with the end winding bus, all are done on the neutral phase of the generator. All sensing fibers

transducer array is shown as **Figure 13**.

**4.1. Details of the field test**

#### *4.2.1. In-slot vibration*

Gas-fired generators are known to produce strong vibration and noise [34]. The in-slot vibration sensor, which has been installed onto the wedge (see **Figure 14**), gave a very clear vibration response as illustrated in **Figure 16**.

We observed a strong line frequency (1xLF) vibration peak together with a strong two times line frequency (2xLF) component. The 1xLF component measures 15-mil peak-to-peak and the 2xLF component measured 10-mil peak-to-peak. This was very high compared with

**Figure 13.** The sensing transducer array. Courtesy of QPS Photronics.

**Figure 14.** The in-slot vibration sensor installed between the Brillouin temperature sensing fiber loop (U-turn not shown). Courtesy of Calpine Corporation and QPS Photronics.

previously experienced 6-mil peak-to-peak in the end winding vibration of coal-fired generators. It is noted that there exists some harmonics, which rolled off normally but then showed a strong peak at 6xLF. In this generator design, the bearing is coupled onto the frame, which became incorporated into the slot vibration. Concern was expressed to Calpine as this seems to be slightly too high. The 1xLF vibration seems to be related to balancing while the 2xLF component to alignment. This excessive vibration appears to validate the visual inspection of the winding sample cutout from the generator before the rewind.

*4.2.2. End winding lead vibration*

*4.2.3. End winding bus vibration*

tent to the in-slot measurement (see **Figure 18**).

was so strong in similar machines that cracks developed.

**4.3. In-slot and end winding temperatures**

Calpine Corporation and QPS Photronics.

would end up coupled.

One single point vibration sensor was installed at the end winding of the neutral lead as depicted in **Figure 17**. The photograph at the right shows the end winding conductor before installation where the colored circles pinpoint where the two additional discrete sensors

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The end winding lead sensor also showed strong vibration spectrum in both 1xLF and 2xLF. There were hardly any harmonics. The 1xLF component measured 15-mil peak-to-peak and the 2xLF component measured 11-mil. They are higher than what we expect but consis-

A third vibration sensor was mounted on the neutral end winding bus (see **Figure 17**). Once

Larger vibration amplitude is shown: 1xLF component measured 16.5-mil peak-to-peak and 2xLF measured 12.1-mil peak-to-peak. It seems that gas-fired generators vibrate much more than the coal-fired generators. Plant experts informed all participants of the field test that it may be related to the generator design where the bearing is coupled to the frame. Vibration

The temperature distribution inside the slot was measured by the Brillouin fiber optic loop sensor. The gathered data brought up a wave-like distribution as demonstrated in **Figure 20**

**Figure 17.** Two discrete sensors coupled at different locations of the neutral bus of the phase C conductor. Courtesy of

again, the vibration observed here is also consistent as depicted below in **Figure 19**.

**Figure 15.** At left, the vibration sensing cavity housed inside a PEEK package. At right, the in-slot vibration sensor bandwidth can even be extended from 10 Hz~1 kHz (standard) to 5 Hz~2 kHz. Courtesy of QPS Photronics.

**Figure 16.** FFT shows in-slot vibration characteristics. Courtesy of QPS Photronics.

#### *4.2.2. End winding lead vibration*

previously experienced 6-mil peak-to-peak in the end winding vibration of coal-fired generators. It is noted that there exists some harmonics, which rolled off normally but then showed a strong peak at 6xLF. In this generator design, the bearing is coupled onto the frame, which became incorporated into the slot vibration. Concern was expressed to Calpine as this seems to be slightly too high. The 1xLF vibration seems to be related to balancing while the 2xLF component to alignment. This excessive vibration appears to validate the visual inspection of

**Figure 15.** At left, the vibration sensing cavity housed inside a PEEK package. At right, the in-slot vibration sensor

bandwidth can even be extended from 10 Hz~1 kHz (standard) to 5 Hz~2 kHz. Courtesy of QPS Photronics.

**Figure 16.** FFT shows in-slot vibration characteristics. Courtesy of QPS Photronics.

the winding sample cutout from the generator before the rewind.

190 Simulation and Modelling of Electrical Insulation Weaknesses in Electrical Equipment

One single point vibration sensor was installed at the end winding of the neutral lead as depicted in **Figure 17**. The photograph at the right shows the end winding conductor before installation where the colored circles pinpoint where the two additional discrete sensors would end up coupled.

The end winding lead sensor also showed strong vibration spectrum in both 1xLF and 2xLF. There were hardly any harmonics. The 1xLF component measured 15-mil peak-to-peak and the 2xLF component measured 11-mil. They are higher than what we expect but consistent to the in-slot measurement (see **Figure 18**).

#### *4.2.3. End winding bus vibration*

A third vibration sensor was mounted on the neutral end winding bus (see **Figure 17**). Once again, the vibration observed here is also consistent as depicted below in **Figure 19**.

Larger vibration amplitude is shown: 1xLF component measured 16.5-mil peak-to-peak and 2xLF measured 12.1-mil peak-to-peak. It seems that gas-fired generators vibrate much more than the coal-fired generators. Plant experts informed all participants of the field test that it may be related to the generator design where the bearing is coupled to the frame. Vibration was so strong in similar machines that cracks developed.

#### **4.3. In-slot and end winding temperatures**

The temperature distribution inside the slot was measured by the Brillouin fiber optic loop sensor. The gathered data brought up a wave-like distribution as demonstrated in **Figure 20**

**Figure 17.** Two discrete sensors coupled at different locations of the neutral bus of the phase C conductor. Courtesy of Calpine Corporation and QPS Photronics.

**Figure 18.** End winding lead frequency domain. Courtesy of QPS Photronics.

**Figure 19.** End winding bus frequency domain. Courtesy of QPS Photronics.

(left). The distribution is not flat; the cooling airflow did not form a perfect balance. The temperature extrema were less than 2°C apart, positively assuring the lack of hot spots.

We tried to line up the temperature profile with the layout of the air ducts by wrapping over at the middle. Valleys coincide very well with the air duct exhaust and the top peak always occurs in the middle between two air ducts where there is minimum cooling airflow as illustrated in **Figure 20** (right). The difference between peak and valley is consistent at different loading levels.

**Figure 21** shows the temperature measurement of the in-slot sensor. The red plot indicates the coarse temperature reading of 70°C at a loading of 170 MVA. The blue line shows the fine temperature variation over a period of 10 min. The sensor was able to detect temperature changes down to 0.1°C.

The end winding lead of **Figure 22** (left) showed a temperature consistent with the slot as well as the end winding bus plotted in **Figure 22** (right). All of them read close to each other, there does not seem to be any problem.

**4.4. Air cooling inside a gas-fired generator**

measurement for about 3 min. Courtesy of QPS Photronics.

**Figure 21.** In-slot temperature measurement for 10 min. Courtesy of QPS Photronics.

duct locations. Courtesy of Oz Optics, Ltd.

The gas-fired generator involved in the field test was manufactured by Siemens [35–38]. **Figure 23** shows a conceptual diagram of the cooling air radial flow. There are resistance

**Figure 22.** At left, end winding lead temperature measurement for about 5 min. At right, end winding bus temperature

**Figure 20.** At left, temperature profiles at different loading intensities. At right, temperature profile line-ups with the air

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**Figure 20.** At left, temperature profiles at different loading intensities. At right, temperature profile line-ups with the air duct locations. Courtesy of Oz Optics, Ltd.

**Figure 21.** In-slot temperature measurement for 10 min. Courtesy of QPS Photronics.

**Figure 22.** At left, end winding lead temperature measurement for about 5 min. At right, end winding bus temperature measurement for about 3 min. Courtesy of QPS Photronics.

#### **4.4. Air cooling inside a gas-fired generator**

(left). The distribution is not flat; the cooling airflow did not form a perfect balance. The tem-

We tried to line up the temperature profile with the layout of the air ducts by wrapping over at the middle. Valleys coincide very well with the air duct exhaust and the top peak always occurs in the middle between two air ducts where there is minimum cooling airflow as illustrated in **Figure 20** (right). The difference between peak and valley is consistent at different

**Figure 21** shows the temperature measurement of the in-slot sensor. The red plot indicates the coarse temperature reading of 70°C at a loading of 170 MVA. The blue line shows the fine temperature variation over a period of 10 min. The sensor was able to detect temperature

The end winding lead of **Figure 22** (left) showed a temperature consistent with the slot as well as the end winding bus plotted in **Figure 22** (right). All of them read close to each other, there

perature extrema were less than 2°C apart, positively assuring the lack of hot spots.

**Figure 18.** End winding lead frequency domain. Courtesy of QPS Photronics.

192 Simulation and Modelling of Electrical Insulation Weaknesses in Electrical Equipment

**Figure 19.** End winding bus frequency domain. Courtesy of QPS Photronics.

loading levels.

changes down to 0.1°C.

does not seem to be any problem.

The gas-fired generator involved in the field test was manufactured by Siemens [35–38]. **Figure 23** shows a conceptual diagram of the cooling air radial flow. There are resistance

as mentioned in an earlier section of this paper. Discussion with the plant technical staff experienced with maintaining these gas-fired generators indicated that troubles seem to start typically after 5–10 years. The distributed temperature sensing Brillouin technology is usually used for oil and gas pipeline and helps to locate small leaks. Best spatial resolution is listed as three centimeters [39], which is much larger than the defects we observed, therefore, no hot spots were found despite our expectations. Given that the generator has been rewound and the insulation is new it would take several years for the defect to grow to a size large enough to be detected by the sensing fiber. We believe it is better to maintain the sensors as a continuous monitoring tool where we can see slight temperature changes down to 0.01°C increments.

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This field test demonstrates consistent strong vibration in both 1xLF and 2xLF. A spectral analysis tool published by SKF USA Inc. [40], the strong vibration is related to misalignment (strong 2xLF amplitude) and also imbalance (strong 1xLF amplitude). Misalignment can be caused by thermal expansion; alignment could be performed when the generator is cold then heat up when it is placed in full operation. Thermal expansion might have caused the rotor to become elliptical instead of being perfectly circular. Misalignment could also be caused by shifting of the foundation or uneven support [41]. On the other hand, imbalance seems to be the cause of the strong 1xLF vibration. Their relative amplitude might indicate the extent of each problem (see [20]).

Referring to **Figure 15** of SKF USA Inc. article (see [40]), their example can be compared to the data of this field test (see **Figures 16**, **18** and **19**) and it seems that both imbalance and misalignment could be affecting the generator. These measured values were two to three times higher than typical measurements on large coal-fired generators. Those large generators do not display any 1xLF vibration signal at all because they are physically much larger in structure.

The fiber optic signal processing system equipped with a gateway device connects the large volume of sensor data with the plant data from the SCADA for parametric correlation between cause (loading, start-stop periods, etc.) and result (level of vibration and temperature). **Figure 24** displays a flowchart of damage diagnostic that would be further quantified

It is now possible to simplify the problem into an intuitive model that does not involve complex mathematics [42–46]. The methodology is described below and can be used to analyze the measured data from these fiber optic sensors. It has been known that gas generators are frequently subjected to many daily start-stop events: the winds are usually stronger during the night and the sun shines more strongly late morning and afternoon. When these renewable energies become abundant, gas turbines must be turned off for the total power to not

When these start-stop cycles occur [47], the inside temperature of a generator would go through significant changes. Frequent start-stop cycles together with strong vibration will potentially cause delamination of the insulation as well as voids at their interface with the conducting copper. They also have different coefficients of expansion. Severe vibration can

**5.1. Analysis and discussion for vibration measurement**

**5.2. Proposed composite damage model for gas-fired generators**

with aforementioned correlation.

overload the power grid (see **Figure 25**).

**Figure 23.** Siemens Westinghouse AeroPac I generator with emphasis on the radial airflow via colored arrows. Courtesy of Calpine Corporation.

temperature detectors (RTDs) installed at different locations along the air path. We will compare our measurement with those coming from the supervisory control and data acquisition (SCADA).

Cooling air flows through many cooling ducts placed approximately 2 inches apart from each other. Blue arrows are meant to represent cooler air entering the core. As it flows through the ducts, the air progressively relieves the stator core of its heat (yellow arrows) and it ends up evacuated (red arrows) via the main central exhaust. Since RTDs carry conducting material, they are mainly installed in the grounded core. This field test represented the first time where in-slot vibration and hot spot measurements were performed.

#### **5. Analysis and discussion for temperature measurement**

The field test was performed after a successful rewind. A close examination of a sample segment of the old winding showed that defects always started extremely small at the scale of few millimeters as seen in the pinholes of **Figure 6**. In the advanced stages of disturbance, there appears a single or multiple small dark depressions inside the exposed mica. These might represent two stages of degradation. If hot spots were to start as point defects, prior observations make sense. These disturbed areas in the carbon paint grew increasingly larger together with the dark spots within them. We could not confirm whether these dark spots represent complete shorts to the copper conductor. The mica insulation is usually quite thick. The dark spot certainly looks like a reduced thickness in the mica. This must be a lengthy process for the electro-etching caused by the plasma of deionized air resulting from a strong in-slot vibration as mentioned in an earlier section of this paper. Discussion with the plant technical staff experienced with maintaining these gas-fired generators indicated that troubles seem to start typically after 5–10 years. The distributed temperature sensing Brillouin technology is usually used for oil and gas pipeline and helps to locate small leaks. Best spatial resolution is listed as three centimeters [39], which is much larger than the defects we observed, therefore, no hot spots were found despite our expectations. Given that the generator has been rewound and the insulation is new it would take several years for the defect to grow to a size large enough to be detected by the sensing fiber. We believe it is better to maintain the sensors as a continuous monitoring tool where we can see slight temperature changes down to 0.01°C increments.

#### **5.1. Analysis and discussion for vibration measurement**

This field test demonstrates consistent strong vibration in both 1xLF and 2xLF. A spectral analysis tool published by SKF USA Inc. [40], the strong vibration is related to misalignment (strong 2xLF amplitude) and also imbalance (strong 1xLF amplitude). Misalignment can be caused by thermal expansion; alignment could be performed when the generator is cold then heat up when it is placed in full operation. Thermal expansion might have caused the rotor to become elliptical instead of being perfectly circular. Misalignment could also be caused by shifting of the foundation or uneven support [41]. On the other hand, imbalance seems to be the cause of the strong 1xLF vibration. Their relative amplitude might indicate the extent of each problem (see [20]).

Referring to **Figure 15** of SKF USA Inc. article (see [40]), their example can be compared to the data of this field test (see **Figures 16**, **18** and **19**) and it seems that both imbalance and misalignment could be affecting the generator. These measured values were two to three times higher than typical measurements on large coal-fired generators. Those large generators do not display any 1xLF vibration signal at all because they are physically much larger in structure.

#### **5.2. Proposed composite damage model for gas-fired generators**

temperature detectors (RTDs) installed at different locations along the air path. We will compare our measurement with those coming from the supervisory control and data acquisition

**Figure 23.** Siemens Westinghouse AeroPac I generator with emphasis on the radial airflow via colored arrows. Courtesy

Cooling air flows through many cooling ducts placed approximately 2 inches apart from each other. Blue arrows are meant to represent cooler air entering the core. As it flows through the ducts, the air progressively relieves the stator core of its heat (yellow arrows) and it ends up evacuated (red arrows) via the main central exhaust. Since RTDs carry conducting material, they are mainly installed in the grounded core. This field test represented the first time where

The field test was performed after a successful rewind. A close examination of a sample segment of the old winding showed that defects always started extremely small at the scale of few millimeters as seen in the pinholes of **Figure 6**. In the advanced stages of disturbance, there appears a single or multiple small dark depressions inside the exposed mica. These might represent two stages of degradation. If hot spots were to start as point defects, prior observations make sense. These disturbed areas in the carbon paint grew increasingly larger together with the dark spots within them. We could not confirm whether these dark spots represent complete shorts to the copper conductor. The mica insulation is usually quite thick. The dark spot certainly looks like a reduced thickness in the mica. This must be a lengthy process for the electro-etching caused by the plasma of deionized air resulting from a strong in-slot vibration

in-slot vibration and hot spot measurements were performed.

194 Simulation and Modelling of Electrical Insulation Weaknesses in Electrical Equipment

**5. Analysis and discussion for temperature measurement**

(SCADA).

of Calpine Corporation.

The fiber optic signal processing system equipped with a gateway device connects the large volume of sensor data with the plant data from the SCADA for parametric correlation between cause (loading, start-stop periods, etc.) and result (level of vibration and temperature). **Figure 24** displays a flowchart of damage diagnostic that would be further quantified with aforementioned correlation.

It is now possible to simplify the problem into an intuitive model that does not involve complex mathematics [42–46]. The methodology is described below and can be used to analyze the measured data from these fiber optic sensors. It has been known that gas generators are frequently subjected to many daily start-stop events: the winds are usually stronger during the night and the sun shines more strongly late morning and afternoon. When these renewable energies become abundant, gas turbines must be turned off for the total power to not overload the power grid (see **Figure 25**).

When these start-stop cycles occur [47], the inside temperature of a generator would go through significant changes. Frequent start-stop cycles together with strong vibration will potentially cause delamination of the insulation as well as voids at their interface with the conducting copper. They also have different coefficients of expansion. Severe vibration can

The proposed model consists of a menu of degradation processes, individually selectable to form a clear picture of health of the air-cooled gas-fired generator, all based on the sensor ability to work inside high voltages, and strong electromagnetic fields. They are also small (0.5″ X 3″ X 0.08″) and can fit inside the stator slots which are approximately 1″ in width. Once installed in place, these sensors will be able to measure both thermal as well as mechanical stresses. The class of insulation material and the operating voltage of the generator have defined the limit of the electrical stress. Thermal stresses can be measured directly inside the slot as well as the end winding overhang where curvature might give rise to current crowding. These sensors are also designed to be thin; they can easily conform to any curved surface yet maintain good thermal contact. The field test described in prior sections represents the first occasion where direct measurement could be done inside the generator slot. Previous work combining the total aging effect consider vibration as related mainly to fatigue, which might not be the case here. From the failure analysis of the extracted winding section prior to the rewind, there was clear evidence of vibration sparking. Defects first appeared as small pinholes in the carbon paint, which grew with time. Once these defects grew into sufficient size while developing local high voltage spots, they cause the air to break down into plasma, whereas the remaining half of the vibration cycle broke the current path and turns the plasma into a source for electro-etching. Vibration sparking together with frequent start-stop cycles drastically shortens the lifespan of the gas-fired generator. This hypothesis was reinforced while working with Calpine maintenance staff: gas-fired generators typically start to have trouble as early as the warranty period within 5–10 years. This information is compared to the prior field tests of coal-fired base load generators monitoring stator end winding (SEW) vibration, where rewind would only be done after 30–40 years of operation. Vibration sparking might come with a threshold related to vibration peak-to-peak amplitudes. In fact vibration amplitudes observed at Hermiston field test showed a magnitude two to three times greater than the ones collected from coal-fired power plants where they are usually less than 150 microns peak-to-peak. There is another possibility. Large coal-fired generators are hydrogen-cooled. Hydrogen does not break down easily, pinhole defects in their old winding samples were rare occurrences. The proposed second field test for Calpine Corporation will take place in their plant of Texas (Hidalgo Energy Center) where the gas-fired generator would be hydrogencooled. We will go through the same analysis and compare the vibration and temperature data there. Fiber optic sensor capturing PD events will be introduced, provided PD first produced the pinhole defects that facilitated the following vibration sparking destruction. Those PD sensors are based on a very long FBG cavity. Cavity length affects sensor performance: the longer the cavity, the denser would be the interference fringes. These fringes will have steeper slopes and therefore more sensitive to small surface acoustic wave perturbations. Fringes are so densely packed spectrally that maintaining the operating point will not be needed. This PD sensor will not be affected by small changes in temperature and will pick up PD events along the full length of the optical fiber cavity. However, it is a dedicated PD sensor and will not be able to measure temperature. The distributed low frequency vibration sensor will be set in the

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same slot as the PD sensor to collect all the data that would complement each other.

The influence of PD on the proposed aging model is that they start to register pinhole formation. As the defects grew both in magnitude and incidence, an indication is given about the size of the carbon paint disturbance. However, the PD sensor would not be able to detect the

**Figure 24.** Damage model that allows residual life predictions.

**Figure 25.** Graph showing gas generators must be turned on and off to adapt to the supply of the renewables.

introduce cracks inside the thick epoxy mica and voids inside the mica facilitating PDs [48–56]. Of course, winding temperature would depend on load which will show up in the temperature trending profile in an online monitoring configuration. Loading changes will be reflected in changes in the 2xLF vibration amplitude, which is affected by the electromagnetic forces interaction between the rotor and the stator. Simultaneously, the temperature will also see small adjustments due to additional heating caused by higher load currents. Hence, smaller temperature changes will reflect the loading conditions and correlation with the control signal from the SCADA system becomes unnecessary. Current stage of sensor development allows the freedom to take advantage of the latest technology like the internet-of-things (IofT) where signals can be analyzed by gateway devices attached to our signal processing system. Then, the partially analyzed data are transmitted to a cloud where higher level calculation is performed to form a residual life model as well as providing warnings for urgent attention and quick remedial decisions. As for the start-stop cycles, they will simply switch the vibration signals on and off. These will be very clear and easy to interpret. Since temperature change is slow, its decrease will come gradually. Again, these data can be derived without any connection to the SCADA system of the power plant (power industry is known to be very concerned about external connection to their SCADA, making it open to external hacking).

The proposed model consists of a menu of degradation processes, individually selectable to form a clear picture of health of the air-cooled gas-fired generator, all based on the sensor ability to work inside high voltages, and strong electromagnetic fields. They are also small (0.5″ X 3″ X 0.08″) and can fit inside the stator slots which are approximately 1″ in width. Once installed in place, these sensors will be able to measure both thermal as well as mechanical stresses. The class of insulation material and the operating voltage of the generator have defined the limit of the electrical stress. Thermal stresses can be measured directly inside the slot as well as the end winding overhang where curvature might give rise to current crowding. These sensors are also designed to be thin; they can easily conform to any curved surface yet maintain good thermal contact. The field test described in prior sections represents the first occasion where direct measurement could be done inside the generator slot. Previous work combining the total aging effect consider vibration as related mainly to fatigue, which might not be the case here.

From the failure analysis of the extracted winding section prior to the rewind, there was clear evidence of vibration sparking. Defects first appeared as small pinholes in the carbon paint, which grew with time. Once these defects grew into sufficient size while developing local high voltage spots, they cause the air to break down into plasma, whereas the remaining half of the vibration cycle broke the current path and turns the plasma into a source for electro-etching. Vibration sparking together with frequent start-stop cycles drastically shortens the lifespan of the gas-fired generator. This hypothesis was reinforced while working with Calpine maintenance staff: gas-fired generators typically start to have trouble as early as the warranty period within 5–10 years. This information is compared to the prior field tests of coal-fired base load generators monitoring stator end winding (SEW) vibration, where rewind would only be done after 30–40 years of operation. Vibration sparking might come with a threshold related to vibration peak-to-peak amplitudes. In fact vibration amplitudes observed at Hermiston field test showed a magnitude two to three times greater than the ones collected from coal-fired power plants where they are usually less than 150 microns peak-to-peak. There is another possibility. Large coal-fired generators are hydrogen-cooled. Hydrogen does not break down easily, pinhole defects in their old winding samples were rare occurrences. The proposed second field test for Calpine Corporation will take place in their plant of Texas (Hidalgo Energy Center) where the gas-fired generator would be hydrogencooled. We will go through the same analysis and compare the vibration and temperature data there. Fiber optic sensor capturing PD events will be introduced, provided PD first produced the pinhole defects that facilitated the following vibration sparking destruction. Those PD sensors are based on a very long FBG cavity. Cavity length affects sensor performance: the longer the cavity, the denser would be the interference fringes. These fringes will have steeper slopes and therefore more sensitive to small surface acoustic wave perturbations. Fringes are so densely packed spectrally that maintaining the operating point will not be needed. This PD sensor will not be affected by small changes in temperature and will pick up PD events along the full length of the optical fiber cavity. However, it is a dedicated PD sensor and will not be able to measure temperature. The distributed low frequency vibration sensor will be set in the same slot as the PD sensor to collect all the data that would complement each other.

introduce cracks inside the thick epoxy mica and voids inside the mica facilitating PDs [48–56]. Of course, winding temperature would depend on load which will show up in the temperature trending profile in an online monitoring configuration. Loading changes will be reflected in changes in the 2xLF vibration amplitude, which is affected by the electromagnetic forces interaction between the rotor and the stator. Simultaneously, the temperature will also see small adjustments due to additional heating caused by higher load currents. Hence, smaller temperature changes will reflect the loading conditions and correlation with the control signal from the SCADA system becomes unnecessary. Current stage of sensor development allows the freedom to take advantage of the latest technology like the internet-of-things (IofT) where signals can be analyzed by gateway devices attached to our signal processing system. Then, the partially analyzed data are transmitted to a cloud where higher level calculation is performed to form a residual life model as well as providing warnings for urgent attention and quick remedial decisions. As for the start-stop cycles, they will simply switch the vibration signals on and off. These will be very clear and easy to interpret. Since temperature change is slow, its decrease will come gradually. Again, these data can be derived without any connection to the SCADA system of the power plant (power industry is known to be very concerned

**Figure 25.** Graph showing gas generators must be turned on and off to adapt to the supply of the renewables.

**Figure 24.** Damage model that allows residual life predictions.

196 Simulation and Modelling of Electrical Insulation Weaknesses in Electrical Equipment

about external connection to their SCADA, making it open to external hacking).

The influence of PD on the proposed aging model is that they start to register pinhole formation. As the defects grew both in magnitude and incidence, an indication is given about the size of the carbon paint disturbance. However, the PD sensor would not be able to detect the

• Ongoing electrical and thermal stresses • Degradation due to vibration sparking

**6. Conclusion**

safety for workers at the plant.

**Acknowledgements**

**Author details**

QPS Photronics Inc., Canada

Peter Kung

• Signature analysis of the observed vibration spectrum indicating structural looseness and

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With the upcoming field tests, this aging model covering generators of various sizes and make will become more sophisticated, turning the monitoring solution into a service business.

Sensors which can survive in a hostile environment are essential to understanding the aging process of turbines. It is quite possible that the key to extending the lifespan of turbines is real

We have proven through this test that real time monitoring of vibration and temperature is both possible and economical. What needs to happen now is that these measurements need to take a part in the day to day operation of generating plants. By sensing internal vibration and temperature we can know when a particular turbine is on the edge of conditions which will lead to lasting damage and a control scheme can be implemented where other turbines will start up to alleviate those conditions before damage happens resulting in increased life span of all the turbines. At the moment these turbines have a very limited lifespan, typically 7 or 8 years. With improved controls there could be a significant improvement in that lifespan. This would have benefits in reducing asset costs, improving reliability to customers and improved

The authors would like to express gratitude to Calpine Corporation for allowing and facilitating the installation of the fiber optic sensing systems at the Hermiston Power Project plant. Likewise, contribution from Oz Optics, Ltd. is highlighted for providing temperature records from their Brillouin distributed temperature technology. At last, acknowledgements are given to the senior consultant George F. Dailey for bringing all participants together and Refined Manufacturing

Acceleration Process (ReMAP) network for providing a substantial financial aid.

Address all correspondence to: peter@qpscom.com

time monitoring using fiber optic sensors for both vibration and temperature.

other mechanical problems like misalignment and unbalancing.

**Figure 26.** Cracking of copper turns and also bowing of the rotor.

onset of electro etching. Arcing experiments will be conducted in laboratory environment to study and relate the signatures associated with the electro-etching process.

In summary, it is a damage model of the insulation. It is related to the accelerated aging of insulation inside the gas-fired generator and additional work will be needed to turn it into a residual life prediction model. Such a model will depend on the design, and manufacturing workmanship, which vary depending on the individual OEMs.

#### **5.3. Influence of start-stop cycles**

Frequent start-stop cycles can cause stresses due to coefficient of expansion differences between different materials used inside the generator (see **Figure 26**).

In the industry, the effect of start-stop cycles is introduced as a life consumption factor. Attempts to use the gas generator to compensate for the intermittent renewables like the solar and wind is one instance. It might have to be turned on and off several times per day. The number of times per day serves to cause an apparent acceleration factor. In a case where it is done three times a day, each additional day of operation in that fashion is equivalent to 3 days of operation, effectively shortening their lifespan. An increasingly accumulated number of start-stop cycles per day correspondingly shorten the life of the generator. This explains the difference between the expected onsets of problems comparing a coal-fired power plant versus the gas-fired unit. Coal-fired plants usually need to be rewound after 30 years, whereas the gas generator needs adjustments at a daily pace depending of the wind and solar, leading to a major maintenance even inside the warranty period. This aging factor ranges from three to six, depending on the frequency of start-stop cycles per day.

So the life of a gas generator depends not only on who manufactured them, but it also depends how owners use them. Using fiber optic sensors it would be possible to get a real-time picture of how the generator is doing and extend its lifespan.

In summary, a new class of fiber optic sensors is introduced and can address the following complex problems:


With the upcoming field tests, this aging model covering generators of various sizes and make will become more sophisticated, turning the monitoring solution into a service business.

### **6. Conclusion**

onset of electro etching. Arcing experiments will be conducted in laboratory environment to

In summary, it is a damage model of the insulation. It is related to the accelerated aging of insulation inside the gas-fired generator and additional work will be needed to turn it into a residual life prediction model. Such a model will depend on the design, and manufacturing

Frequent start-stop cycles can cause stresses due to coefficient of expansion differences

In the industry, the effect of start-stop cycles is introduced as a life consumption factor. Attempts to use the gas generator to compensate for the intermittent renewables like the solar and wind is one instance. It might have to be turned on and off several times per day. The number of times per day serves to cause an apparent acceleration factor. In a case where it is done three times a day, each additional day of operation in that fashion is equivalent to 3 days of operation, effectively shortening their lifespan. An increasingly accumulated number of start-stop cycles per day correspondingly shorten the life of the generator. This explains the difference between the expected onsets of problems comparing a coal-fired power plant versus the gas-fired unit. Coal-fired plants usually need to be rewound after 30 years, whereas the gas generator needs adjustments at a daily pace depending of the wind and solar, leading to a major maintenance even inside the warranty period. This aging factor ranges from three

So the life of a gas generator depends not only on who manufactured them, but it also depends how owners use them. Using fiber optic sensors it would be possible to get a real-time picture

In summary, a new class of fiber optic sensors is introduced and can address the following

• Marginal design and poor choice of material to be indicated by the onset of PD activities • Number of start-stop cycles in its mode of operation as observed in the cycle of large tem-

study and relate the signatures associated with the electro-etching process.

workmanship, which vary depending on the individual OEMs.

**Figure 26.** Cracking of copper turns and also bowing of the rotor.

198 Simulation and Modelling of Electrical Insulation Weaknesses in Electrical Equipment

to six, depending on the frequency of start-stop cycles per day.

of how the generator is doing and extend its lifespan.

complex problems:

perature changes

between different materials used inside the generator (see **Figure 26**).

**5.3. Influence of start-stop cycles**

Sensors which can survive in a hostile environment are essential to understanding the aging process of turbines. It is quite possible that the key to extending the lifespan of turbines is real time monitoring using fiber optic sensors for both vibration and temperature.

We have proven through this test that real time monitoring of vibration and temperature is both possible and economical. What needs to happen now is that these measurements need to take a part in the day to day operation of generating plants. By sensing internal vibration and temperature we can know when a particular turbine is on the edge of conditions which will lead to lasting damage and a control scheme can be implemented where other turbines will start up to alleviate those conditions before damage happens resulting in increased life span of all the turbines. At the moment these turbines have a very limited lifespan, typically 7 or 8 years. With improved controls there could be a significant improvement in that lifespan. This would have benefits in reducing asset costs, improving reliability to customers and improved safety for workers at the plant.

#### **Acknowledgements**

The authors would like to express gratitude to Calpine Corporation for allowing and facilitating the installation of the fiber optic sensing systems at the Hermiston Power Project plant. Likewise, contribution from Oz Optics, Ltd. is highlighted for providing temperature records from their Brillouin distributed temperature technology. At last, acknowledgements are given to the senior consultant George F. Dailey for bringing all participants together and Refined Manufacturing Acceleration Process (ReMAP) network for providing a substantial financial aid.

#### **Author details**

Peter Kung Address all correspondence to: peter@qpscom.com QPS Photronics Inc., Canada

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## *Edited by Ricardo Albarracín Sánchez*

Around 80% of electrical consumption in an industrialised society is used by machinery and electrical drives. Therefore, it is key to have reliable grids that feed these electrical assets. Consequently, it is necessary to carry out pre-commissioning tests of their insulation systems and, in some cases, to implement an online condition monitoring and trending analysis of key variables, such as partial discharges and temperature, among others. Because the tests carried out for analysing the dielectric behaviour of insulation systems are commonly standardised, it is of interest to have tools that simulate the real behaviour of those and their weaknesses to prevent electrical breakdowns. The aim of this book is to provide the reader with models for electrical insulation systems diagnosis.

Published in London, UK © 2018 IntechOpen © FactoryTh / iStock

Simulation and Modelling of Electrical Insulation Weaknesses in Electrical Equipment

Simulation and Modelling

of Electrical Insulation

Weaknesses in Electrical

Equipment

*Edited by Ricardo Albarracín Sánchez*