**4. PILC cables – Diagnostic methods**

Diagnostic methods are nondestructive measurements of the dielectric properties in the purpose of a condition determination. Among electrical diagnostic methods, which refer to the behavior of the insulation in an electrical field, there are also chemical diagnostic methods, optical, acoustical, magnetic, mechanical, etc. defined in several standards (e.g. (High-voltage test techniques – Partial discharge measurements, 2000) (High voltage test techniques - Measurement of partial discharges by electromagnetic and acoustic methods "Proposed Horizontal Standard", 2011)). Electrical diagnostic methods of PILC cables could be divided in two main groups dependent on the type of applied voltage, AC or DC as shown in Figure 3. Measurements of cable performance with an AC test-voltage are Partial Discharge (PD), Oscillating Wave Test System (OWTS) and tan*δ* measurements, where measurements of PDs and tan are usually performed by 0,1Hz (Very Low Frequency (VLF) (IEEE Guide for Field Testing of Shielded Power Cable Systems Using Very Low Frequency (VLF), 2004)), and in some cases at 50Hz. Besides, some diagnostic measurements of tan like e.g. Frequency Response Analyses (FRA) operate in a wide range of frequencies, (Neimanis & Eriksson, 2004). The DC methods are based on the analyses of the diagnostic properties in time domain, (Mladenovic, Determination of the Remaining Lifetime of PILC cables based on PD and tan(δ) diagnosis, 2012, to be published), like in e.g. return voltage measurement (RVM), polarization/depolarization current measurement (PDC) and the less used Decay Voltage Method (DVM) and Isothermal Relaxation Current (IRC).

The only measurement with a local character are measurements of partial discharges, means it can localize the fault in progress, but it does not give the information about the general insulation condition. In (Densley, 2001) diagnostic tests for PILC cable system with their advantages and limitations are listed.

It has to be also mentioned, that a direct comparison of the values measured under different test-conditions e.g. ambient temperature, or even with different diagnostic systems is not always plausible. In any case, the results of the different diagnostic measurements are more or less complementary to one other and summarized could deliver more complete and more reliable information about insulation condition.

**Figure 3.** Principal overview of diagnostic systems for power cable

### **4.1. Dielectric losses and dissipation factor**

256 Dielectric Material

degradation of cellulose is thoroughly described.

both, thermal or oxidative degradation.

**4. PILC cables – Diagnostic methods** 

measurements of PDs and tan

advantages and limitations are listed.

(Emsley & Stevens, 1994) the chemical mechanisms of low-temperature (<200°C)

Moreover, through the transients, short circuits or load variations in field operation, the cable temperature could vary and change very suddenly. As it was shown in (Soares, Caminot, & Levchik, 1995) there is no influence of the temperature increase (heating rate) on activation energy; it was opined that thermal decomposition of the kraft paper mostly proceeds through steady depolymerisation. Anyway, hydrolytic degradation is the most powerful degradation in cellulose, and due to the moisture outcome it could be initiated by

Obviously, ageing is a complex chain cause-reaction-cause process. Summarized, it results in the decrease of DP, increase of moisture content, and appearance of different gasses, resulting in the changing of electrical and mechanical properties of the insulation system.

Diagnostic methods are nondestructive measurements of the dielectric properties in the purpose of a condition determination. Among electrical diagnostic methods, which refer to the behavior of the insulation in an electrical field, there are also chemical diagnostic methods, optical, acoustical, magnetic, mechanical, etc. defined in several standards (e.g. (High-voltage test techniques – Partial discharge measurements, 2000) (High voltage test techniques - Measurement of partial discharges by electromagnetic and acoustic methods "Proposed Horizontal Standard", 2011)). Electrical diagnostic methods of PILC cables could be divided in two main groups dependent on the type of applied voltage, AC or DC as shown in Figure 3. Measurements of cable performance with an AC test-voltage are Partial Discharge (PD), Oscillating Wave Test System (OWTS) and tan*δ* measurements, where

(IEEE Guide for Field Testing of Shielded Power Cable Systems Using Very Low Frequency (VLF), 2004)), and in some cases at 50Hz. Besides, some diagnostic measurements of tan

like e.g. Frequency Response Analyses (FRA) operate in a wide range of frequencies, (Neimanis & Eriksson, 2004). The DC methods are based on the analyses of the diagnostic properties in time domain, (Mladenovic, Determination of the Remaining Lifetime of PILC cables based on PD and tan(δ) diagnosis, 2012, to be published), like in e.g. return voltage measurement (RVM), polarization/depolarization current measurement (PDC) and the less

The only measurement with a local character are measurements of partial discharges, means it can localize the fault in progress, but it does not give the information about the general insulation condition. In (Densley, 2001) diagnostic tests for PILC cable system with their

It has to be also mentioned, that a direct comparison of the values measured under different test-conditions e.g. ambient temperature, or even with different diagnostic systems is not always plausible. In any case, the results of the different diagnostic measurements are more

used Decay Voltage Method (DVM) and Isothermal Relaxation Current (IRC).

are usually performed by 0,1Hz (Very Low Frequency (VLF)

The dissipation factor is the tangent of the loss angle defined as the phase shift between leakage current and the applied test voltage to 90°.

Losses in the dielectric are caused by the moving of the charges under the influence of applied electrical field. The real losses (*P*) are determined through the current component *I* and the reactive power ( *Qc* ) through the component *<sup>c</sup> I* , Figure 4. Thus, dissipation factor tancould be determined as:

$$\tan \delta = \frac{P\_{\mathcal{S}}}{Q\_{\mathcal{c}}}.\tag{2}$$

**Figure 4.** Simplified equivalent circuit and vector diagram of a real dielectric

Hereby two kinds of losses could be defined: polarization losses and losses due to the conductivity:

$$\tan \delta = \frac{\kappa + \alpha \varepsilon\_0 \varepsilon\_r''}{\alpha \varepsilon\_0 \varepsilon\_r'} = \tan \delta\_{\alpha u} + \tan \delta\_{pol} \tag{3}$$

where:

$$\tan \delta\_{con} = \frac{\kappa}{o \varkappa\_0 \varepsilon\_r'} \tag{4}$$

Empiric Approach for Criteria Determination of Remaining Lifetime Estimation of MV PILC Cables 259

Townsend discharges. In the same time a cloud of positive ions, which are heavier and therefore slowly, moves towards the cathode, tending to reduce the cavity voltage under its breakdown value. If the voltage increases more Townsend discharges will occur again, means if the sinusoidal voltage continues to increase exceeding again the breakdown voltage of the gas, new discharge will occur to stabilize the cavity voltage. The process of charge transferring will begin tapering off near the breakdown voltage. Townsend discharges predominantly occur between paper layers and within the butt

On the other hand, if the applied voltage is high enough and the cavity is of enough depth so that the mean free path of the electrons is shorter than the distance between the electrodes, Townsend avalanches can develop into stream-discharges. If one start-electron creates the avalanche of 106 up to 108 electrons, it will come to a change of the electrical field in the surrounding area, since the heavier positive ions remain at the avalanche-tail while the electrons form the tip of the streamer. Through the enhanced electrical field and space charges on the avalanche-tip it will come more rapidly to new avalanches and more photons, accelerating the charge transfer regardless to the reduction of the cavity voltage. If the producing of the free electrons trough the impact ionization is faster than the electron summation, a conducting channel within the cavity will be formed and the transfer will stop as all the charges stored on the faces of the cavity are transferred, i.e. cavity voltage drops to

Even fewer in number, single streamers could cause the bigger damage to the insulation (paper) due to the higher released energy. "The tip of the streamer has a radius of the order of tens of micrometers and the concentration of energy within the dimension results in penetration of the paper layers by pinholes", (Robinson, 1990). Moreover, in (Lemke, 2008) PDs are classified as: "the pulse charge created by glow discharges, oft referred to as Townsend discharges, is usually in the order of few pC. Streamer discharges create pulse charges between about 10pC and some 100 pC. A transition from streamer to leader discharges may occur if the pulse charge exceeds few 1000pC". Dependent on the intensity and the repetition rate of the local breakdowns, its surrounding within the dielectric could be "infected" and thus the weak area could expand, finally leading to the breakdown of the

Beside the strength of applied voltage, relevant for the occurrence and sustain of these discharges, i.e. partial discharges, is the voltage frequency and insulation temperature. The nature of the discharges, its detection, PDs damaging power and insulation degradation exposed to PD activity, detection and measurement of PD, their intensity, repetition rate,

Partial discharges are defined in IEC 60270 as: "localized electrical discharges that only partially bridge the insulation between conductors and which can or cannot occur adjacent to a conductor. Partial discharges are in general a consequence of local electrical stress concentrations in the insulation or on the surface of the insulation. Generally, such

etc. are systematically presented e.g. in (Niemeyer, 1995) (Bartnikas, 2002).

discharges appear as pulses having duration of much less than 1 µs."

gaps, (Mladenovic, 2012, to be published), (Robinson, 1990).

zero, (Küchler, 2009) (Robinson, 1990).

complete dielectric.

and

$$\tan \delta\_{\text{pol}} = \frac{\varepsilon\_r''}{\varepsilon\_r'}. \tag{5}$$

Assuming a very low value of the dissipation loss angle and regarding to (3) it can be approximated that:

$$
\delta \mathcal{S} = \mathcal{S}\_{con} + \mathcal{S}\_{pol}.\tag{6}
$$

It is important to mention that if there is some PD activity within the dielectric, it will cause additional losses:

$$
\tan \mathcal{S} = \tan \mathcal{S}\_{\text{con}} + \tan \mathcal{S}\_{\text{pol}} + \tan \mathcal{S}\_{\text{PD}}.\tag{7}
$$

Analog to (6), it can be approximated:

$$
\delta \mathcal{S} = \delta\_{\rm com} + \delta\_{\rm pol} + \delta\_{\rm PD} \,. \tag{8}
$$

#### **4.2. Partial discharges**

Partial discharges are gas-discharges of Townsend type and streamers which occur under specific conditions in gas filled cavities within the insulation. The presence of the cavities leads to a local change of the electrical permittivity and conductivity, disturbing the homogeneity of the electrical field and therefore to a decrease of electrical stability.

Exposed to an electrical AC field a random ionization process within the cavity will start, which produces a free 'start' electron needed for the development of PD. The electrical field forces the electron moving to the (local) anode within the void, ionizing on the way more gas atoms and producing therefore positive ions and more electrons. In this way the ionization events multiply causing the start of an electron avalanche. These are called Townsend discharges. In the same time a cloud of positive ions, which are heavier and therefore slowly, moves towards the cathode, tending to reduce the cavity voltage under its breakdown value. If the voltage increases more Townsend discharges will occur again, means if the sinusoidal voltage continues to increase exceeding again the breakdown voltage of the gas, new discharge will occur to stabilize the cavity voltage. The process of charge transferring will begin tapering off near the breakdown voltage. Townsend discharges predominantly occur between paper layers and within the butt gaps, (Mladenovic, 2012, to be published), (Robinson, 1990).

258 Dielectric Material

conductivity:

where:

and

approximated that:

additional losses:

**4.2. Partial discharges** 

stability.

Analog to (6), it can be approximated:

Hereby two kinds of losses could be defined: polarization losses and losses due to the

tan tan tan , *<sup>r</sup>*

0 *r*

*r r*

*con pol*

It is important to mention that if there is some PD activity within the dielectric, it will cause

*con pol PD* tan tan tan tan

 

> 

*con pol PD*

 

 

Partial discharges are gas-discharges of Townsend type and streamers which occur under specific conditions in gas filled cavities within the insulation. The presence of the cavities leads to a local change of the electrical permittivity and conductivity, disturbing the homogeneity of the electrical field and therefore to a decrease of electrical

Exposed to an electrical AC field a random ionization process within the cavity will start, which produces a free 'start' electron needed for the development of PD. The electrical field forces the electron moving to the (local) anode within the void, ionizing on the way more gas atoms and producing therefore positive ions and more electrons. In this way the ionization events multiply causing the start of an electron avalanche. These are called

 

*con pol*

 . (7)

. (8)

(3)

(4)

and regarding to (3) it can be

(5)

. (6)

0 0

 

Assuming a very low value of the dissipation loss angle

 

tan

 

*r*

*con*

*pol*

tan . 

On the other hand, if the applied voltage is high enough and the cavity is of enough depth so that the mean free path of the electrons is shorter than the distance between the electrodes, Townsend avalanches can develop into stream-discharges. If one start-electron creates the avalanche of 106 up to 108 electrons, it will come to a change of the electrical field in the surrounding area, since the heavier positive ions remain at the avalanche-tail while the electrons form the tip of the streamer. Through the enhanced electrical field and space charges on the avalanche-tip it will come more rapidly to new avalanches and more photons, accelerating the charge transfer regardless to the reduction of the cavity voltage. If the producing of the free electrons trough the impact ionization is faster than the electron summation, a conducting channel within the cavity will be formed and the transfer will stop as all the charges stored on the faces of the cavity are transferred, i.e. cavity voltage drops to zero, (Küchler, 2009) (Robinson, 1990).

Even fewer in number, single streamers could cause the bigger damage to the insulation (paper) due to the higher released energy. "The tip of the streamer has a radius of the order of tens of micrometers and the concentration of energy within the dimension results in penetration of the paper layers by pinholes", (Robinson, 1990). Moreover, in (Lemke, 2008) PDs are classified as: "the pulse charge created by glow discharges, oft referred to as Townsend discharges, is usually in the order of few pC. Streamer discharges create pulse charges between about 10pC and some 100 pC. A transition from streamer to leader discharges may occur if the pulse charge exceeds few 1000pC". Dependent on the intensity and the repetition rate of the local breakdowns, its surrounding within the dielectric could be "infected" and thus the weak area could expand, finally leading to the breakdown of the complete dielectric.

Beside the strength of applied voltage, relevant for the occurrence and sustain of these discharges, i.e. partial discharges, is the voltage frequency and insulation temperature. The nature of the discharges, its detection, PDs damaging power and insulation degradation exposed to PD activity, detection and measurement of PD, their intensity, repetition rate, etc. are systematically presented e.g. in (Niemeyer, 1995) (Bartnikas, 2002).

Partial discharges are defined in IEC 60270 as: "localized electrical discharges that only partially bridge the insulation between conductors and which can or cannot occur adjacent to a conductor. Partial discharges are in general a consequence of local electrical stress concentrations in the insulation or on the surface of the insulation. Generally, such discharges appear as pulses having duration of much less than 1 µs."

Gas filled cavities, locally increased moisture content; impurities within the dielectric, rifts, etc. are all classified as local weakness. A consequence of their presence is a local decrease of the relative permittivity which results in the decrease of a local breakdown voltage. In Figure 5 a gas filled cavity within a dielectric and its equivalent circuit are shown. *C*<sup>1</sup> is the capacity of the cavity with relative permittivity *<sup>r</sup>*<sup>1</sup> , *C*3 and *C*2 are inner dielectric capacities with relative permittivity *<sup>r</sup>* .

Empiric Approach for Criteria Determination of Remaining Lifetime Estimation of MV PILC Cables 261

Standard", 2011). The PD measurements applicable on the MV and HV cables are based on the electrical detection methods, which are shown in (High-voltage test techniques – Partial discharge measurements, 2000). For diagnostic purposes, the most appropriate are the measurements on the nominal frequency of 50/60Hz. In this way measured inception voltage or PD intensity etc. corresponds to those during normal network operation. Hereby measurements are commonly run on different voltage levels, for example 1, 1,3, 1,7, 2, and 2,5 times the line to ground voltage. Beside 50/60 Hz measurement systems, there are diagnostic systems operating on 0,1Hz, and so-called Oscillating Wave Test System (OWTS) presented in (Petzold & Zakharov, 2005), (Petzold & Gulski, 2006). Newly, many researches and developments are directed to the improving of on-line PD measurements, (A. N. Cuppen, E. F. Steennis, & P. C. J. M. van der Wielen, 2010) (P. A. A. F. Wouters, P. C. J. M. van der Wielen, J. Veen, P. Wagenaars, and E. F. Steennis, 2005) (Boltze, Markalous, Bolliger, Ciprietti, & Chiu, 2009) (Tian, Lewin, Wilkinson, Sutton, & Swingler) (Ambikairajah, Phung,

Finally, presence of PD could accelerate the ageing rapidity hardly and shorten the remaining lifetime of the insulation material, (Robinson, 1990). However, also for the purpose of the interpretation of the PD measurements, there are still no classifications or criteria, and hence the correspondence between PD activity levels and recommended actions. Nevertheless, beside PD magnitudes numerous other related and derived PD quantities are considered for the data interpretation, (Lemke, 2008): PD Inception Voltage (PDIV), PD Extinction Voltage (PDEV), PD magnitudes on different voltage levels, PD repetition rate, PD repetition frequency, phase angle, average discharge current, discharge power, etc. Due to the numerous still unknown influencing factors on the PD activity, the physical interpretation of the

the number of pulses and their amplitudes vary from half-cycle to half-cycle.

pulses per half-cycle until the number increase with cavity size.

 The PD repetition rate per second should increase with a rising of the test-voltage. It could also indicate the size of the source cavities, since small voids yield to very few PD

The temperature, i.e. pressure increase in the cavity could yield to extinguishing of PD

Finally, it is given in (High Voltage - VLF Hipot Instruments): "There is no reliable consensus on what are good versus bad PD levels. Splices can exhibit very high levels of partial discharge yet last for years, while those showing lower PD levels might fail sooner. One must keep in mind what the purpose of the test is. Whether using partial discharge or

techniques, the point of the test is to grade all cables tested on a scale from high quality

are shown in Figure 6. A temperature increase results in an increase of the mobility of the

, the relative permittivity *<sup>r</sup>*

and the conductivity

Ravishankar, Blackburn, & Liu, 2010).

activity.

tan

to low".

measured data is still a very complex process. For example:

**5. Dependencies of the diagnostic parameters** 

The structural dependencies of the tan

**Figure 5.** Equivalent circuit of the cavity within the dielectric and PD activity

Here *u*1 is indicated as the breakdown voltage of the cavity. If, by applying an ACvoltage *u* , the voltage over capacitance *C*1 exceeds ignition voltage *u*<sup>1</sup> , the capacity will discharge as symbolically shown in Figure 5 over spark gap. This process will repeat with the frequency 40 kHz-1MHz (up to 10MHz, (Kuhnert, Wieznerowicz, & Wanser, 1997)), as long as the absolute value of the voltage over *C*1 is higher than the ignition voltage of the void, and it can be notified as discharge pulses in the measured current *i* . Theoretically, PDs are essentially Townsend discharges, although their form and some features could vary as shown in (Bartnikas, 2002). However, in the praxis, measurement systems concern pulse-type discharges, their inception voltage, intensity, repetition rate i.e. frequency, etc. Generally, for an insulation material it can be stated that PD activity could cause progressive deterioration and cause irreversible mechanical weakening through chemical reactions and physical changes, (Mladenovic, Determination of the Remaining Lifetime of PILC cables based on PD and tan(δ) diagnostics, 2012, to be published).

However, in the complex insulation systems like PILC cables, or oil cables, the effect of selfhealing is well known and refers to the e.g. oil/mass refilling of the cavities, or cavities displacing. Still, if PD occurs between the paper layers, they could punctures it, (Robinson, 1990), causing irreparable chemical changes which will influence the surrounded material by exposing it to an increased electrical stress, heating, and so on, until the further carbonization and break down, (Stanka, 2011). In (Bartnikas, 2002), the mechanism and nature of PD is well elaborated, and its detection and measurement systems are chronologically shown and discussed, since first reported measurement methods in 1933.

The measurement or rather detection of these pulses can be done using electrical, mechanical, optical and chemical methods, (High voltage test techniques - Measurement of partial discharges by electromagnetic and acoustic methods "Proposed Horizontal Standard", 2011). The PD measurements applicable on the MV and HV cables are based on the electrical detection methods, which are shown in (High-voltage test techniques – Partial discharge measurements, 2000). For diagnostic purposes, the most appropriate are the measurements on the nominal frequency of 50/60Hz. In this way measured inception voltage or PD intensity etc. corresponds to those during normal network operation. Hereby measurements are commonly run on different voltage levels, for example 1, 1,3, 1,7, 2, and 2,5 times the line to ground voltage. Beside 50/60 Hz measurement systems, there are diagnostic systems operating on 0,1Hz, and so-called Oscillating Wave Test System (OWTS) presented in (Petzold & Zakharov, 2005), (Petzold & Gulski, 2006). Newly, many researches and developments are directed to the improving of on-line PD measurements, (A. N. Cuppen, E. F. Steennis, & P. C. J. M. van der Wielen, 2010) (P. A. A. F. Wouters, P. C. J. M. van der Wielen, J. Veen, P. Wagenaars, and E. F. Steennis, 2005) (Boltze, Markalous, Bolliger, Ciprietti, & Chiu, 2009) (Tian, Lewin, Wilkinson, Sutton, & Swingler) (Ambikairajah, Phung, Ravishankar, Blackburn, & Liu, 2010).

260 Dielectric Material

Gas filled cavities, locally increased moisture content; impurities within the dielectric, rifts, etc. are all classified as local weakness. A consequence of their presence is a local decrease of the relative permittivity which results in the decrease of a local breakdown voltage. In Figure 5 a gas filled cavity within a dielectric and its equivalent circuit are shown. *C*<sup>1</sup> is the

Here *u*1 is indicated as the breakdown voltage of the cavity. If, by applying an ACvoltage *u* , the voltage over capacitance *C*1 exceeds ignition voltage *u*<sup>1</sup> , the capacity will discharge as symbolically shown in Figure 5 over spark gap. This process will repeat with the frequency 40 kHz-1MHz (up to 10MHz, (Kuhnert, Wieznerowicz, & Wanser, 1997)), as long as the absolute value of the voltage over *C*1 is higher than the ignition voltage of the void, and it can be notified as discharge pulses in the measured current *i* . Theoretically, PDs are essentially Townsend discharges, although their form and some features could vary as shown in (Bartnikas, 2002). However, in the praxis, measurement systems concern pulse-type discharges, their inception voltage, intensity, repetition rate i.e. frequency, etc. Generally, for an insulation material it can be stated that PD activity could cause progressive deterioration and cause irreversible mechanical weakening through chemical reactions and physical changes, (Mladenovic, Determination of the Remaining Lifetime of PILC cables based on PD

However, in the complex insulation systems like PILC cables, or oil cables, the effect of selfhealing is well known and refers to the e.g. oil/mass refilling of the cavities, or cavities displacing. Still, if PD occurs between the paper layers, they could punctures it, (Robinson, 1990), causing irreparable chemical changes which will influence the surrounded material by exposing it to an increased electrical stress, heating, and so on, until the further carbonization and break down, (Stanka, 2011). In (Bartnikas, 2002), the mechanism and nature of PD is well elaborated, and its detection and measurement systems are chronologically shown and discussed, since first reported measurement methods in 1933.

The measurement or rather detection of these pulses can be done using electrical, mechanical, optical and chemical methods, (High voltage test techniques - Measurement of partial discharges by electromagnetic and acoustic methods "Proposed Horizontal

, *C*3 and *C*2 are inner dielectric

capacity of the cavity with relative permittivity *<sup>r</sup>*<sup>1</sup>

.

**Figure 5.** Equivalent circuit of the cavity within the dielectric and PD activity

capacities with relative permittivity *<sup>r</sup>*

and tan(δ) diagnostics, 2012, to be published).

Finally, presence of PD could accelerate the ageing rapidity hardly and shorten the remaining lifetime of the insulation material, (Robinson, 1990). However, also for the purpose of the interpretation of the PD measurements, there are still no classifications or criteria, and hence the correspondence between PD activity levels and recommended actions. Nevertheless, beside PD magnitudes numerous other related and derived PD quantities are considered for the data interpretation, (Lemke, 2008): PD Inception Voltage (PDIV), PD Extinction Voltage (PDEV), PD magnitudes on different voltage levels, PD repetition rate, PD repetition frequency, phase angle, average discharge current, discharge power, etc. Due to the numerous still unknown influencing factors on the PD activity, the physical interpretation of the measured data is still a very complex process. For example:


Finally, it is given in (High Voltage - VLF Hipot Instruments): "There is no reliable consensus on what are good versus bad PD levels. Splices can exhibit very high levels of partial discharge yet last for years, while those showing lower PD levels might fail sooner. One must keep in mind what the purpose of the test is. Whether using partial discharge or tan techniques, the point of the test is to grade all cables tested on a scale from high quality to low".
