**Monitoring Power Quality in Small Scale Renewable Energy Sources Supplying Distribution Systems**

Nicolae Golovanov, George Cristian Lazaroiu, Mariacristina Roscia and Dario Zaninelli

Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/53464

### **1. Introduction**

The integration of renewable sources within the existing power system affects its traditional principles of operation. The renewable energy sources (RES) can be used in small, decentral‐ ized power plants or in large ones, they can be built in small capacities and can be used in different locations [1]. In isolated areas where the cost of the extension of the power systems (from utilities point of view) or the cost for interconnection with the grid (from customer's point of view) are very high with respect to the cost of the RES system, these renewable sources are suitable. The RES systems are appropriate for a large series of applications, such as standalone systems for isolated buildings or large interconnected networks. The modularity of these systems makes possible the extension in the case of a load growth.

The increasing penetration rate of RES in the power systems is raising technical problems, as voltage regulation, network protection coordination, loss of mains detection, and RES operation following disturbances on the distribution network [2]. The utilization of these alternative sources presents advantages and disadvantages. The impact of the wind turbines and photovoltaic systems on network operation and power quality (harmonics, and voltage fluctuations) is highly important. The capability of the power system to absorb the power quality disturbances is depending on the fault level at the point of common coupling. [3] In weak networks or in power systems with a high wind generation penetration, the integration of these sources can be limited by the flicker level that must not exceed the standardized limits. The wind generators and PV systems interconnected to the main grid with the help of power electronics converters can cause important current harmonics.

© 2013 Golovanov et al.; licensee InTech. This is an open access article 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. © 2013 Golovanov et al.; licensee InTech. This is a paper 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. Power quality and renewable energy sources**

Nowadays, the renewable sources generation is rapidly developing in Europe. In the last 17th years, the average growth rate is of wind generation is 15.6% annually [4]. As these renewable sources are increasingly penetrating the power systems, the impact of the RES on network operation and power quality is becoming important. The intermittent character of the wind and solar irradiation constrains the power system to have an available power reserve. Due to the output power variations of the wind turbines, voltage fluctuations are produced. In weak networks or in power systems with a high wind generation penetration, the integration of these sources can be limited by the flicker level that must not exceed the standardized limits.

The photovoltaic (PV) installations, interconnected to the mains supply, can be single-phase connected (photovoltaic installations with capacity less than 5 kW) or three-phase connected (photovoltaic installations with capacity greater than 5 kW). The direct-coupled PV systems, without electrical energy storage, inject in the power system a generated power that follows the intermittency of the primary energy source. In this case, important voltage variations can occur at the PCC. The connection of PV systems to the low voltage grid can determine voltage variations and harmonic currents [5].

### **2.1. Voltage fluctuations**

Determination of voltage fluctuations (flicker effect) due to output power variations of renewable sources is difficult, because depend of the source's type, of generator's characteristics and network impedance. For the case of wind turbines, the long term flicker coefficient *Plt* due to commutations, computed over a 120 min interval and for step variations, becomes [6]:

$$P\_{lt} = \frac{8}{S\_{sc}} \cdot N\_{120}^{0.31} \cdot k\_f \left(\psi\_{sc}\right) \cdot \mathbf{S}\_r \tag{1}$$

( ) ψ, . *<sup>r</sup>*

The flicker coefficient *c*(ψsc, υ*a*) for a specified value of the angle ψsc, for a specified value of the wind speed υ*a* and for a certain installation is given by the installation manufacturer, or can be experimentally determined based on standard procedures. Depending on the voltage level where the wind generator (wind farms) is connected, the angle ψsc can take values between 30° (for the medium voltage network) and 85° (for the high voltage network). Flicker evaluation is based on the IEC standard 61000-3-7 [7] which gives guidelines for emission limits for fluctuating loads in medium voltage and high voltage networks. Table 1 reports the recom‐

The flicker evaluation determined by a wind turbine of 650kW is analyzed. The wind turbine has a tower height of 80 meters, the rotor diameter is 47 m, and the swept area is 1735 m<sup>2</sup>

electrical energy production during two winter months is 127095 kWh, respectively 192782 kWh. The average wind speeds, measured at 60m height, during the first monitoring month was 6.37m/s while during march was 7.32m/s. The variation of turbine output power is shown in Fig. 1. The intermittent character of the produced power is clearly highlighted. The tower

12:10:00 AM 10:00:00 PM 8:00:00 PM 4:10:00 PM 2:50:00 PM

*sc*

Monitoring Power Quality in Small Scale Renewable Energy Sources Supplying Distribution Systems

MV HV

. The

*<sup>S</sup>* = = (3)

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95

*st lt sc a*

**Flicker severity factor Planning levels**

0

**Figure 1.** Turbine output power variation during the 1 month monitoring period

200

400

**Turbine power output (kW)**

600

800

*Pst* 0.9 0.8 *Plt* 0.7 0.6

**Table 1.** Flicker planning levels for medium voltage (MV) and high voltage (HV) networks

mended values.

*<sup>S</sup> PPc v*

where *N120* is the number of possible commutations in a 120 min interval, *kf* (ψ*sc*) is the flicker factor defined for angle ψsc = arctan*(Xsc /Rsc)*, *Sr* − rated power of the installation, and *Ssc* − fault level at point of common coupling (PCC).

For a 10 minutes interval, the short-term flicker *Pst* is defined [6]:

$$P\_{lt} = \frac{18}{\mathcal{S}\_{sc}} \cdot \mathbf{N}\_{10}^{0.31} \cdot k\_f \left(\psi\_{sc}\right) \cdot \mathbf{S}\_r \tag{2}$$

where *N10* is the number of possible commutations in a 10 min interval.

The values of flicker indicator for wind turbines, due to normal operation, can be evaluated using flicker coefficient *c*(ψsc, υ*a*), dependent on average annual wind speed, υ*a*, in the point where the wind turbine is installed, and the phase angle of short circuit impedance, ψsc:

Monitoring Power Quality in Small Scale Renewable Energy Sources Supplying Distribution Systems http://dx.doi.org/10.5772/53464 95

$$P\_{st} = P\_{lt} = c \left(\psi\_{sc'} \upsilon\_a\right) \frac{S\_r}{S\_{sc}}.\tag{3}$$

The flicker coefficient *c*(ψsc, υ*a*) for a specified value of the angle ψsc, for a specified value of the wind speed υ*a* and for a certain installation is given by the installation manufacturer, or can be experimentally determined based on standard procedures. Depending on the voltage level where the wind generator (wind farms) is connected, the angle ψsc can take values between 30° (for the medium voltage network) and 85° (for the high voltage network). Flicker evaluation is based on the IEC standard 61000-3-7 [7] which gives guidelines for emission limits for fluctuating loads in medium voltage and high voltage networks. Table 1 reports the recom‐ mended values.


**Table 1.** Flicker planning levels for medium voltage (MV) and high voltage (HV) networks

**2. Power quality and renewable energy sources**

variations and harmonic currents [5].

level at point of common coupling (PCC).

**2.1. Voltage fluctuations**

94 Power Quality Issues

Nowadays, the renewable sources generation is rapidly developing in Europe. In the last 17th years, the average growth rate is of wind generation is 15.6% annually [4]. As these renewable sources are increasingly penetrating the power systems, the impact of the RES on network operation and power quality is becoming important. The intermittent character of the wind and solar irradiation constrains the power system to have an available power reserve. Due to the output power variations of the wind turbines, voltage fluctuations are produced. In weak networks or in power systems with a high wind generation penetration, the integration of these sources can be limited by the flicker level that must not exceed the standardized limits.

The photovoltaic (PV) installations, interconnected to the mains supply, can be single-phase connected (photovoltaic installations with capacity less than 5 kW) or three-phase connected (photovoltaic installations with capacity greater than 5 kW). The direct-coupled PV systems, without electrical energy storage, inject in the power system a generated power that follows the intermittency of the primary energy source. In this case, important voltage variations can occur at the PCC. The connection of PV systems to the low voltage grid can determine voltage

Determination of voltage fluctuations (flicker effect) due to output power variations of renewable sources is difficult, because depend of the source's type, of generator's characteristics and network impedance. For the case of wind turbines, the long term flicker coefficient *Plt* due to

( ) 0.31

factor defined for angle ψsc = arctan*(Xsc /Rsc)*, *Sr* − rated power of the installation, and *Ssc* − fault

( ) 0.31

The values of flicker indicator for wind turbines, due to normal operation, can be evaluated using flicker coefficient *c*(ψsc, υ*a*), dependent on average annual wind speed, υ*a*, in the point where the wind turbine is installed, and the phase angle of short circuit impedance, ψsc:

10

*lt f sc r* ψ

*P Nk S*

*<sup>S</sup>* =× × × (1)

*<sup>S</sup>* =× × × (2)

(ψ*sc*) is the flicker

commutations, computed over a 120 min interval and for step variations, becomes [6]:

120

*P Nk S*

*lt f sc r* ψ

8

*sc*

where *N120* is the number of possible commutations in a 120 min interval, *kf*

18

*sc*

where *N10* is the number of possible commutations in a 10 min interval.

For a 10 minutes interval, the short-term flicker *Pst* is defined [6]:

The flicker evaluation determined by a wind turbine of 650kW is analyzed. The wind turbine has a tower height of 80 meters, the rotor diameter is 47 m, and the swept area is 1735 m<sup>2</sup> . The electrical energy production during two winter months is 127095 kWh, respectively 192782 kWh. The average wind speeds, measured at 60m height, during the first monitoring month was 6.37m/s while during march was 7.32m/s. The variation of turbine output power is shown in Fig. 1. The intermittent character of the produced power is clearly highlighted. The tower

**Figure 1.** Turbine output power variation during the 1 month monitoring period

shadow effect for the wind generator determines a variation of the absorbed energy, which is measured as a power variation at generator terminals. Fig. 2(a) shows the wind generator, while Fig. 2(b) illustrates the tower shadow effect corresponding variation of the generator output power.

The computations based on the values reported in Table 2 and Table 3 lead to the flicker

Monitoring Power Quality in Small Scale Renewable Energy Sources Supplying Distribution Systems

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97

**i.** continuous operation, annual average wind speed υ*a*=7.5, interconnection with the

<sup>300</sup> =0, 00109 ;

<sup>300</sup> =0, 00104 .

<sup>300</sup> =0, 0049 ;

<sup>300</sup> =0, 0047 .

Due to the output power variations of the wind turbines, voltage fluctuations are produced. Voltage fluctuations are produced due to the wind turbine switching operations (start or stop), and due to the continuous operation. The presented voltage fluctuations study, made for one turbine, becomes necessary in large wind farms as the wind power penetration level increases

The variability nature of solar radiation, the weather changes or passing clouds can cause important variation of PV output power [8]. The variation of the power produced by a 30kW PV system is illustrated in Fig. 3. Fig. 4 (a) shows the generation power in a sunny day, while

The connection of these variable renewable sources can determine a voltage rise at PCC and in the grid. The utility has the general obligation to ensure that customer voltages are kept within prescribed limits. A voltage variation Δ*V* between *Vmax* and *Vmin*, can appear on short periods. This voltage variation can highly stress the electrical devices supplied by the power system, and in particular the owner of the photovoltaic facility (as shown in Fig. 5). Fig. 5 illustrates the possible case of a summer mid-day, when the load downstream PCC is relatively

medium voltage network (ψsc=50°, *Ssc* = 300 MVA, Sr = 0.65MVA)

<sup>300</sup> <sup>⋅</sup>3=0.0065

**2.** generator interconnection at rated speed of the wind turbine

**2.** generator interconnection at minimum speed of the wind turbine

=18⋅30,31 <sup>⋅</sup>0, 02<sup>⋅</sup> 0.65

=8⋅350,31 <sup>⋅</sup>0, 02<sup>⋅</sup> 0.65

=18⋅30,31 <sup>⋅</sup>0, 09<sup>⋅</sup> 0.65

=8⋅350,31 <sup>⋅</sup>0, 09<sup>⋅</sup> 0.65

indicator values:

*Pst* <sup>=</sup>*Plt* <sup>=</sup> *Sr*

*Pst* =18⋅ *N*<sup>10</sup>

*Plt* =8⋅ *N*<sup>120</sup>

*Pst* =18⋅ *N*<sup>10</sup>

*Plt* =8⋅ *N*<sup>120</sup>

quickly.

*Ssc*

<sup>⋅</sup> *<sup>c</sup>*(ψ*sc*, *va*) <sup>=</sup> 0.65

*Sr Ssc*

*Sr Ssc*

> *Sr Ssc*

*Sr Ssc*

**2.2. PV impact on steady state voltage variations**

Fig. 4 (b) illustrates the generation power in a cloudy day.

small and the PV output power exceeds the demand.

0,31 <sup>⋅</sup> *kf* (ψ*sc*) <sup>⋅</sup>

0,31 <sup>⋅</sup> *kf* (ψ*sc*) <sup>⋅</sup>

0,31 <sup>⋅</sup> *kf* (ψ*sc*) <sup>⋅</sup>

0,31 <sup>⋅</sup> *kf* (ψ*sc*) <sup>⋅</sup>

The measured values of the flicker coefficient *c*(ψsc, υ*a*) for different values of the annual average wind speed υ*a* and for different network impedance angle ψsc are reported in Table 2. Table 3 reports the flicker coefficient *kf* values for voltage step variations, for the same wind generator.

**Figure 2.** Variation of the wind generator output power due to the tower shadow effect.


**Table 2.** Values of the flicker factor for various values of the wind speed *va* and for various angles ψsc


**Table 3.** Values of the flicker factor *kf*

The computations based on the values reported in Table 2 and Table 3 lead to the flicker indicator values:

**i.** continuous operation, annual average wind speed υ*a*=7.5, interconnection with the medium voltage network (ψsc=50°, *Ssc* = 300 MVA, Sr = 0.65MVA)

$$P\_{st} = P\_{lt} = \frac{S\_r}{S\_{sc}} \cdot c \left(\psi\_{sc\prime} \cdot \upsilon\_a\right) = \frac{0.65}{300} \cdot 3 = 0.0065$$

shadow effect for the wind generator determines a variation of the absorbed energy, which is measured as a power variation at generator terminals. Fig. 2(a) shows the wind generator, while Fig. 2(b) illustrates the tower shadow effect corresponding variation of the generator

The measured values of the flicker coefficient *c*(ψsc, υ*a*) for different values of the annual average wind speed υ*a* and for different network impedance angle ψsc are reported in Table 2. Table

(a) (b)

**Table 2.** Values of the flicker factor for various values of the wind speed *va* and for various angles ψsc

30º 50º 70º 85º

Installation is sized for *N* 10 = 3; *N* 120=35

**Annual wind speed va [m/s] Network impedance angle ψsc [°]** 30º 50º 70º 85º

> 6 3.1 2.9 3.6 4.0 7.5 3.1 3.0 3.8 4.2 8.5 3.1 3.0 3.8 4.2 10 3.1 3.1 3.8 4.2

> > With start at minimum speed 0.02 0.02 0.01 0.01 With start at rated speed 0.12 0.09 0.06 0.06

*P* [u.r.]

**Figure 2.** Variation of the wind generator output power due to the tower shadow effect.

 0.99 0.98 0.97 0.96 0.95 0.94

1

values for voltage step variations, for the same wind generator.

0 1 *t* [s] 2

**Network impedance angle ψsc [°]**

output power.

96 Power Quality Issues

Flicker factor *kf*

for voltage step

variations

**Table 3.** Values of the flicker factor *kf*

3 reports the flicker coefficient *kf*

**2.** generator interconnection at minimum speed of the wind turbine

$$P\_{st} = 18 \cdot N\_{10}^{0.31} \cdot k\_f \left(\psi\_{sc}\right) \cdot \frac{S\_r}{S\_{sc}} = 18 \cdot 3^{0.31} \cdot 0,\ 02 \cdot \frac{0.65}{300} = 0,\ 00109;$$

$$P\_{lt} = 8 \cdot N\_{120}^{0.31} \cdot k\_f \left(\psi\_{sc}\right) \cdot \frac{S\_r}{S\_{sc}} = 8 \cdot 35^{0.31} \cdot 0,\ 02 \cdot \frac{0.65}{300} = 0,\ 00104.$$

**2.** generator interconnection at rated speed of the wind turbine

$$P\_{st} = 18 \cdot N\_{10}^{0.31} \cdot k\_f \text{(}\psi\_{sc}\text{)} \cdot \frac{S\_r}{S\_{sc}} = 18 \cdot 3^{0.31} \cdot 0,\ 09 \cdot \frac{0.65}{300} = 0,\ 0049\text{;}\ 1$$

$$P\_{lt} = 8 \cdot N\_{120}^{0.31} \cdot k\_f \text{(}\psi\_{sc}\text{)} \cdot \frac{S\_r}{S\_{sc}} = 8 \cdot 35^{0.31} \cdot 0,\ 09 \cdot \frac{0.65}{300} = 0,\ 0047\text{.}$$

Due to the output power variations of the wind turbines, voltage fluctuations are produced. Voltage fluctuations are produced due to the wind turbine switching operations (start or stop), and due to the continuous operation. The presented voltage fluctuations study, made for one turbine, becomes necessary in large wind farms as the wind power penetration level increases quickly.

### **2.2. PV impact on steady state voltage variations**

The variability nature of solar radiation, the weather changes or passing clouds can cause important variation of PV output power [8]. The variation of the power produced by a 30kW PV system is illustrated in Fig. 3. Fig. 4 (a) shows the generation power in a sunny day, while Fig. 4 (b) illustrates the generation power in a cloudy day.

The connection of these variable renewable sources can determine a voltage rise at PCC and in the grid. The utility has the general obligation to ensure that customer voltages are kept within prescribed limits. A voltage variation Δ*V* between *Vmax* and *Vmin*, can appear on short periods. This voltage variation can highly stress the electrical devices supplied by the power system, and in particular the owner of the photovoltaic facility (as shown in Fig. 5). Fig. 5 illustrates the possible case of a summer mid-day, when the load downstream PCC is relatively small and the PV output power exceeds the demand.

Figure 3. Variation of PV system power output during a month **Figure 3.** Variation of PV system power output during a month

the load downstream PCC is relatively small and the PV output power exceeds the demand.

The connection of these variable renewable sources can determine a voltage rise at PCC and in the grid. The utility has the general

*V*

[p.u.]

20/0. 4 kV PCC

length

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99

PV

*V*max

⋅ cos(ψ*sc* −φ) (4)

LV line End users LV line

Monitoring Power Quality in Small Scale Renewable Energy Sources Supplying Distribution Systems

*V*min

with PV

Voltage variations can influence the characteristics of the electrical equipment and household appliances (loss of the guaranteed performances, modifications of the efficiency) leading in some cases even to the interruption of operation. The voltage variation at PCC can be expressed

where *SPV* is the power produced by PV, Ssc is the short circuit power at PCC, ψsc = arctan(X/R) is the angle of the network short circuit impedance, φ is the phase angle of the PV output current (we consider that the electric quantities are sinusoidal). In existing power systems, there are measures such that the line voltage to be sinusoidal. In (4), the system

At low and medium voltage levels, the utilities have established limits for the amplitude of the voltage variations, which must not be exceeded during normal operation. Due to the statistical nature of the steady state voltage variations, the standard EN 50160 stipulates statistical limits [9]. In some Countries the limit ±10% established in EN 50160 is applied, while other present guidelines, elaborated in different Countries, impose more restrictive limits for the voltage variations. The relevant variations of the voltage will overlap the voltage's variations caused by load modification and can lead to the widening of the voltage limit bands.

without PV

*ΔV* =

*SPV Ssc*

1.1

1

**Figure 5.** Influence of PV on voltage level

harmonics are not considered.

as:

90.

appear on short periods. This voltage variation can highly stress the electrical devices supplied by the power system, and in particular the owner of the photovoltaic facility (as shown in Fig. 5). Fig. 5 illustrates the possible case of a summer mid-day, when

Voltage variations can influence the characteristics of the electrical equipment and household appliances (loss of the guaranteed performances, modifications of the efficiency) leading in some cases even to the interruption of operation. The voltage variation at

obligation to ensure that customer voltages are kept within prescribed limits. A voltage variation Δ*V* between *Vmax* and *Vmin*, can **Figure 4.** Variation of PV system power output during: (a) sunny day, (b) cloudy day

Figure 5. Influence of PV on voltage level

PCC can be expressed as:

Monitoring Power Quality in Small Scale Renewable Energy Sources Supplying Distribution Systems http://dx.doi.org/10.5772/53464 99

**Figure 5.** Influence of PV on voltage level

Figure 3. Variation of PV system power output during a month

**Figure 3.** Variation of PV system power output during a month

*P* [ Wk ] 25 20 15 10 5 0

98 Power Quality Issues

*P* [ Wk ] 25 20 15 10 5 0

30

30

Figure 4. Variation of PV system power output during: (a) sunny day, (b) cloudy day

**Figure 4.** Variation of PV system power output during: (a) sunny day, (b) cloudy day

Figure 5. Influence of PV on voltage level

PCC can be expressed as:

the load downstream PCC is relatively small and the PV output power exceeds the demand.

0 4 8 12 16 18 24 Time [h]

Voltage variations can influence the characteristics of the electrical equipment and household appliances (loss of the guaranteed performances, modifications of the efficiency) leading in some cases even to the interruption of operation. The voltage variation at

(a)

(b)

Voltage variations can influence the characteristics of the electrical equipment and household appliances (loss of the guaranteed performances, modifications of the efficiency) leading in some cases even to the interruption of operation. The voltage variation at PCC can be expressed as:

$$
\Delta V = \frac{\mathcal{S}\_{PV}}{\mathcal{S}\_{sc}} \cdot \cos(\psi\_{sc} - \varphi) \tag{4}
$$

where *SPV* is the power produced by PV, Ssc is the short circuit power at PCC, ψsc = arctan(X/R) is the angle of the network short circuit impedance, φ is the phase angle of the PV output current (we consider that the electric quantities are sinusoidal). In existing power systems, there are measures such that the line voltage to be sinusoidal. In (4), the system harmonics are not considered.

The connection of these variable renewable sources can determine a voltage rise at PCC and in the grid. The utility has the general obligation to ensure that customer voltages are kept within prescribed limits. A voltage variation Δ*V* between *Vmax* and *Vmin*, can appear on short periods. This voltage variation can highly stress the electrical devices supplied by the power system, and in particular the owner of the photovoltaic facility (as shown in Fig. 5). Fig. 5 illustrates the possible case of a summer mid-day, when At low and medium voltage levels, the utilities have established limits for the amplitude of the voltage variations, which must not be exceeded during normal operation. Due to the statistical nature of the steady state voltage variations, the standard EN 50160 stipulates statistical limits [9]. In some Countries the limit ±10% established in EN 50160 is applied, while other present guidelines, elaborated in different Countries, impose more restrictive limits for the voltage variations. The relevant variations of the voltage will overlap the voltage's variations caused by load modification and can lead to the widening of the voltage limit bands.

### **2.3. Current harmonic perturbations**

Measurement results of a 200 MW wind farm reveals the harmonic current and voltage spectra, active and reactive power variations, and the relationship between wind farm harmonic emission level and output power. It is considered that the harmonics are determined by the converter at the interconnection point with the main power system. In order to connect the PV power systems with the grid, an inverter that transforms the dc output power of the PV to the 50Hz ac power is required. The small capacity PV systems are interconnected to the main grid with the help of simple single-phase inverters, which can cause important current harmonics. General requirements can be found in standards, especially those for the interconnection of distributed generation systems to the grid and for photovoltaic systems [1, 10]. In the standard IEEE 1547, the harmonic current injection of RES at the PCC must not exceed the limits stated in table 4.

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**Fig. 6.** Variation of RMS current, phase a, during the monitoring period

**Fig. 6.** Variation of RMS current, phase a, during the monitoring period

0 5000 10000 15000 20000 25000

Monitoring Power Quality in Small Scale Renewable Energy Sources Supplying Distribution Systems

0 5000 10000 15000 20000 25000

Monitoring Power Quality In Small Scale Renewable Energy Sources Supplying Distribution Systems

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Samples

Samples

0

0

**Figure 6.** Variation of RMS current, phase a, during the monitoring period

**Figure 7.** Variation of *THDI* during the monitoring period

THDI [%]

5

6

7

8

9

4

0

1

2

3

THDI [%]

*P* [W]

*P* [W]

50

RMS

RMS

Current[A]

50

Current[A]

100

150

200

250

300

100

150

200

250

300

**Fig. 7.** Variation of *THDI* during the monitoring period

**Fig. 7.** Variation of *THDI* during the monitoring period

0 5000 10000 15000 20000 25000

0 5000 10000 15000 20000 25000

*THDI* 

[%]

*THDI* 

[%]

Samples

Samples

9

9


**Table 4.** Maximum harmonic current distortion in percent of current *I*, where *I* is the fundamental frequency current at full system output. Even harmonics in these ranges shall be <25% of the odd harmonic limits listed [1].

The current variation on phase *a*, during one week monitoring period of the 200 MW wind farm, is illustrated in Fig. 6. The high current variability function of wind speed can be clearly observed. The variation of total current harmonic distortion (*THDI*) during the monitoring period is illustrated in Fig. 7. The inverse relationship between the RMS electrical current and *THDI* can be seen in Fig. 6 and Fig. 7, at the same time instants. When the generated power is high (large value of RMS electrical current), the fundamental current is high and the *THDI* is small. For small generated powers, the fundamental current is small and the *THDI* is high. From practical point of view, this fact is not highly important as the small current values do not influence the voltage quality at point of common coupling.

Fig. 8 illustrates the output power variation of a PV system and the total current harmonic distortion variation during a day [11]. When the PV system has an output power close to the rated power, the *THDI* is relatively low. During the shadowing period of the PV system, the *THDI* is taking high values.

The analysis of the total harmonic distortion factor has to consider that, for high variability of the primary energy source, the large *THDI* values can lead to inappropriate conclusions. For the periods with small primary energy source, the electric current injected into the grid presents a reduced fundamental component, resulting in a high distortion factor. As the electrical current has small values, the voltage distortion and the voltage drop in the power system are negligible, and thus the voltage waveform at the point of common coupling is not affected.

<sup>100</sup> Power Quality Issues Monitoring Power Quality In Small Scale Renewable Energy Sources Supplying Distribution Systems http://dx.doi.org/10.5772/53464 http://dx.doi.org/10.5772/53464 Monitoring Power Quality in Small Scale Renewable Energy Sources Supplying Distribution Systems http://dx.doi.org/10.5772/53464 101

Monitoring Power Quality In Small Scale Renewable Energy Sources Supplying Distribution Systems

9

9

**Fig. 6.** Variation of RMS current, phase a, during the monitoring period

**Fig. 6.** Variation of RMS current, phase a, during the monitoring period **Figure 6.** Variation of RMS current, phase a, during the monitoring period

9

RMS

Current[A]

**2.3. Current harmonic perturbations**

in table 4.

Measurement results of a 200 MW wind farm reveals the harmonic current and voltage spectra, active and reactive power variations, and the relationship between wind farm harmonic emission level and output power. It is considered that the harmonics are determined by the converter at the interconnection point with the main power system. In order to connect the PV power systems with the grid, an inverter that transforms the dc output power of the PV to the 50Hz ac power is required. The small capacity PV systems are interconnected to the main grid with the help of simple single-phase inverters, which can cause important current harmonics. General requirements can be found in standards, especially those for the interconnection of distributed generation systems to the grid and for photovoltaic systems [1, 10]. In the standard IEEE 1547, the harmonic current injection of RES at the PCC must not exceed the limits stated

**Individual harmonic order h<11 11≤h<17 17≤h<23 23≤h<35 35≤h TDD**

**Table 4.** Maximum harmonic current distortion in percent of current *I*, where *I* is the fundamental frequency current

The current variation on phase *a*, during one week monitoring period of the 200 MW wind farm, is illustrated in Fig. 6. The high current variability function of wind speed can be clearly observed. The variation of total current harmonic distortion (*THDI*) during the monitoring period is illustrated in Fig. 7. The inverse relationship between the RMS electrical current and *THDI* can be seen in Fig. 6 and Fig. 7, at the same time instants. When the generated power is high (large value of RMS electrical current), the fundamental current is high and the *THDI* is small. For small generated powers, the fundamental current is small and the *THDI* is high. From practical point of view, this fact is not highly important as the small current values do

Fig. 8 illustrates the output power variation of a PV system and the total current harmonic distortion variation during a day [11]. When the PV system has an output power close to the rated power, the *THDI* is relatively low. During the shadowing period of the PV system, the

The analysis of the total harmonic distortion factor has to consider that, for high variability of the primary energy source, the large *THDI* values can lead to inappropriate conclusions. For the periods with small primary energy source, the electric current injected into the grid presents a reduced fundamental component, resulting in a high distortion factor. As the electrical current has small values, the voltage distortion and the voltage drop in the power system are negligible, and thus the voltage waveform at the point of common coupling is not

at full system output. Even harmonics in these ranges shall be <25% of the odd harmonic limits listed [1].

not influence the voltage quality at point of common coupling.

*THDI* is taking high values.

affected.

Percent (%) 4 2 1.5 0.6 0.3 5

*THDI* 

[%]

*THDI* 

[%]

**Fig. 7.** Variation of *THDI* during the monitoring period **Figure 7.** Variation of *THDI* during the monitoring period

*P* [W]

*P* [W]

**Author details**

Nicolae Golovanov1

**References**

tics.pdf

systems.

Standard 929-2000, April 2000

, George Cristian Lazaroiu1

2 Department of Design and Technology, University of Bergamo, Italy

3 Dipartimento di Energia, Politecnico di Milano, Italia

IEEE Standard 1547, 2003.

neers, London, 2000, pp. 49-85.

ty, New York: Mc.Graw-Hill, 1996, pp. 1-8.

tersburg, Russia, June 27-30, 2005, pp. 6

1 Department of Power Systems, University Politehnica of Bucharest, Romania

[1] IEEE Standard for interconnecting distributed resources with electric power systems,

[2] N. Jenkins and R. Allan, Embedded generation, The Institution of Electrical Engi‐

[3] R. C. Dugan, H. W. Beaty, and M. F. McGranagham, Electrical power systems quali‐

[4] European Wind Energy Association [Online]. Available: http://www.ewea.org/filead‐ min/files/library/publications/statistics/Wind\_in\_power\_2011\_European\_statis‐

[5] F. Gagliardi, F. Iannone, G.C. Lazaroiu, M. Roscia, D. Zaninelli, Sustainable building for the quality of power supply, in Proc. IEEE PowerTech Conference 2005, Sankt Pe‐

[6] IEC 61400-21, Wind turbine generator systems - Measurement and assessment of

[7] IEC 61000-3-7, Assessment of emission limits for the connection of fluctuating load

[8] M. Simonov, M. Mussetta, F. Grimaccia, S. Leva, R. E. Zich, "Artificial Intelligence forecast of PV plant production for integration in smart energy systems," Interna‐

[9] EN 50160/2010, Voltage characteristics of electricity supplied by public distribution

[10] IEEE Recommended Practice for Utility Interface of Photovoltaic (PV) Systems, IEEE

power quality characteristics of grid connected wind turbines, 2001

tional Review of Electrical Engineering, vol. 7, no. 1, pp. 3454-3460

installation to MV, HV and EHV power systems, 2008.

, Mariacristina Roscia2

Monitoring Power Quality in Small Scale Renewable Energy Sources Supplying Distribution Systems

and Dario Zaninelli3

http://dx.doi.org/10.5772/53464

103

**Figure 8.** Variation of PV system output power and variation of THDI, during a day.

### **3. Conclusions**

The renewable sources interconnected with the main supply can influence the power quality at the point of common coupling and can pollute the electrical network with harmonic components that must not exceed the stipulated limits. The existing trend of installing more and more small capacity sources implies the establishment as accurate as possible of their impact on power system operation.

The voltage fluctuations determined wind power variations are analyzed, both for the wind turbine switching operations (start or stop), as well as for the continuous operation. The voltage flicker study becomes necessary as the wind power penetration level increases quickly. The connection of variable renewable sources, like photovoltaic systems, can determine a voltage rise at PCC and in the grid which can affect the electrical characteristics of the equipments.

The wind generators and photovoltaic sources, connected to the power system through power electronic converters, can pollute the electrical network with harmonic components that must not exceed the stipulated limits. A better characterization, from the practical point of view, of the total current harmonic distortion determined by renewable energy sources interconnected to the mains supply through power electronic converters is necessary.

### **Author details**

Nicolae Golovanov1 , George Cristian Lazaroiu1 , Mariacristina Roscia2 and Dario Zaninelli3


### **References**

*THDI*

*THDI* [%]

20 0

40

60

80

100

Output power

**Figure 8.** Variation of PV system output power and variation of THDI, during a day.

Time [h] 0 4 8 12 16 18 24

The renewable sources interconnected with the main supply can influence the power quality at the point of common coupling and can pollute the electrical network with harmonic components that must not exceed the stipulated limits. The existing trend of installing more and more small capacity sources implies the establishment as accurate as possible of their

The voltage fluctuations determined wind power variations are analyzed, both for the wind turbine switching operations (start or stop), as well as for the continuous operation. The voltage flicker study becomes necessary as the wind power penetration level increases quickly. The connection of variable renewable sources, like photovoltaic systems, can determine a voltage rise at PCC and in the grid which can affect the electrical characteristics of the equipments.

The wind generators and photovoltaic sources, connected to the power system through power electronic converters, can pollute the electrical network with harmonic components that must not exceed the stipulated limits. A better characterization, from the practical point of view, of the total current harmonic distortion determined by renewable energy sources interconnected

to the mains supply through power electronic converters is necessary.

*P*

200

400

600

800

1000

102 Power Quality Issues

0

**3. Conclusions**

impact on power system operation.

[W]


[11] G. Chicco, J. Schlabbach, F. Spertino, Characterisation and Assessment of the Har‐ monic Emission of Grid-Connected Photovoltaic Systems, in Proc. IEEE Power Tech 2005, St. Petersburg, Russia, June 27-30, 2005, pp.7

**Chapter 5**

L1) contains harmonics. The har‐

*<sup>S</sup>* ) produce a non-linear voltage drop (∆*v*) in the line impedance,

© 2013 Afonso et al.; licensee InTech. This is an open access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use,

© 2013 Afonso et al.; licensee InTech. This is a paper 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.

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

**Active Power Conditioners to Mitigate Power Quality**

Non-linear loads are commonly present in industrial facilities, service facilities, office build‐ ings, and even in our homes. They are the source of several Power Quality problems such as harmonics, reactive power, flicker and resonance [1-3]. Therefore, it can be observed an in‐ creasing deterioration of the electrical power grid voltage and current waveforms, mainly due to the contamination of the system currents with harmonics of various orders, including inter-harmonics. Harmonic currents circulating through the line impedance produces distor‐ tion in the system voltages (see Figure 1). Moreover, since many of the loads connected to the electrical systems are single-phase ones, voltage unbalance is also very common in threephase power systems [2]. The distortion and unbalance of the system voltages causes several power quality problems, including the incorrect operation of some sensitive loads [4,5]. Fig‐ ure 1 presents a power system with sinusoidal source voltage (*vS* ) operating with a linear

which distorts the load voltage (*vL* ). Since the load voltage is distorted, even the current at

The problems caused by the presence of harmonics in the power lines can be classified into two kinds: instantaneous effects and long-term effects. The instantaneous effects problems are associated with interference problems in communication systems, malfunction or per‐ formance degradation of more sensitive equipment and devices. Long-term effects are of

**Problems in Industrial Facilities**

Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/53189

**1. Introduction**

monics in the line-current (*i*

the linear load (*i*

João L. Afonso, J. G. Pinto and Henrique Gonçalves

and a non-linear load. The current of the non-linear load (*i*

L2) becomes non-sinusoidal.
