**2.4. Harmonics**

The harmonics distortion caused by non-linear load such as electric arc furnaces, variable speed drives, large concentrations of arc discharge lamps, saturation of magnetization of transformer and a distorted line current. The current generated by such load interact with power system impedance and gives rise to harmonics. The effect of harmonics in the power system can lead to degradation of power quality at the consumer's terminal, increase of power losses, and malfunction in communication system. The degree of variation is assessed at the point of common connection, where consumer and supplier area of responsibility meet. The harmonics voltage and current should be limited to acceptable level at the point of wind turbine connection in the system. This fact has lead to more stringent requirements regarding power quality, such as Standard IEC 61000-3-2 or IEEE-519. Conventionally, passive LC resonant filters have been used to solve power quality problems. However, these filters have the demerits of fixed compensation, large size, and the resonance itself. To overcome these drawbacks, active filters appear as the dynamic solution.

The IEC 61000-3-6 gives a guideline and harmonic current limits. According to standard IEC 61400-21 guideline, harmonic measurements are not required for fixed speed wind turbines where the induction generator is directly connected to grid. Harmonic measurements are required only for variable speed turbines equipped with electronic power converters. In general the power converters of wind turbines are pulse-width modulated inverters, which have carrier frequencies in the range of 2-3 kHz and produce mainly inter harmonic currents.

The harmonic measurement at the wind turbine is problem due to the influence of the already existing harmonic voltage in the grid. The wave shape of the grid voltage is not sinusoidal. There are always harmonics voltages in the grid such as integer harmonic of 5th and 7th order which affect the measurements.

Today's variable speed turbines are equipped with self commutated PWM inverter system. This type of inverter system has advantage that both the active and reactive power can be controlled, but it also produced a harmonic current. Therefore filters are necessary to reduce the harmonics.

The harmonic distortion is assessed for variable speed turbine with a electronic power converter at the point of common connection. The total harmonic voltage distortion of voltage is given as in (4).

$$V\_{THD} = \sqrt{\frac{40}{\sum\_{h=2}^{} V\_{\mathbf{1}}^2} 100} \tag{4}$$

*Vh*- hth harmonic voltage and *V1* –fundamental frequency 50 Hz. The THD limit for various level of system voltages are given in the table 1.0


**Table 1.** Voltage Harmonics Limit

26 An Update on Power Quality

standstill.

currents.

and 7th order which affect the measurements.

**2.4. Harmonics** 

occur. The maximum number of above specified switching operation within 10-minute

Voltage sag is a phenomenon in which grid voltage amplitude goes below and then returns to the normal level after a very short time period. Generally, the characteristic quantity of voltage sag is described by the amplitude and the duration of the sags. The IEEE power quality standards define the voltage sag when the amplitude of voltage is 0.1–0.9 p.u. value and its duration is between 10 ms and 1 min. A voltage sag is normally caused by shortcircuit faults in the power network or by the starting up of Induction Generator/Motors.

The bad weather conditions, such as thunderstorm, single-phase earthed faults are the causes of voltage sags. In addition, large electric loads such as large electrical motors or arc furnaces

The adverse consequences are the reduction in the energy transfer of electric motors. The disconnection of sensitive equipments and thus the industrial process may bring to a

The harmonics distortion caused by non-linear load such as electric arc furnaces, variable speed drives, large concentrations of arc discharge lamps, saturation of magnetization of transformer and a distorted line current. The current generated by such load interact with power system impedance and gives rise to harmonics. The effect of harmonics in the power system can lead to degradation of power quality at the consumer's terminal, increase of power losses, and malfunction in communication system. The degree of variation is assessed at the point of common connection, where consumer and supplier area of responsibility meet. The harmonics voltage and current should be limited to acceptable level at the point of wind turbine connection in the system. This fact has lead to more stringent requirements regarding power quality, such as Standard IEC 61000-3-2 or IEEE-519. Conventionally, passive LC resonant filters have been used to solve power quality problems. However, these filters have the demerits of fixed compensation, large size, and the resonance itself. To

The IEC 61000-3-6 gives a guideline and harmonic current limits. According to standard IEC 61400-21 guideline, harmonic measurements are not required for fixed speed wind turbines where the induction generator is directly connected to grid. Harmonic measurements are required only for variable speed turbines equipped with electronic power converters. In general the power converters of wind turbines are pulse-width modulated inverters, which have carrier frequencies in the range of 2-3 kHz and produce mainly inter harmonic

The harmonic measurement at the wind turbine is problem due to the influence of the already existing harmonic voltage in the grid. The wave shape of the grid voltage is not sinusoidal. There are always harmonics voltages in the grid such as integer harmonic of 5th

can also cause voltage sags during the startup phase with serious current distortion.

overcome these drawbacks, active filters appear as the dynamic solution.

period and 2-hr period are defined in IEC 61400-3-7 Standard.

THD of current ITHD is give as in (5)

$$I\_{THD} = \sqrt{\sum\_{h=2}^{40} \frac{I\_h^2}{I\_1} 100} \tag{5}$$

Where *Ih* - hth harmonic current and *I1* –fundamental frequency (50) Hz. The acceptable level of THD in the current is given in table 2.


**Table 2.** Current Harmonic Limit

Various standards are also recommended for individual consumer and utility system for helping to design the system to improve the power quality. The characteristics of the load and level of power system significantly decides the effects of harmonics. IEEE standards are adapted in most of the countries. The recommended practice helps designer to limit current and voltage distortion to acceptable limits at point of common coupling (PCC) between supply and the consumer.

1. IEEE standard 519 issued in 1981, recommends voltage distortion less than 5% on power lines below 69 kV. Lower voltage harmonic levels are recommended on higher supply voltage lines.

2. IEEE standard 519 was revised in 1992, and impose 5% voltage distortion limit. The standards also give guidelines on notch depth and telephone interface considerations.

Power Quality and Grid Code Issues in Wind Energy Conversion System 29

It is assumed that wind turbines under study is running at normal operation; hence, the long-term flicker index (Plt), which is based on a 120-min time interval, is equal to Pst and, therefore, Pst is only considered in this work. The normalized response of the flicker meter

1 10 100

(6)

Hz

A quite small voltage fluctuation at certain frequency (8.8 Hz) can be irritable. The flicker level (Pst ≤ 1) is a threshold level for connecting wind turbines to low voltage. The measurements are made for maximum number of specified switching operation of wind

( ) *Sn P C lt <sup>K</sup> SK*

Where *lt p* - Long term flicker. ( ) *C <sup>K</sup>* **-** Flicker coefficient calculated from Rayleigh distribution of the wind speed. The Limiting Value for flicker coefficient is about 0.4, for

Traditional wind turbines are equipped with induction generators. Induction generator is preferred because they are inexpensive, rugged and requires little maintenance. Unfortunately induction generators require reactive power from the grid to operate. The interactions between wind turbine and power system network are important aspect of wind generation system. When wind turbine is equipped with an induction generator and fixed capacitor are used for reactive compensation then the risk of self excitation may occur

2

**Figure 2.** Influence of frequency on the perceptibility of sinusoidal voltage change

turbine with 10-minutes period and 2-hour period are specified, as given in (6)

described in Figure 2.0.

average time of 2 hours.

**2.6. Reactive power** 

1

0.1

10

Voltage fluctuation %

3. ANSI/IEEE Standard C57.12.00 and C57.12.01 limits the current distortion to 5% at full load in supply transformer.

In order to keep power quality under limit to a standards it is necessary to include some of the compensator. Modern solutions for active power factor correction can be found in the forms of active rectification (active wave shaping) or active filtering.
