**2.5. Flickers**

Flicker is the one of the important power quality aspects in wind turbine generating system. Flicker has widely been considered as a serious drawback and may limit for the maximum amount of wind power generation that can be connected to the grid. Flicker is induced by voltage fluctuations, which are caused by load flow changes in the grid. The flicker emission produced by grid-connected variable-speed wind turbines with full-scale back-to-back converters during continuous operation and mainly caused by fluctuations in the output power due to wind speed variations, the wind shear, and the tower shadow effects. The wind shear and the tower shadow effects are normally referred to as the 3p oscillations. As a consequence, an output power drop will appear three times per revolution for a threebladed wind turbine. There are many factors that affect flicker emission of grid connected wind turbines during continuous operation, such as wind characteristics and grid conditions. Variable-speed wind turbines have shown better performance related to flicker emission in comparison with fixed-speed wind turbines.

The flicker study becomes necessary and important as the wind power penetration level increases quickly. The main reason for the flicker in fixed speed turbines is to wake of the tower. Each time a rotor blade passes the tower, the power output of the turbine is reduced. This effect cause periodical power fluctuations with a frequency of about ~1 Hz. The power fluctuation due to the wind speed fluctuation has lower frequencies and thus is less critical for flicker. In general, the flicker of fixed speed turbines reaches its maximum at high wind speed. Owing to smoothing effect, large wind turbine produced lower flicker than small wind turbines, in relation to their size.

Several solutions have been proposed to mitigate the flicker caused by grid-connected wind turbines. The mostly adopted technique is the reactive power compensation. It can be realized by the grid-side converter of variable-speed wind turbines or the Static synchronous compensator connected at the point of common coupling (PCC). Also, some papers focus on the use of active power curtailment to mitigate the flicker [5].

The flicker level depends on the amplitude, shape and repetition frequency of the fluctuated voltage waveform. Evaluating the flicker level is based on the flicker meter described in IEC 61000-4-15. Two indices are typically used as a scale for flicker emission, short-term flicker index, Pst and long-term flicker index, Plt. Plt is estimated by certain process of the Pst values.

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 described in Figure 2.0.

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

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

$$P\_{lt} = \mathbf{C}(\boldsymbol{\Psi}\_K) \frac{S\_{\mathcal{U}}}{S\_K} \tag{6}$$

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 average time of 2 hours.

#### **2.6. Reactive power**

28 An Update on Power Quality

**2.5. Flickers** 

load in supply transformer.

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

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

Flicker is the one of the important power quality aspects in wind turbine generating system. Flicker has widely been considered as a serious drawback and may limit for the maximum amount of wind power generation that can be connected to the grid. Flicker is induced by voltage fluctuations, which are caused by load flow changes in the grid. The flicker emission produced by grid-connected variable-speed wind turbines with full-scale back-to-back converters during continuous operation and mainly caused by fluctuations in the output power due to wind speed variations, the wind shear, and the tower shadow effects. The wind shear and the tower shadow effects are normally referred to as the 3p oscillations. As a consequence, an output power drop will appear three times per revolution for a threebladed wind turbine. There are many factors that affect flicker emission of grid connected wind turbines during continuous operation, such as wind characteristics and grid conditions. Variable-speed wind turbines have shown better performance related to flicker

The flicker study becomes necessary and important as the wind power penetration level increases quickly. The main reason for the flicker in fixed speed turbines is to wake of the tower. Each time a rotor blade passes the tower, the power output of the turbine is reduced. This effect cause periodical power fluctuations with a frequency of about ~1 Hz. The power fluctuation due to the wind speed fluctuation has lower frequencies and thus is less critical for flicker. In general, the flicker of fixed speed turbines reaches its maximum at high wind speed. Owing to smoothing effect, large wind turbine produced lower flicker than small

Several solutions have been proposed to mitigate the flicker caused by grid-connected wind turbines. The mostly adopted technique is the reactive power compensation. It can be realized by the grid-side converter of variable-speed wind turbines or the Static synchronous compensator connected at the point of common coupling (PCC). Also, some

The flicker level depends on the amplitude, shape and repetition frequency of the fluctuated voltage waveform. Evaluating the flicker level is based on the flicker meter described in IEC 61000-4-15. Two indices are typically used as a scale for flicker emission, short-term flicker index, Pst and long-term flicker index, Plt. Plt is estimated by certain process of the Pst values.

papers focus on the use of active power curtailment to mitigate the flicker [5].

forms of active rectification (active wave shaping) or active filtering.

emission in comparison with fixed-speed wind turbines.

wind turbines, in relation to their size.

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 during off grid operation. Thus the sensitive equipments may be subjected to over/under voltage, over/under frequency operation and other disadvantage of safety aspect. According to IEC Standard, reactive power of wind turbine is to be specified as 10 min average value as a function of 10-min. output power for 10%, 20% … 100% of rated power. The effective control of reactive power can improve the power quality and stabilize the grid. Although reactive power is unable to provide actual working benefit, it is often used to adjust voltage, so it is a useful tool for maintaining desired voltage level. Every transmission system always has a reactive component, which can be expressed as power factor. Thus the some method is needed to manage the reactive power by injecting or absorbing VAr as necessary in order to maintain optimum voltage level and enable real power flow. Until recently, this has been especially difficult to effectively accomplish at a wind farms due to the variable nature of wind*.* The suggested control technique in the thesis is capable of controlling reactive power to zero value at point of common connection (PCC).The mode of operation is referred as unity power factor.

Power Quality and Grid Code Issues in Wind Energy Conversion System 31

**Loads**

Lower value of voltage band

**WTGS**

**Loads Loads**

**LV LV**

**PCC**

**1**

The impact of the wind generation on the power system will no longer be negligible if high penetration levels are going to be reached. The extent to which wind power can be integrated into the power system without affecting the overall stable operation depends on the technology available to mitigate the possible negative impacts such as loss of generation for frequency support, voltage flicker, voltage and power variation due to the variable speed

0 150 700 1500 3000

msec

**MV**

**HV**

**Figure 3.** WTGS coupled to MV transmission-line.

**2.8. Low voltage ride through capability** 

15

Fault Ocuured

**Figure 4.** Low voltage ride through (LVRT) capability

30

45

60

75

90 100

Line-to-Line Voltage %
