**2. Need of power quality studies**

The power quality studies are of importance to wind turbine as a individual units can be large up to 5 MW, feeding into distribution circuit with high source impedance and with customer connected in close proximity.

With the advancement in fast switching power devices there is a trend for power supply size reduction. The current harmonics due to switching converters makes supply current distorted. The increase of electronic controllers in drives, furnaces, household equipments and SMPS are increasing the harmonic content and reactive power in electric supply. The distribution transformers apart from reactive loads draw reactive current from the supply to meet the magnetizing current. The ever-increasing demand for power is not fulfilled by increase in generation and particularly in distribution for various reasons such as environmental issues, increasing cost of natural fuel, opposition to nuclear power plants, etc. This puts excessive burden on the electric supply resulting in poor power quality. The term power quality here refers to the variation in supply voltage, current and frequency. The excessive load demand tries to retard the turbines at generation plant. This results in reduction in voltage and more severely reduction in the supply frequency. The authorities are working for power quality improvement by using reactive compensators and active filters on supply side and penalizing consumers for polluting the power grid.

The increasing problems and advances in power electronic technology, has forced to change the traditional power system concepts. Use of fast reactive power compensators can improve the power system stability and hence, the maximum power transfers through the electric system.

The reactive power in its simpler form, for a single phase sinusoidal voltages and current can be defines as the product of a phase current (reactive component) and the supply voltage. There is a simple right angle triangle relation between active power, reactive power and apparent power. But, this definition of the reactive power is not sufficient for non-linear loads where fundamental current and fundamental voltage may not have any phase difference. However, for such loads, power factor is still less than unity. The power factor definition is modified to accommodate for non-linear loads.

The overall power factor has two parts, the displacement power factor and distortion power factor. The displacement power factor defined as cosine of phase shift between fundamental supply current and voltage.

Distortion power factor "DF" or harmonic factor is defined as the ratio of the RMS harmonic content to the RMS value of fundamental component expressed as percentage of the fundamental.

$$DF = \sqrt{\frac{\text{sum of squares of amplitudes of all harmonics}}{\text{square of amplitude of fundamental}}} \text{\*} 100\% \tag{1}$$

$$DF(for \text{ current}) = \frac{\sqrt{\frac{\sum\_{i}^{2} I\_{i}^{2}}{I\_{1}}}}{I\_{1}} \tag{2}$$

#### **2.1. Issue of voltage variation**

24 An Update on Power Quality

further section.

electric system.

supply current and voltage.

fundamental.

prepared by IEC- Technical Committee 88.

**2. Need of power quality studies** 

customer connected in close proximity.

Today the measurement and assessment of the power quality characteristics of the gridconnected wind turbines is defined by IEC Standard 61400-21 (wind turbine system)

The need of power quality in wind integration system and its issues are highlighted in

The power quality studies are of importance to wind turbine as a individual units can be large up to 5 MW, feeding into distribution circuit with high source impedance and with

With the advancement in fast switching power devices there is a trend for power supply size reduction. The current harmonics due to switching converters makes supply current distorted. The increase of electronic controllers in drives, furnaces, household equipments and SMPS are increasing the harmonic content and reactive power in electric supply. The distribution transformers apart from reactive loads draw reactive current from the supply to meet the magnetizing current. The ever-increasing demand for power is not fulfilled by increase in generation and particularly in distribution for various reasons such as environmental issues, increasing cost of natural fuel, opposition to nuclear power plants, etc. This puts excessive burden on the electric supply resulting in poor power quality. The term power quality here refers to the variation in supply voltage, current and frequency. The excessive load demand tries to retard the turbines at generation plant. This results in reduction in voltage and more severely reduction in the supply frequency. The authorities are working for power quality improvement by using reactive compensators and active

filters on supply side and penalizing consumers for polluting the power grid.

definition is modified to accommodate for non-linear loads.

The increasing problems and advances in power electronic technology, has forced to change the traditional power system concepts. Use of fast reactive power compensators can improve the power system stability and hence, the maximum power transfers through the

The reactive power in its simpler form, for a single phase sinusoidal voltages and current can be defines as the product of a phase current (reactive component) and the supply voltage. There is a simple right angle triangle relation between active power, reactive power and apparent power. But, this definition of the reactive power is not sufficient for non-linear loads where fundamental current and fundamental voltage may not have any phase difference. However, for such loads, power factor is still less than unity. The power factor

The overall power factor has two parts, the displacement power factor and distortion power factor. The displacement power factor defined as cosine of phase shift between fundamental

Distortion power factor "DF" or harmonic factor is defined as the ratio of the RMS harmonic content to the RMS value of fundamental component expressed as percentage of the If a large proportion of the grid load is supplied by wind turbines, the output variations due to wind speed changes can cause voltage variation, flicker effects in normal operation. The voltage variation can occur in specific situation, as a result of load changes, and power produce from turbine. These can expected in particular in the case of generator connected to the grid at fixed speed. The large turbine can achieve significantly better output smoothing using variable speed operation, particularly in the short time range. The speed regulation range is also contributory factor to the degree of smoothing with the large speed variation capable of suppressing output variations.

#### **2.2. Issue of voltage dips**

It is a sudden reduction in the voltage to a value between 1% & 90 % of the nominal value after a short period of time, conventionally 1ms to 1 min. This problem is considered in the power quality and wind turbine generating system operation and computed according to the rule given in IEC 61400-3-7 standard, "Assessment of emission limit for fluctuating load". The start up of wind turbine causes a sudden reduction of voltage. The relative % voltage change due to switching operation of wind turbine is calculated as

$$d = 100K\_u(\Psi\_k)\frac{\mathcal{S}\_n}{\mathcal{S}\_k^\*}\tag{3}$$

Where d - Relative voltage change,

( ) *u k k* - Voltage change factor,

*<sup>n</sup> <sup>S</sup>* - Rated apparent power of wind turbine and \* *<sup>k</sup> S* short circuit apparent power of grid. The voltage dips of 3% in most of the cases are acceptable. When evaluating flicker and power variation within 95% of maximum variation band corresponding to a standard deviation are evaluated.

#### **2.3. Switching operation of wind turbine on the grid**

Switching operations of wind turbine generating system can cause voltage fluctuations and thus voltage sag, voltage swell that may cause significant voltage variation. The acceptances of switching operation depend not only on grid voltage but also on how often this may occur. The maximum number of above specified switching operation within 10-minute period and 2-hr period are defined in IEC 61400-3-7 Standard.

Power Quality and Grid Code Issues in Wind Energy Conversion System 27

(4)

(5)

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 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

*Vh VTHD <sup>h</sup> <sup>V</sup>*

*Vh*- hth harmonic voltage and *V1* –fundamental frequency 50 Hz. The THD limit for various

System Voltage (kV) Total Harmonic Distortion (%) 400 2.0 220 2.5 132 3.0

2 40

*h I THD <sup>I</sup> <sup>h</sup>*

Voltage level 66 kV 132kV ITHD 5.0 2.5

Where *Ih* - hth harmonic current and *I1* –fundamental frequency (50) Hz. The acceptable level

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

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

2 1

*I*

100

2 40

2 1

100

the harmonics.

voltage is given as in (4).

**Table 1.** Voltage Harmonics Limit

THD of current ITHD is give as in (5)

of THD in the current is given in table 2.

**Table 2.** Current Harmonic Limit

supply and the consumer.

supply voltage lines.

level of system voltages are given in the table 1.0

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

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 standstill.
