**2.7. Location of wind turbine**

The way of connecting wind turbine into the electric power system highly influences the impact of the wind turbine generating system on the power quality. As a rule, the impact on power quality at the consumer's terminal for the wind turbine generating system (WTGS) located close to the load is higher than WTGS connected away, that is connected to H.V. or EHV system.

Wind turbine generator systems (WTGS) are often located in the regions that have favorable wind conditions and where their location is not burdensome. These regions are low urbanized, which means that the distribution network in these regions is usually weak developed. Such situation is typical for all countries developing a wind power industry.

The point of common coupling (PCC) of the WTGS and the power network parameter and structure of grid is of essential significance in the operation of WTGS and its influence on the system. WTGS can be connected to MV transmission line and to HV networks.

The WTGS connected to the existing MV transmission line, which feeds the existing customers is presented in Figure 3.

The distance between WTGS and PCC is usually small up to a few kilometers. Such connections are cheap as compare to other types of connection but greatly affected on consumers load (power quality).

If the location of WTGS is connected to an MV bus in feeding an HV/MV substation through a separate transmission line (position 1), the connection has some advantages related to low influence of WTGS on customers load. Such connection are expensive than presented above.

The location of WTGS connected to HV bus through a separate transmission line, when a relatively large rated WTGS has to be connected in the power network, where the MV network is weak. This type of connection are most expensive than other presented.

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

unity power factor.

EHV system.

**2.7. Location of wind turbine** 

customers is presented in Figure 3.

consumers load (power quality).

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

The way of connecting wind turbine into the electric power system highly influences the impact of the wind turbine generating system on the power quality. As a rule, the impact on power quality at the consumer's terminal for the wind turbine generating system (WTGS) located close to the load is higher than WTGS connected away, that is connected to H.V. or

Wind turbine generator systems (WTGS) are often located in the regions that have favorable wind conditions and where their location is not burdensome. These regions are low urbanized, which means that the distribution network in these regions is usually weak developed. Such situation is typical for all countries developing a wind power industry.

The point of common coupling (PCC) of the WTGS and the power network parameter and structure of grid is of essential significance in the operation of WTGS and its influence on

The WTGS connected to the existing MV transmission line, which feeds the existing

The distance between WTGS and PCC is usually small up to a few kilometers. Such connections are cheap as compare to other types of connection but greatly affected on

If the location of WTGS is connected to an MV bus in feeding an HV/MV substation through a separate transmission line (position 1), the connection has some advantages related to low influence of WTGS on customers load. Such connection are expensive than presented above. The location of WTGS connected to HV bus through a separate transmission line, when a relatively large rated WTGS has to be connected in the power network, where the MV

network is weak. This type of connection are most expensive than other presented.

the system. WTGS can be connected to MV transmission line and to HV networks.

#### **2.8. Low voltage ride through capability**

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

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

of the wind and the risk of instability due to lower degree of controllability. Many countries in Europe and other parts of the world are developing or modifying interconnection rules and processes for wind power through a grid code. The grid codes have identified many potential adverse impacts of large scale integration of wind resources. The risk of voltage collapse for lack of reactive power support is one of the critical issues when it comes to contingencies in the power system. The low voltage ride through (LVRT) capability, which is one of the most demanding requirement that have been included in the grid codes and shown in Fig. 4.

Power Quality and Grid Code Issues in Wind Energy Conversion System 33

inter harmonic in the IEC 61000-3-6 are applicable to wind turbines. The inter harmonics that are not a multiple of 50 Hz, since the switching frequency of the inverter is not constant but varies, the harmonic will also vary. Consequently, the grid codes has been define to specify the requirements that the wind turbines must meet in order to be connected to the grid, including the capabilities of contributing to frequency and voltage control by adjusting

The Electricity Grid Code is a regulation made by the Central Commission and it to be follow by various persons and participants in the system to plan, develop, maintain, and operate the power system grid in the most secure, reliable, economic and efficient manner,

The first grid code was focused on the distribution level, after the blackout in the United State in August 2003. The United State wind energy industry took a stand in developing its own grid code for contributing to a stable grid operation. The rules for realization of grid operation of wind generating system at the distribution network is defined as - per IEC-61400-21.The grid quality characteristics and limits are given for references that the customer and the utility grid may expect. According to Energy-Economic Law, the operator of transmission grid is responsible for the organization and operation of interconnected system. The grid code also covers some of the technical standards for

To ensure the safe operation, integrity and reliability of the grid is utmost important. It is mentioned that reactive power compensation should ideally be provided locally by generating reactive power as close to the reactive power consumption as possible. The regional entity except generating stations, expected to provide local VAr compensation/generation such that they do not draw VArs from the grid, particularly under low-voltage condition. Indian grid code commission mentions that the charge for VArh shall

The wind farms must be able to run at rated voltage at a specified voltage range. The voltage range depends on the level of the voltage on the transmission system, which varies

The wind farms shall have a closed loop voltage regulation system. The voltage regulation system shall act to regulate the voltage at the point by continuous modulation of the reactive power output within its reactive power range, and without violating the voltage step

Voltage fluctuations at a point of common coupling with a fluctuating load directly connected to the transmission system shall not exceed 3% at any time. The flicker

contributions Pst and Plt are defined in IEC 61000-3-7 (Electromagnetic compatibility).

be at the rate of 25 paise/kVArh w.e.f.1.4.2010, for VAr interchanges.

while facilitating healthy competition in the generation and supply of electricity.

the active and reactive power supplied to the transmission system.

**3. Grid code for wind farms** 

connection to the grid.

from country to country.

emissions.

It defines the operational boundary of a wind turbine connected to the network in terms of frequency, voltage tolerance, power factor, fault ride through is regarded as the main challenges to the wind turbine manufactures. The wind turbine should remain stable and connected during the fault while voltage at the PCC drop to 15% of the nominal value i.e. drops of 85% for the part of 150 msec. Only when the grid voltage fall below the curve, the turbine is allowed to disconnected from the grid.

Significant barriers to interconnection are being perceived already with the requirements of the new grid codes and there it is a need for a better understanding of the factors affecting the behavior of the wind farm under severe contingencies such as voltage sags. Wind farms using squirrel cage induction generators directly connected to the network will suffer from the new demands, since they have no direct electrical control of torque or speed, and would usually disconnect from the power system when the voltage drops more than 10–20% below the rated value. In general, fulfillment of LVRT by reactive compensation will require fast control strategies for reactive power in wind turbines/farms with cage induction generators. The LVRT requirement, although details are differing from country to country, basically demands that the wind farm remains connected to the grid for voltage dips as low as 5%.

#### **2.9. IEC recommendation**

For consistent and replicable documentation of power quality characteristic of wind turbine, the international Electro-technical Commission IEC-61400-21 was developed and today, most of the large wind turbine manufactures provide power quality characteristic data accordingly.IEC 61400-21 describe the procedures for determine the power quality characteristics of wind turbines. It is a guideline for power quality measurements of wind turbine. The methodology of IEC standard consists of three analyses. The first one is the flickers analyses. IEC 61400-21 specified a method that uses current and voltage time series measured at the wind turbine terminals to simulate the voltage fluctuation on a fictitious grid with no source of voltage fluctuations other that wind turbine switching operation. The second one is regarding the switching operation. The voltage and current transients are measured during the switching operation of wind turbine. The last one is the harmonic analysis which is carried out by FFT algorithms. Recently harmonic and inter harmonic are treated in the IEC 61000-4-7 and IEC 61000-3-6. The method for summing harmonics and inter harmonic in the IEC 61000-3-6 are applicable to wind turbines. The inter harmonics that are not a multiple of 50 Hz, since the switching frequency of the inverter is not constant but varies, the harmonic will also vary. Consequently, the grid codes has been define to specify the requirements that the wind turbines must meet in order to be connected to the grid, including the capabilities of contributing to frequency and voltage control by adjusting the active and reactive power supplied to the transmission system.
