**1. Introduction**

20 An Update on Power Quality

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The aim of the electric power system is to produce and deliver to the consumer's electric energy of defined parameters, where the main quantities describing the electric energy are the voltage and frequency. During normal operation of system the frequency varies as a result of the variation of the real power generated and consumed. At the same time, because of voltage drops in the transmission lines and transformers it is impossible to keep the voltage at the nominal level in all the nodes of the power system. It is also impossible to keep an ideal sinusoidal shape of the voltage or current waveform due to the nonlinearities in many devices use for electric energy generation, transmission and at end users. That is why the electric power system require to keep the quantities near the nominal value[1]-[5].

Recently, the deregulated electricity market has also opened the door for customers own distributed generation due to economical and technical benefit. The liberalization of the grid leads to new management structures, in which the trading of energy is important. The need to integrate the renewable energy like wind energy into power system is to minimize the environmental impact on conventional plant of generation. The conventional plant uses fossil fuels such as coal & petroleum products to run the steam turbines and generate the thermal power. The fossil fuel consumption has an adverse effect on the environment and it is necessary to minimize the polluting and exhausting fuel. The penetration of renewable energy especially wind has been increasing fast during the past few years and it is expected to rise more in near future. Many countries around the world are likely to experience similar penetration level. During the last decade of the twentieth century, worldwide wind energy capacity is doubled approximately every three years.

Today's trends are to connect all size of generating units like wind farm,solar farm,biogas generation and conventional source like coal,hydro,nuclear power plant in to the grid system shown in Fig.1.0

© 2013 Mohod and Aware, licensee InTech. This is an open access chapter 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 The Author(s). Licensee InTech. This chapter is 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.

Power Quality and Grid Code Issues in Wind Energy Conversion System 23

The critical power quality issues related to integration of wind farms have been identified by team of Riso National Laboratory and Danish Utility Research Institute, Denmark and Electronic Research and Development Centre, India in Nov.1998 .The power quality in relation to a wind turbine describe the electrical performance of wind energy generating system. It reflects the generation of grid interference and the influence of a wind turbine on power and voltage quality of grid. The issue of power quality is of great importance to the wind turbines. There has been an extensive growth and quick development in the exploitation of wind energy in recent years [6]-[7]. The individual units can be of large capacity up to 5 MW, feeding into distribution network, particularly with customers connected in close proximity. However with rapidly varying voltage fluctuations due to the nature of wind, it is difficult to improve the power quality with simple compensator. Advance reactive power compensators with fast control and power electronic have emerged

It has been suggested that today's industrial development are related with generalized use of computers, adjustable speed drives and other microelectronic loads. It also becomes an increasing concern with power quality to the end customer. The presence of harmonic and reactive power in the grid is harmful, because it will cause additional power losses and malfunction of grid component. The massive penetration of electronically controlled devices and equipments in low voltage distribution network is responsible for further worsening of

The problems are related to the load equipment and devices used in electric energy generation. Now a days the transmission and distribution system become more sensitive to power quality variation than those used in the past. Many new devices contain microprocessor based controls and electronics power elements that are sensitive to many types of disturbances. The wind turbine generating systems are the highly variable sources

The wind power in the electric grid system affects the voltage quality. To assess this effect, the knowledge of about the electrical characteristic of wind turbine is needed. The electrical characteristics of wind turbine are manufacturer's specification and not site specification. This means that by having the actual parameter values for a specific wind turbine the expected impact of the wind turbine on voltage quality is important. The need for consistent and replicable documentation of the power quality characteristics of wind turbines, the International Electro-technical Commission (IEC) started work to facilitate for power quality in 1996. As a result, IEC 61400-21 was developed and today most wind turbines manufacturers provide power quality characteristic data accordingly. Wind turbines and their power quality will be certified on the basis of measurements according to national or international guidelines. These certifications are an important basis for utilities to evaluate

The power quality is defined as set of parameters defining the properties of the power supply as delivered to user in normal operating condition in terms of continuity of supply

of energy and wind turbine are belonging to the source of such problem.

the grid connection of wind turbines and wind farms.

and characteristics of voltage, frequency.

to supersede the conventional reactive compensator [8]-[9].

power-quality problem [10]-[13].

**Figure 1.** Grid integration of interconnected system

The critical power quality issues related to integration of wind farms have been identified by team of Riso National Laboratory and Danish Utility Research Institute, Denmark and Electronic Research and Development Centre, India in Nov.1998 .The power quality in relation to a wind turbine describe the electrical performance of wind energy generating system. It reflects the generation of grid interference and the influence of a wind turbine on power and voltage quality of grid. The issue of power quality is of great importance to the wind turbines. There has been an extensive growth and quick development in the exploitation of wind energy in recent years [6]-[7]. The individual units can be of large capacity up to 5 MW, feeding into distribution network, particularly with customers connected in close proximity. However with rapidly varying voltage fluctuations due to the nature of wind, it is difficult to improve the power quality with simple compensator. Advance reactive power compensators with fast control and power electronic have emerged to supersede the conventional reactive compensator [8]-[9].

22 An Update on Power Quality

**Figure 1.** Grid integration of interconnected system

It has been suggested that today's industrial development are related with generalized use of computers, adjustable speed drives and other microelectronic loads. It also becomes an increasing concern with power quality to the end customer. The presence of harmonic and reactive power in the grid is harmful, because it will cause additional power losses and malfunction of grid component. The massive penetration of electronically controlled devices and equipments in low voltage distribution network is responsible for further worsening of power-quality problem [10]-[13].

The problems are related to the load equipment and devices used in electric energy generation. Now a days the transmission and distribution system become more sensitive to power quality variation than those used in the past. Many new devices contain microprocessor based controls and electronics power elements that are sensitive to many types of disturbances. The wind turbine generating systems are the highly variable sources of energy and wind turbine are belonging to the source of such problem.

The wind power in the electric grid system affects the voltage quality. To assess this effect, the knowledge of about the electrical characteristic of wind turbine is needed. The electrical characteristics of wind turbine are manufacturer's specification and not site specification. This means that by having the actual parameter values for a specific wind turbine the expected impact of the wind turbine on voltage quality is important. The need for consistent and replicable documentation of the power quality characteristics of wind turbines, the International Electro-technical Commission (IEC) started work to facilitate for power quality in 1996. As a result, IEC 61400-21 was developed and today most wind turbines manufacturers provide power quality characteristic data accordingly. Wind turbines and their power quality will be certified on the basis of measurements according to national or international guidelines. These certifications are an important basis for utilities to evaluate the grid connection of wind turbines and wind farms.

The power quality is defined as set of parameters defining the properties of the power supply as delivered to user in normal operating condition in terms of continuity of supply and characteristics of voltage, frequency.

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) prepared by IEC- Technical Committee 88.

Power Quality and Grid Code Issues in Wind Energy Conversion System 25

(2)

sum of squares of amplitudes of all harmonics \*100% square of amplitude of fundamental

( current) =

**2.1. Issue of voltage variation** 

capable of suppressing output variations.

**2.2. Issue of voltage dips** 

Where d - Relative voltage change,

*<sup>n</sup> <sup>S</sup>* - Rated apparent power of wind turbine and \*

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

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

deviation are evaluated.

*DF for <sup>I</sup>*

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

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 %

\* 100 ( ) *<sup>n</sup> u k*

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

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

*<sup>S</sup> d K*

*k*

(3)

*<sup>k</sup> S* short circuit apparent power of grid.

*S*

voltage change due to switching operation of wind turbine is calculated as

*DF* (1)

 

2 2 1

*h h I*

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