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State of the art in research on wind energy conversion system: A review.

INTRODUCTION

Wind is one of the most abundant renewable sources of energy in nature. The research for wind power industry started to be improved in the last century because of the oil crisis and natural resources ripening which demands additional transmission capacity and better means of maintaining system reliability. To have sustainable growth and social progress, it is necessary to meet the energy need by utilizing the renewable energy resources like wind which having tremendous environmental, social, and economic benefits. The variable speed system are more efficient than the fixed speed system at low wind speed sites and at high wind speed sites it is less efficient than the constant speed system. Synchronous generator equipped with a gear is used for the variable speed applications [1]. Integration of the wind energy gives minimum environmental impact on conventional plant.

A. Wind energy:

In recent years wind energy is most preferable in the power system due to its less carbon emission and no depleting nature. The winds result from the large scale movements of air masses in the atmosphere. These movements of air are created by differential solar heating of the earth's atmosphere. The wind energy production technology plays a major role in the electrical system in recent days due to the increasing demand and high cost and less availability of the conventional energy sources and also the transportation cost and storage problem also the reason for the shift of the conventional energy sources to the renewable energy source. The wind energy technology uses the kinetic energy available from the wind to produce electricity by using turbine generator set. By the electromechanical conversion process the power production obtained from the wind source. At the earlier days, windmills were used for grinding grain and pumping water. Charles F. Brush erected the first wind turbine in 1888. In which, he designed the system with 144 blades made of cedar wood and 17 meters rotor diameter and obtained the power rating of 12 kWatt. Around the 14th century it became the main source of energy. At the turn of the 21st century the use of wind energy for the electrical power generation is rapidly increased. In the year 2004, the global installed capacity has raised to twenty percentage. The group of wind turbines named as the wind farm.

B. Wind availability:

The wind has variable nature in the both space and time. The wind variation classified in to three types according to the time scale. First one is, the large time scale variability. In which, the variation of wind from one year to another year taken. The next one is the medium time scale. It covers the periods up to a year and usually assessed in terms of monthly variations, covering one year. So it is otherwise called as monthly variation. The third one is the short term time scale variability, covering time scales of minutes to seconds. To analyze the wind distribution, Weibull distribution is commonly used. The Weibull distribution is a two-parameter function. The Weibull distribution function is mostly used in statistical analysis. It is given by Seguro and Lambert (2000).

P(v<vi<v+dv)=P(v>0)(k/c)(vi/c)k-1exp[-(vi/c)k] (1)

The parameters, c-Weibull scale parameter, k-Weibull shape parameter, v- is the wind speed, vi- is a particular wind speed, dv- is the wind speed increment. In which parameter c-has the units equal to the wind speed unit and k-unit less quantity (v<vi<v+dv)-is the probability that the wind speed is between v and v+dv and P (v>0)-is the probability that the wind speed exceeds zero. The cumulative distribution function given as below equation,

P(v<vi)=P(v>0){1-exp[-(vi/c)k]} (2)

The two Weibull parameters and the average wind speed are related by

[bar.v]=c.[GAMMA]+lk (3)

In which [bar.v]- is the average wind speed and [GAMMA]-is the complete gamma function. At some special type when k= 2, the Weibull distribution comes like a Rayleigh distribution. At this situation the factor [GAMMA]1+ 1k has the value of 0.8862.

The effect of the parameter k on the probability density function is given in fig 1.1.It used to analyze the wind distribution. In which the scale factor value kept in constant. It means the variation of the hourly mean speed around the annual mean is small as k is higher. The scale factor c denotes the how the windy locations present.

The influence of the scale factor on the probability density function is presented in Fig 1.2, with the shape factor kept constant. In this graphical representation the shape factor maintained at the constant value. So the variation in the speed depends upon the scale factor.

II. Principle of wind machine:

The wind machine used to provide the electrical power from the available wind power. The principle of the wind machine is the electromechanical conversion process.

The components present in the wind energy conversion system are the turbine, gearbox and the generator. The kinetic energy available in the moving wind strikes the blades of the turbine and produce the two types of forces namely lift force and the drag force. The lift force only used for the power conversion (kinetic energy to mechanical energy).This lift force direction is opposite to the direction of blade rotation. The lift force acts perpendicular to the wind direction.

The achieved mechanical energy from the wind turbine given to the gearbox. The gearbox used for converting the low speed high torque into the high speed low torque. The main use of gearbox is matching of the turbine rotor and the generator rotor speed. Then the mechanical power given to the generators to produce the electrical energy (mechanical to electrical energy).The generators maybe synchronous or the asynchronous machines. According to the application requirements the generator type is selected. If the wind system is connected with the grid then the transformers are used to give the proper voltage level to the grid.

III. Wind energy conversion system:

The WECS consists of wind turbine and generator. A wind turbine is a device which is used to convert the kinetic energy present in the moving air into the mechanical energy. The wind generator used to convert the mechanical energy from the wind turbine into the electrical energy.

In WECS the key technologies include wind turbine technology, power electronics technology, and system control technology.

A Wind turbine technology:

In wind turbine technology there are many types of turbines are used based on the orientation and operation speed is controllable.

The turbine rotation is parallel to the ground known as horizontal-axis wind turbines, otherwise verticalaxis turbines. In industry horizontal-axis wind turbines used because of higher wind energy conversion efficiency..

Fixed-speed wind turbines which are simple, robust, and require lower construction and maintenance cost and variable speed wind turbines. Its operation speed is fixed or constant and cannot be controlled with the different wind speed, which gives in lower energy conversion efficiency so the variable-speed wind turbine are applied in industry.

B. Power electronics technology:

The wind energy conversion system uses the generator for the conversion of the mechanical energy obtained from the wind turbine into the electrical energy. The generators used for the WECS (Wind Energy Conversion System) maybe the synchronous generator or the asynchronous generators. The asynchronous generators also called as the induction generators. The synchronous generators used for the constant speed applications and it also can be used for variable speed applications by the suitable converters. The asynchronous generators used for the variable speed applications .The classifications are

1 Doubly-fed induction generator

2 squirrel-cage induction generator

3 wound-rotor synchronous generator

4 Permanent magnet synchronous generator

In the DFIG WECSs, only 30% of the rated power is processed by the power converters, which greatly reduces the cost of the converters while preserving the capability to control the speed of the generator in the range of about of its rated speed. The DFIG (Doubly Fed

Induction Generator) is the type of asynchronous machine. The term 'doubly fed' refers that the voltage on the stator can get the energy from the grid and the voltage on the rotor is induced by the power converter. By using DFIG we can extract maximum energy from the wind energy at low wind speeds by the optimization of the turbine speed and extract the optimum energy at gusts of wind by minimizing mechanical stresses on the turbine. The optimum energy obtained from the wind turbine is directly proportional to the wind speed. The major advantage of the DFIG is ,it can either generate or absorb reactive power by using the power electronic converters and it doesn't requires, capacitor banks as in the case of squirrel -cage induction generator. The DFIG used for the variable speed operation by the presence of the power electronic converter. The converter used for the compensation, if there is difference occurs between the mechanical and electrical frequency, which is done by injecting a rotor current with a variable frequency. The behavior of the generator at normal operation and faults are governed by the power converter and its controllers. The two type of converters presented in the power electronic converter, one is rotor-side converter and another is grid-side converter. The control mechanism of the converters are independently of each other. Their advantages are control of reactive power and to decouple active and reactive power control by independently controlling the excitation current of rotor. The DFIG has the advantage of it can be magnetized from the power grid, and magnetized from the rotor circuit also. It is also capable of generating reactive power and the generated reactive power can be delivered to the stator by the grid-side converter

The synchronous generator consists of stator and rotor. Stator is a stationary part and the rotor is a rotating part. The selection of synchronous generator maybe WRSG (Wound Rotor Synchronous Generator).The stator of WRSG directly connected with the grid so the rotational speed of the generator fixed by the grid frequency. The rotor gets excited by direct current using slip rings and brushes or with a brushless exciter with a rotating rectifier. The main advantage of synchronous generatoris, it don't requires reactive power compensation system and no need of gearbox. By the DC supply the rotor get excited and rotates at synchronous speed. The speed of the synchronous generator is determined by the frequency of the rotating field and by the number of pole pairs of the rotor. It is given by,

N = 120fp (4)

N--Rotational speed of the generator, f--frequency, p--number of poles

The speed and torsional oscillations even occurs in the direct driven wind energy system, by the elimination of assistant damping device The active damping strategy used for the speed and torsional oscillations suppression occurs in the permanent--synchronous generator (PMSG) based wind energy conversion system (WECS) [2]..In SCIG, WRSG and PmSg WECSs, full-capacity power converters are needed to process the power generated by the generators up to the rated power of the systems. With the application of the full-capacity power converters, the generators are fully decoupled from the grid, and are able to operate in the full speed range.

IV. Power quality and its issues:

The power quality problems are the variation in the voltage, frequency, real power, reactive power, Flickering and harmonics.

The reason for migrate from the conventional sources to the renewable sources is to face the demand, depletion of fossil fuels and the protection of environment from the air pollution. So the better utilization of the wind required to meet the above problems. The wind is fluctuating in nature. The variable nature of the wind causes the power quality problems. It causes the problems in maintenance of power factor. The power quality problems are variation in real and reactive power, changes in sinusoidal nature of the waveform, voltage and frequency changes. This changes makes the power quality problems and produce the harmonics. This harmonics produce the heating effect. The impact of harmonics on the system is the stability and reliability problems. This injection of variable power output obtain from the windsystem given to the grid affects that grid stability and reliability.

The power quality is an essential customer- focused measure and is greatly affected by the operation of a distribution and transmission network. Good power quality is benefit to the operation of electrical equipment, but poor power quality will produce great harm to the power system. Therefore injection of the wind power into an electric grid affects the power quality. The important factors to be considered in power quality measurement are the active power, reactive power, variation of voltage, flicker, harmonics, and electrical behavior of switching operation.

The power quality problems are Interruption, Voltage sag, Voltage surge, Frequency variation, Transients, Harmonics. The interruption of power supply called power outage. This caused by Ice storms, lightning, wind, utility equipment failure. Voltage Fluctuations are the variation in the system voltage above or below the rated level. It maybe voltage sag or swell. The causes for voltage fluctuations-Large equipment start-up or shut down, sudden change in load. The sudden changes in the system parameters mainly in magnitude known as transients. It also called as surges. It caused by lighting, equipment start-up and shutdown, welding equipment. The harmonic refers the distortion in the pure sinusoidal nature of waveform. It caused by the equipment not operate at 50Hz and also by electronic ballasts, non-linear loads, variable frequency drives.

V. Power quality improvement:

The control scheme regulates the measured frequency to this reference frequency by injecting the real power to the HVDC system. The fault-ride through also can be met by the reduction of injection of real power [2] .The synchronous generator based wind energy conversion system with battery bank and the PWM technique provides better transient response. The battery system used for storing the excess wind power to manage the powerimbalance between the generating system and the load and PID controller used to provide the better transient response with the reduction of the peak overshoot and the settling time compared to the PI controller [8].

To meet the power quality problems and to maintain the stability and reliability of the system the FACTS (Flexible Alternating Current Transmission Systems) used. Due to the real and reactive power control and the cost effective advantages of the STATCOM (Static Synchronous Compensator) used. The STATCOM is applied to the synchronous and asynchronous machine with grid system to improve the power quality under abnormal conditions. The series static synchronous compensator (SSSC) and a series vectorial compensator (SVeC) used in the synchronous generator based offshore wind farm (OWF) for the damping process [3].

For using of soft computing techniques FACTS devices to get better performance such us FUZZY NEURANAL and ANFIS etc.

Conclusion:

This paper has reviewed the wind energy conversion system with different types of wind turbine and generator systems have quite different performances and controllability The power output obtained from the wind energy conversion system has the fluctuating nature. The wind has variable nature in the both space and time. The main issue of the integration of the wind energy system with the grid is the power quality problems because of the variable wind speed conditions which is reduced by using of controller such as PID PI and FACTS devices. Soft computing techniques also used for the better performance.

REFERENCES

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(1) S. Shenbaga priya and (2) G. Elatharasan

(1) Research scholar, University college of Engineering, Pattukkottai, Rajamadam.

(2) Assistant professor, Department of Mechanical Engineering, University College of Engineering Pattukkottai Anna University, Rajamadam-614701. Tamilnadu, India.

Received 28 February 2017; Accepted 22 May 2017; Available online 6 June 2017

Address For Correspondence: G. Elatharasan, Assistant professor, Department of Mechanical Engineering, University College of Engineering Pattukkottai Anna University, Rajamadam-614701.Tamilnadu,India E-mail: elatharasan@yahoo.co.in; Contact +9109791940476

Caption: Fig. 1: Weibull distributions with constant c

Caption: Fig. 2: Weibull distributions with constant k

Caption: Fig. 3: Block diagram
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Author:Priya, S. Shenbaga; Elatharasan, G.
Publication:Advances in Natural and Applied Sciences
Article Type:Report
Date:Jun 1, 2017
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