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Int. J. Elec&Electr.Eng&Telecoms. 2013 V Sujatha and M Bhaskar Reddy, 2013

MITIGATION OF POWER QUALITY PROBLEMS INGRID CONNECTED WIND GENERATION PLANT BY

USING STATCOM

V Sujatha1* and M Bhaskar Reddy1

*Corresponding Author: V Sujatha,[email protected]

This paper demonstrate the power quality problem due to installation of wind turbin with electicgrid. The performance of the wind turbine and thereby power quality are determined on thebasis of measurements as the active power, reactive power, variation of voltage, flicker, harmonics,and electrical behavior of switching operation. In this scheme STATCOM is connected at a pointof common coupling with a Battery Energy Storage System (BESS) to reduce the power qualityissues. The battery energy storage is integrated to sustain the real power source under fluctuatingwind power. The power quality enhancement in wind generation plant by STATCOM controller issimulated using MATLAB/SIMULINK in power system block set. The effectiveness of this schemerelives the main supply source from the reactive power demand of the load and the inductiongenerator.

Keywords: Power quality, Wind Generating Plant (WGP), STATCOM BESS

INTRODUCTIONTo have sustainable growth and socialprogress, it is necessary to meet the energyneed by utilizing the renewable energyresources like wind, biomass, hydro, co-generation, etc. In sustainable energy system,energy conservation and the use of renewablesource are the key paradigm. The need tointegrate the renewable energy like windenergy into power system is to make itpossible to minimize the environmental impact

ISSN 2319 – 2518 www.ijeetc.comVol. 2, No. 4, October 2013

© 2013 IJEETC. All Rights Reserved

Int. J. Elec&Electr.Eng&Telecoms. 2013

1 Faculty of Engineering, University Malaysia Sarawak (UNIMAS), Kota Samarahan 94300, Sarawak, Malaysia.

on conventional plant. The integration of windenergy into existing power system presents atechnical challenges and that requiresconsideration of voltage regulation, stability,power quality problems. The power quality isan essential customer-focused measure andis greatly affected by the operation of adistribution and transmission network. Theissue of power quality is of great importanceto the wind turbine. There has been anextensive growth and quick development in the

Research Paper

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exploitation of wind energy in recent years. Theindividual units can be of large capacity up to2 MW, feeding into distribution network,particularly with customers connected in closeproximity. Today, more than 28000 windgenerating turbine are successfully operatingall over the world.

In the fixed-speed wind turbine operation,all the fluctuation in the wind speed aretransmitted as fluctuations in the mechanicaltorque, electrical power on the grid and leadsto large voltage fluctuations. During the normaloperation, wind turbine produces a continuousvariable output power. These power variationsare mainly caused by the effect of turbulence,wind shear, and tower-shadow and of controlsystem in the power system. Thus, the networkneeds to manage for such fluctuations. Thepower quality issues can be viewed withrespect to the wind generation, transmissionand distribution network, such as voltage sag,swells, flickers, harmonics, etc. However thewind generator introduces disturbances intothe distribution network. One of the simplemethods of running a wind generating systemis to use the induction generator connecteddirectly to the grid system. The inductiongenerator has inherent advantages of costeffectiveness and robustness. However;induction generators require reactive powerfor magnetization. When the generated activepower of an induction generator is varied dueto wind, absorbed reactive power and terminalvoltage of an induction generator can besignificantly affected. A proper control schemein wind energy generation system is requiredunder normal operating condition to allow theproper control over the active powerproduction. In the event of increasing grid

disturbance, a battery energy storage systemfor wind energy generating system is generallyrequired to compensate the fluctuationgenerated by wind turbine. A STATCOM-based control technology has been proposedfor improving the power quality which cantechnically manages the power levelassociates with the commercial wind turbines.The proposed STATCOM control scheme forgrid connected wind energy generation forpower quality improvement has followingobjectives.

• Unity power factor at the source side.

• Reactive power support only fromSTATCOM to wind Generator and Load.

• Simple bang-bang controller for STATCOMto achieve fast dynamic response.

POWER QUALITYSTANDARDS, ISSUES ANDITS CONSEQUENCESVoltage VariationThe voltage variation issue results from thewind velocity and generator torque. Thevoltage variation is directly related to real andreactive power variations. The voltagevariation is commonly classified as under:

• Voltage Sag/Voltage Dips.

• Voltage Swells.

• Short Interruptions.

• Long duration voltage variation.

The voltage flicker issue describesdynamic variations in the network caused bywind turbine or by varying loads. Thus thepower fluctuation from wind turbine occursduring continuous operation. The amplitudeof voltage fluctuation depends on grid

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strength, network impedance, and phase-angle and power factor of the wind turbines.It is defined as a fluctuation of voltage in afrequency 10-35 Hz.

HarmonicsThe harmonic results due to the operationof power electronic converters. Theharmonic voltage and current should belimited to the acceptable level at the point ofwind turbine connection to the network. Toensure the harmonic voltage within limit,each source of harmonic current can allowonly a limited contribution, The rapidswitching gives a large reduction in lowerorder harmonic current compared to the linecommutated converter, but the output currentwill have high frequency current and can beeasily filter-out.

Wind Turbine Location in PowerSystemThe way of connecting the wind generatingsystem into the power system highlyinfluences the power quality. Thus theoperation and its influence on power systemdepend on the structure of the adjoining powernetwork.

Self Excitation of Wind TurbineGenerating SystemThe self excitation of Wind TurbineGenerating System (WTGS) with anasynchronous generator takes place afterdisconnection of Wind Turbine GeneratingSystem (WTGS) with local load. The risk ofself excitation arises especially when WTGSis equipped with compensating capacitor.The capacitor connected to inductiongenerator provides reactive power

compensation. However the voltage andfrequency are determined by the balancingof the system. The disadvantages of selfexcitation are the safety aspect and balancebetween real and reactive power.

Consequences of the IssuesThe voltage variation, flicker, harmonicscauses the malfunction of equipmentsnamely microprocessor based controlsystem, programmable logic controller;adjustable speed drives, flickering of lightand screen. It may leads to tripping ofcontractors, tripping of protection devices,stoppage of sensitive equipments likepersonal computer, programmable logiccontrol system and may stop the processand even can damage of sensit iveequipments. Thus it degrade the powerquality in the grid.

GRID COORDINATION RULEThe American Wind Energy Association(AWEA) led the effort in the united state foradoption of the grid code for the inter-connection of the wind plants to the utilitysystem. The United State wind energy industrytook a stand in developing its own grid codefor contributing to a stable grid operation. Therules for realization of grid operation of windgenerating system at the distribution network.The grid quality characteristics and limits aregiven for references that the customer andthe utility grid may expect. According toEnergy-Economic Law, the operator oftransmission grid is responsible for theorganization and operation of interconnectedsystem.

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Voltage Rise (u): The voltage rise at the pointof common coupling can be approximated asa function of maximum apparent power Smax

of the turbine, the grid impedances R and X atthe point of common coupling and the phaseangle , given in (1)

u = Smax(Rcos Xsin)/U2 ...(1)

where u—voltage rise, Smax—max. apparentpower, —phase difference, U—is the nominalvoltage of grid. The Limiting voltage rise valueis <2%.

Voltage Dips (d): The voltage dips is due tostart up of wind turbine and it causes a suddenreduction of voltage. It is the relative % voltagechange due to switching operation of windturbine. The decrease of nominal voltagechange is given in (2).

K

nu S

SKd ...(2)

where d is relative voltage change, Sn ratedapparent power, SK short circuit apparentpower, and Ku sudden voltage reduction factor.The acceptable voltage dips limiting value is3%.

Flicker: The measurements are made formaximum number of specified switchingoperation of wind turbine with 10-min periodand 2-h period are specified, as given in (3)

K

nKt S

SCP 1 ...(3)

where P1t—Long term flicker. C(K)—Flickercoefficient calculated from Rayleighdistribution of the wind speed. The LimitingValue for flicker coefficient is about 0.4, foraverage time of 2 h.

Harmonics: The harmonic distortion isassessed for variable speed turbine with a

electronic power converter at the point ofcommon connection. The total harmonicvoltage distortion of voltage is given as in (4):

40

2 1

2

100h

nTHD V

VV ...(4)

where Vn is the nth harmonic voltage and V1 isthe fundamental frequency (50) Hz. The THDlimit for 132 KV is < 3%.

THD of current ITHD is given as in (5)

1001

III n

THD ...(5)

where In is the nth harmonic current and I1 isthe fundamental frequency (50) Hz. The THDof current and limit for 132 KV is < 2.5%.

Grid Frequency: The grid frequency in Indiais specified in the range of 47.5-51.5 Hz, forwind farm connection. The wind farm shallable to withstand change in frequency up to0.5 Hz/s.

TOPOLOGY FOR POWERQUALITY IMPROVEMENTThe STATCOM based current control voltagesource inverter injects the current into the gridin such a way that the source current areharmonic free and their phase-angle withrespect to source voltage has a desired value.The injected current will cancel out the reactivepart and harmonic part of the load andinduction generator current, thus it improvesthe power factor and the power quality. Toaccomplish these goals, the grid voltages aresensed and are synchronized in generating thecurrent command for the inverter. Theproposed grid connected system isimplemented for power quality improvement

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at Point of Common Coupling (PCC), asshown in Figure 1.

The grid connected system in Figure 1,consists of wind energy generation system andbattery energy storage system withSTATCOM.

coefficient Cp of the wind turbine, and is givenin (7).

windpmech PCP ...(7)

where Cp is the power coefficient, depends ontype and operating condition of wind turbine.This coefficient can be express as a functionof tip speed ratio and pitch angle . Themechanical power produce by wind turbine isgiven in (8)

pwindmech CVRP 32

21 ...(8)

where R is the radius of the blade (m).

BESS-STATCOMThe Battery Energy Storage System (BESS)is used as an energy storage element for thepurpose of voltage regulation. The BESS willnaturally maintain dc capacitor voltageconstant and is best suited in STATCOM sinceit rapidly injects or absorbed reactive powerto stabilize the grid system. It also control thedistribution and transmission system in a veryfast rate. When power fluctuation occurs in thesystem, the BESS can be used to level thepower fluctuation by charging and dischargingoperation. The battery is connected in parallelto the dc capacitor of STATCOM.

The STATCOM is a three-phase voltagesource inverter having the capacitance on itsDC link and connected at the point of commoncoupling. The STATCOM injects acompensating current of variable magnitudeand frequency component at the bus ofcommon coupling.

System OperationThe shunt connected STATCOM with batteryenergy storage is connected with the

Figure 1: Grid Connected System forPower Quality Improvement

Wind Energy Generating SystemIn this configuration, wind generations arebased on constant speed topologies with pitchcontrol turbine. The induction generator is usedin the proposed scheme because of itssimplicity, it does not require a separate fieldcircuit, it can accept constant and variableloads, and has natural protection against shortcircuit. The available power of wind energysystem is presented as under in (6).

3

21

windwind AVP ...(6)

where (kg/m3) is the air density and A(m3) isthe area swept out by turbine blade, Vwind isthe wind speed in mtr/s. It is not possible toextract all kinetic energy of wind, thus it extracta fraction of power in wind, called power

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interface of the induction generator and non-linear load at the PCC in the grid system. TheSTATCOM compensator output is variedaccording to the controlled strategy, so as tomaintain the power quality norms in the gridsystem. The current control strategy isincluded in the control scheme that definesthe functional operation of the STATCOMcompensator in the power system. A singleSTATCOM using insulated gate bipolartransistor is proposed to have a reactivepower support, to the induction generator andto the nonlinear load in the grid system. TheSIMULINK Model for the proposed schemeis shown in Figure 2.

CONTROL SCHEMEThe control scheme approach is based oninjecting the currents into the grid using “bang-bang controller”. The controller uses ahysteresis current controlled technique. Usingsuch technique, the controller keeps the controlsystem variable between boundaries ofhysteresis area and gives correct switchingsignals for STATCOM operation.

The control system scheme for generatingthe switching signals to the STATCOM.

The control algorithm needs themeasurements of several variables such asthree-phase source current iSabc, DC voltage

Figure 2: SIMULINK Model for the Proposed Scheme

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Vdc, inverter current iiabc with the help of sensor.The current control block, receives an input ofreference current *

Sabci and actual current iSabc

are subtracted so as to activate the operationof STATCOM in current control mode.

Grid SynchronizationIn three-phase balance system, the RMSvoltage source amplitude is calculated at thesampling frequency from the source phasevoltage (Vsa, Vsb, Vsc) and is expressed, assample template Vsm, sampled peak voltage,as in (9).

21

222

32

scsbsasm VVVV ...(9)

The in-phase unit vectors are obtained fromAC source-phase voltage and the RMS valueof unit vector usa, usb, usc as shown in (10).

sm

Scsc

sm

Sbsb

sm

Sasa V

VuvVu

VVu ,, ...(10)

The in-phase generated reference currentsare derived using in-phase unit voltagetemplate as, in (11)

ScScSbSbSaSa uIiuIiuIi .,.,. *** ...(11)

where I is proportional to magnitude of filteredsource voltage for respective phases. Thisensures that the source current is controlled tobe sinusoidal. The unit vectors implement theimportant function in the grid connection for thesynchronization for STATCOM. This methodis simple, robust and favorable as comparedwith other methods.

Bang-Bang Current ControllerBang-Bang current controller is implementedin the current control scheme. The referencecurrent is generated as in (11) and actual

current are detected by current sensors andare subtracted for obtaining a current error fora hysteresis based bang-bang controller. Thusthe ON/OFF switching signals for IGBT ofSTATCOM are derived from hysteresiscontroller.

The switching function SA for phase ‘a’ isexpressed as (12).

0* Asasa SHBii

1* Asasa SHBii ...(12)

where HB is a hysteresis current-band,similarly the switching function SB, SC can bederived for phases “b” and “c”.

SYSTEM PERFORMANCEThe proposed control scheme is simulatedusing SIMULINK in power system block set.The system parameter for given system isgiven Table 1.

The system performance of proposedsystem under dynamic condition is alsopresented.

Table 1: System Technical Parameters

Voltage Source Current Control-Inverter OperationThe three phase injected current into the gridfrom STATCOM will cancel out the distortion

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caused by the non-linear load and windgenerator. The IGBT based three-phaseinverter is connected to grid through thetransformer. The generation of switchingsignals from reference current is simulatedwithin hysteresis band of 0.08. The choice ofnarrow hysteresis band switching in thesystem improves the current quality. Thecontrol signal of switching frequency within itsoperating band, as shown in Figure 3.

The choice of the current band depends onthe operating voltage and the interfacing

transformer impedance. The compensatedcurrent for the nonlinear load and demandedreactive power is provided by the inverter. Thereal power transfer from the batteries is alsosupported by the controller of this inverter. Thethree phase inverter injected current are shownin Figure 4.

STATCOM—Performance UnderLoad VariationsThe wind energy generating system isconnected with grid having the nonlinearload. The performance of the system ismeasured by switching the STATCOM attime s in the system and how theSTATCOM responds to the step changecommand for increase in additional load at1.0 s is shown in the simulation. WhenSTATCOM controller is made ON, withoutchange in any other load condit ionparameters, it starts to mitigate for reactivedemand as well as harmonic current. Thedynamic performance is also carried out bystep change in a load, when applied at 1.0s. This additional demand is fulfill bySTATCOM compensator. Thus, STATCOMcan regulate the available real power fromsource. The result of source current, loadcurrent are shown in Figures 5a and 5brespectively. While the result of injectedcurrent from STATCOM are shown in Figure5c and the generated current from windgenerator at PCC are depicted in Figure 5d.

The DC link voltage regulates the sourcecurrent in the grid system, so the DC linkvoltage is maintained constant across thecapacitor as shown in Figure 6a. The currentthrough the dc link capacitor indicating thecharging and discharging operation as shownin Figure 6b.

Figure 3: Switching Signal Withina Control Hysteresis Band

Figure 4: Three Phase Injected InverterCurrent

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Figure 5: (a) Source Current, (b) Load Current, (c) Inverter Injected Current, (d) WindGenerator (Induction Generator) Current

Figure 6: (a) DC Link Voltage, (b) Current Through Capacitor

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The above tests with proposed scheme hasnot only power quality improvement feature butit also has sustain capability to support the loadwith the energy storage through the batteries.

CONCLUSIONThe paper presents the STATCOM-basedcontrol scheme for power quality improvementin grid connected wind generating system andwith non linear load. The power quality issuesand its consequences on the consumer andelectric utility are presented. The operation ofthe control system developed for theSTATCOM-BESS in MATLAB/SIMULINK formaintaining the power quality is simulated. Ithas a capability to cancel out the harmonicparts of the load current. It maintains the sourcevoltage and current in-phase and support thereactive power demand for the wind generatorand load at PCC in the grid system, thus it givesan opportunity to enhance the utilization factorof transmission line. The integrated windgeneration and STATCOM with BESS haveshown the outstanding performance. Thus theproposed scheme in the grid connectedsystem fulfills the power quality norms.

Figure 7: STATCOM Output Voltage

Figure 8: Supply Voltage and Currentat PCC

Power Quality ImprovementIt is observed that the source current on thegrid is affected due to the effects of nonlinearload and wind generator, thus purity ofwaveform may be lost on both sides in thesystem. The inverter output voltage underSTATCOM operation with load variation isshown in Figure 7. The dynamic load doesaffect the inverter output voltage. The sourcecurrent with and without STATCOM operationis shown in Figure 8. This shows that the unitypower factor is maintained for the sourcepower when the STATCOM is in operation. Thecurrent waveform before and after theSTATCOM operation is analyzed. The Fourieranalysis of this waveform is expressed and theTHD of this source current at PCC withoutSTATCOM is 4.71%, as shown in Figure 8.

The power quality improvement is observedat point of common coupling, when thecontroller is in ON condition. The STATCOMis placed in the operation at 0.7 s and sourcecurrent waveform is shown in Figure 8 with itsFFT. It is shown that the THD has beenimproved considerably and within the normsof the standard.

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REFERENCES1. Billinton R and Gao Y (2008), “Energy

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2. Gutierrez J J, Ruiz J, Leturiondo L andLazkano A (2009), “Flicker MeasurementSystem for Wind Turbine Certification”,IEEE Trans. Instrum. Meas., Vol. 58,No. 2, pp. 375-382.

3. Han C, Huang A Q, Baran M, BhattacharyaS and Litzenberger W (2008), “STATCOMImpact Study on the Integration of a LargeWind Farm into a Weak Loop PowerSystem”, IEEE Trans. Energy Conv.,Vol. 23, No. 1, pp. 226-232.

4. Heier S (2007), Grid Integration of WindEnergy Conversions, pp. 256-259, Wiley,Hoboken, NJ.

5. Hook K S, Liu Y and Atcitty S (2006),“Mitigation of the Wind GenerationIntegration Related Power Quality Issues byEnergy Storage”, EPQUJ, Vol. XII, No. 2.

6. Manel J (2006), “Power ElectronicSystem for Grid Integration of RenewableEnergy Source: A Survey”, IEEE Trans.Ind. Electron., Vol. 53, No. 4, pp. 1002-1014, Carrasco.

7. Sannino A (2004), “Global PowerSystems for Sustainable Development”,in IEEE General Meeting, June, Denver,CO.

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9. Yao D L, Choi S S, Tseng K J and Lie T T(2009), “A Statistical Approach to theDesign of a Dispatchable Wind Power—Battery Energy Storage System”, IEEETrans. Energy Conv., Vol. 24, December,No. 4.

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