Optimal Design of Islanded Microgrid Using Genetic

OptimalDesignofIslandedMicrogridUsingGeneticAlgorithm*FarzadRazavi,**RezaTorani,***ImanAskarian,****AlirezaAsgharizadeh,*NimaMasoomi*DepartmentofElectricalEngineering,QazvinBranch,IslamicAzadUniversity,Qazvin**DepartmentofElectricalEngineering,TafreshUniversity***DepartmentofElectricalEngineering,TehranUniversity****DepartmentofElectricalEngineering,AmirkabirUniversity(Correspondingauthor:FarzadRazavi,tell:[phone];email:[e-mail])Abstract—Thispaperfocusesonanoptimizationmethodforadroopcontrolledmicrogridinislandingoperation.Theultimategoalistooptimizedroopcoefficienttominimizefrequencyvariation.Also,gainsofPIcontrollersareoptimizedtoensuregoodbehaviorofthecontroller.OptimizationsareimplementedinMATLABsoftwareusingGeneticAlgorithm(GA).StabilityofoptimizedPIgainsofvoltageandcurrentcontrollersareanalyzed.Keywords-Microgrid;DistributedGeneration(DG);DroopControl;Islandingoperation;V/fcontrol;powersharing;VoltageSourceInverter(VSI);GeneticAlgorithm(GA).I.INTRODUCTIONAdvancementinDistributedGenerations(DGs)systemsandpowerelectronicdevicesledtoconceptofMicrogrid.ItcanintegraterenewableenergyandotherformsofDGandalsoincreasereliabilityandefficiency[1].ManyformsofDGsuchasfuelcellandphotovoltaicareinterfacedtothenetworkthroughpowerelectronicdevices.Theseinterfaceddevicesmakethesystemmoreflexibleintheiroperationandtheircontrolcomparedtotheconventionalelectricalmachine[2-4].Consequently,controlstrategyofparallelconnectedinvertersisimportantformicrogridoperating.Thebasicobjectiveinmicrogridcontrolistoachieveanaccuratepowersharingandregulationofthemicrogridvoltageandfrequency.Centralizedcontrolofmicrogridisproposedin[5].However,thismethodisimpracticalandcostlyinmicrogridwithlongdistancebetweenDGunits.Toovercomethislimitation,decentralizedcontrollersbasedonthedroopcontrolmethodareproposed[6-9].Thismethoddoesnotneedanycommunication.Innormalcondition,Microgridoperatesingrid-connectedmodeandthemaingridcansupportthesystemfrequencyandvoltagesbysupplyingthepowermismatchimmediately.Whenafaultoccursinthemaingrid,Microgridneedstodisconnectfromthemaingridtoprovideuninterruptedpowertotheloads.Inislandingoperationmode,DGsoperateinVfcontrolforsupplyingmicrogridloadandcontrollingvoltageandfrequency[10].Tothispurpose,droopcontrolthatassignstheamountofpowersharingforchangesofloadwithoutcommunicationisused[11].Aftertransitionfromgrid-connectedtoislanding,thefrequencyofmicrogridisdeterminedbythedroopcoefficientsofDGs.Sincethedroopcontrolchangesthesystemfrequencytosupplythepowermismatch,thefrequencyvariationoccurs.Tomaintainfrequencyclosetothenominalvalue,droopcoefficientsshouldbedeterminedproperly.Thedroopcoefficientandsharingratiomaybedictatedbyeconomicinterestedofthesystemoperators[12].Thechoiceofdroopcoefficientsinsuchcaseisanalyzedin[13].Themainproblemsforcontrollerparameteroptimizationarenonlinearityandcomplexityofthesystem.Smallsignallinearizationisausualmethodfordesigningofcontrollerparameters.Butthismethoddependsontheoperationpoint[14].Hence,inthispapersimulationmodelinMATLAB/Simulinkisemployedasareplacementforsmallsignalmethodforoptimization.Thispaperconcentratesonoptimizationofmicrogridcontroller.Geneticalgorithmisusedtooptimizethedroopcoefficients.Theproportionalandintegralgainsofvoltageandcurrentcontrollerareoptimizedtoachievethesystemstability.II.MICROGRIDCONFIGURATIONAmicrogridconfigurationwithtwoDGisshowninFig.1.EachDGconsistsofDCsource,voltagesourceinverter(VSI)andLCfilterforrejectinghighfrequencyharmonics.Load1issensitiveload.Load2isnonsensitiveload.Undernormaloperation,themicrogridisapartofmaingrid.Inthismode,DGsinjectedpredefinedactiveandreactivepowersandmaingridregulatevoltageandfrequencyofmicrogrid.Whendisturbancesuchasfaultoccursinthemaingrid,theswitchkopensandmicrogridoperateinislandingmode.Hence,increasethereliabilityofthemicrogrid.Inislandingoperationmode,duetoabsenceofmaingrid,DGsshouldbeabletosharethepowermismatchtosupplyloadsandtomaintainpowerquality.Inthissituation,DGsoperateinV/fcontrolforcontrollingthevoltageandfrequencyofmicrogridandfeedingtheloads.Fig.1.MicrogridConfigurationIII.CONTROLSTRATEGYThissectionpresentsthecontrolstrategyforislandingoperationmodeofmicrogrid.Fig.2showstheV/fcontrollerandFig.3showsthepowercontrollerthatconsistsofpowercalculationanddroopcontroller.ThedroopcontrolisusedforsharingpowerbetweenDGsinislandingoperation.Fig.2.V/fControlPowercalculationblockcalculatesactiveandreactivepowersfrommeasuredinstantaneousvaluesofdaxisandqaxisvoltageandcurrent.Equations1and2showthecalculationprocedureofpowers:(1)()cododoqoqcwPVIVISw=++(2)()codoqoqodcwQVIVISw=+Thebasicideabehindthedroopcontrolistomimicthegovernorofsynchronousgenerator.Inaconventionalpowersystem,synchronousgeneratorswillshareanyincreaseintheloadbydecreasingthefrequencyaccordingtotheirgovernordroopcharacteristic.Thisprincipleisimplementedininvertersbydecreasingthereferencefrequencywhenthereisanincreaseintheload.Similarly,reactivepowerissharedbyintroducingadroopcharacteristicinthevoltagemagnitude.Inislandingoperation