翻译 改.docx

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翻译改

毕业设计（论文）外文参考资料及译文

译文题目：

PreventionofIslandinginGrid-connectedPhotovoltaicSystems

光伏系统中孤岛效应的预防

学生姓名：

陆煜斌学号：

1204102001

专业：

电气工程及其自动化

所在学院：

机电学院

指导教师：

王海祥

职称：

副教授

2016年3月4日

PreventionofIslandinginGrid-connectedPhotovoltaicSystems

Recentlytherehasbeenaresurgenceofconcernaboutislandingofgrid-connectedphotovoltaic（PV）systems.ThisconditionoccurswhenthePVsystemcontinuestoenergizeasectionofthegridafterthatsectionhasbeenisolatedfromthemainutilityvoltagesource.Generally,islandingisundesirablebecauseitposesasafetyhazardtoutilityservicepersonnel,andalsobecauseitcanleadtoasynchronousreclosurewhichcandamageequipment.ItisthereforeimportantthatPVsystemsincorporatemethodstopreventislanding.Thepurposeofthispaperisthreefold:

（1）tocriticallyreviewtheliteratureonislandingpreventionmethodsforPVsystemsanddiscusstheirstrengthsandshortcomings;

（2）toreviewandanalyzetheislandingbehavioroffourconverterswhichareprominentintheliteratureinordertodemonstratetheimplementationandeffectivenessofsomeislandingpreventionmethods;and（3）tointroduceanewislandingpreventionscheme,activefrequencydriftwithpositivefeedback,whichovercomesmanyoftheshortcomingsofexistingschemes.Itisconcludedthatno`perfect'islandingpreventionmethodyetexists,butalsothatmanyexistingmethodsorcombinationsthereofworkverywellinpracticalsituations.Finally,itisnotedthananinvestigationofwhatconstitutes‘sufficient’islandingpreventionisneeded.

INTRODUCTION

Inmanycountriesthroughouttheworld,programsarebeingimplementedtoencouragetheinstallationofgrid-connectedphotovoltaic（PV）systems.Theproliferationofsuchsystemshasledtoaresurgenceofconcernabouttheproblemofislanding.IslandingoccurswhenaPVsystemfeedspowerintoasectionoftheutilitysystemwhichhasbeenisolatedfromtheutilityvoltagesource.APVsystemconnectedtoafeederlinewhichisinturnconnectedtotheutilitygridthroughatransformerandsomesortofswitch（arecloser,breaker,fuseetc.）.ThePVsystemconsistsofaPVarrayandapowerconditioningunit（PCU）.Alocalloadisalsoconnectedtothefeederline.Iftheswitchwereopened,undercertainconditionsitispossibleforthePVPCUtocontinuetoenergizetheisolatedsectionofthegridandsupplypowertothelocalload.Thisisislanding,andtheisolatedsectionoftheutilitybeingpoweredbythePVsystemisreferredtoasanislandofsupplyorsimplyanisland.Utilitiesfrequentlyusetheterm‘renewableenergyisland'todifferentiatethissituationfromothertypesofislands.Althoughthisdistinctionisfrequentlyimportant,forbrevitywewillusetheterm`island'throughoutthispaperwithoutambiguity.

TheamountoftimebetweenthedisconnectionoftheutilityandtheshutdownofthePCUisreferredtoastherun-ontime.Islandingeventsaretypicallysubdividedintotwocategories:

long-term,withrun-ontimesof1secormore,andshort-term,withrun-ontimesoflessthan1sec.

Theprimaryconcernwithlong-termislandingisoneofsafety.Maintenanceorrepairpersonnelarrivingtoservicetheisolatedfeedermaybeunawarethatitisstillenergized,whichcouldleadtopersonalinjury.Thisisofparticularlygreatconcerninthecaseofscheduledmaintenance,whentheswitchwouldbemanuallyoperatedbyservicepersonnelwhowillimmediatelycommenceworkontheisolatedsystem.Inthiscase,islandingofevenafewtensofsecondscouldbedangerous.

Anotherproblemassociatedwithbothlong-andshort-termislandingisthatthePVsystem,whichreliesontheutilityvoltagetoprovideaphaseandfrequencyreferenceforitsoutputcurrent,maylosesynchronismwiththeutilitywhiletheswitchisopen.TheutilitycouldthenrecloseonaPVsystemwhichisoutofphase.MostPVPCUsaretwoquadrantdevices,designedforunidirectionalpowerflowfromtheDCtoACsideonly.Thisisdoneforeconomicreasons;two-quadrantconvertersarelessexpensivethanfour-quadrantconverters.However,duringanout-of-phasereclosure,thereareintervalsinwhichthepolaritiesofthevoltageandcurrentareopposite.Duringtheseintervals,theconverterisabsorbingpowerfromboththePVandutilitysides,whichcanleadtodestructivecomponentfailuresinthePCU.

Ithasbeenpostulatedthatanotherpossibleproblemwithshort-termislandingisthatitcaninterferewiththearc-clearingfunctionofprotectiverelays.9However,thereismuchdebateoverwhetherthisisasignificantissue.Thissubjectwillbediscussedinmoredetailshortly.

Onefinalproblemwhichisincreasinginrelevanceisthatsomeislandingpreventionmethodsinterferewitheachother,leadingtolongerrun-ontimesandpossiblyfailuretodetectislandingifseveralPVsystemsarepresentintheisland.Insomecases,thiscanhappenevenifallthePVsystemsintheislandareusingthesameislandingpreventionscheme.Thissituationhasbeentermedthe`multi-invertercase',anditcouldbecomeincreasinglycommonwiththeproliferationofsmall,roof-mountedPVsystemsandthedevelopmentofPCUsforACPVarrays,inwhichcasetherecouldbetensorevenhundredsofPCUsinanisland.

Aconsiderablebodyofworkexistsonislandingandislandingpreventionmethods.Thepurposeofthispaperisthreefold:

first,tocollectandreviewtheexistingmethods,discusstheirstrengthsandweaknesses,andcomparethem;second,toreviewandanalyzethebehavioroffourexistingconverterstodemonstratetheeffectivenessandimplementationofthesemethods;andthird,toproposeanovelislandingpreventionscheme,activefrequencydriftwithpositivefeedback（AFDPF）,whichlargelyovercomestheshortcomingsoftheexistingmethods.

ThereactanceinsertionmethodisuniqueamongislandingpreventionmethodsinthatitdoesnotrelyonthePCUtodetecttheislandingcondition.Instead,alargereactance,usuallyacapacitorbank,isinstalledontheutilitysysteminsidethepotentialislandatpointbasshowninFigure10.Theswitchisnormallyopen.Whentherecloseropens,thecapacitorbankswitchclosesafterashortdelay.InthecasewhereDPandDQwerenearlyzero,thesuddenadditionofalargereactiveimpedancewillimbalancethereactivepowerrequirement,causingafrequencydecreasewhichtheUFRcandetect.Thismethodothersseveraladvantages.Itishighlyeffectiveinpreventingislanding1,2aslongasthesmalldelayisallowedbetweenthetimeofrecloseropeningandthetimeofcapacitorinsertion.ThisdelayisnecessarytoensurethatinsertionofthecapacitorwillnotactuallycreateabalancedsituationbetweenthePVsystemandalaggingload.Capacitorsofthistypearereadilyavailable,andutilitieshaveagreatdealofexperiencewiththem.Thesamecapacitorbankcouldalsobeusedforreactivepowersupport,withonlyminorchangesintheswitchinglogictoallowittomaintainitsunit-islandingfunction.However,tworeadily-apparentdrawbackstothereactanceinsertionmethodare:

（1）theremaybemultipleswitchesinseriesleadingintothepotentialisland,meaningthateachseriesswitchmightneedtobeequippedwithaswitchablecapacitorbank（dependingontheloadconfiguration）;

（2）thismethodcannotpreventshort-termislanding,partiallybecauseofthespeedofactionofthecapacitorswitchesandpartiallybecauseofthenecessarydelayinswitching.Thismethodactuallyhasathirdproblemwhichismorepoliticalthantechnical:

itrequirestheinstallationofequipmentontheutilitysideofthepointofcommoncoupling,whichisusuallytakentobetheutility'selectricmeter.Utilitiesgenerallylookunfavourablyonsuchanarrangement.

METHODSOFELIMINATINGTHENDZ

PassivemethodsforislandingpreventioninvolvemonitoringthePCU'sterminalvoltageforsomeconditionwhichindicatesislanding.Thesemethodsarediscussedbelow.

Thevoltageharmonicmonitoringmethoddoesnotrelyonarealorreactivepowermismatchforislandingdetection.Instead,thePVPCUmonitorsthetotalharmonicdistortion（THD）ofvaandshutsdownifthisTHDexceedssomethreshold.Therearetwomechanismswhichcancausetheharmonicsinvatoincreasewhenislandingbegins.OneoftheseisthePVPCUitself.APVPCUwillproducesomecurrentharmonicsinitsACoutputcurrent,asallswitchingpowerconvertersdo.Atypicalrequirementforagrid-connectedPVPCUisthatitproducenomorethan5%THDofitsfullratedcurrent.Summingatnodea,weseethatunderthisconditiontheharmoniccurrentsproducedbythePCUwillflowoutintothegrid.Whentherecloseropens,theharmoniccurrentsproducedbythePCUwillflowintotheload,andbyOhm'slawandsuperposition,thesewillproduceharmonicsinva.18ThesevoltageharmonicscanbedetectedbythePCU,whichcanthenassumethatthePVsystemisislandinganddiscontinueoperation.

Iftheswitchwhichdisconnectstheutilityvoltagesourcefromtheislandisontheprimarysideofthetransformer,thesecondaryofthetransformerwillbeexcitedbytheoutputcurrentofthePVsystem.However,becauseofthemagnetichysteresisofthetransformer,thevoltageresponseofthetransformertothis（approximately）sinusoidalexcitationishighlydistorted.Inparticular,itcontainsalargethirdharmoniccomponent.Thisphenomenonhasbeenverifiedexperimentally;investigatorshavefoundthatthethirdharmonicvoltageatnodeacanincreasebyafactoroffiveormorewhenislandingbeginsbecauseofthedistortionintroducedbystandardpole-mountedtransformers.

Intheory,thevoltageharmonicmonitoringmethodpromisestobehighlysuccess