含风电场的电力系统静态电压稳定性分析翻译.docx

含风电场的电力系统静态电压稳定性分析翻译.docx

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含风电场的电力系统静态电压稳定性分析翻译

燕山大学

本科毕业设计（论文）英文翻译

课题名称：

含风电场的电力系统静态电压稳定性研究

学院（系）：

电气工程学院

年级专业：

09级电力四班

学生姓名：

张建春

指导教师：

王珺

完成日期：

2013.5.8

一Theinterductionofsteady-StateCharacteristics

thespeed-torquecharacteristicisquitelineararoundsynchronousspeed.Iftherotorspeedisbelowsynchronousspeed，theinductionmachineisoperatingasamotorandiftherotorspeedisabovesynchronousspeed，theinductionmachineisrunningasagenerator.themechanicalpowerandthemechanicaltorquearegivenbytheslip,rotorresistanceandtherotorcurrent.Thespeed-torquecharacteristicoftheinductionmachineisquitelineararoundsynchronousspeed.

thetorqueisproportionaltotheinverseoftherotorresistance.Thisimpliesthatitispossibletohaveexternalrotorresistancesconnectedinserieswiththeexistingrotorresistancesofawound-rotorinductionmachine.Bychangingthevalueoftheexternalrotorresistanceitispossibletochangetheslopeofthespeed-torquecharacteristic.Onedisadvantagewiththismethodisthatitisonlypossibletoincreasetheslipusingtheexternalrotorresistances.Thisimpliesthatiftheinductionmachineisrunningasamotor,thenanincreasedrotorresistancewilldecreasetherotorspeed.Ontheotherhand,iftheinductionmachineisrunningasagenerator,theniftherotorresistanceincreases,therotorspeedwillalsoincrease.

Beforesemiconductorswereavailable,onewayofadjustingtheslipwastointroduceexternalrotorresistances.Theexternalrotorresistancewillcauseadditionallossesintherotorcircuit.Whensemiconductorsbecameavailableitwaspossibletorecovertheslipotherwisedissipatedintheexternalrotorresistance.Thus,theslippowercanberecoveredintomechanicalorelectricalenergy;therefore,thismethodiscalled"slippowerrecovery."Therotorcurrentmustberectifiedwithadioderectifier.Formotoroperation,therotorcircuitwillseethedioderectifierasaresistanceandthereforethismethodwillworkapproximatelyinthesamewayasfortheexternalrotorresistances.Notethatthedioderectifiercannotbeusedingeneratoroperation.

Therectifiedcurrentcouldbeconvertedtomechanicalpowerusingadcmotorcoupledtotheshaftoftheinductionmotororfedbackintothegrid.SinceKramerdriverequireanextradcmotoritisofnointerest,whiletheScherbiusdriveisstillinuse.Themainadvantageofthisconfigurationcomparedtotheexternalrotorresistanceisthatthelossesoftheexternalrotorresistancecanberecovered.Ifbothstatorvoltageandfrequencycanbeadjustedbyaninverter,thetorque-speedcharacteristiccanbeeasilychanged.Whenthespeedisincreasedsothatthestatorvoltagereachesmaximumvoltage,thereisneedforfieldweakening,thestatorvoltageiskeptconstantwhilethefrequencyisstillincreased.

二Theinterductionofdoubly-FedInductionMachines

Doubly-fedmachinescanbeusedinvariable-speedconstant-frequencyapplications,suchaswindturbines.Themainadvantageofadoubly-fedmachinecomparedtoasingly-fedforavariable-speedsystemisthereducedratingoftheconverter'spowerrating.Thereductioninpowerratingisdependentonthespeedrangeofthedrive.Thestandarddoubly-fedinductionmachineisawoundrotorinductionmachineequippedwithsliprings.Thestatorcircuitisconnecteddirectlytothegridwhiletherotorcircuitiscontrolledbyaninverterviasliprings.

Thecascadeddoubly-fedinductionmachineconsistsoftwodoubly-fedinductionmachineswithwoundrotors,thatareconnectedmechanicallythroughtherotorandelectricallythroughtherotorcircuits.Thestatorcircuitofoneofthemachinesisdirectlyconnectedtothegridwhiletheothermachine'sstatorisconnectedviaaninvertertothegrid.Sincetherotorvoltagesofbothmachinesareequal,itispossibletocontroltheinductionmachinethatisdirectlyconnectedtothegridwiththeotherinductionmachine.

Itisdoubtfulwhetheritispracticaltocombinetwoindividualmachinestoformacascadeddoubly-fedinductionmachine,eventhoughitisthebasicconfigurationofdoubly-fedinductionmachinearrangement.Duetoalargeamountofwindings,thelossesareexpectedtobehigherthanforastandarddoubly-fedinductionmachineofacomparablerating.

三StabilityofaPowerSystem

Powersystemstabilityisunderstoodastheabilitytoregainanequilibriumstateafterbeingsubjectedtoaphysicaldisturbance.threequantitiesareimportantforpowersystemoperation:

（i）anglesofnodalvoltages,alsocalledpowerorloadangles;（ii）frequency;and（iii）nodalvoltagemagnitudes.Thesequantitiesareespeciallyimportantfromthepointofviewofdefiningandclassifyingpowersystemstability.Hencepowersystemstabilitycanbedividedinto:

（i）rotor（orpower）anglestability;（ii）frequencystability;and（iii）voltagestability.Aspowersystemsarenonlinear,theirstabilitydependsonboththeinitialconditionsandthesizeofadisturbance.Consequently,angleandvoltagestabilitycanbedividedintosmall-disturbanceandlarge-disturbancestability.

Powersystemstabilityismainlyconnectedwithelectromechanicalphenomena.However,itisalsoaffectedbyfastelectromagneticphenomenaandslowthermodynamicphenomena.Hence,dependingonthetypeofphenomena,onecanrefertoshort-termstabilityandlong-termstability.

四ConnectionsofWindFarms

Althoughthemajorityofwindturbinesaresituatedonland,thereisagrowingdemandforwindturbinestobeplacedoffshorewithsomelargewindfarmsnowoperational.Thisdoesnotmeanthatoffshoresitesarealwaysbetterthanthoseonshore,assomeonshoresiteshavebetterwindregimesthansitesoffshore.Acommonproblemtoalloffshoreenergyconversionsystemsistheelectricalcableconnectiontotheonshoresubstation.andthisthenraisesdistanceissuesbecauseallACcableshavehighcapacitanceandthelinechargingcurrentforlongcablerunscanbeveryhigh.whileanumberofindependentcablerunsmaybenecessaryinordertotransmittherequiredpowerfromanoffshorewindfarm.

BecauseofthelargecablecapacitanceACcablesarecurrentlylimitedtoadistanceundertheseaofabout100-150kmwiththemaximumratingofthree-coresubmarinecablescurrentlybeingabout200MWat145kV,althoughlargerratingsareunderdevelopment.Generallytheoutputsofanumberofturbinesarecollectedtogetheratanoffshoresubstationforonwardtransmissiontoshore.Oncetheoutputofanumberofturbineshasbeencollected,analternativetoACtransmissiontoshoreistouseDCtransmission.NewDCtransmissiontechnologyusesIGBTvoltagesourceconvertersatthesendingend（andpossiblyalsoatthereceivingend）allowingtotalcontrolatthesendingend.Forhigherpowers,conventionalDCtechnologyusingGTOscanbeused.

CurrentlyoffshorewindfarmsaresufficientlyclosetoshorethatACcablescanbeused,althoughanumberofcablesmaybenecessarytotransmittherequiredpower.Onepracticalpointtonoteisthatthedistancetoshorealsoincludestheshore-basedcableruntotheshoresubstation.Insomesituationsthiscanbesubstantial.

Theproblemsassociatedwithtransferringelectricalpowertoshorefromoffshorewindfarmsisalsofacedbytidalstreamgeneratorsandwavegenerators.Tidalstreamgeneratorstendtoberelativelyclosetoshore,althoughlayingcablesinthestrongcurrentswheretheseturbinesaresituatedisnotstraightforward.Waveenergyisinitsinfancywiththelargeamountsofresourceavailable.Harnessingthisenergyandtransferringittoshoreposesasignificantchallenge.

五InfluenceofWindGeneratorsonPowerSystemStability

Thesynchronousgeneratorisstifflyconnectedtothepowersystemandexhibitsaninherentlyoscillatoryresponsetoadisturbancebecauseitspoweroutputisapproximatelyproportionaltothesineoftherotorangle.Forsmallvaluesoftherotorangle,powerisproportionaltotheangleitselfwhichproducesspring-likeoscillations.Ontheotherhand,squirrel-cage（fixed-speed）inductiongeneratorsarecoupledtothegridlessstifflythansynchronousgenerators.thetorqueofafixedspeedinductiongeneratorisproportionaltothespeeddeviation（slip）henceprovidinginherentdampingofoscillations.Thispositiveinfluenceiscounteractedbythevulnerabilityoffixed-speedinductiongeneratorstosystemfaults.DampingduetovariablespeedDFIGsdependsverymuchontheparticularcontrolstrategyemployed.theDFIGshavegoodcontrolcapabilitiesduetothepossibilityofcontrollingboththemagnitudeandphaseoftheinjectedvoltage.Thismakesitpossibletodesignapowersystemstabilizerthatimprovesthedampingofpowerswingswithoutdegradingthequalityofvoltagecontrolprovided.

Fullyratedconvertersystemseffectivelydecouplethegeneratorfromthegrid,sotheyofferaverygoodpossibilityofimprovingthedampingofpowerswings.Hencethegeneralconclusionisthatapartialreplacementoftraditionalthermalplantsemployingsynchronousgenerators,whichexhibitarelativepoornaturaldamping,byrenewablegenerators,whichexhibitabetterdamping,willimprovethedampingofelectromechanicalswings.Thiseffectwillbecounterbalancedtosomeextentbythehighlyvariablenatureofrenewablesourcesthemselves,suchaswind,marineorsolar,buttheirvariabilitymaybeeffectivelymanagedbyeitherusingenergystorageorpartloadingoneoftheturbinesinafarmandusingitssparecapacitytosmoothpoweroscillations.

Thenetworkeffectofreplacinglargetraditionalgeneratorsbyrenewableoneswilllargelydependonthesysteminquestion.Recallthatthestabilityofsynchronousgenerator