某变电所毕业设计中英文对照中英文翻译.docx
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某变电所毕业设计中英文对照中英文翻译
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A1.1原文
TRANSFORMER
1.INTRODUCTION
Thehigh-voltagetransmissionwasneedforthecaseelectricalpoweristobeprovidedatconsiderabledistancefromageneratingstation.Atsomepointthishighvoltagemustbereduced,becauseultimatelyismustsupplyaload.Thetransformermakesitpossibleforvariouspartsofapowersystemtooperateatdifferentvoltagelevels.Inthispaperwediscusspowertransformerprinciplesandapplications.
2.TOW-WINDINGTRANSFORMERS
Atransformerinitssimplestformconsistsoftwostationarycoilscoupledbyamutualmagneticflux.Thecoilsaresaidtobemutuallycoupledbecausetheylinkacommonflux.
Inpowerapplications,laminatedsteelcoretransformers(towhichthispaperisrestricted)areused.Transformersareefficientbecausetherotationallossesnormallyassociatedwithrotatingmachineareabsent,sorelativelylittlepowerislostwhentransformingpowerfromonevoltageleveltoanother.Typicalefficienciesareintherange92to99%,thehighervaluesapplyingtothelargerpowertransformers.
Thecurrentflowinginthecoilconnectedtotheacsourceiscalledtheprimarywindingorsimplytheprimary.Itsetsupthefluxφinthecore,whichvariesperiodicallybothinmagnitudeanddirection.Thefluxlinksthesecondcoil,calledthesecondarywindingorsimplysecondary.Thefluxischanging;therefore,itinducesavoltageinthesecondarybyelectromagneticinductioninaccordancewithLenz’slaw.Thustheprimaryreceivesitspowerfromthesourcewhilethesecondarysuppliesthispowertotheload.Thisactionisknownastransformeraction.
3.TRANSFORMERPRINCIPLES
WhenasinusoidalvoltageVpisappliedtotheprimarywiththesecondaryopen-circuited,therewillbenoenergytransfer.TheimpressedvoltagecausesasmallcurrentIθtoflowintheprimarywinding.Thisno-loadcurrenthastwofunctions:
(1)itproducesthemagneticfluxinthecore,whichvariessinusoidallybetweenzeroand
φm,whereφmisthemaximumvalueofthecoreflux;and
(2)itprovidesacomponenttoaccountforthehysteresisandeddycurrentlossesinthecore.Therecombinedlossesarenormallyreferredtoasthecorelosses.
Theno-loadcurrentIθisusuallyfewpercentoftheratedfull-loadcurrentofthetransformer(about2to5%).Sinceatno-loadtheprimarywindingactsasalargereactanceduetotheironcore,theno-loadcurrentwilllagtheprimaryvoltagebynearly90º.ItisreadilyseenthatthecurrentcomponentIm=I0sinθ0,calledthemagnetizingcurrent,is90ºinphasebehindtheprimaryvoltageVP.Itisthiscomponentthatsetsupthefluxinthecore;φisthereforeinphasewithIm.
Thesecondcomponent,Ie=I0sinθ0,isinphasewiththeprimaryvoltage.Itisthecurrentcomponentthatsupplies
thecorelosses.Thephasorsumofthesetwocomponentsrepresentstheno-loadcurrent,or
I0=Im+Ie
Itshouldbenotedthattheno-loadcurrentisdistortesandnonsinusoidal.Thisistheresultofthenonlinearbehaviorofthecorematerial.
Ifitisassumedthattherearenootherlossesinthetransformer,theinducedvoltageIntheprimary,Epandthatinthesecondary,Escanbeshown.Sincethemagneticfluxsetupbytheprimarywinding,therewillbeaninducedEMFEinthesecondarywindinginaccordancewithFaraday’slaw,namely,E=NΔφ/Δt.Thissamefluxalsolinkstheprimaryitself,inducinginitanEMF,Ep.Asdiscussedearlier,theinducedvoltagemustlagthefluxby90º,therefore,theyare180ºoutofphasewiththeappliedvoltage.Sincenocurrentflowsinthesecondarywinding,Es=Vs.Theno-loadprimarycurrentI0issmall,afewpercentoffull-loadcurrent.ThusthevoltageintheprimaryissmallandVpisnearlyequaltoEp.Theprimaryvoltageandtheresultingfluxaresinusoidal;thustheinducedquantitiesEpandEsvaryasasinefunction.Theaveragevalueoftheinducedvoltagegivenby
Eavg=turns×
whichisFaraday’slawappliedtoafinitetimeinterval.Itfollowsthat
Eavg=N
=4fNφm
whichNisthenumberofturnsonthewinding.Formaccircuittheory,theeffectiveorroot-mean-square(rms)voltageforasinewaveis1.11timestheaveragevoltage;thus
E=4.44fNφm
Sincethesamefluxlinkswiththeprimaryandsecondarywindings,thevoltageperturnineachwindingisthesame.Hence
Ep=4.44fNpφm
and
Es=4.44fNsφm
whereEpandEsarethenumberofturnontheprimaryandsecondarywindings,respectively.Theratioofprimarytosecondaryinducedvoltageiscalledthetransformationratio.Denotingthisratiobya,itisseenthat
a=
=
Assumethattheoutputpowerofatransformerequalsitsinputpower,notabadsumptioninpracticeconsideringthehighefficiencies.Whatwereallyaresayingisthatwearedealingwithanidealtransformer;thatis,ithasnolosses.Thus
Pm=Pout
or
VpIp×primaryPF=VsIs×secondaryPF
wherePFisthepowerfactor.Fortheabove-statedassumptionitmeansthatthepowerfactoronprimaryandsecondarysidesareequal;therefore
VpIp=VsIs
fromwhichisobtained
=
≌
≌a
Itshowsthatasanapproximationtheterminalvoltageratioequalstheturnsratio.Theprimaryandsecondarycurrent,ontheotherhand,areinverselyrelatedtotheturnsratio.Theturnsratiogivesameasureofhowmuchthesecondaryvoltageisraisedorloweredinrelationtotheprimaryvoltage.Tocalculatethevoltageregulation,weneedmoreinformation.
Theratiooftheterminalvoltagevariessomewhatdependingontheloadanditspowerfactor.Inpractice,thetransformationratioisobtainedfromthenameplatedata,whichlisttheprimaryandsecondaryvoltageunderfull-loadcondition.
WhenthesecondaryvoltageVsisreducedcomparedtotheprimaryvoltage,thetransformationissaidtobeastep-downtransformer:
conversely,ifthisvoltageisraised,itiscalledastep-uptransformer.Inastep-downtransformerthetransformationratioaisgreaterthanunity(a>1.0),whileforastep-uptransformeritissmallerthanunity(a<1.0).Intheeventthata=1,thetransformersecondaryvoltageequalstheprimaryvoltage.Thisisaspecialtypeoftransformerusedininstanceswhereelectricalisolationisrequiredbetweentheprimaryandsecondarycircuitwhilemaintainingthesamevoltagelevel.Therefore,thistransformerisgenerallyknowsasanisolationtransformer.
Asisapparent,itisthemagneticfluxinthecorethatformstheconnectinglinkbetweenprimaryandsecondarycircuit.Insection4itisshownhowtheprimarywindingcurrentadjustsitselftothesecondaryloadcurrentwhenthetransformersuppliesaload.
Lookingintothetransformerterminalsfromthesource,animpedanceisseenwhichbydefinitionequalsVp/Ip.From
=
≌
≌a,wehaveVp=aVsandIp=Is/a.IntermsofVsandIstheratioofVptoIpis
=
=
ButVs/IsistheloadimpedanceZLthuswecansaythat
Zm(primary)=a2ZL
Thisequationtellsusthatwhenanimpedanceisconnectedtothesecondaryside,itappearsfromthesourceasanimpedancehavingamagnitudethatisa2timesitsactualvalue.Wesaythattheloadimpedanceisreflectedorreferredtotheprimary.Itisthispropertyoftransformersthatisusedinimpedance-matchingapplications.
4.TRANSFORMERSUNDERLOAD
Theprimaryandsecondaryvoltagesshownhavesimilarpolarities,asindicatedbythe“dot-making”convention.
Thedotsneartheupperendsofthewindingshavethesamemeaningasincircuittheory;themarkedterminalshavethesamepolarity.Thuswhenaloadisconnectedtothesecondary,theinstantaneousloadcurrentisinthedirectionshown.Inotherwords,thepolaritymarkingssignifythatwhenpositivecurrententersbothwindingsatthemarkedterminals,theMMFsofthetwowindingsadd.
Sincethesecondaryvoltagedependsonthecorefluxφ0,itmustbeclearthatthefluxshouldnotchangeappreciablyifEsistoremainessentiallyconstantundernormalloadingconditions.Withtheloadconnected,acurrentIswillflowinthesecondarycircuit,becausetheinducedEMFEswillactasavoltagesource.ThesecondarycurrentproducesanMMFNsIsthatcreatesaflux.Thisfluxhassuchadirectionthatatanyinstantintimeitopposesthemainfluxthatcreateditinthefirstplace.Ofcourse,thisisLenz’slawinaction.ThustheMMFrepresentedbyNsIstendstoreducethecorefluxφ0.ThismeansthatthefluxlinkingtheprimarywindingreducesandconsequentlytheprimaryinducedvoltageEp,ThisreductionininducedvoltagecausesagreaterdifferencebetweentheimpressedvoltageandthecounterinducedEMF,therebyallowingmorecurrenttoflowintheprimary.ThefactthatprimarycurrentIpincreasesmeansthatthetwoconditionsstatedearlierarefulfilled:
(1)thepowerinputincreasestomatchthepoweroutput,and
(2)theprimaryMMFincreasestooffsetthetendencyofthesecondaryMMFtoreducetheflux.
Ingeneral,itwillbefoundthatthetransformerreactsalmostinstantaneouslytokeeptheresultantcorefluxessentiallyconstant.Moreover,thecorefluxφ0dropsveryslightlybetweennoloadandfullload(about1to3%),anecessaryconditionifEpistofallsufficientlytoallowanincreaseinIp.
Ontheprimaryside,Ip’isthecurrentthatflowsintheprimarytobalancethedemagnetizingeffectofIs.ItsMMFNpIp’setsupafluxlinkingtheprimaryonly.Sincethecorefluxφ0remainsconstant.I0mustbethesamecurrentthatenergizesthetransformeratnoload.TheprimarycurrentIpisthereforethesumofthecurrentIp’andI0.
Becausetheno-loadcurrentisrelativelysmall,itiscorrecttoassumethattheprimaryampere-turnsequalthesecondaryampere-turns,sinceitisunderthisconditionthatthecorefluxisessentiallyconstant.