外文翻译-电机保护原理

英文原文：MotorProtectionPrinciplesArijitBanerjee,ArvindTiwariJakovVico,CraigWesterGEGlobalResearch,GEMultilinBangalore,IndiaMarkham,Ontario,CanadaAbstractthispaperdiscussesthefundamentalsofmotorprotectionprinciples.everymotorisdesignedforaspecificoperatingtemperaturedependinguponitsinsulation.oncethislimitisexceededitslifedecreasesdrastically.overheatingconditionmayariseduetoseveralfactorslikesupplysystemdisturbance,electricalunbalanceconditions,faults,hispaperanoverviewofexistingprotectionmethodsformotorsisreviewedwithaspecialconsiderationofthermalprotectionofmotors.I.IntroductionThreephasemotorscanbeclassifiedintotwotypes:inductionandsynchronous.inductionmotorsaretheworkhorsesofmostindustrialsystems.inductionmotorsareusedonfans,blowers,conveyors,crushers,compressors,cranes,pumps,shredders,extruders,refinersandchillers.despiteheirruggednessandsimplicityinconstruction,themotorfailurerateperyearissignificant1.motorfailurerateisconservativelyestimatedas3-5%peryear,butinsomeindustrieslikemining,andpulpandpaper,motorfailureratecanbeashighas12%.motorfailurecostcontributorsare,repairorreplacement,removalandinstallationandlossofproduction.allfailurescanbedividedinthreegroups.onethirdoffailuresareelectricallyrelated,onethirdaremechanicallyrelatedfailuresandremainingonethirdoffailuresarerelatedtoenvironment,maintenanceandtheotherreasons.duetooccurrenceofsuchoutages,theindustryhastoreplaceorrepairthemotorandincurhugedowntime.thisnecessitatestherequirementofpropertheprotectionofthemachinetoensureproductivityunplannedoutages.mostofthemotorfailurecontributorsandfailedmotorcomponentsarerelatedtomotoroverheating.thermalstresscanpotentiallycausethefailureofallthemajormotorparts:stator,rotor,bearings,shaftandframe.therearetwomainrisksforanoverheatedmotor:statorwindingsinsulationdegradation(forstatorlimitedmotors)androtorconductorsdeformingormelting(forrotorlimitedmotors,wheremotorthermallimitisdefinedbyallowedmotorstalltime).Insulationlifetimedecreasesbyhalfifthemotoroperatingtemperatureexceedsitsthermallimitby10c(figure2).apartfromoverheating,thereareanumberofconditionsthatcanresultindamagetothree-phasemotorsalso.thesedamagesmaybeduetooperatingconditionsorinternalorexternalfaults.Externalfaultsandoperatingconditionsinclude:under-voltage,asymmetricalloading,phaseandgroundfaultsonthemotorfeederandoverloadingduringstartingandrunningoperation.Internalfaultsinclude:groundfaults,faultsbetweenwindingsandinter-turnfaults.Themotorprotectiondevice(MPD)mustbeabletopreventanydamageoccurringtothemotor.Inthefollowingsections,therequirementsofseveralprotectionmethodsarebrieflydiscussed.II.OvervoltageprotectionWhenaninductionmotorrunsinanovervoltagecondition,slip(beinginverselyproportionaltosquareofthevoltage)decreasesresultingindecreaseinloadcurrent.Simultaneously,magnetizingcurrentincreasesexponentiallyresultinginpoorerpowerfactor.Decreaseinloadcurrentresultsinlowercopperlosswhileincreaseinmagnetizingcurrentresultsinhighercoreloss.Althougholdmotorshadrobustdesign,newmotorsaredesignedclosetosaturationpointforbetterutilizationofcorematerialsandincreasingtheV/Hzratiocouldcausesaturationofairgapfluxleadingmotorheating.Thisleadstohigheroverallloss,lessefficiencyandhigheroperatingtemperatureofthemotor2.Inaddition,thedielectricstressontheinsulationofthemotorishigher.Thisnecessitatesrequirementofovervoltageprotectionforaninductionmotor.Theovervoltageelementshouldbesetto110%ofthemotorsnameplateunlessotherwisestatedinthemotordatasheets.III.Under-voltageprotectionWhenaninductionmotoroperatingatfullloadissubjectedtoanundervoltagecondition,slipincreases,powerfactorisincreasedandfullloadcurrentIncreases.Althoughstatorcorelossdecreases,rotorcopperlossandstatorcopperlossincreases.Thisleadstopoorerefficiencyandhighertemperatureriseforthemotor2.Theundervoltageprotectionelementcanbethoughtofabackupprotectionforthethermaloverloadelement.Insomecases,ifanundervoltageconditionexistsitmaybedesirabletotripthemotorfasterthantheoverloadelement.Theundervoltagetripshouldbesetto90%ofnameplateunlessotherwisestatedonthemotordatasheets.Motorsthatareconnectedtothesamesourcemayexperienceatemporaryundervoltage,whenoneofmotorsstarts.Tooverridethistemporaryvoltagesags,atimedelaysetpointshouldbesetgreaterthanthemotorstartingtime.IV.UnbalanceprotectionAvoltageimbalanceinamotorimpliesthatunequallinevoltagesarebeingappliedtothemotor.thiscanhappenduetoseveralreasonslikeunbalancesupply,faultyoperationofautomaticpowerfactorconnection,unevendistributionofsinglephaseloadonthesamefeederetc2.aspersymmetricalcomponenttheory,unbalancesetofpastorscanbedecomposedinbalancedsetsofpositive,negativeandzerosequencecomponents(figure3appearanceofnegativesequencecomponentclearlyindicatesunbalanceinthesupply.thenegative-sequencereactanceofthethree-phasemotoris5to7timessmallerthanpositivesquincereactance,andevenasmallunbalanceinthepowersupplywillcausehighnegativesequencecurrents.forexample,foraninductionmotorwithastartingcurrentsixtimesthefullloadcurrent,anegativesequencevoltagecomponentof1%correspondstoanegativesequencecurrentcomponentof6%.thenegativesequencecurrentinducesafieldintherotor,whichrotateswithdoublespeedintheoppositedirectiontothemechanicaldirectionandcausestorquepulsations,increasedmechanicalstressonthemotor,andadditionaltemperaturerise.itisessentialtomeasurethevoltageunbalanceandifitexceeds1%themotormustfederated.ifvoltageunbalanceexceeds5%themotorhastobeshutdown3.howeverforsmallandmediumsizedmotoritisoftencustomarytohaveonlycurrenttransformers(CTS)andnovoltagetransformers(VTS).thusinsteadofmeasuringvoltageunbalance,itisbettertodirectlymeasurecurrentunbalanceandprotectthemotor.apartfromsystemvoltageunbalance,currentunbalancecanalsobepresentduetolooseconnections,incorrectphaserotationconnection,statorturn-to-turnfaultsandsystemvoltagedistortion.theunbalancesettingsaredeterminedbyexaminingthemotorapplicationandmotordesign.theheatingeffectcausedbytheunbalancewillbeprotectedbyenablingtheunbalanceinputtothethermalmemory;describedlater.inaddition,separatecurrentunbalancetripandalarmsettingsmaybeapplied.forexample,asettingof10%xFLAforthecurrentunbalancealarmwithadelayof10secondsandatriplevelsettingof25%xFLAforthecurrentunbalancetripwithadelayof5secondswouldbeappropriate.V.GroundfaultprotectionGroundfaultsinamotorgenerallyoccurwhenitsphaseconductorsinsulationisdamagedforexampleduetovoltagestress,moistureoraninternalfaultoccurbetweentheconductorandground.agroundfaultcanbedetectedbyazerosequenceCTapproach,whichhashighsensitivityandinherentnoiseimmunity(figure4).AllphaseconductorsarepassedthroughthewindowofasingleCTreferredtoasazerosequenceCT4.Undernormalcircumstances,thethreephasecurrentswillsumtozeroresultinginanoutputofzerofromthezerosequenceCTssecondary.Ifoneofthemotorsphaseswereshortedtoground,thesumofthephasecurrentswouldnolongerequalzerocausingacurrenttoflowinthesecondaryofthezerosequenceCT.themotorprotectiondevice(MPD)detectsthiscurrentanddeclaresagroundfault.forlargecablesthatcannotbefitthroughthezerosequenceCTswindow,theresidualgroundfaultconfigurationcanbeused(figure6).thisconfigurationisinherentlylesssensitivethanthatofthezerosequenceconfigurationsandhastwomajordrawbacks.Duringstartingofamotor,themotorsphasecurrentsarenotonlyveryhigh(typically6timesthemotorsfullloadcurrent)butalsohaveasymmetry.consequently,thisresultsinadcoffsetcurrent.astheCTSarenotperfectlymatched,aslightmismatchoftheCTScombinedwiththerelativelylargephasecurrentmagnitudesalsoproduceafalseresidualcurrent.theseoffsetcurrentsasseenbytheMPDcanbemisinterpretedasagroundfaultunlessthegroundfaultelementspickupissethighenoughtodisregardthiserrorduringstarting.VI.DifferentialprotectionDifferentialprotectionmaybeconsideredthefirstlineofprotectionforinternalphase-to-phaseorphasetogroundfaults.Intheeventofsuchfaults,thequickresponseofthedifferentialelementmaylimitthedamagethatmayotherwiseoccurredtothemotor.thisprotectionfunctionismostlyusedtoprotectinductionandsynchronousmotorsagainstphase-to-phasefaults.Bothendsofthephaseconductormustbeavailabletousethisprotection.Thisfunctioncanbeimplementedintwoways.Inoneconfiguration,twosetsofCTs,oneatthebeginningofthemotorfeeder,andtheotherattheneutralpointcanbeused.alternatively,onesetofthreecore-balanceCTScanalsobeusedasshowninfigure8fordetection.Thedifferentialelementsubtractsthecurrentcomingoutofeachphasefromthecurrentgoingintoeachphaseandcomparestheresultordifferencewiththedifferentialpickuplevel.Ifthisdifferenceisequaltoorgreaterthenthepickuplevelatripwilloccur.ifsixCTSareusedinasummingconfigurationasshowninfigure9,duringmotorstarting,thevaluesfromthetwoCTSoneachphasemaynotbeequalastheCTSarenotperfectlyidenticalandasymmetricalcurrentsmaycausetheCTSoneachphasetohavedifferentoutputs.topreventnuisancetrippinginthisconfiguration,thedifferentiallevelmayhavetobesetlesssensitive,orthedifferentialtimedelaymayhavetobeextendedtoridethroughtheproblemperiodduringmotorstarting.therunningdifferentialdelaycanthenbefinetunedtoanapplicationsuchthatitrespondsveryfastandissensitivetolowdifferentialcurrentlevels.abiaseddifferentialprotectionmethod(figure10&11)allowsfordifferentratiosforsystem/lineandtheneutralCTs.thismethodhasadualslopecharacteristic.Themainpurposeofthepercent-slopecharacteristicistopreventamisspredationcausedbyunbalancesbetweenCTSduringexternalfaults.CTunbalancesariseasaresultofCTaccuracyerrorsorCTsaturation.thischaracteristicallowsforverysensitivesettingswhenthefaultcurrentislowandlesssensitivesettingswhenthefaultcurrentishighandCTperformancemayproduceincorrectoperatingsignals.VII.ShortcircuitprotectionTheshortcircuitelementprovidesprotectionforexcessivelyhighovercurrentfaults.Whenamotorstarts,thestartingcurrent(whichistypically6timestheFullLoadCurrent)hasasymmetricalcomponentsasshowninfigure12.Theseasymmetricalcurrentsmaycauseonephasetoseeasmuchas1.7timestheRMSstartingcurrent.Theruleofthumbistosettheshortcircuitprotectionpickuptoavalue,whichisatleast1.7timesthemaximumexpectedsymmetricalstartingcurrentofthemotor.Thisallowsthemotortostartwithoutnuisancetripping.Theshortcircuittripshouldbesetabovethemaximumlockedrotorcurrent,butbelowtheshortcircuitcurrentofthefuses.Forexamplemotordatasheetsindicateamaximumlockedrotorcurrentof540%FLCor5.4xFLC.Asettingof6xFLCwithainstantaneoustimedelaywillbeideal,butnuisancetrippingmayresultduetotheasymmetricalstartingcurrentsandDCoffset.Ifasymmetricalstartingcurrentslimitsthestartingcap-ability,itisrecommendedtosettheshortcircuittriplevelhighertoamaximumof9.2xFLCtooverridethiscondition(1.7x5.4=9.2)where1.7isthemaximumDCoffsetforanasymmetricalcurrent).With300:5CTandFLCof297A,9.2xFLC=9.2x297/300=9.10CT.Itisveryimportanttonotethat,whenanMPDdetectsashortcircuititgivesatripsignaltothebreakerorcontactorofthemotor.Thebreakerorcontactormusthaveaninterruptingcapacityequaltoorgreaterthanthemaximumavailablefaultcurrentotherwiseitmaycausepotentialdamagetotheequipmentordersonnet.Ifthebreakerorcontactordoesnothaveaninterruptingcapacityequaltoorgreaterthanthemaximumavailablefaultcurrent,itisrecommendedthatyouturnoffshortcircuitprotectionandletanupstreamdevicethatisratedtointerruptthefaulttoopenthecircuit.VIII.StatorRTDProtectionAsimplemethodtodeterminetheheatingwithinthemotoristomonitorthestatorwithRTDS.StatorRTDtriplevelshouldbesetatorbelowthemaximumtemperatureratingoftheinsulation.Forexample,amotorwithclassinsulationthathasatemperatureratingof155Ccouldhavethestatorrtdtriplevelbesetbetween140Cto150C,with150Cbeingthemaximum.thestatorRTDalarmlevelcouldbesettoaleveltoprovideawarningthatmotortemperatureisrising.IX.OverloadprotectionTheprotectionphilosophysofardealtwithfaultsinthemotororfaultinthesupply.howevercasesmayariseinwhichnosuchfaultoccurbutthemotorrunscontinuouslyinoverloadcondition.thetemperatureriseofthemotordictatesitssafeoperation.amotorcanrunonoverloadforshortperiodsoftimeprovideditstemperaturelimitisnotreached.directmonitoringofthetemperatureriseofthemotorcanprovidethermalprotection,howeverithasitsowninherentdrawbacks.firstly,thetemperaturesensorsitselfhassometimeconstantandresponsetime.Furtheritsisnotalwayspossibletofittemperaturesensorsinstatorofsquirrelcageinductionmotor.hencethereisaneedforincorporatingthermalprotectionbasedonefficientpredictionofmotortemperatureinMPD.temperatureriseinamotoriscausedduetopowerlossesinthemotor.Totalpowerdissipatedandthermalimpedancesofthemotorsareneededforproperestimationoftemperaturerise.powerlossesinaninductionmotorcanbebroadlyclassifiedintofollowwingcategories:1)Statorcopperloss2)Statorcoreloss3)Rotorcopperloss4)Rotorcoreloss5)Frictionloss6)Windageloss7)StrayloadlossThermalmodelingrangesfromthermalanalysisofthemachineusingFEM,lumpedparametermodel8,andusingsoftwarelikeMotor-CAD10.thesedetailedmodelingtoolsaredefinitelyvaluableforthemotordesignengineers,butforaprotectionengineertheprotectionphilosophyshouldbesimpleyeteffective.theprimarymotorprotectiveelementoftheMPDisthethermaloverloadelementandthisisaccomplishedthroughmotorthermalimagemodeling.thismodelmustaccountforthermalheatinginthemotorwhilethemotorisstarting,runningatnormalload,runningoverloadedandstopped.analgorithmofthethermalmodelintegratesbothstatorandrotorheatingintoasinglemodel.themotorthermallimitisgenerallypresentedintheformofatimecurrentcurveforthreepossiblemotoroverloadconditions:lockedrotororstallcondition,accelerationandrunningoverloadasshowninfigure13.thecurverepresentsthemaximumallowablesafetimeatastatorcurrentabovenormalloadforwhichthemotorcanoperate.a,bandcrepresentsthethermallimitcurvesatdifferentvoltagelevels.thesearenecessaryformotorsthataredesignedforlowvoltagestarting.ideally,time-currentcurveshavebeenprovidedforbothahotandcoldmotor.ahotmotorisdefinedasonethathasbeenrunningforaperiodoftimeatfullloadsuchthatthestatorandrotortemperatureshavesettledattheirratedtemperature.conversely,acoldmotorisdefinedasamotorthathasbeenstoppedforaperiodoftimesuchthattherotorandstatortemperatureshavesettledatambienttemperature.Formostmotors,hisparticularexample(figure13),therearethreeaccelerationcurvesshown:thecurveaistheaccelerationcurveat100%ratedvoltagewhilethecurvecistheaccelerationat80%ofratedvoltage.asoftstarteriscommonlyusedtoreducetheamountofinrushcurrentduringstarting.ascanbeseenonthecurveshown,sincethevoltageandcurrentarelower,ittakeslongerforthemotortostart.thereforestartingthemotoronaweaksystemcanresultinvoltagedepression,providingthesameeffectasasoft-start.thethermalmodeldevelopedinaMPDhefollowingsection,athermalmodelthateffectivelycorrelateswithstandardoverloadcurvesisreviewed.standardoverloadcurvesimplyrunningoverloadcurvesforthemotorsthathavestartingtimewellwithinthesafestalltime.thethermalmodelhasflexibilitysothatitcanincorporateprotectionfordifferentmotorsincludingmotorswithhighinertialoadormotorswithcyclicload.X.SingletimeconstantthermalmodelThetotalheatgeneratedinthemotorequalsthesummationoftotalheatstoredinthemotorandtotalheatdissipated.Mathewmetrically,wherec=thermalcapacitanceofthemotor,t(t)=temperatureriseaboveambient,i(t)=motorcurrent,r=electricalresistanceandh=runningheatdissipationfactor.ifcurrentandtemperatureareexpressedinperunitwithrespecttoratedconditionnamely,Irated(ratedcurrentforthemotor)andtmax(motortemperatureatthermallimittripcondition)respectively,(1)reducesthemaximumtemperaturerelatedtotheratedcurrentbythefollowingEquation(2)isrewrittenwiththehelpofisdefinedasmotorthermaltimeconstant.theabovedifferentialcanbeanalyzedforsteadyoverloadcondition.solutionoftheabovedifferentialequationatsteadyoverloadconditionisgivenby.whenforamotoritsthermallimitis,reachedT(t)=1and(5)canbeusedtoobtainthemaximumtime,denotedastmax(I),forwhichthemotorcanoperateatthegivencurrentlevelbeforethemotorreachesthermallimit.equation(6)canfurtherbesimplified,withaTaylorsseriesapproximationparticularlyforlargevaluesofcurrent,asshownin(7)andrepresentsstandardoverloadcurve.when(6)and(7)isplottedtheyareinalmostexactmatchathighercurrentsasinfigure14.thedeviationatlowercurrentvalueisnotimportant,aschangeintemperatureisslow.Howeverinpractice,boththecurvesareanapproximationtotheactualsystemwhereinacomplexmultipletimeconstantmotorhasbeenmodeledassingletimeconstantmodel.Eithercurvecanbeshiftedhorizontallyorverticallyandcanbematchedtomanufacturerspublishedcurve.inordertoincorporatethecomplexbehaviorofthemotor,flexibilityinthemodelmustbeprovidedsuchthatvariousfactorsareaccountedinthemodel.thethermaloverloadpickuphastobesettothemaximumvalueallowedbytheservicefactorofthemotor.itiscomm.onpracticetosetthethermaloverloadpickuptonomorethantheratedmotorfullloadcurrentplusnomorethan8-10%oftheservicefactorunlessthereisanotherindependentmeasureofmotortemperaturesuchasstatorRTDs.whenstatorRTDSaremonitoredandusedwithinthethermalmodel,thethermaloverloadpickupcanbesetashighas1.25formotorswith1.15SF(1.15formotorswith1.0SF).ifthemotorswindingtemperatureisalsobeingdirectlymonitoredbystatorRTDbiasingfunctiontothethermalmodel,thethermaloverloadpickupcanbesafelyincreasedtothemaximumallowablevalueforthatmotor.Themotorfeedercablesarenormallysizedat1.25timesthemotorsfullloadcurrentrating,whichwouldlimitthemotorthermaloverloadpickupsettingtoamaximumof125%.aparametercalledthermalcapacityused(TCU)thatisexpressedaspercentageofthethermallimitusedduringmotoroperationisusedfordefiningthethermalcontentofthemotor.TCUisincrementallyupdatedevery100millisecondsandtheintegratedvalueofTCUspecifiesthethermalstateofthemachinewhenTCUreachesunityor100%,themotorreachesthethermallimitandithastobetrippedorshutdown.Inordertoprovideacompletetherma