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《HighVoltageEquipmentTesting》LeakageCurrentandDCWithstandVoltageTest主講人:何發(fā)武(副教授)OperatingWorksheet:TableofContents01Followme02Workshop03Training01FollowmeⅠ.MeasurementPurposeTheprincipleofmeasuringleakagecurrentissimilartothemeasuringinsulationresistance,andthedetectabledefectsareroughlythesame.However,sincethemeasurementofleakagecurrentrequiresahighertestvoltagethaninsulationresistancetesting,themicroammetercanmonitortheleakagecurrentinthecircuitwithhighersensitivity.Therefore,itcandetectinsulationdefectsthatareonlyexposedunderhighervoltage.Theinsulationresistancevaluecalculatedfromtheleakagecurrentshouldbesimilartothevaluemeasuredbyamegohmmeter.Forexample,iftheleakagecurrentofatransformerincreasesfrom15Ato490A,a30-foldincrease,itwasfoundthatthecausewaswateringressduetoimperfectcasingsealing.Ⅰ.MeasurementPurposeDCwithstandvoltagetestingcaneffectivelydetectoveralldefectssuchasinsulationmoistureanddirt,andcanalsoidentifylocaldefectsininsulationthroughtherelationshipcurvebetweenvoltageandleakagecurrent.DCwithstandvoltagetestingcauseslessdamagetoinsulation,requiressmallertestingequipmentcapacity,andisconvenienttocarry.Ⅰ.MeasurementPurposeACwithstandvoltagetestingcaneffectivelydetectmoredangerousconcentrateddefects.Itisthemostdirectmethodfordeterminingtheinsulationstrengthofelectricalequipmentandhasadecisivesignificanceindeterminingwhetherelectricalequipmentcanbeputintooperation.Itisalsoanimportantmeanstoensureequipmentinsulationlevelandpreventinsulationaccidents.BothDCwithstandvoltagetestingandACwithstandvoltagetestingcaneffectivelydetectinsulationdefects,buttheyhavetheirowncharacteristics.Therefore,thesetwomethodscannotreplaceeachother.Ifnecessary,bothmethodsshouldbecarriedoutsimultaneouslytocomplementeachother.Ⅱ.TestDataStandardsTakingtheexampleofthezincoxidearrester,theDCleakagecurrenttestisconducted.TherequiredtestdataforU1mAand0.75U1mAofthezincoxidearresterareasfollows:ThemeasuredvalueofU1mAshouldhaveavariationoflessthan5%comparedtothefactoryorinitialvalue,andtheleakagecurrentat0.75U1mAshouldnotexceed50A.Otherwise,thezincoxidearrestermaybeagedordampened.Ⅱ.TestDataStandardsThepurposeofmeasuringthevoltageatU1mAinDCistofindthecriticalvalueatwhichthezincoxidearresterbreaksdown,checkifthevalvedisksaredamporaged,anddetermineiftheiroperationalperformancemeetstherequirements.ThepurposeofmeasuringtheDCleakagecurrentatthebreakdownvoltageof0.75U1mAistoinspecttheinsulationconditionofthezincoxidearresterwhenithasnotyetbrokendown.The0.75U1mADCvoltageisgenerallyslightlyhigherthanthemaximumworkingphasevoltage.Atthisvoltage,theprimaryobjectiveistotestwhetherthelong-termallowableoperatingcurrentcomplieswithregulations.Ⅱ.TestDataStandardsThesetwotestsarebeneficialforexaminingtheDCreferencevoltageofthemetal-oxidearresteranditschargingstateduringnormaloperation.Theyplayacrucialroleindeterminingthenumberofvalvedisks,assessingtherationalityoftheratedvoltageselection,andevaluatingtheagingstatus.Becausethiscurrentisdirectlyrelatedtothelifespanofthemetal-oxidearrester,theleakagecurrentandlifespanareinverselyproportionalatthesametemperature.Ⅱ.TestDataStandardsInthefield,thearrestermonitorisusedinserieswiththearrestertorecordtheworkingconditionsofthearrester.Thisisespeciallysuitableforarresterapplicationsinpowerstationsandlinesof35
kVandabove.Itcanalsobeusedinconjunctionwiththree-phasecombinedovervoltageprotectivedevices.Whenthearresteroperates,thedischargecountisaccumulatedandrecordedbyacounter.Thecounterusesathree-digitelectromagneticcounterthatautomaticallyresetsafterreachingthefullcount.Itoperatesinacycliccountingmodewithoutclearingtozero,asshowninFigure2-4.Leakagecurrentteststandardsasshownintable2-3.Ⅱ.TestDataStandards
(a)Hy5ws-17/50zincoxidelightningarrester
(b)110
kVlinearrester
(MetalOxideArrester,MOA)(c)JCQ8arrestermonitor
Figure2-4
ArresterⅡ.TestDataStandardsTable2-3
Leakagecurrentteststandards02WorkshopⅠ.DCHighVoltageGeneratorThesequenceandlocationformeasuringleakagecurrentonthedual-windingandthree-windingtransformerareshowninTable2-4.Figure2-5illustratestheconnectiondiagramfortheleakagecurrenttestprinciple.Figure2-6depictsthefunctionalschematicdiagramoftheDCgenerator.Table2-4
ThesequenceandlocationformeasuringleakagecurrentinTransformerWindingsⅠ.DCHighVoltageGeneratorFigure2-5
ConnectionprincipleschematicdiagramforleakagecurrenttestⅠ.DCHighVoltageGenerator
Figure2-6
FunctiondiagramofDCgeneratorcomponentsⅠ.DCHighVoltageGenerator
ThestandardreferenceforthetestvoltageisshowninTable2-5.Table2-5
StandardtestvoltagesforleakagecurrenttestingRatedVoltageofWinding/kV36–1520–35Over35DCTestVoltage/kV5102040Ⅰ.DCHighVoltageGeneratorDuringthemeasurement,afterthevoltageisraisedtothetestvoltage,thecurrentvalueobtainedbyreadingafter1minuteistheleakagecurrentvalue.Toensureaccuratereadings,themicroammetershouldbeconnectedtothehigh-voltageside.Itshouldalsobenotedthatforthetransformerwithoutoil,thevoltageappliedtothetransformerwhenmeasuringtheleakagecurrentshouldbe50%ofthevalueshowninthetable.Table2-6providesreferencevaluesforDCleakagecurrentintheoil-immersedpowertransformerwinding.Ⅰ.DCHighVoltageGeneratorTable2-6
ReferencevaluesforDCleakagecurrentinoil-immersedpowertransformerwindingsⅡ.TestConnectionTherearetwomethodsforconnectingtheDCleakagecurrentmeasurement:low-voltageconnectionmethodandhigh-voltageconnectionmethod.Thelow-voltageconnectionmethodinvolvesconnectingthemicroammetertothetailendofthehigh-voltagewindingofthetesttransformer.Sincethemicroammeterisonthelow-voltageside,itissaferandmoreconvenientforreading.However,thismethodcannoteliminatethemeasurementerrorscausedbyleakagecurrentsoninsulationsurfacesandcoronacurrentsonhigh-voltageleads.Therefore,thehigh-voltageconnectionmethodiscommonlyusedforon-sitetests.Figure2-7showsaschematicdiagramofthelow-voltageconnectionmethodformeasuringleakagecurrentwithamicroammeter.Ⅱ.TestConnectionFigure2-7
Schematicdiagramoflow-voltageconnectionmethodforleakagecurrentmeasurementwithamicroameterⅡ.TestConnectionThehigh-voltageconnectionmethodinvolvesconnectingthemicroammeterbeforethetestsample.Inthisconnectionmethod,sincethemicroammeterislocatedonthehigh-voltageside,itisplacedontheshieldingframeandconnectedtotheshieldingringofthetestsampleviaashieldingwire.Thisavoidsmeasurementerrorscausedbythewiring.However,placingthemicroammeteronthehigh-voltagesidecanbeinconvenientforreadingthemeasurements.Figure2-8showstheschematicdiagramofthehigh-voltageconnectionmethodformeasuringleakagecurrentwithamicroammeter.Ⅱ.TestConnectionFigure2-8
Schematicdiagramofhigh-voltageconnectionmethodforleakagecurrentmeasurementwithamicroammeterⅢ.DCLeakageCurrentTestProcessTestSteps:(1)RemoveordisconnectallexternalconnectionsoftheMOA(MetalOxideArrester)andgrounddischargeit.(2)Cleanthesurfaceandmaketheconnections.Afterverifyingthecorrectness,removethegroundingwireandstartthetest.(3)Confirmthatthevoltageoutputisatzero.TurnonthepowersupplyandstarttheDCgenerator.Graduallyadjustthevoltageboostknobuntiltheammeterreadingreaches1
mA.RecordtheDChigh-voltagevalueU1
mA(thecurrentreadingfromA1meterformeasuringtheuppersectionandlowersection,andfromA2meterformeasuringthemiddlesection).Ⅲ.DCLeakageCurrentTestProcess(4)Thesystemautomaticallyreducesthevoltageto0.75U1
mA.Readandrecordtheleakagecurrentvalueatthispoint,thenreducethevoltagetozero.(5)Disconnectthetestpowersupplyanduseadischargerodtogroundthetestsample.(6)Removethetestconnectionsandcleanthesite.Forthetestresults,theyaremainlyjudgedbycomparingthemwithprevioustestdata.Itisrequiredthatthereshouldbenosignificantchangeswhencomparedwithpreviousdata.Ingeneral,themeasuredvalueofthecurrentyearshouldnotexceed150%ofthemeasuredvaluefromthepreviousyear.Ifthevaluesincreaseyearbyyear,Payattentionthatitmayindicateinsulationdegradationovertime.Ifthevaluesuddenlyincreasescomparedtopreviousyears,itmayindicateaseriousdefect,andthecauseshouldbeinvestigated.Ⅳ.SpecialNotesForon-sitewithstandvoltagetestsofthecross-linkedpolyethylene(XLPE)powercable,directcurrent(DC)withstandvoltagemethodsshouldnotbeused.DCwithstandvoltagetestscanbeperformedonthepaper-insulatedcable.DuetothepresenceofinsulationdefectsinsidetheXLPEcable,thereisahighriskofpartialdischarge.ApplyingDCvoltageatthistimewillfurtheraccelerateinsulationagingandcauseelectricaltreeing.ThestructureoftheXLPEpowercablehasa“memoryeffect.”TheytendtoaccumulateresidualchargesunderDCvoltage,whichtakesalongtimetodissipate.Ⅳ.SpecialNotesIfthecablesareputintooperationbeforethechargesarecompletelydissipated,theDCbiaswillbesuperimposedontheACvoltage,leadingtoinsulationdamage.InsulationbreakdownpointsdetectedduringDCwithstandvoltagetestsareoftendifficulttobreakdownunderACoperatingconditions,whilepointspronetoinsulationbreakdowninACconditionsarefrequentlyundetectableinDCwithstandvoltagetests.Ⅳ.SpecialNotesTherefore,eventheXLPEpowercablethathaspassedDCwithstandvoltagetestsoftenexperiencesinsulationbreakdownaccidentsduringoperation.Therefore,0.1Hzverylowfrequency(VLF)testequipmentshouldbeusedfortheXLPEcable.DChighvoltagetestsarehalf-wavetestsandcannoteffectivelydetectinsulationdefectssuchaswatertreesincross-linkedpolyethylene(XLPE)insulation.Ⅳ.SpecialNotesItcanalsoleadtoinsulationbreakdownshortlyafterthehigh-voltagecableisputintooperationdespitepassingtheDCtest,thusfailingtoachievethedesiredtestingpurposeeffectively.Therefore,fortheXLPEcable,itismorecommontousevariablefrequencyseriesresonantwithstandvoltagetestequipmentforACwithstandvoltagetestsoncapacitivetestsamples.Thismethodhastheadvantagesofgoodpowerfrequencyequivalence,highfaultdetectionrate,minimaldamagetothetestsample,smallsize,andlightweightofthetestequipment,makingitsuitableforon-sitetransportation.Ⅳ.SpecialNotes(1)AftertheDCtest,itisnecessarytodischargetheresidualcharges.Thedischargeprocessconsistsofgroundingthrougharesistorandthendirectgrounding.Forlarge-capacitytestspecimenssuchaslargecapacitors,largemotors,longcables,etc.,thedischargetimeshouldnotbelessthan5minutestoensuresufficientdischargeofchargesonthetestspecimen.Eachstageofthemulti-stagevoltage-boostingrectifierdeviceshouldbefullydischargedbeforechangingthewiring.Ⅳ.SpecialNotes(2)Ifthereisanabnormalleakagecurrentduringthemeasurementprocess,methodssuchasdryingorshieldingcanbeusedtoassesstheinsulationstatusoftheequipment.Shieldedwiresshouldbeusedforhigh-voltageleadstopreventleakagecurrentfromaffectingtheresults.High-voltageleadsshouldnotproducecoronadischarge,andmicroammetersshouldbeusedtomeasurethehigh-voltageend.Ifasharpincreaseinleakagecurrent,ins
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