Electrical Notes & Articles _ Sharing Abstracts,Notes on Various Electrical Engineering Topics

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SharingAbstracts,NotesonvariousElectricalEngineeringTopics.

SelectionofSurgeProtectiveDevice(SPD)(Part3)

JULY6,2015 2COMMENTS(HTTPS://ELECTRICALNOTES.WORDPRESS.COM/2015/07/06/SELECTIONOFSURGEPROTECTIVEDEVICESPDPART3/#COMMENTS)

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TypeofSPD:

Type1SPD:

Protectionfor:TransientOvervoltagesduetoDirectLightningStrokesLocation:Itisinstalledatanylocationbetweenthesecondaryoftheutilityservicetransformerandtheserviceentranceprimarydisconnection

Itisinstalledinthemainelectricalswitchboardwhenthebuildingisequippedwithalightningprotectionsystem.Itprotectsagainstexternalsurgescausedbylightningorutilitycapacitorbankswitching.Thesedevicestodischargingaveryhighlightningcurrentfromearthtothepowerdistributionsystem.

Currentratings:10Kato35Ka10/350swaveform.

RequiredDedicatedFuse/CircuitBreakerforSPD:NoRiskFactor:VerystrongriskArea

Type2SPD:

Protectionfor:TransientOvervoltagesduetoSwitchingandIndirectLightningStroke.Location:Itisinstalledinthemaindistributionswitchboard.

Itisdesignedtodischargethecurrentsgeneratedbyindirectlightningstrokesandcausinginducedorconductedovervoltageonthepowerdistributionnetwork.Itprotectsagainstresiduallightningenergy,motordrivensurgesandotherinternallygeneratedsurges.

Currentratings:5Kato200Ka8/20swaveform.RequiredDedicatedFuse/CircuitBreakerforSPD:MayorMayNot

RiskFactor:CommonriskArea

Type3SPD:

Protectionfor:SensitiveLoads.ItisinstalledasasupplementtoType2devicesandtoreducetheovervoltageattheterminalsofsensitiveequipment.Theircurrentdischargecapacityisverylimited.Asaconsequencetheycannotbeusedalone.Installedatminimumconductorlengthof10meters(30feet)fromtheelectricalservicepaneltothepointofutilizationProvidespointofuseprotection,easilyreplaceableanditprovidesthelastlineofdefenseagainstalightningstrike.RiskFactor:Verystrong&commonriskArea

ConnectionofSPDinDistributionBox.

Incommonmode:PhasetoearthorneutraltoearthIndifferentialmode:Phasetophaseorphasetoneutral

(https://electricalnotes.files.wordpress.com/2015/07/0.png)

FactorseffectonSPDPerformance:

(1)LocationofSurgeProtectionDevice:

LightningprotectionshouldbeinstalledonaoverallviewpointofProtection.Forlargeindustrialplants,datacenters,hospitals,ariskassessmentmethodmustbeusedtoguideinchoosingoptimaldistance.Inothercaseslikehousing,offices,buildingsWherethereisnotorlesssensitiveindustrialrisks,wemayadoptfollowingprincipletoselectSPD.Type2surgeprotectivedeviceshouldbeinstalledintheelectricalinstallationsincomingMainswitchboard.Ifthedistancebetweenthatsurgeprotectivedeviceandtheequipmenttobeprotectedismorethan30meters,thanadditionalsurgeprotectivedevice(Type2orType3)shouldbeinstalledneartheequipment.


Whenthebuildingisequippedwithalightningprotectionsystem,aType1surgeprotectivedevicemustbeinstalledattheincomingMainSwitchBoard.ThereexistsurgeprotectivedevicescombiningType1andType2inthesameenclosure.


TheLightningrodshavetobelocatedonthehighestpointsofthestructure,takingintoaccountthelocationofthegrounding,andthatthepathofthedownconductorsareasshortandstraightaspossible.

(2)SizeofDownConductor:

Lightningisaphenomenonthatgeneratesahighfrequencyvoltage.Thelengthofthecablesmustbetakenintoaccountincasesofhighfrequency.Thedownconductorsmaybetapes,strandedwireorsolidround.Theminimumcrosssectionmustbe1meterofcablecrossedbyalightningcurrentgeneratesanovervoltageof1,000V.MandatoryinStandardIEC603645534:L(lengthofcables)

(4)NumberofDownConductor

Atleastonedownconductorforeverylightningrod.Aminimumoftwodownconductorwhen,(1)ThehorizontalProjectionlengthoftheconductorexceedsitsverticalprojectionlength.(2)Theheightofthestructureisgreaterthan28meter.Equipotentialbondingwillbemadebetweentheconductorsatgroundlevelandevery20meters.AccordingtoUNE21186:(1)Eachlightningrodshallbegroundedbytwodownconductor.(2)Itwillbenecessary4downconductorsonbuildingshigherthan60meters.(3)Itshouldbeplacedwheneverpossibleinthe4cornersofthebuilding.

(5)PathofDownConductor

Thedownconductorrouteswilltheshortestpath,straightanddirecttogrounding.Weshouldavoidingelevationsabove40cmwithslopeequaltoorgreaterthan45.Theradiiofcurvesshallnotbelessthan20cmanddirectionchangeslessthan90.Theroutewillbechosensoastoavoidproximitytoelectricalconduits,telephone,dataanditscrossingwiththem.Inanycase,whenwecannotavoidanintersectionconduitsmustbeplacedinsideametallicshieldthatextended1mtoeachsideofthecrossing,andtheshieldshouldbindtothedownconductor.

(6)SafetyDistance:

AccordingCTESU8:Safetydistance(m)=0.1xLL=verticaldistancefromthepointwhereitisconsideredtheproximitytothegroundingofthemetalmassSafetydistancetooutdoorgaspipelines5m.AccordingtoUNE21186:Safetydistancefor1noofdownconductor(m)=0.16xLSafetydistancefor2Noofdownconductors(m)=0.08xLSafetydistancefor4Noofdownconductors(m)=0.04xLL=lengthofthedownconductorfromthepointwhereitisconsideredtheseparationdistancetothepointwhereislocatedthenearestequipotentialpoint.

(7)Earthing:

Therewillbe1groundingsystemforeachdownconductor.

(8)LightingCounter:

Thelightningcountermustbeinstalledoverthemoredirectdownconductor,abovethejointcontrol,andinallcases,about2metersabovetheground.

(9)SizeofSurgeProtectionDevice:

Atype2surgeprotectivedevicedependsmainlyontheexposurezone(moderate,medium,high).Type2SPDhasdischargecapacity(Imax)of20kA,40kA,65kA(8/20s).Type1SPDhasminimumdischargecapacity(Imax)of12.5kA(10/350).Highervaluesmayberequiredbytheriskassessmentwhenitsrequired.Forresidentialorlightcommerciallocations:asurgecurrentratingof20kAto70kA(8/20s)perphaseshouldbesufficient.InstallationsinForhighlightningareas:SPDswithhighersurgecurrentratingsof40kAto120kA,toprovidealongerservicelifeandhigherreliability.

FILEDUNDERUNCATEGORIZED


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SizeofSurgeProtectionDevice(SPD)dependsuponLocationofPanel:

Panellocationwithintheelectricalsystemismoreimportantthanthepanelssize.Thelocationofthepanelwithinthefacilityismuchmoreimportant.IEEEC62.41.2definesthetypesofexpectedsurgeswithinafacilityas:CategoryC:ServiceEntrance,moresevereenvironment:10kV,10kAsurgeCategoryB:Downstreammorethan30feetfromcategoryC,lesssevereenvironment:6kV,3kAsurgeCategoryA:Furtherdownstream,morethan60feetfromcategoryC,leastsevereenvironment:6kV,0.5kAsurgeWhenselectingtheappropriatekAratingforanSPD.CategoryC:100kAto200kAperphaseCategoryB:50kAto100kAperphaseCategoryA:50kAto100kAperphase

LargeSizeofSurgeProtectionDevice(SPD)doesnotgivebetterProtection:

MostSPDsuseametaloxidevaristor(MOV)asthemainlimitingdevice.IfanMOVisratedfor10kAandhavinga10kAsurge,itwoulduse100%ofitscapacity.ThesurgewilldegradetheMOValittlebit.Nowifweuse20KASPDsothisSPDhastwo10kAMOVsinparallel.TheMOVswillequallysplitthe10kAsurge,soeachwouldtake5kA.Inthiscase,eachMOVhaveonlyused50%oftheircapacitywhichdegradestheMOVmuchlessthan10KASPDAgainItistotallymisleadingthattwoparallelpath(in20KASPD)absorbsurgefasterorbetterthansinglepathSPD(like10KASPD)ofsamerating.ThemainpurposeofhavingMOVsinparallelistoincreasethelongevityorLifeoftheSPD.Again,Itisneedtoclearthatitissubjectiveandatsomepointweareonlyaddingcostby

incorporatingmoreMOVsandreceivinglittlebenefit.LargerkAratingsareforredundancy&longerlifeonly.

SPDcannotgive100%ProtectionagainstAllTypesofelectricaldisturbance

ThereisamisconceptionaboutSPDsisthattheyaredesignedtoprotectagainstallElectricalproblems.SPDisnotdesignedtoprotectagainstexcessivevoltageatthefundamentalpowerfrequency.Itisdesigntogiveprotectionagainstsurges(bydirectlightingorvoltagesurgesinlineatremotelocation).SPDcannotgiveProtectionagainstPoorPowerQuality(Harmonics)SomeSPDscontainfilteringtoremovehighfrequencynoise(50kHzto250kHz),ButSPDcannotfilterharmonicloads(3rdthrough50thharmonicequals180to3000Hz).SPDcannotgiveProtectionagainstUnderVoltage.SPDcannotgiveprotectionagainstundervoltageproblems.SPDcannotgiveProtectionagainstdirectlightingStrikes.AnSPDcannotpreventdamagecausedbyadirectlightningstrike.AdirectlightningstrikecausesinducedsurgesonthepowerlinethatarereducedbytheSPDButSPDcannotProtectagainstLightingStrikesnearSPDLocation.SPDcannotgiveprotectionagainsttemporaryovervoltage.Temporaryovervoltageiscausedbyaseverefaultintheutilitypowerorduetoproblemswiththeground(poorornonexistentNGbond).TemporaryovervoltageoccurswhentheVoltageexceedsthenominalvoltageforashortduration(millisecondtoafewminutes).Ifthevoltageexceeds25%ofthenominalsystemvoltage,theSPDandotherloadsmaybecomedamaged.

SelectionofSurgeProtectiondevice(SPD):

TheSize,performanceandspecificationofSPDdependonfollowingcharacteristics

CurrentcharacteristicofSPD

I:SurgeCurrentRating(KA),In:NominalDischargeCurrent(In),Imax:MaximumdischargeCurrent(Imax)

ShortCircuitCurrentRating(SCCR).

VoltagecharacteristicofSPD

Uc:MaximumContinuousOperatingVoltage(MCOV),Up:VoltageProtectionRating(VPR)orsurgevoltagerating(SVR)orClampingVoltage.TOV:TemporaryOverVoltage.

(1)SurgeCurrentRatings(I):

ThepeaksurgecurrentratingsofSPDaregenerallybasedonthesumofLineneutralandLinegroundcurrent.Apeakampereratingperphase.(I.e.LN100kA,LG100kAprovides200kA/phase).OtherSpecificationlikeMCOV,VPR,InandSCCRthathaveclearlydefinedtestcriteria,butforSurgeCurrentthereisnospecifiedTestCriteriaorindustrystandardhencedifferentSPDmanufacturerstocreatetheirowndefinitionsofpeakamperesurgecurrentratings.PleasenotethatselectionofHigherSurgeCurrentRatingsdontalwaysgivesBetterProtectionbutitisprovidelonerlife.IEEEClearlystatesthatTheselectionofasurgecurrentratingforanSPDshouldbematchedtotheexpectedsurgeenvironmentandtheexpectedordesiredusefullifeofthedevice.SelectionofSurgeRatingforanSPDdependsonThelocationoftheSPDwithintheelectricaldistribution&environmentalsurroundingsconditionofSite.FollowingsurgecurrentratingsbasedonSPDlocationwithintheelectricaldistribution.

SurgecurrentratingsbasedonSPDlocation

Location SurgeCurrent

ServiceEntranceLocations 240kA

DistributionLocations 120KAto160kA

BranchLocations 50KAto120kA

(2)Nominaldischargecurrentrating(In):

TheNominalDischargeCurrentisthepeakvalueofsurgecurrentconductedthroughtheSPD.Ithas8/20sImpulsecurrentWaveform.TheSPDmustfunctionafter15appliedsurges.

NominaldischargeCurrentshowsdurabilityofSPD.Thehighestnominaldischargecurrentratingis20kA.Example:calculateInforMaximumpeakcurrent(SurgeCurrent):I=200kA(themaximumlevelofnaturallightningwhere5%ofstrikesarebiggerthan100kA)Assumethatforperfectcurrentsharing50%togroundand50%totheelectricalnetworkNetworkconfigurationis3Phases+Neutral(n=4)In=SurgeCurrentXCurrentpathtoGround(%)/NoofPath=200x0.5/4=25kATheNominaldischargecurrentvalues,witha8/20swaveshapeasperUL1449areType1SPD(In)=10KAor20kAType2SPD(In)=3KA,5KA,10KAor20kATheNominaldischargecurrentvalueasperIEEEC62.41is200Ato10KA.TheNominaldischargecurrentvalueasperNFPAis20KA

(3)Maximumdischargecurrent(lmax):

ThemaximumsurgecurrentbetweenanyonephaseandneutralthattheSPDcanwithstandforasinglestrikeof8/20sor10/350scurrentiscalledMaximumdischargecurrentofSPD.Thisisthemaximumvalueofasurgecurrentthatcanbedivertedbythesurgeprotectivedevice.currentsurgeshavetwodifferentwaveshapesLightningcurrentsisalongwaveshape(10/350s)whichrepresentsdirectlightningstrike.Shortwaveshape(8/20s)whichrepresentsaindirectstrike;lmaxisthemaximumvalueofashortwaveshapecurrentandlimpisthevalueofalongwaveshapecurrent;thevaluelmaxorlimphastobeadaptedtotheexpectedvalueofthepossiblelightningcurrents.Imax>In

(4)Shortcircuitcurrentrating(SCCR):

Maximumsymmetricalfaultcurrent,atratedvoltage,thattheSPDcanwithstandwithoutsustainingdamageiscalledSCCRofSPD.Everyelectricalsystemhasanavailableshortcircuitcurrent.Thisistheamountofcurrentthatcanbedeliveredbythesystemataparticularpointinashortcircuitsituation.SCCRshoesthatMeasureofhowmuchcurrenttheelectricalutilitycansupplyduringafaultcondition.SCCRisnotasurgeratingbutitisthemaximumallowablecurrentaSPDcaninterruptintheeventofafailure.NECArticle285.6saysthattheSPDtobeinstalledwheretheavailablefaultcurrentislessthantheSCCRratingoftheSPDunit.

Typicalavailableshortcircuitcurrents

Load shortcircuitcurrentsofSPD

Residential 5KAto10kA

Smallcommercial 14KAto42kA

Largecommercial/industrial 42kAto65kA

Largeindustrial/utility/downtowninlargecities 100kAto200kA

Atasubpanel 120kAto160kAprovidesgoodprotectionandlife

PointofuseSPDs80kAto100kAperformwell

(5)CalculatingMaximumContinuousOperatingVoltage(MCOVorUc):

WhenSurgeProtectorareinstalledtoprotectsystemsfromlightningorswitchingsurges,itshouldbeinstalledbetweenthephaseandearth.HenceMCOVoftheinstalledarrestermustbeequalorhighertothecontinuousvoltagebetweenthephaseandearth.Onthreephasesystems,thelinetogroundvoltageisequaltothephasetophasevoltagedividedby1.73Forexample:ona440kVtransmissionsystem,thenominalsystemphasetophasevoltageis440kVthereforethelinetoearthvoltagewouldbe440/1.73=254kV.Sinceallsystemshavesomeregulationerror.Iftheregulationis10%,thenthelinetogroundvoltagecouldbe254x1.10=280kV.TheMCOVorUcoranarresterforthissystemataminimumshouldbe280kV.

TypicalMCOVs

System MCOV

120Vsystem 150VMCOV

240Vsystem 320VMCOV

480Vsystem 550VMCOV

SelectingSPDwithtoolowofavoltageratingwillresultinSPDfailureSelectingSPDwithtoohighofavoltageratingwillresultinreducedprotection

(6)CalculatingLinetoGroundVoltage:

ThemaximumrmsvoltagethatcanbeappliedtoeachmodeoftheSPDiscalledMCOVWhenathreephasepowersystemhaveafaultbetweenoneofthePhasetoearth,theVoltageoftwohealthyphasestogroundincrease.SinceArrestorismostlyconnectbetweenPhaseandEarthhenceVoltageacrossLAterminalsalsoIncrease.Thisincreaseinvoltagewillremainacrossthearresteruntilasystembreakeroperatesandbreaksorinterruptsthefault.Thisisaverysignificanteventinthelifeofanarresterandmustbeaccountedforduringthevoltageratingselectionofanarrester.Therearesomerulesofthumbandgraphsthatcanbeused,butthesearequitcrudeanddifficultatbesttouse.AnnexCofIEEEstandardC62.22andAnnexAofIEC600995coverthissubject.Fordistributionsystemswherethesystemandtransformerimpedancesarerelativelyunknown,aworstcasescenarioisusedforeachtypeofsystem.Thevoltageriseduringafaultinthesecasesisdeterminedbymultiplyingthelinetogroundvoltageby

TypeofSystem GroundFaultFactor

SolidlyGrounded4wiresystems 1.25

Unigrounded3wiresystems 1.4

Impedancegroundedsystems 1.73

IsolatedGroundSystemsandDeltaSystems 1.73

Forexample:Ina440kVmultigroundedsystem,themaximumcontinuouslinetogroundvoltage=PhasetoPhaseVoltage/1.73=440/1.73=254kV.Thevoltageduringagroundfaultontheunfaultedphasescanreach254x1.25or=318kVrms.Thisisthevoltageanarresterwillseeacrossitsterminalsforaslongasthefaultexists.

(7)Voltageprotectionlevel(UPatIn):

ThisisthemaximumvoltageacrosstheterminalsoftheSPDwhenitisactive.ThisvoltageisreachedwhenthecurrentflowingintheSPDisequaltoNominaldischargecurrent(In).Thevoltageprotectionlevelmustbebelowtheovervoltagewithstandcapabilityoftheloads.Intheeventoflightningstrokes,thevoltageacrosstheterminalsoftheSPDgenerallyremainslessthanUp.WhiledivertingthesurgecurrenttothegroundVoltageProtectionLevel(Up)mustnotexceed

thevoltagewithstandvalueoftheequipmentconnecteddownstream.SuppressedVoltageRating(SVR)waspartofanearlierversionofUL1449EditionandisnolongerusedintheUL1449standard.TheSVRwasreplacedbyVPR.

(8)TemporaryOverVoltage(TOV):

ItisusedtodescribetemporarySurgewhichcanariseasafaultoffaultswithinmedium&Lowvoltage.UTov=1.45XUo,whereUo=NominalLinetoearthVoltage.For230/440VSystemUTov=1.45X230=333.33Volt



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Introduction:

AdevicewhichdivertsorlimitssurgecurrentiscalledSurgeprotectivedevices(SPD).SPDprotectelectricalequipmentagainstovervoltagescausedbylightningorSwitching.Itiswiredinparalleltotheequipmentwhichisneededtobeprotected.OncethesurgevoltageexceedsSPDsratingitstartstoconductenergydirectlytotheelectrical

groundingsystem.AnSPDhasaverylowresistanceduringthistimeandgivelowresistancepaththeenergytoground.Oncethesurgeisoveritgiveshighresistancepathtocurrent.SPDispreviouslyknownasTransientVoltageSurgeSuppressors(TVS)orSecondarySurgeArresters.Underwriterlaboratories,UL1449ListedSPDsarenowdesignatedaseitherType1,Type2orType3andintendedforuseonACpowersystemsratedLessthan1000vrms

Principle:

SPDisusedtolimittransientovervoltagesofatmosphericorSwitchingSurgeandgivespathtotheexcessivecurrenttoearthhencelimittheovervoltagetoavaluethatisnothazardousfortheelectricalinstallation.

CausesofSurges:

(1)ExternalSurge:lightningstrikes:DirectStroke,IndirectStroke(2)InternalSurge:SwitchingSurge:Switchingon/offofinductiveloads.Trippedcircuitbreakersandfuses.Shortcircuits.Malfunctionscausedbythepowercompany.InsulationFailures:ArcingGround:Ignitionandinterruptiontoelectricarc.

DifferencebetweenSurgearrestor(LightingArrestor)andSurgeSuppressor:

SurgearrestersandSurgeSuppressorbothareusedtoprotectequipmentfromsurges.But,thereisconfusionbetweentheapplicationofsurgearrestors/Lightingarrestorandsurgesuppressors.Themaindifferencesbetweenalightningarresterandasurgearresterareitsfaultclearingtimeanditsposition

Botharedoingthesamejob,butstillbotharenotsame.

LightingArrestor/SurgeArrestor:

SurgeArrestersarewidelyalsoknownLightningarresters.SurgearrestersaredevicesinstalledonOverheadlines,substationsetctoavoidaLightingsurgeandotherSurgesofanadditionalcurrent/voltage/chargeduetovariousfaultsoccurring.Inthepastyearwhennonlinear/solidstatedevices(computers,PLCanddrives)werenotused.TheElectricalLoadismostlyLinearLoad.Utilitycompaniesandenduserswereconcernedwithhowtoprotectelectricaldistributionsystemsfromlightningsurgestoensurethatvoltagesurgesdidnotexceedthebasicinsulationlevel(BIL)oftheconductorwires,transformersandotherequipment.HenceSurgearrestors/Lightingarrestorsweredevelopedforuseinlow,mediumandhighvoltageapplicationsatvariouspointsinthetransmissionanddistributionsystem.SurgeArrestorprovidelowresistancepathbetweenthephaseconductorandground.LAdidnotconcernwiththeloadsifitclearedwithinafewcycles.Arrestorsarestillusedintheelectricalindustryprimarilyalongthetransmissionlinesandupstreamofafacilitysserviceentrance.Arrestorsareavailableinvariousclassesdependingupontheirwithstandcapability(e.g.,stationvs.distributionclass).Attheserviceentrancelocationonlowvoltagesystems(600Vandbelow),Lightningarrestorsweredesignedtoprotecttheelectricaldistributionsystemandnotthesensitivesolidstateequipment.Economically,surgearrestersarebetterthansurgeDifferentsurgearrestersareavailablebasedontheirwithstandingcapability.Themainproblemwiththemisthattheyaredesignedforprotectinglargeelectricaldistributionsystemsfromlightningsurges,andnotforsensitivesolidstateequipment.Applications:Thesurgearresterisbesttoprotectinsulationoftransformers,panelboards,andwirings.However,itdoesntworkwellforsolidstatecomponents.

SurgeSuppressor/SurgeProtector(calledTVSS):

Intodayswemostlyusesolidstate(nonlinear)loadslikeelectronicequipment,drives,PLCs,computers,electronicballasts,telecommunicationequipment.NonLinearisabout70%ofutilityloads.Thesolidstatecomponentswillbedamagedbythesurges.UsingSurgesuppressorsattheserviceentranceandkeybranchpanels,thesurgewillbeeffectivelyreducedtounder100V.IfaTVSSandlightningarrestorarebothusedataserviceentranceswitchboard,theTVSSwillturnonearlierandshuntmostofthesurgecurrent.Manywatertreatmentplants,telecommunicationfacilities,hospitals,schoolsandheavyindustrialplantsutilizeTVSSsinsteadofsurgearrestorstoprovideprotectionagainsttheeffectsoflightning,utilityswitching,switchingelectricmotors.Applications:Theyareusedinwatertreatmentplants,hospitals,schools,andtelecommunicationfacilities.

SizeofSurgeProtectionDevice(SPD)doesnotdependonPanelSize:

ThekAratingofanSPD(surgerating)isoneofthemostmisleadingterms.Wenormallyuse50KASPDtoprotect50KApanel.ThekAratingofthesurgearrestershasnothingtodowiththefaultcurrentratingofelectricaldistributionboard.Wecanfita40kAsurgearresterinadomesticboardwithafaultcurrentratingoflessthan5kAWhenasurgeentersapanel,itdoesnotknowthesizeofthepanel.SoItistotallymiscalculationforuse50KASPDfor50KAPanelThereisanormalPracticethatlargerpanelsneedlargerSPD,butsurgesareindifferenttopanelsize.Thelargestsurgethatcanenterabuildingswiringis10kA,asexplainedintheIEEEC62.41standard.SowhywouldweneedaSPDratedfor100KAor200kA.


VariousRoutineTestofPowerTransformer(Part4)

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(9)MagneticBalanceTest

TestPurpose:

Magneticbalancetestoftransformerisconductedonlyonthreephasetransformerstochecktheimbalanceinthemagneticcircuit.

TestInstrument:

Multimeter.MillAmmeter

TestCircuitDiagram:

(https://electricalnotes.files.wordpress.com/2014/09/untitled10.png)

TestProcedure:

Firstkeepthetapchangeroftransformerinnormalposition.Nowdisconnectthetransformerneutralfromground.Thenapplysinglephase230VACsupplyacrossoneoftheHVwindingterminalsandneutralterminal.MeasurethevoltageintwootherHVterminalsinrespectofneutralterminal.Repeatthetestforeachofthethreephases.Incaseofautotransformer,magneticbalancetestoftransformershouldberepeatedforIVwindingalso.Therearethreelimbssidebysideinacoreoftransformer.Onephasewindingiswoundinonelimb.Thevoltageinducedindifferentphasesdependsupontherespectivepositionofthelimbinthecore.Thevoltageinducedindifferentphasesoftransformerinrespecttoneutralterminalsgiveninthetablebelow.415V,TwophasesupplyistobeappliedtoanytwophasesterminalsonHVsideofPowertransformerandvoltagesinothertwophasecombinationaretobemeasuredwithLTopen.SumoftheResultanttwovaluesshallbeequaltothevoltageapplied.

AppliedVoltage(415V) MeasuredVoltage(V1)

MeasuredVoltage(V2) Result

RY YB BR V=V1+V2

YB RY BR V=V1+V2

BR YB RY V=V1+V2

(10)HighVoltagetestsonHV&LVWinding:

TestPurpose:

TocheckstheinsulationpropertybetweenPrimarytoearth,SecondarytoearthandbetweenPrimary&Secondary.

TestInstrument:

HighVoltagetester(100KV&3KV)

TestCircuitDiagram:

(https://electricalnotes.files.wordpress.com/2014/09/untitled11.png)TestProcedure:

HVhighvoltagetest:LVwindingconnectedtogetherandearthed.HVwindingconnectedtogetherandgivenFollowingHVSupplyfor1minute.

LVhighVoltagetest:HVwindingconnectedtogetherandearthed.LVwindingconnectedtogetherandgivenFollowingHVSupplyfor1minute.433VWinding=3KVHighVoltage11KVWinding=28KVHighVoltage22KVWinding=50KVHighVoltage33KVWinding=70KVHighVoltage.

(11)DielectricalTest:

TestPurpose:

TochecktheabilityofmaininsulationtoearthandbetweenwindingTocheckstheinsulationpropertybetweenPrimarytoearth,SecondarytoearthandbetweenPrimary&Secondary.

TestInstruments:

3PhaseVariableVoltage&FrequencySource.AutoTransformer.

TestProcedure:

ThefollowingDielectrictestsareperformedinordertomeetthetransformerinsulationstrengthexpectations.Switchingimpulsetest:toconfirmtheinsulationofthetransformerterminalsandwindingstotheearthedpartsandotherwindings,andtoconfirmtheinsulationstrengthinthewindingsandthroughthewindings.Lightningimpulsetest:toconfirmthetransformerinsulationstrengthincaseofalightninghittingtheconnectionterminalsSeparatesourceACwithstandvoltagetest:toconfirmtheinsulationstrengthofthetransformerlineandneutralconnectionterminalsandtheconnectedwindingstotheearthedpartsandotherwindings.InducedACvoltagetest(shortdurationACSDandlongdurationACLD):toconfirmtheinsulationstrengthofthetransformerconnectionterminalsandtheconnectedwindingstotheearthedpartsandotherwindings,bothbetweenthephasesandthroughthewinding.Partialdischargemeasurement:toconfirmthepartialdischargebelowadeterminedlevelpropertyofthetransformerinsulationstructureunderoperatingconditions.

MethodNo1(separatesourcevoltagewithstandtest)


Alltheterminalsofthewindingundertestshouldbeconnectedtogetherandthevoltageshouldbeapplied.Thesecondarywindingsofbushingtypecurrenttransformersshouldbeconnectedtogetherandearthed.Thecurrentshouldbestableduringtestandnosurgesshouldoccur.Asinglephasepowerfrequencyvoltageofshapeapproximatelysinusoidalisappliedfor60secondstotheterminalsofthewindingundertest.Thetestshallbeperformedonallthewindingsonebyone.Thetestissuccessfulifnobreakdowninthedielectricoftheinsulationoccursduringtest.DuringtheSeparatesourceACwithstandvoltagetest,thefrequencyofthetestvoltageshouldbeequaltothetransformersratedfrequencyorshouldbenotlessthan80%ofthisfrequency.Inthisway,60Hztransformerscanalsobetestedat50Hz.Theshapeofthevoltageshouldbesinglephaseandsinusoidalasfaraspossible.Thistestisappliedtothestarpoint(neutralpoint)ofuniforminsulatedwindingsandgradual(nonuniform)insulationwindings.Everypointofthewindingwhichtestvoltagehasbeenappliedisacceptedtobetestedwiththisvoltage.Thetestvoltageismeasuredwiththehelpofavoltagedivider.Thetestvoltageshouldbereadfromvoltmeteraspeakvaluedividedby2.Testperiodis1minute.

MethodNo2(Inducedsourcevoltagewithstandtest)


TheaimofthistestistochecktheinsulationbothbetweenphasesandbetweenturnsofthewindingsandalsotheinsulationbetweentheinputterminalsofthegradedinsulationwindingsandearthDuringtest,normallythetestvoltageisappliedtothelowvoltagewinding.MeanwhileHVwindingsshouldbekeepingopenandearthedfromacommonpoint.Sincethetestvoltagewillbemuchhigherthanthetransformersratedvoltage,thetestfrequencyshouldnotbelessthantwicetheratedfrequencyvalue,inordertoavoidoversaturationofthetransformercore.Thetestshallstartwithavoltagelowerthan1/3thefulltestvoltageanditshallbequicklyincreaseduptodesiredvalue.ThetestvoltagecaneitherbemeasuredonavoltagedividerconnectedtotheHVterminaloronavoltagetransformerandvoltmeterwhichhavebeensettogetherwiththisvoltagedividerattheLVside.Anothermethodistomeasurethetestvoltagewithapeakvaluemeasuringinstrumentatthemeasuringtapendofthecapacitortypebushing(ifany).Testperiodwhichshouldnotbelessthan15seconds.Itiscalculatedaccording,Testperiod=120secondsx(Ratedfrequency/Testfrequency)Thedurationofthetestshallbe60second.Thetestisacceptedtobesuccessfulifnosurges,voltagecollapsesorextremeincreasesinthecurrenthaveoccurred.

AcceptanceCriteria:

Thetestissuccessfulifnobreakdownoccursatfulltestvoltageduringtest.

MethodNo3LightingImpulseTest:

AllthedielectrictestschecktheinsulationleveloftheTransformer.Impulsegeneratorisusedtoproducethespecifiedvoltageimpulsewaveof1.2/50microsecondswaveOneimpulseofareducedvoltagebetween50to75%ofthefulltestvoltageandsubsequentthreeimpulsesatfullvoltage.Forathreephasetransformer,impulseiscarriedoutonallthreephasesinsuccession.Thevoltageisappliedoneachofthelineterminalinsuccession,keepingtheotherterminalsearthed.Thecurrentandvoltagewaveshapesarerecordedontheoscilloscopeandanydistortioninthewaveshapeisthecriteriaforfailure.



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(5)ShortCircuitTest

TestPurpose:

ThevalueoftheshortcircuitimpedanceZ%andtheload(copper)losses(I2R)areobtained.Thistestshouldbeperformedbeforetheimpulsetestifthelaterwillbeperformedasaroutinetestinordertoavoidreadingserrors

TestInstrument:

MeggerorMultimeter.CT,PT

TestProcedure:

SuitableLowVoltage(3phase415V,50Hz)willbeappliedtotheterminalsofonewinding(usuallytheH.V.)withtheotherwindingshortcircuitedwith50sq.mm.Coppercable.(UsuallytheL.V.)Theappliedvoltageisadjustedtopasstheneededcurrentintheprimary/secondary.Inordertosimulateconditionsnearesttofullload,itiscustomarytopass100%,50%oratleast25%offullloadcurrent.Voltagetobeincreasedgraduallytillthecurrentintheenergizedwindingreachestherequiredvalue(50%to100%ratedcurrent).Measurethe3Phaselinecurrentsatalltapposition.IfthetapswitchisanOffCircuittapswitch,thesupplyhastobedisconnectedbeforechangingthetap.Aconsistenttrendintheincreaseordecreaseofcurrent,asthecasemaybe,confirmsthehealthinessofthetransformer.Iftransformerisequippedwithatapchanger,tappingregulationsareapplied.(1)Iftappingrangewithin5%andratedpowerlessthan2500kAV,loadlossguaranteerefer

totheprincipaltaponly.(2)Iftappingrangeexceeds5%orratedpowerabove2500kAV,itshallbestatedforwhichtappingbesidetheprincipaltaptheloadlosseswillbeguaranteedbythemanufacturer.ThreephaseLTsupplyisappliedonHVsideofpowertransformeratnormaltapwithratedcurrentonHVsideandcurrentsmeasuredinallthephasesonHVsideandphases&neutralonLVsidevaluesnoted.Readingstobetakenasquicklyaspossibleasthewindingswarmupandthewindingresistanceincreases.Hence,thelossesvaluewillincreaseaccordingly.Usingappropriateinstruments(conventionalthreewattmetermethodordigitalwattmeterwithammeters&voltmeters)measurementsofvoltage,currentsandpowercanberecorded.


ShortCircuitTest(WithoutusingCT,PT)ToavoidCTsandPTs,thismethodcanbeusedatcurrentlevelsof2to5Aandmeasurementofloadlossesisdoneatthiscondition.Thismeasuredloadlossisthenextrapolatedtoactualloadcurrentstoobtainloadlossesattheoperatingcurrent.Example:11kV/433V,1000kVAtransformerwith5%impedance,thevoltagetobeappliedonH.V.sideduringloadtestisestimatedbelow.V.sidefullloadcurrent(I1)=(KVAx1000/1.732xLineVoltage)V.sidefullloadcurrent(I1)=(10001000/1.73211000)=52.5AmpLinetolinevoltagetobeappliedonH.Vsideforgetting5AonH.V.side,LinetolinevoltagetobeappliedonH.VsideVisc=(LineVoltagex1000xZx5/0.866xI1x100)LinetolinevoltagetobeappliedonH.VsideVisc=(11x1000x5xx/x0.86652.5100)=60.5volts.SincethecurrentdrawnonH.V.sideisonlyabout5Ainthistest,CTscanbeavoidedandhencephaseangleerrorisnotapplicable.


ShortCircuitTest(WithusingCT,PT)

(https://electricalnotes.files.wordpress.com/2014/09/untitled8.png)AcceptanceCriteria:

Measuredimpedancetobewithinguaranteedvalueandnameplatevalue.Loadlossestobewithinguaranteedvalues.

Testcandetect:

Windingdeformation.Deviationinnameplatevalue.

(6)OpenCircuit/NoLoadTest

TestPurpose:

Inthistest,thevalueofNoLoadpower(Po)&theNoLoadcurrent(Io)aremeasuredatratedvoltage&frequency.

TestInstruments:

Wattmeters.Ammeter,VoltmeterorPoweranalyses

TestProcedure:

Testisperformedatratedfrequency.ThreephaseLTVoltageof415VappliedonHVsideofPowertransformerkeepingLTopenTwovoltmetersareconnectedtotheenergizedwinding,oneismeasuringthevoltagemeanvalueandtheotherisfortheVoltageR.M.Svalue.Voltageappliedtowinding(usuallytoH.V.windings).Itwillbeinarangefrom90%ofwindingratedvoltageto110%ofthesameinsteps,eachof5%(i.e.fora33/11kVtransformer,appliedvoltagevalueswillbe29.7kV,31.35kV,36.3kV)Readingsofwattmeters,Voltmeters&AmmetersarerecordedtoobtainthevaluesofV(r.m.s),Vmean,PoandIoateachvoltagestep.Testresultsareconsideredsatisfactoryifthereadingsofthetwoareequalwithin3%.Ifitsmorethan3%,thevalidityofthetestissubjectedtoagreement.Measuredvalueofpowerlossiscorrectedaccordingtothefollowingformula:Pc=Pm(1+d)D=(VmeanVr.m.s)/VmeanMeasurethelossinallthethreephaseswiththehelpof3wattmetermethod.Totalnoloadlossorironlossofthetrf=W1+W2+W3

TestCaution:

Thistestshouldbeperformedbeforetheimpulsetestifthelaterwillbeperformedasaroutinetestinordertoavoidreadingserrors

AcceptanceCriteria:

NoLoadlossestobewithinguaranteedvalues.

(7)Continuitytest:

PurposeofTest:

Toknowthecontinuityofwindingsofthetransformer.

TestInstruments:

MeggerorMultimeter.

TestProcedure:

CheckContinuityofTransformerbyusingmultimeterorbyMeggerbetweenfollowingTerminals

Transformer PP PP PP Result

HVSide RY YB BR ZeroMegaohmorcontinuity

LVSide ry yb br ZeroMegaohmorcontinuityTestcandetect:

Opencircuit/looseconnectionofwinding

(8)MagneticCurrentTest

TestPurpose:

Magnetizingcurrenttestoftransformerlocatesthedefectsinthemagneticcorestructure,shiftingofwindings,failureinturntoturninsulationorproblemintapchangers.Theseconditionschangetheeffectivereluctanceofthemagneticcircuit,thusaffectingtheelectriccurrentrequiredtoestablishfluxinthecore.

TestInstrument:

Multimeter.MillAmmeter

TestCircuitDiagram:


ThreephasesLTVoltageof415VappliedonHVsideofPowertransformerandcurrentsaretobemeasuredwithmillammeter.Thevalueshallbe=(1to2percentofratedfullloadcurrentofTC/HTKV)XVoltage

Applied

TestProcedure:

FirstofallkeepthetapchangerinthelowestpositionandopenallIV&LVterminals.Thenapplythreephase415Vsupplyonthelineterminalsforthreephasetransformersandsinglephase230Vsupplyonsinglephasetransformers.Measurethesupplyvoltageandelectriccurrentineachphase.Nowrepeatthemagnetizingcurrenttestoftransformertestwithkeepingtapchangerinnormalposition.Andrepeatthetestwithkeepingthetapathighestposition.Generallytherearetwosimilarhigherreadingsontwoouterlimbphasesontransformercoreandonelowerreadingonthecenterlimbphase,incaseofthreephasetransformers.Anagreementtowithin30%ofthemeasuredexcitingcurrentwiththeprevioustestisusuallyconsideredsatisfactory.Ifthemeasuredexcitingcurrentvalueis50timeshigherthanthevaluemeasuredduringfactorytest,thereislikelihoodofafaultinthewindingwhichneedsfurtheranalysis.

TestCaution:

ThismagnetizingcurrenttestoftransformeristobecarriedoutbeforeDCresistancemeasurement.



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(3)TurnsRatio/VoltageRatioTest:

TestPurpose:

TurnsRatioTest/VoltageRatioTestaredoneinTransformertofindoutOpenCircuitedturns,ShortCircuitedturnsinTransformerwinding.Thevoltageratioisequaltotheturnsratioinatransformer(V1/V2=N1/N2).Usingthisprinciple,theturnsratioismeasuredwiththehelpofaturnsratiometer.Ifitiscorrect,thenthevoltageratioisassumedtobecorrectThistestshouldbemadeforanynewhighvoltagepowertransformeratthetimeitisbeinginstalled.WithuseofTurnsRatiometer(TTR),turnsRatiobetweenHV&LVwindingsatvarioustapstobemeasured&recorded.TheturnsratioismeasureoftheRMSvoltageappliedtotheprimaryterminalstotheRMSVoltagemeasuredatthesecondaryterminals.R=Np/NsWhere,R=VoltageratioNp=Numberofturnsatprimarywinding.Ns=NumberofturnsatsecondaryWinding.Thevoltageratioshallbemeasuredoneachtappinginthenoloadcondition.

TestInstruments:

TurnsRatiometer(TTR)toenergiesthetransformerfromalowvoltagesupplyandmeasuretheHVandLVvoltages.WheatstoneBridgeCircuit

MethodNo1TurnsRatioTesting:

TestProcedure:

TransformerTurnsRatioMeter(TTR):TransformerratiotestcanbedonebyTransformerTurnsRatio(TTR)Meter.Ithasinbuiltpowersupply,withthevoltagescommonlyusedbeingverylow,suchas8,10Vand50Hz.TheHVandLVwindingsofonephaseofatransformer(i.e.RY&rn)areconnectedtotheinstrument,andtheinternalbridgeelementsarevariedtoproduceanullindicationonthedetector.Valuesarerecordedateachtapincaseoftappedwindingsandthencomparedtocalculated

ratioatthesametap.Theratiometergivesaccuracyof0.1percentoveraratiorangeupto1110:1.Theratiometerisusedinabridgecircuitwherethevoltagesofthewindingsofthetransformerundertestarebalancedagainstthevoltagesdevelopedacrossthefixedandvariableresistorsoftheratiometer.Adjustmentofthecalibratedvariableresistoruntilzerodeflectionisobtainedonthegalvanometerthengivestheratiotounityofthetransformerwindingsfromtheratiooftheresistors.BridgeCircuit:


Aphasevoltageisappliedtotheoneofthewindingsbymeansofabridgecircuitandtheratioofinducedvoltageismeasuredatthebridge.Theaccuracyofthemeasuringinstrumentis

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