Portland State University
PDXScholar
Dissertations and Theses
Dissertations and Theses
1-1-2011
Proposal and Analysis of Demagnetization Methods of High
Voltage Power System Transformers and Design of an Instrument
to Automate the Demagnetization Process
Nathanael Jared Makowski
Portland State University
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Makowski, Nathanael Jared, "Proposal and Analysis of Demagnetization Methods of High Voltage Power System Transformers and
Design of an Instrument to Automate the Demagnetization Process" (2011). Dissertations and Theses. Paper 431.
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ProposalandAnalysisofDemagnetizationMethodsofHighVoltagePower
SystemTransformersandDesignofanInstrumenttoAutomatethe
DemagnetizationProcess
by
NathanaelJaredMakowski
Athesissubmittedinpartialfulfillmentofthe
requirementsforthedegreeof
MasterofScience
in
ElectricalandComputerEngineering
ThesisCommittee:
MartinSiderius,Chair
BetsyNatter
BranimirPejcinovic
PortlandStateUniversity
©2011
i
ABSTRACT
Presentdemagnetizationmethodsforlargepowersystemtransformersare
timeconsumingandcanbedangeroustopersonsperformingdemagnetization.The
workofthisthesiswastodevelopimproveddemagnetizationmethodsandto
constructanautomatedinstrumentthatwouldimplementthemethodsdeveloped.
Onepreviouslydevelopedmethodwasanalyzedforeffectiveness.Then,two
newmethodsfordemagnetizationweredevelopedandalsoanalyzedfor
effectiveness.Anautomatedtestinstrumentprototypewasredesignedtobeableto
accommodatethesemethodsandtoimprovethesafetyoftheuser.
Thepreviouslydevelopedmethodattemptsdemagnetizationbasedoncurrent
flowbehaviorcharacteristics.Thefirstnewmethodisamagneticfluxestimation
basedonsaturationtime.Thesecondnewmethodisalsobasedonmeasuring
saturationtime,modifiedtoaccountforthevariablevoltagelossduetowire
resistance.
Thesecondofthetwonewmethodsdevelopedprovedtobethemosteffective
fordemagnetizationandwasabletodemagnetizeatransformerwithinanerror
marginof2%.Theinstrumentdesignedtoperformthedemagnetizationwiththis
newroutineisnowinearlyproductionstagesforanexpandedfieldtrialwith
transformermaintenanceteams.
ii
Acknowledgements
IamgratefultomyadvisorDr.MartinSideriusforhisguidanceandencouragement
topreparethisthesiswiththoroughattentiontodetailandprofessionalism.
ItisanhonorformetohaveDr.BranimirPejcinovicandProf.BetsyNatter
participateonmythesiscommittee;mytimeworkingtogetherwitheachwas
enjoyableandofferedmanyofmymostinterestinglearningexperiencesatPSU.
IwouldliketothankthepeopleatBonnevillePowerAdministrationforthechance
toworkonthisproject;RonDenisformuchoftheinspirationthatbegantheproject
andJeffHildrethforprojectsupportandguidance.
Iwouldalsoliketothankmywifeforherencouragementandconstantsupportto
completethiswork.
Mostofall,IthankGodforbringingitalltogether.
iii
TableofContents
1
2
Abstract................................................................................................................................................................i
Acknowledgements.......................................................................................................................................ii
ListofFigures.................................................................................................................................................iv
Motivation.........................................................................................................................................................1
IntroductionofProblem..............................................................................................................................4
2.1 WindingResistanceTest...................................................................................................................4
2.1.1
IEEEStandardProcedure........................................................................................................4
2.1.2
TestCurrentMagnitude...........................................................................................................5
2.3 SignificanceofTransformerSaturation......................................................................................6
2.3.1
TransformerHealth...................................................................................................................7
2.3.2
PowerSystemProtection........................................................................................................7
2.3.3
SystemPowerQuality...............................................................................................................9
3 TheoryandPrincipalsofPowerTransformerOperation...........................................................10
3.1 History.....................................................................................................................................................10
3.2 MagneticProperties..........................................................................................................................11
3.3 ElectricSteelMagnetization&Hysteresis................................................................................13
3.4 PresentlyUsedDemagnetizationMethods..............................................................................15
4 PreviousWork...............................................................................................................................................18
4.1 InstrumentDesign.............................................................................................................................18
4.2 DemagnetizationAlgorithm...........................................................................................................20
5 DemagnetizationMethods.......................................................................................................................23
5.1 PermeabilityMethod........................................................................................................................23
5.2 TimeBasedMethod...........................................................................................................................24
5.3 IntegrationMethod............................................................................................................................26
6 DemagnetizationDeviceDesignRequirements..............................................................................29
6.1 UserSafety.............................................................................................................................................29
6.2 MeasurementSystem.......................................................................................................................33
6.3 SystemReliability&Protection....................................................................................................34
6.4 Automation&Usability....................................................................................................................35
7 DeviceConstructedforDemagnetizationTesting..........................................................................36
7.1 Controller...............................................................................................................................................36
7.2 DevicePower........................................................................................................................................37
7.3 MeasurementComponents............................................................................................................38
7.4 System&UserProtection...............................................................................................................40
8 Results..............................................................................................................................................................43
8.1 MethodsforDeterminingtheStateofResidualMagnetization......................................43
8.2 PermeabilityMethod........................................................................................................................43
8.3 TimeBasedMethod...........................................................................................................................44
8.4 IntegrationMethod............................................................................................................................46
9 Conclusions.....................................................................................................................................................47
10
WorksCited...............................................................................................................................................49
iv
ListofFigures
Figure1‐IEEEResistanceMeasurementCircuit.......................................................................................4
Figure2‐InrushCurrentexample...................................................................................................................6
Figure3‐MagnetizationComparisonofSiliconSteeltoIron.............................................................12
Figure4–ExampleofCurrentMagnitudeDuringDemagnetizationRoutine..............................16
Figure5–MeggerMTO210DemagnetizationRoutine(16)................................................................17
Figure6‐PrototypeTestInstrument...........................................................................................................18
Figure7‐DeviceDesignforPreviousWork...............................................................................................18
Figure8‐PresentWindingResistanceandDemagnetizationTestset..........................................19
Figure9‐TheoreticalmagnetizingCurrentandMagneticFluxforonecycle.............................21
Figure10‐PermeabilityDemagnetizationRoutine................................................................................24
Figure11–Coremagnetizationovertimewhenafixedvoltageisappliedtothewinding...25
Figure12‐TimeIntegrationDemagnetizationRoutine.......................................................................26
Figure13‐ModifiedTimeIntegrationDemagnetizationRoutine....................................................27
Figure14‐CompletedTestSet........................................................................................................................40
Figure15‐TestInstrumentInternalCircuitry.......................................................................................41
Figure16‐TestInstrumentSchematic.......................................................................................................42
Figure17‐ExampleCurrentFlowTimelineafterVoltagePolarityisReversed.........................44
Figure18‐DeltaTransformerMagneticFieldduringDCEnergization.........................................45
Figure19–ComparisonofDemagnetizationMethods:MaximumError.......................................47
1
1
MOTIVATION
Presentdemagnetizationmethodsforlargepowersystemtransformersare
timeconsumingandcanbedangeroustopersonsperformingdemagnetization.
Demagnetizationafterroutinemaintenanceisanimportantpracticeforthehealth
andlonglifeofatransformer.Highvoltagepowertransformersareanessentialpart
ofanytransmissionsystem.AtBPA(BonnevillePowerAdministration),withabout
630transformersrepresentingapproximately$1billioninassets,managingand
prolongingtheservicelifeoftransformersiscritical.Towardsthisend,manynew
toolsarebeingintroducedfortransformertestingandconditionanalysisincluding
frequencyresponseanalysisandultrasonicfailurelocatingandprediction.
Additionally,advancingtechnologiesinotherfieldshavehelpedreducethestress
andwearontransformerassets.
BPA’ssuccessfulmaintenanceprogramhelpstokeepfailureratesfarbelow
theaveragefailureratefoundinamajor10‐yearstudy(1).Oneoftheteststhatare
performediscalledawindingresistancetest.Thistestmeasurestheohmic
resistanceofthewindingmaterialinhighvoltagetransmissiontransformers.Thisis
achievedbysaturatingthecorewithaDCvoltagesourceinordertoobtainasteady
statecurrentandthenmeasuringthevoltagedropacrossthewinding.However,
thistestcanleavethetransformerinastateofheightenedsusceptibilitytolarge
inrushcurrents.
2
Whenapowertransformerwithresidualmagnetismleftinthecoreis
energized,inrushcurrentsoccurthatcanbepotentiallydamagingtovarious
portionsofthepowersystem.Thesecurrentscanexceedtheratedcurrentbyan
orderofmagnitudeandmore.Studieshaveshownthatthehighmechanicalforces
andresultingvibrationsduetothesecurrentscausesincreasedwearonthe
insulationoftransformerwindings(2).Inamajor10‐yearstudyitwasfoundthat
linesurges,likethatofinrushcurrent,andInsulationdegradationarethenumber
oneandtwocausesoftransformerfailuresrespectively;cumulativelythese
representedalmost35%offailures(1).Muchefforthasbeenmadetoreducethe
likelihoodandmagnitudeofinrushcurrents(3),(4),(5).
Existingguidelinesandtechniques(6)torestoreapowertransformertoa
neutralmagneticstatearetimeconsumingandpotentiallydangeroustountrained
personnel.Primaryinstructiontextsmakethesemethodsevenmoreproblematicby
givinginstructioninaqualitativemannerwhichaddsalevelofuncertaintytothe
accuracyofdemagnetizationandaugmentstheassociateddangersaswell.
Theworkthisthesiswastodevelopademagnetizationmethodthatwould
decreasethetimerequirementfordemagnetizationandtodevelopaprototypetest
setthattakesadvantageofadvancedtechnologyinmeasurementandhighspeed
digitalprocessingtoautomatethedemagnetizationprocess.Theinnovativedevice
automatesthewindingresistancetestandleavesthetransformerinastatethat
minimizesinrushcurrentsuponenergization.Thisprototypereducestheaverage
3
timeneededtoaccuratelyprepareatransformerbankforenergizationbyafull
hourcomparedtosomepreviousmethodsandprovidesanadditionallevelof
personnelsafetywhenperformingthistransformerdiagnostictest.Also,with
furtherdevelopment,theflexibilityoftheadvancedhardwarecouldallowthe
integrationofadditionaltestsintothesameunitandreducethetimenecessaryto
setupthevariousteststhatmustbeperformed.
Thedeploymentofthisnewtestsetwillimprovetheaccuracyandefficiencyof
routinetransformerdiagnostictests.ItwillalsoextendthelifeofBPA’stransformer
assetstherebyimprovingreliabilityanddecreasingcapitalcosts.
4
2
INTRODUCTIONOFPROBLEM
2.1 WINDINGRESISTANCETEST
Oneofthemanytestsperformedduringroutinemaintenanceofapower
transformeristhewindingresistancetest.Thistesthelpstogaugethehealthof
internalconnectionswithinthetransformerbycomparingthemtovaluesmeasured
bythemanufactureruponbeingconstructed.Thisisanimportantbenchmark;
withinatransformerthereareoftenanumberofdifferent“tap”connectionsthat
canbemadetoadjusttheratio
V
Thesetappositionsonlarge
Current Shunt
Make‐Before‐Break
Battery
asafractionofonepercent.
V
powertransformersareoften
controlledremotelyby
Transformer Winding
ofthetransformerbyaslittle
FIGURE1‐IEEERESISTANCEMEASUREMENTCIRCUIT
dispatcherswhomonitorandmakeadjustmentstomaintainsystembalance.The
reliabilityoftheseconnectionsiscriticaltosystemoperations.
2.1.1 IEEESTANDARDPROCEDURE
ThistestisperformedaccordingtothedirectionsgiveninIEEE62‐1995,
(section6.1.1.1)(6)byinjectingacurrentintothewindingofatransformeras
displayedinFigure1.Oncethewindinginductancehasbeenovercome,ohmslaw
canbeusedtocalculatetheresistance.
Whiletheprocedureissimple,thelastingeffectsofsaturatingtheinductance
ofthewindingcanbesignificant.Itisimportantthattheybeconsideredbefore
5
connectingthetransformertothepowersystem,orperformingothertests,andis
specificallysuggestedinIEEE62beforeperformingthe‘ExcitingCurrent’test
(section6.1.3.)ThestateofmagnetizationalsoaffectstheresultsofFrequency
ResponseAnalysis(FRA)tests,whichisrapidlygainingpopularityasadiagnostic
tool.Saturationeffectsarediscussedmoreinsection2.2
2.1.2 TESTCURRENTMAGNITUDE
Forthewindingresistancetest,thestandardrecommendsusingacurrentof
lessthan15%ofthenormalcurrentratingforthetransformerwithnominimalor
targetcurrentspecified.Amorespecifictargetformeasurementcurrent,suggested
bytransformermanufacturers,is1%ofthetransformersnormalcurrentrating.
Thisrecommendationseemstobeacompromisewithconsiderationsof
measurabilityandprecision,testingtime,andaccuracy.Theyfurthersuggestthat
exceeding10%ofthetransformer’sratedcurrentforawindingresistancetestmay
affectthetemperatureofthewindingsignificantlyenoughtogiveerroneous
readings.Theyalsorecommendnolessthanaminimumcurrentof0.1%ofthe
transformer’sratedcurrentbecauseofthedifficultyindeterminingwhetherthe
currenthasreachedasteadystateornot.
6
2.3 SIGNIFICANCEOFTRANSFORMERSATURATION
Asmentionedintheprevioussection,saturationofthetransformercoreis
necessaryforthewindingresistancetest.Thisissignificantbecauseoftheresulting
effectsofresidualmagnetizationduetosaturation.Thissectionwillexplainthe
significanceandconsequencesof
theresidualmagnetizationand
thephysicsoftheresidual
magnetizationwillbeconsidered
insection4.
Figure2showsanexample
ofthemagnitudeofcurrentthat
canpassthroughthetransformer
FIGURE2‐INRUSHCURRENTEXAMPLE
windingswhenthecoreofthetransformergoesintosaturationduetotheresidual
magnetization.TEST
Inthisgraphthecurrentisgivenin“PerUnit”quantitiesandisgivenbythe
relationship:P.U.Current=ActualCurrent/NormalRatedCurrent.Thus,inthis
example,thecurrentpassingthroughthewindingismorethan23timesthenormal
currentforthefirstcycleand5timesthenormalcurrentforthesecondcycle.
Thistransientinrushcurrentcandisturbtheentiresystemwithpotentially
damagingconsequences.Theseconsequencesaregenerallygroupedintooneof
7
threemaincategories:TransformerHealth,SystemProtectionPlanning,andOverall
SystemPowerQuality.
2.3.1 TRANSFORMERHEALTH
Thefirstconcerniswiththeeffectsonthetransformeritself.Sincethe
mechanicalforceonawindingisproportionaltothesquareofthecurrent,inrush
currentscauseasignificantincreaseofmechanicalstressforcesontransformer
windingsaswellasresultinharmonicvibrationsthatincreasedegradationof
insulation(1),(2).Transformerfailuremodeslinkedtoinsulationdegradationare
oftenverydestructiveinnaturewhichcaneffectnearbycomponentsaswell.
Becauseofthesignificantinvestmenteachindividualtransformerrepresents,the
adverseeffectofinrushcurrentontheinternalcomponentsofatransformerwith
specificregardtoservicelifereductionhasledthepowersystemindustryto
researchandapplymanyproceduralchangestothewaythattransformersare
energizedandde‐energizedincludingcontrolledclosing(energizingthe
transformerataspecificpointintime)andtheuseofsurgesuppressionresistors
(3).
2.3.2 POWERSYSTEMPROTECTION
Thesecondissuethatarisesconcernspowersystemprotectionplans.The
highinrushcurrentscancausethepowersystemprotectionandcontrolcircuitry
thatmonitorssystemfaults1tomistakenlyoperate(6)(7).Sincetransformersare
takenoutofserviceregularlyforroutinemaintenance,ifaprotectionrelaysystem
1line‐to‐groundorline‐to‐lineshortcircuitcondition
8
mistakenlyrecognizesafaultconditionwhenattemptingtobringthetransformer
backon‐linethetransformermaybeautomaticallytakenbackoffline.Whenthis
happensitisverydifficulttodeterminewhetherthetransformerwastakenoffline
duetotheeffectsofresidualmagnetism,duetoafailurewithinthetransformer,or
becauseofamaintenanceoversight.Insomecases,attemptingtoenergizethe
transformerasecondtimecouldhaveverydamagingconsequences,includingcase
rupturesandfire.Beforeattemptingtobringthetransformeronlineasecondtime
thereislikelytobeaninvestigationofthesituationwhichwillcausethelossof
manyhoursofoperationtimeaswellasincreasedlaborcosts.
Forexample,asimilarsituationoccurredafteratransformerhadundergone
someextendedmaintenanceandrepairandwasreadytobereinsertedontothe
powergrid.Thesubstationoperatorinchargetriedtoenergizethetransformer
twice,however,theinrushcurrentsweresogreatthattheautomatedprotection
measuresimmediatelydisabledthetransformerbothtimes.Fearingtherewas
internalproblemstherewashesitationtoattemptathirdtimeandwereinclinedto
takethetransformeraparttoensurethatsomeaspectofrepairwasn’toverlooked.
Itwasdecidedtocontactthefieldservicesandtestingdepartment,whichsentout
anexpertwithexperienceindemagnetizingtransformers.Afterperformingthe
demagnetization,thetransformerwassuccessfullyenergizedonthesystemupon
thefirstattempt.
9
2.3.3 SYSTEMPOWERQUALITY
Thethirdmainissuedealswiththeeffectonoverallsystempowerquality.
Theeffectsobservedincludeincreasesanddecreasesinthermsvoltagecalled
resonantharmonicvoltageswells(8)andvoltagesags(9).Theseeventslast16msto
60sindurationandarecharacterizedbylowfrequencyoscillationofrmsvoltage
amplitudesthatcoincidewithresonantpointsinthepowertransmissionsystem.
Also,sincetheseeffectsunbalancethecurrentflowofthepowersystem,thiscan
haveadetrimentaleffectondistributedgenerationcomponents(10):When
generatorsaredistributedacrosslargeserviceareasthepowerdemandplacedon
anindividualgeneratormaybegreaterthanothers,thiscanresultinhigh
temperaturesinarelativelyshortamountoftimeandhighriskoffailure,(11).
Anotherconsequenceisthatitcandisturbtheresultsofotherroutinemaintenance
tests.(12)
10
3
THEORYANDPRINCIPALSOFPOWERTRANSFORMEROPERATION
3.1 HISTORY
SiliconSteel,alsoknownasElectricalSteel,isthestandardforpower
transformercorematerial.Morethan125yearsago,theeffectsofaddingelements
invariousquantitiestothesteelalloymixturewereperformedusingsystematic
routinesofexperimentationbymanyentities.Itwasthroughthisactivitythatthe
basisformodernelectricalsteelwasdiscovered.
In1886RobertHadfieldfiledforthepatentsonthealloymixtureforSilicon
Steelbecauseofitsmechanicalpropertiesbeingusefulforspringsandsomefine
blades.Thefirsttransformerusingthiscorematerialwasnotbuiltuntilin1913,
almost2decadeslater.
TheproductionofSiliconSteelfortransformerswaslikelymotivatedbythe
increasedindustrializationandmanufacturingrequiredbytheFirstWorldWar.
Hadfield’spatentstoproducethehardManganeseSteelaswellasSiliconSteel
allowedhisbusinesstoflourishduringthistime.Employingasmanyas15000
peoplebytheendofthewar,Hadfieldwasinaprimepositiontoadvancethe
expansionoftheElectricalPowerGrid.
Yetevennow,themagnetizationofSiliconSteelisnotwellunderstood(13).
Becauseitsmagneticpermeability(µr)isbothnonlinearandmultivaluedrelativeto
themagneticfieldstrengthapplied,thequalitiesandcharacteristicsofferrous
materialsmustbeobtainedforindividualsamplesthroughexperimentandtesting.
11
MuchinthesamemannerthatHadfieldsystematicallyusedsomanyyearsago
whenhefirstdevelopedit.
3.2 MAGNETICPROPERTIES
Forthecaseofpowertransformerconstruction,coredesignhasbeenhighly
optimizedusingmaterialswithmagneticdomainsinthecrystallinestructurethat
alignparalleltotheedgesofthecrystal(the[001]vector).ForSiliconSteel,thisis
achievedbythecombinationofapproximately4%Siliconto96%Ironwhichresults
inabodycenteredcubiccrystallatticewherethecubeedgesprovidetheeasiest
directionofmagnetization.
Thesiliconinfusedsteelisrolledintothinsheetsandcoatedwithathin
coatingofinsulation.Atransformercoreisconstructedofmanylayersofthis
materialbeingpressedtogether.Thisreinforcesthattheprimarydirectionforeasy
magnetizationwillbealongthedesiredpath:inthedirectionthatthewindings
aroundthetransformercorewillnaturallydrivethemagneticfieldwhencurrent
passesthroughthem.
Withthistypeofcore,transformersobtainanincreasedleveloffluxdensity
withalowermagnetizingforce(amp‐turnspermeter)thanthatofotherferrous
materials.Thisishelpfulforpowertransformersbecausethisallowsmoreenergyto
betransferredthroughthemagneticfieldforaspecificamountofdrivingenergy
(loss).Theefficiencydifferencebetweensiliconsteelandironcorematerialsisvery
12
significantandisillustratedinFigure3wheretheareaofthehysteresisloopis
representativeoftheenergylosses.
FIGURE3‐MAGNETIZATIONCOMPARISONOFSILICONSTEELTOIRON
AsyoucanseeinFigure3,theB‐Hrelationshipoftransformercorematerials
isverylinearuntilcoresaturationisreached,wheretherelationshipchangesvery
quickly.Becauseofthislinearityandthedramaticchangeatsaturation,power
transformerscanbedesignedtouseaminimumamountofcorematerialand
operateclosetothesesaturationpointswhilestillmaintainingenergyconversion
efficiency.Thisfactoralsohelpstosimplifycalculationsintheanalysisofmagnetic
saturationcharacteristicsandwillbeusedinthenextsectiontoestimatethe
maximumfluxdensityofthecore.
13
3.3 ELECTRICSTEELMAGNETIZATION&HYSTERESIS
Asmentionedintheprevioussection,SiliconSteelfunctionsatveryhigh
magneticfluxdensitiesandexhibitshighlydirectionalmagneticdomains.
Consequently,whenconsideringhowtodemagnetizeatransformer,thisthesis
proposesthattakingtheseconditionsintoaccountcanprovideinsightforthe
explorationofmoreefficientmethodsfordemagnetization.
Conventionaldemagnetizationmethodsemploytheuseofadiminishing
alternatingmagneticfieldwhichhastheeffectofrandomizingthemagnitudeand
directionofthemagneticdomainswithinthecrystalstructure.Thismethodis
effectiveandhasbeenusedsuccessfullyinabroadrangeofapplicationsinthe
historyofelectronicsaswellasotherfieldssuchasgeology,paleontology,and
archeologywhereitisutilizedindateclassification(14).
However,inthecaseofSiliconSteel,wherethemagneticdomainshavea
strongtendencytoalign,evenifthemagnetizationdirectionvectorswereabletobe
randomized,theywouldquicklyandeasilyrevertbacktotheprimaryaxisof
magnetization.Thisthesisproposesthatthedensityofthemagneticfluxalongthe
primarymagneticpathistheonlyremainingsignificantfactorfordemagnetization.
Byrecognizingthatthissetofcircumstancesexistsforpowertransformers,thereis
potentialforimprovingtheefficiencyandsafetyoftransformerdemagnetization
routines.
14
Onemethodtotestourabilitytoaccuratelyestimatethemagneticfluxdensity
andtheconfidenceofassumptionsistouseafewknowndesignvaluestopredict
measurablequantities.Forexample,itwasusefultohaveanestimateforthe
saturationtimeofatransformergivenacertainappliedDCvoltage.
BeginningwithFaraday’sLaw,whenappliedtothegeometriesofa
transformer,simplifiesto:
[1]2
WhereVrepresentsthevoltageacrossthetransformerterminalsandΦrepresents
themagneticfluxinWebers
Next,sincetransformersareusuallydesignedtooperatewithamagneticflux
densityjustbelowthesaturationpointofthecore(15),thisconditioncanbeusedto
estimatethetotalfluxlinkage,whichisdefinedasNϕ.Byintegratingbothsidesof
theequationforhalfofonecycle,themaximumamountoffluxlinkagedeliveredto
thecorecanbefound.Thusforatransformerwithaspecificvoltagerating:
[2]
1
√2
.
2 60
√2
120
WhereVratedrepresentsthedesignedoperatingvoltageofthespecificindividual
transformerwindingundertest.
2Fitzgerald,A.E.ElectricMachinery(23)
[3]
15
Inthecaseofthewindingresistancetest,thevoltageappliedtothewindingis
almostconstant,suchthattheintegrationofthevoltageovertimesimplifiesto:
[4]
Bycombiningequations3and4anapproximatetimetoreachsaturationfora
giventestvoltagecanbefound:
√2
120 [5]
Wheretsatrepresentsthetimeittakestoreachthemagneticfluxsaturationdensity
ofthetransformercoreandVtestrepresentstheconstantDCvoltageappliedtothe
windingduringaresistancetest.
Asanexample,foratransformerwitha230kVratingtestedat12V,the
saturationtimewouldbearound71secondsaccordingtothisrelationship.
3.4 PRESENTLYUSEDDEMAGNETIZATIONMETHODS
Whenperformed,thepresentmethodmostcommonlyusedfor
demagnetizationofatransformerisbasedonthestandardfoundinIEEE62‐1995
(section6.1.3.5)(6)whichdirectsonetoalternatethepolarityofafixedvoltage
withdecreasingapplicationtimeperalternationofpolarity.Witheachalternation,
thevoltageisapplieduntilthecurrentflowhasreversedandis“slightlylower”in
absolutemagnitudethanthecurrentinthepreviousapplicationsimilartothe
16
methodshowninFigure5.Thisiscontinueduntilthenexttargetcurrentlevelis
Current
zero.
time
FIGURE4–EXAMPLEOFCURRENTMAGNITUDEDURINGDEMAGNETIZATIONROUTINE
Thismethodinvolvesthemanual,forced,interruptionofthecircuitwhile
significantlevelsofcurrentarepassingthroughthetransformerwinding.Thiscan
createveryhighvoltagesandarcingdischargeswhichisdangeroustoboth
personnelandequipment.Additionally,dependingoninterpretationofthe
instructions,thisprocesscantakeasignificantamountoftime.
AnothermethodisusedbytheMTO210TransformerOhmmetertestutility
producedbyMegger®,aproviderofelectricaltestequipmentandmeasuring
instrumentsforelectricalpowerapplications.Thismethodisanautomatedmethod
looselybasedontheIEEEstandardmethod.
TheMTO210alsoaccomplishesdemagnetizationbyapplyinganalternatingDC
potentialtothewindings(seeFigure5.)First,theapplicationoftheDCpotential
wouldbeusedfortheinitialwindingresistancetest.Thevoltagepotentialwould
17
thenbereverseduntilthecurrentisequalinmagnitudebutintheoppositepolarity.
Oncethatmagnitudeofcurrentisreached,thevoltagepotentialisthenrevertedto
theoriginalpolarityuntilthecurrentis20%oftheoriginaltestcurrent,atwhich
timethevoltagepotentialisreversedagainuntilthecurrentis20%oftheoriginal
testcurrentintheoppositedirectionofthecurrentthanthefirstapplicationofthe
voltagepotential.Thisprocessisrepeatedforcurrentsat4%oftheoriginaltest
currentandagainfor1%oftheoriginaltestcurrent.
+
Current
time
‐
FIGURE5–MEGGERMTO210DEMAGNETIZATIONROUTINE(16)
Thisunitwasnotavailableforevaluationsoadetailedcomparisonwillnotbe
made.However,evenassumingthattheroutineusedbytheMTO210does
sufficientlydemagnetizethetransformer,fromtheinformationpresented,this
demagnetizationroutinewouldappeartotakesignificantlylongertoexecutethan
theroutinesproposedinthisthesis.
Othermethodsofdemagnetizationhavebeenproposedwhichinvolvethe
applicationofanultra‐low‐frequencysquare‐wavevoltagesource(17)canbeused,
however,theyalsorequireextensivedemagnetizationtime.
18
4
PREVIOUSWORK
Thisthesisisacontinuationofworkdoneforapreviousdesignproject(18).
Thegoalofthepreviousprojectwastoprovideaproof‐of‐conceptforthe
constructionofthetestinstrument(Figure6)thatwouldbeabletoperforma
transformerwindingresistancetest
andalsobeabletodemagnetizethe
transformerautomatically.This
sectiondetailstheworkthatwas
accomplishedduringthatproject.
FIGURE6 ‐ PROTOTYPETESTINSTRUMENT
4.1 INSTRUMENTDESIGN
FIGURE7‐DEVICEDESIGNFORPREVIOUSWORK
Figure7detailsthedesignfortheinstrumentattheendoftheproject;itwas
intendedtobeforanautomatedversionofthestandardIEEEtestcircuitshownin
19
Figure1.Theinstrumentdesignforthatprojectwaslargelybasedoncomponents
thatwerealreadyavailableonhandandestablishedcriteriaforthefollowing:

OperatingVoltage

CurrentLimiting

ProtectionComponents
Forthefirstcriteria,anoperating
voltageof12VDCwasselectedinthe
originaldesignbecausevehiclebatteries
arereadilyavailableinthefield.For
manyyears,automotivebatterieswere
theprimarypowersourceforwinding
resistancemeasurementsinthefield.
FIGURE8 ‐ PRESENTWINDINGRESISTANCE
ANDDEMAGNETIZATIONTESTSET
Originally,asingle12‐voltbatterywas
connectedtothetestinstrumentinFigure8andwasalwaysfoundadequate.
Forthesecondcriteria,thecurrentlimitingselectionwasdecidedby
comparisonandanalysisofthereadilyavailablecomponentswithexpectedwinding
resistancevalues.Inordertoobtainaccuratefieldvoltagemeasurementsand
maintainquicksaturationtimes,itwasdecidedthatthevoltageacrossthecurrent
limitingresistorshouldnotbemuchgreaterthanthevoltagedropacrossthe
windingresistance.Oftheresistorsthatwereavailable,twowire‐wound0.2Ω
resistorswereaddedinseriestoproducealimitoftheshortcircuitcurrentto30A.
20
Forthethirdcriteria,protectionfromhighvoltagesduetotransformer
inductancewasaccomplishedwithacombinationofhighpowerresistors.Forthis
applicationabalancebetweenthedesirestolimitthepeakvoltagesthatcould
appearacrosstheanalog‐to‐digitalconverter(ADC),yetalsotoquicklydissipatethe
energyinthetransformer.Lowerresistancesallowforusageofresistorswitha
lowerpowerrating,however,thesetakemuchlongertodissipateenoughenergyfor
thetransformertobedisconnected.Forthisproofofconceptaresistanceof6Ωwith
600wattsofdissipationwasselectedsinceacurrentmagnitudegreaterthan10
ampswasnotexpected.
Onecomponentthatwaschosenbaseduponitscapabilitiesratherthan
availabilitywasthecontroller.TheQ‐screen,asingleboardcomputerwithabuiltin
touchscreenLCDinterfacewasselectedbecauseofitseaseofprogramming(C‐
based)andexpandabilitythroughtheadditionofoptionalmodulesthatwereableto
fulfilladditionalrequirementsofthetestsystem.
4.2 DEMAGNETIZATIONALGORITHM
Inthepreviouswork,attentionwasalsogiventothedevelopmentofan
algorithmfordemagnetization.Inthatwork,atheorywasdevelopedwhichshowed
that,bymonitoringthechangeincurrentthroughthetransformerwindings,a
neutralmagnetizationstateofatransformercoremaybeextrapolated.The
procedureattemptedtoidentifythepointofneutralmagnetizationbythe
21
relationshipofthemagnetizingcurrenttotherelativepermeabilityaswellastothe
totalfluxinthecore.
Figure9illustratestherelationshipofthecurrentofatransformertothe
magneticfluxinthecorewhenpoweredbyasinusoidalvoltage.Inthegraph,where
themagneticfluxiszero,thecurrentexhibitstwodistinguishablefeatures.Thefirst
featureisthatthechangeincurrentovertimeisalocalminimumwhenthefluxis
zero.Thesecondnoticeablefeatureisthatthecurrentalsopassesthroughthezero
whenthefluxdoes.However,thissecondfeatureisnotasusefulsincethisisonly
thecasewhenpoweredbyasinusoidalvoltage.
Bymonitoringthecurrentandcalculatingthetimederivativeofthecurrent
afteraconstantvoltagepotentialisapplied,alocalminimumin asthe
transformermagnetizationswingsbetweenpolaritiescanbeidentified.Itwas
surmisedthatifthepowersourceisremovedattheappropriatetimethenthecore
shouldbeleftinastateofneutralmagnetization.
v
Φ
i
Local minimum(di/dt)
FIGURE9‐THEORETICALMAGNETIZINGCURRENTANDMAGNETICFLUXFORONECYCLE
22
Thefinalcircuitdesignforthatprojectwassuccessfulinthatitwascapableof
measuringthedesiredquantitiesandcontrollingtheflowofcurrentthroughthe
winding.Howevertheaccuracyofthemeasurementswasultimatelyfoundlacking
onceconstructed.Also,aftertestingtheproposeddemagnetizationmethodona
115kV‐230kV,singlephasetransformer,itwasapparentthatthedemagnetization
methodneededimprovement.
23
5
DEMAGNETIZATIONMETHODS
Thisthesisbeganwiththeintenttoevaluateandimproveupontheprevious
workinbothdemagnetizationmethodandinstrumentoperation.Weaknessesof
theinstrumentanddemagnetizationmethoddevelopedinpreviousworkbecame
apparentinpreliminarytesting,whenevaluatingthedemagnetizationofsmaller
distributiontransformers(singlephase,13.8kV‐240V)andonelargertransmission
transformer(singlephase,345kV‐115kV).
Thefirststepforthisworkwastoidentifyalternatedemagnetizationmethods.
Then,sincetheprototypeinstrumentfailedtotakeintoaccountcertaintransient
voltagesthatweredamagingtothesensorsandelectronicsoftheinstrument,the
secondstepwouldbetheredesignoftheinstrument.Beingsecond,thisalso
providedopportunitytoensurethatallthenecessarydesignrequirementswere
knownwhentheinstrumentwasre‐designed.
5.1 PERMEABILITYMETHOD
Stillneedingevaluationatthestartofthisthesis,thedemagnetizationmethod
developedinthepreviousworkandintroducedinsection4.2willbediscussedhere
first.Thismethodwasexpectedtobethemostdirectandquickestdemagnetization
methodsinceitonlyrequiredthevoltagetobeappliedonceforsaturationandthen
reversedoncefordemagnetization.However,itsaccuracyandeffectiveness
regardingdemagnetizationisdependentonmanyassumptionsabouttheproperties
ofthetransformer.Forexample,itrequiresthatthemagnetichysteresisbehave
24
similarlytothatofanironcoreinductor.Additionalfactorsthatcouldreducethe
effectivenessofthismethodwouldbetestenvironmentconditions.Sincethe
substationsinwhichthesemeasurementsandtestsareperformedhavelarge
electromagneticfieldinterference,theabilitytomakethesensitivemeasurements
necessarytoidentifythemomentthat beginstoincrease.Thisroutineis
illustratedinFigure10‐PermeabilityDemagnetizationRoutine.
Transformer
Winding Test
Apply Voltage to
Transformer
Winding
Begin Timer and
Reverse Voltage
Polarity
Remove Voltage
from Transformer
Winding
Measure Current
Through Winding
Measure Current
Through Winding
& Calculate di/dt
Run Lead
Removal
Preparation
Routine
NO
YES
Is Current
Increasing
NO
YES
IsIs di/dt di/dt at a
peak
Increasing Demagnetization
Complete
FIGURE10‐PERMEABILITYDEMAGNETIZATIONROUTINE
5.2 TIMEBASEDMETHOD
Thebasisforsecondmethodofdemagnetizationcomesfromaproposalbythe
sponsorofthepreviouswork.Themethodproposedisatimebasedmethodwhich
estimatesthemagneticfluxinthetransformercoreviaFaradaysLaw.Rearranging
equation[1]foraconstantvoltageVgives:
25
∆ ∝∆ [6]
Therefore,wherethemagneticfluxisdirectlyproportionaltotheamountof
timethataconstantvoltageisappliedtothewinding,bymeasuringthetimeneeded
forthemagneticstateofthecoretoswitchfrombeingsaturatedinonedirectionto
becomingsaturatedintheoppositedirectionwecandeterminehowlongaconstant
voltageneedstobeappliedtothewindingthatissaturatedinordertoreachthe
neutralpoint.
Forexample,themagneticfluxofatransformershowninFigure11is
saturatedinareversepolaritybecauseofacurrentflowingthroughitswinding.At
Forward
Saturation
Reverse
Saturation
FIGURE11–COREMAGNETIZATIONOVERTIMEWHENAFIXEDVOLTAGE
ISAPPLIEDTOTHEWINDING
26
t=0avoltageisappliedtothetransformerwindingthatopposestheflowofcurrent.
Oncethecorehasreachedforwardsaturation,thevoltagecanthenbereversedand
appliedforhalfthetimerequiredtoreachthepointofreversesaturation.
Thismethodassumesthattheenergylossduetowindingresistancecanbe
considerednegligiblecomparedtotheenergythatdrivesthemagnetizationofthe
core.TheflowchartillustratingthestepsforthismethodisshowninFigure12
Transformer
Winding Test
Apply Voltage to
Transformer
Measure Current
Through Winding
Is Current
Increasing
Store Timer
Value: tsat
Remove Voltage
Potential
Begin Timer and
Reverse Voltage
Polarity
Clear Timer and
Reverse Voltage
Polarity
Run Lead
Removal
Prep Routine
Measure Current
Through Winding
NO
YES
Store Current
Magnitude:
Isat
Measure Current
Through Winding
Demagnetization
Complete
YES NO
NO
Is Current
Magnitude
equal to Isat
Is Timer = ½ tsat
FIGURE12‐TIMEINTEGRATIONDEMAGNETIZATIONROUTINE
5.3 INTEGRATIONMETHOD
Thefinalmethodissimilartothesecondmethod;however,duringtestingit
wasfoundthatthemagnetizationcharacteristicsoftransformercorescangreatly
varydependingontransformerageandhowthetransformeriswound.Depending
27
onthesharpnessofthetransitiontomagneticsaturationasthemagneticfield
intensityincreases,energylossduetothewindingresistanceincreases.
Store Current
Magnitude:
Isat
Transformer
Winding Test
Store Timer
Value: tsat
Measure Current
Through Winding
NO
Begin Timer and
Reverse Voltage
Polarity
Apply Voltage to
Transformer
Measure Current
Through Winding
Is Timer = ½ tsat
NO
Measure Current
Through Winding
Measure Current
Through Winding
NO
YES
Is Current
Increasing
Is Current =
Isat
YES
NO
YES
Remove Voltage
Potential
YES
Clear Timer and
Reverse Voltage
Polarity
Is Current =
Isat * .632
Run Lead
Removal
Prep Routine
Demagnetization
Complete
FIGURE13‐MODIFIEDTIMEINTEGRATIONDEMAGNETIZATIONROUTINE
Ratherthanassumingthatthevoltagedrivingthegenerationofmagneticflux
withinthetransformercoreremainsconstantatalltimesasinEquation[6],the
accuracyofestimationcanbeincreasedbytakingintoaccountvoltagelossesdueto
thecopperwireresistanceascurrentincreases.Beevaluatingtheintegralformof
Faraday’sLawwithrespecttoformofthetransformerwindingsgives:
Φ
3Fitzgerald,A.E.ElectricMachinery(23)
[7]3
28
whereVLrepresentsthevoltagedropacrossthewindingduetoself‐inductance
effects.
ThevoltageappliedduringtheresistancetestisaDCstepof12voltsand
standardoperatingprocedureistoallowthetransformerwindingtobecome
saturated.Assumingthatthevoltagedropduetoinductancedecayswiththenatural
timeconstantofthecircuit,theintegralofVLcanbereduced:
Φ
0
[8]
WhereVL(t=0)representstheinitialvoltageappliedacrossthewindingofthe
transformer.Inthecaseofequation[8]inordertoobtainthetotalchangein
magneticflux,thetimethatittakesforthecurrenttoswingfrom63.7%ofthe
saturationvalueintheinitialdirectionto63.7%ofthesaturationvalueinthe
reversedirectioncanbesubstitutedfor .Sincethesaturationcurrentisalready
knownfromthewindingresistancetest,thetimerequiredtodemagnetizethe
transformerusingaconstantappliedvoltagefrommagneticsaturationiseasyto
determine.TheflowchartillustratingthesechangesisshowninFigure13.
29
6
DEMAGNETIZATIONDEVICEDESIGNREQUIREMENTS
Amajorcomponentofthisthesiswastheredesign,construction,and
programmingoftheautomatedcontrolsystemcapableofoperatinginafield
environment.Fourmaindesigncategorieswereidentifiedfromtheanalysisofthe
device’sintendeduseandworkingenvironment:

UserSafety

MeasurementAccuracy

SystemReliability/Protection

Usability/Automation
6.1 USERSAFETY
Largepowertransformershavetheabilitytoobtainveryhighlevelsof
magneticfluxdensitywithcoresthatareofconsiderablevolume.Thisresultsinan
energystoragecomponentofthetransformerwhichisimportanttotakeinto
consideration.Inordertoensurethatthisenergyissafelycontrolled,reasonable
estimatesoftheexpectedenergylevelstobeencounteredareessential.Energy
storedinamagneticfieldisgivenby:
1
2
Where( )representsthevolumeofthecorecontainingthemagneticfield.
4FundamentalsofAppliedElectromagnetics,F.Ulaby
[9]4
30
Unfortunately,thereisnoconvenientwayofmeasuringthemagneticfield
strengthordeterminingthevolumeofthecorewithoutobtainingdesign
informationfromthemanufacturer.Thus,thevaluemustbeestimatedbysome
combinationofknownormeasureablevalues.
Themostdirectwaytoevaluatetheenergystoredintheinductoristo
calculateitbasedonwhatpowerwasdeliveredtotheinductoroveraspecific
periodoftime:
[10]
Whilethesevaluescanbemeasuredforaspecifictransformer,theycanalso
beapproximatedbasedonequation[5]andutilizingtwotrendsobservedwhile
performingexperiments.
Thefirsttrendisduetothenatureofthecorematerial’spermeability:for
80%ofthetimethecurrenttakestoreachsaturation,vLremainsconstantat
approximatelyVApplied.ThesecondtrendobservedisthatiLisapproximately5%of
Isaturation.Thus:
. 05
.8
[11]
Thefinal20%ofthetimeittakestosaturatethewinding and respond
accordingtoanaturalcurveassociatedwithanaircoreinductancesuchthat:
31
[12]
1
[13]
Experiencehasshownthatonaverage,whena230kVclasstransformeris
energizedwitha12Vbattery,ittakesapproximately60secondstoreachthe
saturationcurrentofatransformerwinding.
Thusintegratingequation[10]usingequations[12]and[13]forthefinal20%
ofsaturationtime:
0
2
[14]
Where representsthetimeittakesforthecurrenttoincreasefromitsmagnitudeat80%
ofthetotalsaturationtimeto63.2%ofthesaturationcurrentmagnitude.
Sincewindingresistancedatacanbefoundfromthemanufacturer’sinitial
tests:
[15]
Then,thetotalenergystoredistheadditionofequations[11]and[14]
Withtheseestimates(andtheexpectedwindingresistancevaluesranging
between2Ωto0.01Ω),thehighvoltagepowertransformerscouldstoreenergywith
amagnitudeofhundredsofjoulesinthemagneticfieldwhenenergizedwitha12V
source.Duringthetestingoftheworkinthisthesis,energyrangesashighas530
32
jouleswereobserved.Forreference,thethresholdforcardiacventricular
fibrillation(afatalelectricshock)isbetween10‐50joules(19).
Forthisreasonspecialcaremustbetakentoensuretheoperatorknowsnotto
attempttodisconnectthetransformerleadswhilethetestisinoperation.Whilethe
batterycanonlydelivertheenergyat12Vofpotential,ifthetransformeris
interruptedwhileenergizedandatfullsaturation,theinductanceofthewindingis
sufficienttogenerateextremelyhighvoltagesthatareeasilyabletoovercomethe
electricalresistanceofaperson’sbody.
Sincevoltageterminalsmustcomedirectlythroughthetestunitinorderto
connectthetransformertotherelaysthatcontroltheapplicationandpolarityofthe
voltage,thecomponentsmustbeproperlyselectedtopreventfailureswhichcould
leadtooverheatingandarcing.
Previousworkhaduseda600Watt,6Ohmresistanceforthedischarge
resistoronthebasisofthecontinuouswattagerating.Additionalanalysisofthe
expectedmaximumenergiesaboverevealedthatresistorsclassifiedwith600Watts
ofdissipationwouldbesufficientforenergylevelsexpected.Thusinregardtouser
safety,thevalueofresistanceissomewhatflexibleaslongasthepowerratingis
adequate.
SafetystandardsforDCvoltageexposurewerealsoconsidered.Astandard
commonlyusedbymanyindustriesfordirectcontactsafetyconsiderationsrequires
voltagestobelessthan60V(20).OtherstandardssuchasinECMA‐287(21)allow
33
forvoltagesashighas60V.Inordertomaintainvoltagesontheorderofthese
magnitudesforwindingcurrentsof30Amaximum,thedissipationresistanceshould
belessthan1.4Ohms.However,duetothelongtimeconstantforthepower
dissipationatthisvalueofresistancefurtherconsiderationsweretaken.
Sincecontactwiththesystemduringthetypeofeventwherethesehigh
voltageswouldbegeneratedisrelativelysmall,theacceptabilityofcontactwith
highervoltageswasalsoconsidered.TheNationalInstituteforOccupationalSafety
andHealthrecognizesthatbodilyelectricalresistancecanbeashighas100,000
Ohms.ThestandardthresholdofinvoluntarymusclecontractionforDCcurrentis
75mA(22).Thus,aslongastheskinremainsunbrokenanddry,apotentialofless
than7,500Vmaybesufficient,however,conditionsinthefieldcanvarygreatly.
Beforefinalselectionofthedissipationresistance,voltagelimitationsofthe
controlandmeasurementsystemsweretakenintoaccount.Thisisdetailedin
section6.3.
6.2 MEASUREMENTSYSTEM
Thesecondaryconcernpertainstotheaccuracyofmeasurementsmadebythe
system.TheIEEEstandardcallsforfieldmeasurementsthatshouldbewithin5%of
theinitialmeasurementsmadebythemanufacturerwhenthetransformerwasfirst
built.However,whenBPA’sfieldservicesteamtakesmeasurementson
transformersinBonneville’ssystem,measurementsarepreferredtobewithinthe
factoryerrormarginof0.5%.
34
Thewindingresistancemeasurementsystemneedstobeabletocollecttwo
fundamentalmeasurementsofthecircuit:thevoltageappliedtothewindingofthe
transformerandalsothecurrentpassingthroughthewindingsofthetransformer.
Duetothehighstandardsfortheerrormargininresistancemeasurementfor
BPA’stests,itwasdesiredtomakemeasurementswithina0.1%margin.Fordata
acquisition,thisrequiresaresolutionof14bitsforafullscalemeasurement.
However,forthevoltagemeasurement,sincethevoltagewillvaryfrom1to12volts
forreasonsthatwillbecoveredinsection0,theresolutionwillneedtobe.005%of
fullscale,or15bits.
Characteristicsofthemeasurementofcurrentinthewindingwasmore
difficulttomanagebecausethevaluescouldrangeanywherefrom0.5Ato30A.To
obtainanaccuracyof0.1%atthelowerbound,themeasurementresolutionmustbe
nogreaterthan500µA/step.Thisresultsinadataacquisitionresolution
requirementof19bits.
Anotherconsiderationofthemeasurementsystemistheaccuracyofthe
measurement.Thermalderating,thermalnoise,componentaccuracy,and
calibrationuncertaintyareafewofthefactorsthatwerealsoconsidered.
6.3 SYSTEMRELIABILITY&PROTECTION
Protectingthesensitivedataacquisitionmodulealsorequiresspecial
consideration.Therearemanyconditions,includingoperationsofthecontrol
system,whichcouldresultinhighvoltagesacrossvariouscomponentsofthetest
35
unit.Specifically,whenrelayoperationsperformswitchingofpolaritiesand
disconnectionofthevoltagesourcefromthelargeinductanceofthetransformer
winding,highvoltagescouldbegeneratedacrossthevoltagemeasurement
terminalsofthedataacquisitioncomponent.
Anothersituationthatmaygeneratehighvoltagesacrossthevoltage
measurementcomponentleadsisifthecurrentcarryingleadsareremovedwhen
thereisstillcurrentflowingthroughthetransformerwinding.Evenafewmilliamps
ofcurrentcanleadtothousandsofvoltsifthereisanattempttoabruptlyinterrupt
thecurrent.
6.4 AUTOMATION&USABILITY
Inordertoperformtheautomatedtasksdesiredforthisinstrument,asuitable
controllerwasnecessary.Thiscontrolsystemneededtobeabletoacquireandstore
thedataassociatedwithcurrentandvoltagemeasurements.Thesystemwouldalso
needtoperformhighspeed,real‐time,calculationandmanipulationoperations.
Additionally,thesystemwouldneedtobeabletoprovidecontrolsignalsforrelay
operationstobedirectedbythespecifiedroutinesandresultsofthecalculations.
Theabilityforeasyuserinteractionwiththecontrolsystemforoperationof
thetestsetwasalsodesired.Anidealsystemwouldbeabletoprompttheuserfor
inputaswellasbecapableofpresentinginformation,directions,andfeedbackboth
textuallyandgraphically.
36
7
DEVICECONSTRUCTEDFORDEMAGNETIZATIONTESTING
7.1 CONTROLLER
PreviousworkhadidentifiedasingleboardcomputermanufacturedbyMosaic
industries,theQscreenControllerTM,toactasthecontrolmoduleforthissystem.
Thissystemwasfoundtobeflexibleandrobustforthepurposeofthisthesis
project.TheQscreenisdrivenbyaMotorolaHC11processorandprovidesthebuilt
infacilityofatouch‐screendisplayforauser‐interface.Additionally,therearemany
optionalanduser‐configurablecomponents,termed“wildcardmodules”bythe
manufacturer,designedtoeasilyconnectandcommunicatewiththecontroller.
Oneofthesewildcardmodulesisa7channel24bitanalog‐to‐digitalconverter
dataacquisitionboard.Itiscapableof20bitseffectiveresolutionwitha30Hz
samplerateandhasaninputvoltagerangefrom‐30mVto5.03Vaswellasa
precision2.5Vreference.
Thesetwocomponentsprovidedthecorefortheinstrument;incorporating
thesecomponentsrequiredmorethan3900linesofcodeinordertotakeinto
accounttheuniqueconditionsthattheoperatingenvironmentdemands.Whilethis
isonlygivenapassingmentionhereitrepresentsasignificanttimecomponentof
thisproject.
37
7.2 DEVICEPOWER
Whileatypicalsubstationgenerallyhasnumerous110voutlets,thistestset
wasintendedtobeamobileunit.Assuch,itwasdecidedthatpoweringthesetfrom
a12vbatterywouldprovidethemostflexibility.
Anotherrelatedconsiderationfordevicepoweristhevoltageappliedtothe
transformerwindingsinordertosaturatethecore.Therearefurthertimeefficiency
benefitsthatcouldbegainedbysteppingthevoltagetohigherpotentialsduringthe
saturationphaseofthetestandthenreducingthevoltagetocorrespondwiththe
desiredcurrentoutput.However,asexperiencehasshownthatusing12Vgenerally
keepssaturationwithinreasonablelengthsoftime,thismethodwasnot
implemented.
Whiletheuseofa12Vbatterytosupplythepowerfortheentiretestset
simplifiesthepowersourceneeds,thisincreasesthecomplexityfortakingboth
voltageandcurrentmeasurements.Thechallengeariseswhenthepolarityofthe
appliedvoltageacrossthewindingsmustbereversed:sincethevoltageoftheADC
moduleissuppliedbytheQscreen,thenegativeterminalofthebatteryistreatedas
thecommonterminal.Inthiscase,thereisashortcircuitpathforthebattery
throughtheADCmodulewhenthepolarityoftheconnectionsfrombatterytothe
transformerwindingisswitched.
Tocompensateforthissituation,anisolatedDC‐DCconverterwasusedto
powertheadditionalcomponentsofthetestsetwhichmainlyconsistoftheQscreen
38
andrelaycontrollines.Usingthismodification,itwaspossiblegreatlysimplify
measurementoffsetsbyconnectingtheappropriateendofthetransformerwinding
tothebuiltin2.5voltagereferenceoftheADCmodule.
Mostpowersystemtransformershaveratedcurrentsinthehundredsor
thousandsofamperes(veryfewreachinggreaterthan2.5kA).Thus,inaccordance
withmanufacturersuggestionsof1%‐10%ofratedcurrentforcoremagnetization
saturation,theselectionofthetargetmaximumcurrentwas30A.Easilysupplied
withacarbattery,thislimitwasregulatedbya0.4Ω,600W,seriesresistance.This
practicaladditionalsochangesthecalculationforenergystoredinthemagnetic
fieldofthecore,since,asthecurrentincreases,thevoltageappliedtothewindings
isreduced.However,itissufficienttobeawarethatregardlessofthischange,the
energystoredinthemagneticfieldisstillverylargethustheprotectioncircuitry
detailedinsection7.4wascarefullyselected.
7.3 MEASUREMENTCOMPONENTS
Asmentionedinsection7.1,dataacquisitionisaccomplishedthroughthe
implementationofthe24bit,sevenchannel,ADCdesignedforusewiththeQscreen.
Dataacquisitionwasperformedatarateof60samplespersecondwhichreduced
theeffectiveresolutionto20bits.Thisprovidedthefoundationformeasurements
withtheabilitytoproducemeasurementswithahighlevelofprecision.
VoltagemeasurementsweremadeusingthefullydifferentialmodeoftheADC.
Thismodeallowsfordifferentialvoltagemeasurementstobemadeanywhere
39
within±2.5voltsofthedesignatedreferencepotential.Asmentionedinsection0,
the2.5VreferenceoftheQscreenwasconnectedtoterminal1ofthetransformer
outputconnections.Thevoltagewasmeasuredacrossa4:1voltagedividernetwork
inordertoreducetheexpected12Vdifferencebetweenthetwoterminalsofthe
outputresultingina2.4VmaxinputtotheADC.Theresistorsofthisdividerwere
chosenforlowthermaldriftandnoisesusception.
Twooptionswereconsideredforcurrentmeasurement.Onemethodmadeuse
ofHallEffectcurrentsensors,toisolatethepotentialdifferencesofthecircuit.This
helpedtoreducethecomplicationsofobtainingmeasurementsignalswithinthe
voltagerangelimitationsoftheADC.Byusinga±30Arangesensoranda±5Arange
sensor,accuracywasexpectedtobebetterthan0.1%.Inpracticehowever,these
devicesprovedpronetooffsetdrifterrorsandahighsusceptibilitytoexternal
electromagneticnoise.
Thesecondchoicewastousea50Acurrentshunt,aprecision1mΩresistor
whichprovidesanoutputof1mV/A.Utilizinganamplifierforthissignalwithagain
of64,producedasignalthatwas80%oftheADCinputrangeatthefullrated
currentofthetestset.Forthismethod,itisimportantthattheresistanceofthe
currentcarryingleadsandconnectionsbenogreaterthan15mΩ,sinceanygreater
resistancemightshiftthevoltagereferencetoofarfromthevoltagesbeing
measuredacrosstheshunt.
40
AllcalibrationwasprogrammedintothesystemusinganRFLIndustries
AC/DCV‐ASourceModel828asareferencesource.
7.4 SYSTEM&USERPROTECTION
Inordertosafelydischargetheappliedcurrentinthecaseofatestsetor
powersourcefailure,afixedprotectionresistorwasaddedinparallelwiththe
transformerwindingsatalltimesduringthetest.Thevalueofthisprotection
resistorwascoordinatedwiththevoltagedividernetworknecessarytoallowthe
ADCtomeasurevoltageswithinthe±12Vrangeaswellaslimittransformer
dischargeeventstolessthanthecontinuousovervoltageprotectionof±70Vbuilt
intotheADC.
Withconsiderationofthevoltagedividernetwork,themaxvoltageacrossthe
transformerwindingterminalsbecomes
350V.Sincethemaxcurrentflowingthrough
thewindingwillbe30Athelargestresistance
forthissafetyresistorshouldbeabout12
Ohms.However,duringtestingitwasfound
thatthecurrentdecaywasveryslow.Since
thedischargeresistordidnotreducethe
currentveryquickly,itwasfoundthatthere
wasstillahighpossibilityfordamagetobe
donetotheADCifthevoltagemeasurement
FIGURE14 ‐ COMPLETEDTESTSET
41
leadswerenotdisconnectedintheproperorder.
Accordingly,toincreasetherateofcurrentdecay,theresistancewasincreasedto24
Ohms.Also,aMOSFETswitchwith1500VofisolationwasaddedbetweentheADC
andthevoltagedividernetwork.Thisprovidedtheadditionalisolationneededfor
theincreasedvoltagethatwouldbeseenonthetransformerwindingterminals.It
alsoreducedthelikelihoodofdamagetotheADCintheeventofuntimelylead
disconnection.
WiththeadditionoftheMOSFETswitch,theprimaryconcernforfailurewas
theprotectionresistor.Basedontheexpectedenergycalculatedinsection6.1and
giventhe5‐secondover‐currentratingsoftheOhmite280seriesresistors,a
minimumofratingof240Wattswouldbenecessary.Toprovideadditionalmargin,
two300Watt,12Ohmresistorswereconnectedinseries.
FIGURE15‐TESTINSTRUMENTINTERNALCIRCUITRY
42
FIGURE16‐TESTINSTRUMENTSCHEMATIC
43
8
RESULTS
8.1 METHODSFORDETERMININGTHESTATEOFRESIDUALMAGNETIZATION
Ofsignificantconcernforthesetestsisthedeterminationoftheeffectiveness
ofdemagnetization.Thisstatewasattemptedtobequalitativelydeterminedintwo
ways,primarilybyrepeatedcomparisonofthesaturationtimeforaspecificDC
inputvoltageatbothpolaritiesafterademagnetizationroutinewascompleted.
Thesecondarymethodusedwastoenergizeanunloadedtransformerand
observethemagnitudeofinrushcurrenttothetransformer.Unfortunately,this
methodwasdeterminedtobeunreliableduetotiminglimitationsandcontact
bouncingoftheswitchingapparatus.
8.2 PERMEABILITYMETHOD
Thismethodusedtherelationshipofthechangeincurrentovertimetothe
amountofmagneticfluxinthecoreinordertoidentifytheneutralmagnetization
state.However,itprovedtobemuchmorecomplicatedwhendealingwithreal‐
worldsystemsthanthetheoreticalmodels.Thereliabilityofthemethodinthe
previousworkwasdifficulttoimplementbecauseofthehighlylinearnatureof
siliconsteelhysteresischaracteristics.Additionally,itwassuspectedthatlossesdue
tomagneticfluxleakageoutsidethecorecausethelocalminimumof andthe
neutralmagnetizationpointofthecoretobeoutofphase.
Whentestingthismethodfordemagnetization,itwasfoundthattheneutral
magnetizationstatewasovershotbymagnitudesof20‐30%.
44
Current
Time
FIGURE17‐EXAMPLECURRENTFLOWTIMELINEAFTERVOLTAGEPOLARITYISREVERSED
8.3 TIMEBASEDMETHOD
Whereisolatedtestingofawindingwaspossible(Wye‐Wye&Delta‐Wye),the
integrationmethodfordemagnetizationwasmuchmoreeffectivethantheprevious
method.Forthesetypesoftransformers,thismethodwasabletoachieveaneutral
magneticstatewitha7%maximumobservedmarginoferror.
Theareathatprovedanobstacleforthismethodwasthedemagnetizationof
transformerswherethewindingscannotbeisolated.Fortransformerwindings
connectedinaDeltaconfiguration,whenapotentialvoltagedifferenceisapplied
betweentwoofthethreeterminalstheresultisthatwhiletheprimarywinding
buildsfluxaccordingtothevoltageapplied,theothertwowindingswillonlysee
halftheappliedvoltage.Thus,assumingtheresistancesofallthreewindingsare
comparable,thecurrentflowingthroughthesecondandthirdwindingsisonehalf
thecurrentflowingthroughtheprincipalwindingundertest.Duetothedirectionof
thevoltagepolarityandthewaythatthewindingsareplacedonthecore,the
45
magneticfieldduetothiscurrentworkstoreinforcethemagneticfieldgeneratedby
thecurrentflowingintheprimarywindingasillustratedinFigure18.
Complementary
windings
PrincipalWinding
FIGURE18‐DELTATRANSFORMERMAGNETICFIELDDURINGDCENERGIZATION
Inadditiontothepreviousfactor,itappearsthatthereducedvoltageacross
bothcomplementarywindingsresultsinalongersaturationtimeforthe
complementarywindingsthantheprincipalwinding.Thismayoccurbecausethe
permeabilityofthecore(aswellastheapparentchangeininductanceovertime)
dependsontheamountofcurrent.Theendresultisthatthesaturationtimeforthe
wholetransformerislongerthanthesaturationtimeoftheprimarywindingusedin
thepreviouscalculations.
Afterimplementingthisroutine,theresidualmagnetizationofdelta‐wound
transformerstestedexhibiteda15%‐25%overshootoftheneutralmagnetization
state.
46
8.4 INTEGRATIONMETHOD
Modifyingthetargetforthemagneticfluxintegrationtimenotonlyhelpedto
accountfortheeffectsofcoremagnetizationbutwasalsofoundtogreatlyincrease
theaccuracyofdemagnetizingtransformerswithDeltaconfigurationwindings.
Compensationofleakagelosseswasaccomplishedbyadjustingtheintegration
intervaltobeginthemomentthevoltagepotentialisreversedandtoendwhenthe
currentthroughthetransformerreaches63.2%ofthesaturationcurrentinthe
oppositedirectionofcurrentflow.Thisresultedinreachinganeutralmagnetization
statewithamaximumobservederrorof3%fortransformerswithisolated
windings.
ThereasonforthisincreaseinaccuracyforDelta‐woundtransformersisdue
tothefactthatthecorematerialofthesecondarywindingssaturatesataslower
ratethantheprimarywinding.Itwasfoundthat,whenthetotalcurrentthroughthe
systemis63.2%ofsaturationcurrent,thecomplementarywindingshavenotyet
goneintosaturationandtheprincipalwindingisjustreachingsaturation.Thisgives
anapproximationforanintegrationintervalthatisreasonablyeffective.This
demagnetizationroutineexhibiteda3%‐8%overshootoftheneutralmagnetization
pointforthesetypesoftransformers.
47
9
CONCLUSIONS
Asexpectedfromobservationsoftransformercharacteristicsintheprevious
work,thepermeabilitymethodfordemagnetizingthetransformercorewasnot
veryeffectivecomparedtotheothertwomethods.Thetimebasemethodfor
estimationofthemagneticstateofatransformercorewasfoundtobeeffectivein
predictingandattainingdemagnetizationofpowertransformerswhichonlyhada
windingforasinglephase.
Max. Demagnetzation Error
Single Phase
30%
Three Phase
25%
8%
20%
7%
Permeability
Based
Time Based
3%
Integration
Based
Three Phase
Single Phase
FIGURE19–COMPARISONOFDEMAGNETIZATIONMETHODS:MAXIMUMERROR
Theintegrationbasedmethodwasthemethodselectedforfutureuse.This
methodwasfoundtohaveimprovedaccuracyoverthetimebasedmethodwhen
demagnetizingtransformerswithwindingsforallthreephases.Whilenotasfastas
thePermeabilitymethod,thismethodconsiderablyreducedthetimerequiredfor
demagnetization.Thedemagnetizationmethoddevelopedduringthisthesisisnow
goingthroughthepatentprocessbydesignatedstaffatBPAandtheU.S.Department
ofEnergy.
48
Theinstrumentdesignedfortheautomationofthisdemagnetizationroutine
waseffectiveinimprovingthesafetyoftheoperatorbyautomatingmanytasks
wheretherewaspotentialtocomeintocontactwithhighvoltages.Thisinstrument
isnowinearlyproductionstagesforanexpandedfieldtrialwithtransformer
maintenanceteams.
End. End.
49
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