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Dissertations and Theses Dissertations and Theses

1-1-2011

Proposal and Analysis of Demagnetization Methods Proposal and Analysis of Demagnetization Methods

of High Voltage Power System Transformers and of High Voltage Power System Transformers and

Design of an Instrument to Automate the Design of an Instrument to Automate the

Demagnetization Process Demagnetization Process

Nathanael Jared Makowski Portland State University

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Recommended Citation Recommended Citation 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. https://doi.org/10.15760/etd.431

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ProposalandAnalysisofDemagnetizationMethodsofHighVoltagePower

SystemTransformersandDesignofanInstrumenttoAutomatethe

DemagnetizationProcess

by

NathanaelJaredMakowski

Athesissubmittedinpartialfulfillmentoftherequirementsforthedegreeof

MasterofSciencein

ElectricalandComputerEngineering

ThesisCommittee:MartinSiderius,Chair

BetsyNatterBranimirPejcinovic

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.

iiiTableofContents

Abstract................................................................................................................................................................i Acknowledgements.......................................................................................................................................ii ListofFigures.................................................................................................................................................iv1 Motivation.........................................................................................................................................................12 IntroductionofProblem..............................................................................................................................42.1 WindingResistanceTest...................................................................................................................42.1.1 IEEEStandardProcedure........................................................................................................42.1.2 TestCurrentMagnitude...........................................................................................................5

2.3 SignificanceofTransformerSaturation......................................................................................62.3.1 TransformerHealth...................................................................................................................72.3.2 PowerSystemProtection........................................................................................................72.3.3 SystemPowerQuality...............................................................................................................9

3 TheoryandPrincipalsofPowerTransformerOperation...........................................................103.1 History.....................................................................................................................................................103.2 MagneticProperties..........................................................................................................................113.3 ElectricSteelMagnetization&Hysteresis................................................................................133.4 PresentlyUsedDemagnetizationMethods..............................................................................15

4 PreviousWork...............................................................................................................................................184.1 InstrumentDesign.............................................................................................................................184.2 DemagnetizationAlgorithm...........................................................................................................20

5 DemagnetizationMethods.......................................................................................................................235.1 PermeabilityMethod........................................................................................................................235.2 TimeBasedMethod...........................................................................................................................245.3 IntegrationMethod............................................................................................................................26

6 DemagnetizationDeviceDesignRequirements..............................................................................296.1 UserSafety.............................................................................................................................................296.2 MeasurementSystem.......................................................................................................................336.3 SystemReliability&Protection....................................................................................................346.4 Automation&Usability....................................................................................................................35

7 DeviceConstructedforDemagnetizationTesting..........................................................................367.1 Controller...............................................................................................................................................367.2 DevicePower........................................................................................................................................377.3 MeasurementComponents............................................................................................................387.4 System&UserProtection...............................................................................................................40

8 Results..............................................................................................................................................................438.1 MethodsforDeterminingtheStateofResidualMagnetization......................................438.2 PermeabilityMethod........................................................................................................................438.3 TimeBasedMethod...........................................................................................................................448.4 IntegrationMethod............................................................................................................................46

9 Conclusions.....................................................................................................................................................4710 WorksCited...............................................................................................................................................49

ivListofFiguresFigure1‐IEEEResistanceMeasurementCircuit.......................................................................................4Figure2‐InrushCurrentexample...................................................................................................................6Figure3‐MagnetizationComparisonofSiliconSteeltoIron.............................................................12Figure4–ExampleofCurrentMagnitudeDuringDemagnetizationRoutine..............................16Figure5–MeggerMTO210DemagnetizationRoutine(16)................................................................17Figure6‐PrototypeTestInstrument...........................................................................................................18Figure7‐DeviceDesignforPreviousWork...............................................................................................18Figure8‐PresentWindingResistanceandDemagnetizationTestset..........................................19Figure9‐TheoreticalmagnetizingCurrentandMagneticFluxforonecycle.............................21Figure10‐PermeabilityDemagnetizationRoutine................................................................................24Figure11–Coremagnetizationovertimewhenafixedvoltageisappliedtothewinding...25Figure12‐TimeIntegrationDemagnetizationRoutine.......................................................................26Figure13‐ModifiedTimeIntegrationDemagnetizationRoutine....................................................27Figure14‐CompletedTestSet........................................................................................................................40Figure15‐TestInstrumentInternalCircuitry.......................................................................................41Figure16‐TestInstrumentSchematic.......................................................................................................42Figure17‐ExampleCurrentFlowTimelineafterVoltagePolarityisReversed.........................44Figure18‐DeltaTransformerMagneticFieldduringDCEnergization.........................................45Figure19–ComparisonofDemagnetizationMethods:MaximumError.......................................47

11 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

3timeneededtoaccuratelyprepareatransformerbankforenergizationbyafull

hourcomparedtosomepreviousmethodsandprovidesanadditionallevelof

personnelsafetywhenperformingthistransformerdiagnostictest.Also,with

furtherdevelopment,theflexibilityoftheadvancedhardwarecouldallowthe

integrationofadditionaltestsintothesameunitandreducethetimenecessaryto

setupthevariousteststhatmustbeperformed.

Thedeploymentofthisnewtestsetwillimprovetheaccuracyandefficiencyof

routinetransformerdiagnostictests.ItwillalsoextendthelifeofBPA’stransformer

assetstherebyimprovingreliabilityanddecreasingcapitalcosts.

42 INTRODUCTIONOFPROBLEM

2.1 WINDINGRESISTANCETEST

Oneofthemanytestsperformedduringroutinemaintenanceofapower

transformeristhewindingresistancetest.Thistesthelpstogaugethehealthof

internalconnectionswithinthetransformerbycomparingthemtovaluesmeasured

bythemanufactureruponbeingconstructed.Thisisanimportantbenchmark;

withinatransformerthereareoftenanumberofdifferent“tap”connectionsthat

canbemadetoadjusttheratio

ofthetransformerbyaslittle

asafractionofonepercent.

Thesetappositionsonlarge

powertransformersareoften

controlledremotelyby

dispatcherswhomonitorandmakeadjustmentstomaintainsystembalance.The

reliabilityoftheseconnectionsiscriticaltosystemoperations.

2.1.1 IEEESTANDARDPROCEDUREThistestisperformedaccordingtothedirectionsgiveninIEEE62‐1995,

(section6.1.1.1)(6)byinjectingacurrentintothewindingofatransformeras

displayedinFigure1.Oncethewindinginductancehasbeenovercome,ohmslaw

canbeusedtocalculatetheresistance.

Whiletheprocedureissimple,thelastingeffectsofsaturatingtheinductance

ofthewindingcanbesignificant.Itisimportantthattheybeconsideredbefore

Current Shunt Make‐Before‐Break

V

V

Transform

er W

inding

Battery

FIGURE1‐IEEERESISTANCEMEASUREMENTCIRCUIT

5connectingthetransformertothepowersystem,orperformingothertests,andis

specificallysuggestedinIEEE62beforeperformingthe‘ExcitingCurrent’test

(section6.1.3.)ThestateofmagnetizationalsoaffectstheresultsofFrequency

ResponseAnalysis(FRA)tests,whichisrapidlygainingpopularityasadiagnostic

tool.Saturationeffectsarediscussedmoreinsection2.2

2.1.2 TESTCURRENTMAGNITUDEForthewindingresistancetest,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.

62.3 SIGNIFICANCEOFTRANSFORMERSATURATION

Asmentionedintheprevioussection,saturationofthetransformercoreis

necessaryforthewindingresistancetest.Thisissignificantbecauseoftheresulting

effectsofresidualmagnetizationduetosaturation.Thissectionwillexplainthe

significanceandconsequencesof

theresidualmagnetizationand

thephysicsoftheresidual

magnetizationwillbeconsidered

insection4.

Figure2showsanexample

ofthemagnitudeofcurrentthat

canpassthroughthetransformer

windingswhenthecoreofthetransformergoesintosaturationduetotheresidual

magnetization.TEST

Inthisgraphthecurrentisgivenin“PerUnit”quantitiesandisgivenbythe

relationship:P.U.Current=ActualCurrent/NormalRatedCurrent.Thus,inthis

example,thecurrentpassingthroughthewindingismorethan23timesthenormal

currentforthefirstcycleand5timesthenormalcurrentforthesecondcycle.

Thistransientinrushcurrentcandisturbtheentiresystemwithpotentially

damagingconsequences.Theseconsequencesaregenerallygroupedintooneof

FIGURE2‐INRUSHCURRENTEXAMPLE

7threemaincategories:TransformerHealth,SystemProtectionPlanning,andOverall

SystemPowerQuality.

2.3.1 TRANSFORMERHEALTHThefirstconcerniswiththeeffectsonthetransformeritself.Sincethe

mechanicalforceonawindingisproportionaltothesquareofthecurrent,inrush

currentscauseasignificantincreaseofmechanicalstressforcesontransformer

windingsaswellasresultinharmonicvibrationsthatincreasedegradationof

insulation(1),(2).Transformerfailuremodeslinkedtoinsulationdegradationare

oftenverydestructiveinnaturewhichcaneffectnearbycomponentsaswell.

Becauseofthesignificantinvestmenteachindividualtransformerrepresents,the

adverseeffectofinrushcurrentontheinternalcomponentsofatransformerwith

specificregardtoservicelifereductionhasledthepowersystemindustryto

researchandapplymanyproceduralchangestothewaythattransformersare

energizedandde‐energizedincludingcontrolledclosing(energizingthe

transformerataspecificpointintime)andtheuseofsurgesuppressionresistors

(3).

2.3.2 POWERSYSTEMPROTECTIONThesecondissuethatarisesconcernspowersystemprotectionplans.The

highinrushcurrentscancausethepowersystemprotectionandcontrolcircuitry

thatmonitorssystemfaults1tomistakenlyoperate(6)(7).Sincetransformersare

takenoutofserviceregularlyforroutinemaintenance,ifaprotectionrelaysystem

1line‐to‐groundorline‐to‐lineshortcircuitcondition

8mistakenlyrecognizesafaultconditionwhenattemptingtobringthetransformer

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.

92.3.3 SYSTEMPOWERQUALITY

Thethirdmainissuedealswiththeeffectonoverallsystempowerquality.

Theeffectsobservedincludeincreasesanddecreasesinthermsvoltagecalled

resonantharmonicvoltageswells(8)andvoltagesags(9).Theseeventslast16msto

60sindurationandarecharacterizedbylowfrequencyoscillationofrmsvoltage

amplitudesthatcoincidewithresonantpointsinthepowertransmissionsystem.

Also,sincetheseeffectsunbalancethecurrentflowofthepowersystem,thiscan

haveadetrimentaleffectondistributedgenerationcomponents(10):When

generatorsaredistributedacrosslargeserviceareasthepowerdemandplacedon

anindividualgeneratormaybegreaterthanothers,thiscanresultinhigh

temperaturesinarelativelyshortamountoftimeandhighriskoffailure,(11).

Anotherconsequenceisthatitcandisturbtheresultsofotherroutinemaintenance

tests.(12)

103 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.

11MuchinthesamemannerthatHadfieldsystematicallyusedsomanyyearsago

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

12significantandisillustratedinFigure3wheretheareaofthehysteresisloopis

representativeoftheenergylosses.

FIGURE3‐MAGNETIZATIONCOMPARISONOFSILICONSTEELTOIRON

AsyoucanseeinFigure3,theB‐Hrelationshipoftransformercorematerials

isverylinearuntilcoresaturationisreached,wheretherelationshipchangesvery

quickly.Becauseofthislinearityandthedramaticchangeatsaturation,power

transformerscanbedesignedtouseaminimumamountofcorematerialand

operateclosetothesesaturationpointswhilestillmaintainingenergyconversion

efficiency.Thisfactoralsohelpstosimplifycalculationsintheanalysisofmagnetic

saturationcharacteristicsandwillbeusedinthenextsectiontoestimatethe

maximumfluxdensityofthecore.

133.3 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

√22 60

.√2120

[3]

WhereVratedrepresentsthedesignedoperatingvoltageofthespecificindividual

transformerwindingundertest.

2Fitzgerald,A.E.ElectricMachinery(23)

15

Inthecaseofthewindingresistancetest,thevoltageappliedtothewindingis

almostconstant,suchthattheintegrationofthevoltageovertimesimplifiesto:

[4]

Bycombiningequations3and4anapproximatetimetoreachsaturationfora

giventestvoltagecanbefound:

√2120 [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

16methodshowninFigure5.Thisiscontinueduntilthenexttargetcurrentlevelis

zero.

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

time

Current

17thenbereverseduntilthecurrentisequalinmagnitudebutintheoppositepolarity.

Oncethatmagnitudeofcurrentisreached,thevoltagepotentialisthenrevertedto

theoriginalpolarityuntilthecurrentis20%oftheoriginaltestcurrent,atwhich

timethevoltagepotentialisreversedagainuntilthecurrentis20%oftheoriginal

testcurrentintheoppositedirectionofthecurrentthanthefirstapplicationofthe

voltagepotential.Thisprocessisrepeatedforcurrentsat4%oftheoriginaltest

currentandagainfor1%oftheoriginaltestcurrent.

FIGURE5–MEGGERMTO210DEMAGNETIZATIONROUTINE(16)

Thisunitwasnotavailableforevaluationsoadetailedcomparisonwillnotbe

made.However,evenassumingthattheroutineusedbytheMTO210does

sufficientlydemagnetizethetransformer,fromtheinformationpresented,this

demagnetizationroutinewouldappeartotakesignificantlylongertoexecutethan

theroutinesproposedinthisthesis.

Othermethodsofdemagnetizationhavebeenproposedwhichinvolvethe

applicationofanultra‐low‐frequencysquare‐wavevoltagesource(17)canbeused,

however,theyalsorequireextensivedemagnetizationtime.

Current

time

+

‐

184 PREVIOUSWORK

Thisthesisisacontinuationofworkdoneforapreviousdesignproject(18).

Thegoalofthepreviousprojectwastoprovideaproof‐of‐conceptforthe

constructionofthetestinstrument(Figure6)thatwouldbeabletoperforma

transformerwindingresistancetest

andalsobeabletodemagnetizethe

transformerautomatically.This

sectiondetailstheworkthatwas

accomplishedduringthatproject.

4.1 INSTRUMENTDESIGN

FIGURE7‐DEVICEDESIGNFORPREVIOUSWORK

Figure7detailsthedesignfortheinstrumentattheendoftheproject;itwas

intendedtobeforanautomatedversionofthestandardIEEEtestcircuitshownin

FIGURE6 ‐ PROTOTYPETESTINSTRUMENT

19Figure1.Theinstrumentdesignforthatprojectwaslargelybasedoncomponents

thatwerealreadyavailableonhandandestablishedcriteriaforthefollowing:

OperatingVoltage

CurrentLimiting

ProtectionComponents

Forthefirstcriteria,anoperating

voltageof12VDCwasselectedinthe

originaldesignbecausevehiclebatteries

arereadilyavailableinthefield.For

manyyears,automotivebatterieswere

theprimarypowersourceforwinding

resistancemeasurementsinthefield.

Originally,asingle12‐voltbatterywas

connectedtothetestinstrumentinFigure8andwasalwaysfoundadequate.

Forthesecondcriteria,thecurrentlimitingselectionwasdecidedby

comparisonandanalysisofthereadilyavailablecomponentswithexpectedwinding

resistancevalues.Inordertoobtainaccuratefieldvoltagemeasurementsand

maintainquicksaturationtimes,itwasdecidedthatthevoltageacrossthecurrent

limitingresistorshouldnotbemuchgreaterthanthevoltagedropacrossthe

windingresistance.Oftheresistorsthatwereavailable,twowire‐wound0.2Ω

resistorswereaddedinseriestoproducealimitoftheshortcircuitcurrentto30A.

FIGURE8 ‐ PRESENTWINDINGRESISTANCEANDDEMAGNETIZATIONTESTSET

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

21relationshipofthemagnetizingcurrenttotherelativepermeabilityaswellastothe

totalfluxinthecore.

Figure9illustratestherelationshipofthecurrentofatransformertothe

magneticfluxinthecorewhenpoweredbyasinusoidalvoltage.Inthegraph,where

themagneticfluxiszero,thecurrentexhibitstwodistinguishablefeatures.Thefirst

featureisthatthechangeincurrentovertimeisalocalminimumwhenthefluxis

zero.Thesecondnoticeablefeatureisthatthecurrentalsopassesthroughthezero

whenthefluxdoes.However,thissecondfeatureisnotasusefulsincethisisonly

thecasewhenpoweredbyasinusoidalvoltage.

Bymonitoringthecurrentandcalculatingthetimederivativeofthecurrent

afteraconstantvoltagepotentialisapplied,alocalminimumin asthe

transformermagnetizationswingsbetweenpolaritiescanbeidentified.Itwas

surmisedthatifthepowersourceisremovedattheappropriatetimethenthecore

shouldbeleftinastateofneutralmagnetization.

FIGURE9‐THEORETICALMAGNETIZINGCURRENTANDMAGNETICFLUXFORONECYCLE

v Φ

i

Local minimum(di/dt)

22

Thefinalcircuitdesignforthatprojectwassuccessfulinthatitwascapableof

measuringthedesiredquantitiesandcontrollingtheflowofcurrentthroughthe

winding.Howevertheaccuracyofthemeasurementswasultimatelyfoundlacking

onceconstructed.Also,aftertestingtheproposeddemagnetizationmethodona

115kV‐230kV,singlephasetransformer,itwasapparentthatthedemagnetization

methodneededimprovement.

235 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

24similarlytothatofanironcoreinductor.Additionalfactorsthatcouldreducethe

effectivenessofthismethodwouldbetestenvironmentconditions.Sincethe

substationsinwhichthesemeasurementsandtestsareperformedhavelarge

electromagneticfieldinterference,theabilitytomakethesensitivemeasurements

necessarytoidentifythemomentthat beginstoincrease.Thisroutineis

illustratedinFigure10‐PermeabilityDemagnetizationRoutine.

FIGURE10‐PERMEABILITYDEMAGNETIZATIONROUTINE

5.2 TIMEBASEDMETHOD

Thebasisforsecondmethodofdemagnetizationcomesfromaproposalbythe

sponsorofthepreviouswork.Themethodproposedisatimebasedmethodwhich

estimatesthemagneticfluxinthetransformercoreviaFaradaysLaw.Rearranging

equation[1]foraconstantvoltageVgives:

Transformer Winding Test

Apply Voltage to Transformer

Winding

Measure Current Through Winding

Is Current Increasing

YES

Begin Timer and Reverse Voltage

Polarity

NO

Measure Current Through Winding & Calculate di/dt

Is di/dt at a peak

NO

Remove Voltage from Transformer

Winding

YES

Run Lead Removal

Preparation Routine

Demagnetization CompleteIs di/dt

Increasing

25

∆ ∝ ∆ [6]

Therefore,wherethemagneticfluxisdirectlyproportionaltotheamountof

timethataconstantvoltageisappliedtothewinding,bymeasuringthetimeneeded

forthemagneticstateofthecoretoswitchfrombeingsaturatedinonedirectionto

becomingsaturatedintheoppositedirectionwecandeterminehowlongaconstant

voltageneedstobeappliedtothewindingthatissaturatedinordertoreachthe

neutralpoint.

Forexample,themagneticfluxofatransformershowninFigure11is

saturatedinareversepolaritybecauseofacurrentflowingthroughitswinding.At

FIGURE11–COREMAGNETIZATIONOVERTIMEWHENAFIXEDVOLTAGEISAPPLIEDTOTHEWINDING

ReverseSaturation

ForwardSaturation

26t=0avoltageisappliedtothetransformerwindingthatopposestheflowofcurrent.

Oncethecorehasreachedforwardsaturation,thevoltagecanthenbereversedand

appliedforhalfthetimerequiredtoreachthepointofreversesaturation.

Thismethodassumesthattheenergylossduetowindingresistancecanbe

considerednegligiblecomparedtotheenergythatdrivesthemagnetizationofthe

core.TheflowchartillustratingthestepsforthismethodisshowninFigure12

FIGURE12‐TIMEINTEGRATIONDEMAGNETIZATIONROUTINE

5.3 INTEGRATIONMETHODThefinalmethodissimilartothesecondmethod;however,duringtestingit

wasfoundthatthemagnetizationcharacteristicsoftransformercorescangreatly

varydependingontransformerageandhowthetransformeriswound.Depending

Transformer Winding Test

Apply Voltage to Transformer

Measure Current Through Winding

Is Current Increasing

YES

Begin Timer and Reverse Voltage

Polarity

NO

Measure Current Through Winding

Is Current Magnitude

equal to Isat

Store Current Magnitude:

Isat

Store Timer Value: tsat

YESNO

Clear Timer and Reverse Voltage

Polarity

Measure Current Through Winding

Is Timer = ½ tsat

NO

Remove Voltage Potential

Run Lead Removal

Prep Routine

Demagnetization Complete

27onthesharpnessofthetransitiontomagneticsaturationasthemagneticfield

intensityincreases,energylossduetothewindingresistanceincreases.

FIGURE13‐MODIFIEDTIMEINTEGRATIONDEMAGNETIZATIONROUTINE

Ratherthanassumingthatthevoltagedrivingthegenerationofmagneticflux

withinthetransformercoreremainsconstantatalltimesasinEquation[6],the

accuracyofestimationcanbeincreasedbytakingintoaccountvoltagelossesdueto

thecopperwireresistanceascurrentincreases.Beevaluatingtheintegralformof

Faraday’sLawwithrespecttoformofthetransformerwindingsgives:

Φ [7]3

3Fitzgerald,A.E.ElectricMachinery(23)

Transformer Winding Test

Apply Voltage to Transformer

Measure Current Through Winding

Is Current Increasing

YES

Begin Timer and Reverse Voltage

Polarity

NO

Measure Current Through Winding

Is Current = Isat * .632

Store Current Magnitude:

Isat

Store Timer Value: tsat

YESNO

Clear Timer and Reverse Voltage

Polarity

Measure Current Through Winding

Is Current = Isat

NO

Remove Voltage Potential

Run Lead Removal

Prep Routine

Demagnetization Complete

Is Timer = ½ tsat

YES

Measure Current Through Winding

YES

NO

28whereVLrepresentsthevoltagedropacrossthewindingduetoself‐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.

296 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:

12

[9]4

Where( )representsthevolumeofthecorecontainingthemagneticfield.

4FundamentalsofAppliedElectromagnetics,F.Ulaby

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:

02 [14]

Where representsthetimeittakesforthecurrenttoincreasefromitsmagnitudeat80%

ofthetotalsaturationtimeto63.2%ofthesaturationcurrentmagnitude.

Sincewindingresistancedatacanbefoundfromthemanufacturer’sinitial

tests:

[15]

Then,thetotalenergystoredistheadditionofequations[11]and[14]

Withtheseestimates(andtheexpectedwindingresistancevaluesranging

between2Ωto0.01Ω),thehighvoltagepowertransformerscouldstoreenergywith

amagnitudeofhundredsofjoulesinthemagneticfieldwhenenergizedwitha12V

source.Duringthetestingoftheworkinthisthesis,energyrangesashighas530

32jouleswereobserved.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

33forvoltagesashighas60V.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

35unit.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.

367 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.

377.2 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

38andrelaycontrollines.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

39within±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

41leadswerenotdisconnectedintheproperorder.

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

438 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

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

Current

Time

45magneticfieldduetothiscurrentworkstoreinforcethemagneticfieldgeneratedby

thecurrentflowingintheprimarywindingasillustratedinFigure18.

FIGURE18‐DELTATRANSFORMERMAGNETICFIELDDURINGDCENERGIZATION

Inadditiontothepreviousfactor,itappearsthatthereducedvoltageacross

bothcomplementarywindingsresultsinalongersaturationtimeforthe

complementarywindingsthantheprincipalwinding.Thismayoccurbecausethe

permeabilityofthecore(aswellastheapparentchangeininductanceovertime)

dependsontheamountofcurrent.Theendresultisthatthesaturationtimeforthe

wholetransformerislongerthanthesaturationtimeoftheprimarywindingusedin

thepreviouscalculations.

Afterimplementingthisroutine,theresidualmagnetizationofdelta‐wound

transformerstestedexhibiteda15%‐25%overshootoftheneutralmagnetization

state.

PrincipalWindingComplementarywindings

468.4 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.

479 CONCLUSIONS

Asexpectedfromobservationsoftransformercharacteristicsintheprevious

work,thepermeabilitymethodfordemagnetizingthetransformercorewasnot

veryeffectivecomparedtotheothertwomethods.Thetimebasemethodfor

estimationofthemagneticstateofatransformercorewasfoundtobeeffectivein

predictingandattainingdemagnetizationofpowertransformerswhichonlyhada

windingforasinglephase.

FIGURE19–COMPARISONOFDEMAGNETIZATIONMETHODS:MAXIMUMERROR

Theintegrationbasedmethodwasthemethodselectedforfutureuse.This

methodwasfoundtohaveimprovedaccuracyoverthetimebasedmethodwhen

demagnetizingtransformerswithwindingsforallthreephases.Whilenotasfastas

thePermeabilitymethod,thismethodconsiderablyreducedthetimerequiredfor

demagnetization.Thedemagnetizationmethoddevelopedduringthisthesisisnow

goingthroughthepatentprocessbydesignatedstaffatBPAandtheU.S.Department

ofEnergy.

Single Phase

Three Phase

PermeabilityBased

Time Based IntegrationBased

20%

7% 3%

30%25%

8%

Max. Demagnetzation Error

Single Phase Three Phase

48

Theinstrumentdesignedfortheautomationofthisdemagnetizationroutine

waseffectiveinimprovingthesafetyoftheoperatorbyautomatingmanytasks

wheretherewaspotentialtocomeintocontactwithhighvoltages.Thisinstrument

isnowinearlyproductionstagesforanexpandedfieldtrialwithtransformer

maintenanceteams.

End.

End.

49

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