非晶合金铁芯.docx

非晶合金铁芯.docx

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非晶合金铁芯

Performanceofsinglewireearthreturntransformerswithamorphousalloycoreinaruralelectricenergydistributionsystem

BeneditoAntonioLucianoI, *;RenatoCucatoInácioII;PablodePaulaeSilvaII;FranciscodasChagasFernandesGuerraI;RaimundoCarlosSilvérioFreireI

IDepartamentodeEngenhariaElétrica,UniversidadeFederaldeCampinaGrande-UFCG,RuaAprígioVeloso,882,CEP58429-900,CampinaGrande,PB,Brasil 

IICompanhiadeEnergiaElétricadoEstadodeTocantins,Palmas,TO,Brasil

ABSTRACT

Inthispaperarepresentedsomeconsiderationsabouttheperformanceofsinglewireearthreturnamorphousalloycoretransformersincomparisonwithconventionalsiliconsteelsheetscorestransformersusedinruralelectricenergydistributionnetwork.Ithasbeenrecognizedthatamorphousmetalcoretransformersimproveelectricalpowerdistributionefficiencybyreducingtransformercorelosses.Thisreductionisduetosomeelectromagneticpropertiesoftheamorphousalloyssuchas:

highmagneticpermeability,highresistivity,andlowcoercivity.Experimentalresultsobtainedwithsomesingle-phase,60 Hz,5 kVAamorphouscoretransformersinstalledinaruralareaelectricdistributionsysteminNorthernBrazilhavebeenconfirmingtheirsuperiorperformanceincomparisontoidenticalnominalratedtransformersbuiltwithconventionalsiliconsteelcores,particularlywithregardtotheexcitationpowerandtotheno-loadlosses.

Keywords:

 amorphousalloy,corelosses,energyefficiency,transformer

1.Introduction

Singlewireearthreturn（SWER）coretransformersarecharacterizedbyhavingonlyonebushingonthehighvoltagesideandtwoormorebushingsonthelowvoltageside.Generally,thesetransformersaredesignedandconstructedtobeusedinruralelectricitydistribution.UsuallySWERcoretransformersarelow-costequipmentandhaveapparentpowerof5,10and15 kVA.

Thedistributionofelectricitypowerinruralareashasgenerallyhighcostofoperationfortheconcessionarycompaniesduetotheseasonalityoftheirloads.Undertheseconditions,thelossesinthecoresofthetransformersmanufacturedwithconventionalgrainorientedsiliconsteelbecomesignificantwhenthesetransformersoperateinconditionsoflightloadornoload.Insuchcircumstances,partofthetotalcurrentabsorbedbytheelectricsystemisusedinthemagnetizingcoretransformersprocess.However,theselossescanbereduced,whichcanbeachievedbyreplacingtheconventionalbytransformersforamorphousmetal-basedtransformers,becauseithasbeenrecognizedthatamorphousmetalcoretransformersimproveelectricalpowerdistributionefficiencybyreducingexcitationcurrentandcorelosses1.Thisreductionisduetosomeelectromagneticpropertiesoftheamorphousalloyssuchas:

highmagneticpermeability,highresistivity,andlowcoactivity2.

InthispaperarepresentedsomeexperimentalresultsobtainedwithSWER,60 Hz,5 kVAamorphouscoretransformersinstalledinaruralareaelectricdistributionsystem,distant50 kmfromPalmas（capitalofthestateofTocantins）,locatedintheAmazonregionofBrazil.

Fromtheseresultsitcanbeobservedthesuperiorperformanceofamorphouscoretransformersincomparisontoidenticaltransformersmanufacturedwithconventionalcold-rolledgrain-oriented（CRGO）siliconsteelcores,particularlywithregardtotheexcitationcurrentandtotheno-loadlosses.Beforebeinginstalled,alltransformersweresubmittedtotestasstatedatBrazilianStandards.

2.FromAcademicResearchtoElectricDistributionSystems

Chronologically,thedevelopmentofamorphousalloyspotentiallyapplicableintransformercoresbeganin1975.However,theintroductionofthesematerialsinthemarketonlyhappenedin1976,withthefirstamorphousmetaldistributiontransformer（AMDT）,builtatMIT'sLincolnLaboratory（USA）.Inthatopportunity,a30 kVAtransformerprototype,withanamorphousmaterialcore,wascomparedtoaconventionalsiliconsteelcoretransformerofequalnominalratedpower.Oncethecomparativetestswerecarriedout,theresultsobtainedwerefavorabletoAMDT,particularlyintheaspectsrelativetothelossesandtheexcitationcurrent:

therewasareductionof60%inthetotallosses（87.5%inthecorelossesand21%inthecopperlosses）andtheexcitationcurrentwasreducedfrom2.5to0.12 A.However,inwhatconcernstheweight,AMDTpresentedanincreaseof15%incomparisontotheconventionalsiliconsteelcoretransformer3.

Intheearly1980's,EPRI（ElectricPowerResearchInstitute,USA）andGeneralElectriclaunchedaprojectinwhich25unitsofpre-prototype25 kVA,15 kVamorphousmetalcoredistributiontransformers（AMDT）weretestedfortwoyears,followedbydesigningandtesting1000unitsofthesamesizeAMDTsontheirdistributionsystems.Thepurposeofthefieldtrialwastoconferthelong-termstabilityofcorelossinamorphousmetaltransformersunderactualoperationconditions.Tenyearslater,therewasanestimated60000to70000 AMDTinstalledthroughouttheworld.

InBrazil,thefirststudiesandexperimentaltestshadtheirorigininlaboratoriesofsomedifferentinstitutions,intheearly1990's4,passing,soonafter,toassemblybyindustrialmanufacturers5.

3.ExperimentalProcedure

In Figure 1 areshownsomeunitsof5 kVASWERtransformerswithamorphousalloycore（Fe78B13Si9）manufacturedbyBrazilianindustriesITB.Seventeenofthesetransformersweresubmittedtoopencircuitandshortcircuittestsinthelaboratoryofthefactory,inJanuary2009.

Thefollowingarelistedthemainelectriccharacteristicsoftransformersundertest:

•Ratedpower kVA = 5;

•Phases = 1;

•Frequency = 60 Hz;

•Highvoltageside（kV）:

20.90to18.19;

•Lowvoltageside（V）:

440/220;

•Ratedcurrentinhighvoltageside:

0.25 A;

•Ratedcurrentinlowvoltageside:

11.36 A;

•Subtractivepolarity.

AllroutinetestscarriedoutinaccordancewithspecificBrazilianStandards.Butbecausethemaininterestisdirectlyrelatedtotheenergyefficiencyinthispaperareonlypresentedtwotests:

no-loadandshortcircuit.

4.Results

In Table 1 arepresentedthecomparativevaluesconcerninglossesandexcitationcurrentinsingle-phase5 kVAtransformerswithmaximumtension24.2and36.2 kV,accordingtoBrazilianStandardNBR54406.

Theno-loadlossesandtheexcitationcurrentmeasurementswereperformedatnominalfrequency,applyingratedvoltagetothelowvoltagewindingandleavingthehighvoltageinopencircuit,asrequiredbyBrazilianStandardNBR53567.

In Figure 2 arepresentedingraphform,theresultsofcomparativetestsbetweenseventeentransformerswithamorphousalloycoreandseventeentransformerswithCRGOsiliconsteelcore,allwiththesamenominalpowerrating（5 kVA）.

5.Discussion

Sincetheexcitationcurrentissmall,theJoulelossesinthewindingsarenegligible;thelossesresultingfromtheprocessofmagnetizationanddemagnetizationofthecorearesignificant.Inanalyticalterms,theselossesarecharacterizedashysteresis,eddycurrentsandanomalouslosses.

Thehysteresislossisdirectlyproportionaltothefrequencyofsupplyvoltageandeddycurrentlossisdirectlyproportionaltothesquareofthefrequencyofthesource.Thisisthereasonwhythistestshouldbeconductedatnominalfrequency.

Physically,thereasonfortheexcitationcurrentissmallcomparedtothenominalcurrentofthewinding,duetothefactthattheno-loadtransformercanberepresentedbyahigh-impedanceloadwithrespecttothepowersupply.Andthisimpedancewillbegreaterthelargerthemagneticpermeabilityandelectricalresistivityofcorematerial.

InthespecificcaseofamorphousFe78B13Si9 usedinthecoresoftransformersthesevaluesarehigherthanthevaluesforGOFeSialloysusedinconventionaltransformers.Therefore,themaximumrelativemagneticpermeabilityofamorphousFe78B13Si9,60 Hz,isaround70000,whilethevalueofthisquantityfortheGOFeSiat60 Hz,isaround40000.SincethevalueoftheelectricalresistivityofamorphousFe78B13Si9 isbetween130to135 µΩ.m,whilethevalueofthismagnitudefortheGOFeSiisaround10to47 µΩ.m.Notethatthehighvalueofrelativepermeabilityprovidesabettermagneticcouplingandthehighvalueofelectricalresistivityresultsinsmallereddycurrentlossesinthecore.

NotealsothatalthoughbothtransformershavevaluesofexcitationcurrentandloadlossvaluesbelowtheBrazilianStandardNBR 5440recommended,itappearsthatthevaluesoftheexcitationcurrent（0.13 A）andno-loadlosses（8 W）fortransformerswithamorphouscoretransformersarelowerthanvaluesoftheconventionaltransformers,thatare0.36 Aand36 W,respectively.

Themeasurementofshortcircuitlosseswasperformedatnominalfrequency,connectingthehighvoltageterminalstothesourceandatthesametimekeepingthelowvoltagewindingsshort-circuited,asshowninFigure 3.

In Table 2 arepresentedthecomparativevaluesconcerningshort-circuittestinsingle-phase5 kVAtransformerswithmaximumtension24.2and36.2 kV,accordingtoBrazilianStandardNBR 54406.

Ascanbeseenin Table 2,therearevirtuallynodifferencesbetweenthevaluesobtainedinshort-circuittestforbothtypesoftransformers.This,infact,wasexpected,sincetheactivelossesrelatetotheJouleeffectinthewindingsandhasnodependenceonthemagneticmaterialcoreandshort-circuitcurrentdependsontheimpedanceofashortcircuit.

Aftersubmittedtoroutinetestsinthelaboratory,theseventeen5 kVASWERamorphouscoretransformerswereinstalledinaruralareaelectricdistributionsysteminNorthernBrazil,