lightning impulse breakdown characteristic of sf6 alternative gases for gas insulated switchgear

lightning impulse breakdown characteristic of sf6 alternative gases for gas insulated switchgear

LightningimpulsebreakdowncharacteristicsofSF6alternativegasesforgas-insulatedswitchgear

Y.Hoshina,M.Sato,M.Shiiki,M.HanaiandE.Kaneko

Abstract:InvestigationsofthelightningimpulsebreakdowncharacteristicsofaSF6/N2gasmixture,N2gasandCO2gashavebeencarriedout.InthecaseoftheSF6/N2gasmixturethe50%breakdown(electric) eldstrengthforthepositiveimpulsevoltagewasslightlylowerthanthatforthenegativeimpulsevoltage,andthepressureoftheSF6/N2gasmixturemustbeincreasedtoapproximatelytwicethepressureofSF6gastoobtainthesamedielectricstrengthasthatofSF6gas.ForCO2gas,the50%breakdown eldstrengthforthenegativeimpulsevoltagewaslowerthanthatforthepositiveimpulsevoltageinthecaseofSF6gas.However,thedielectricstrengthofCO2gas,evenattwicethepressureofSF6gas,wasapproximately60%ofthestrengthofSF6gas,sothattheCO2gas-insulatedbusbarmaybeapproximately1.6timesaslargeastheSF6gas-insulatedbusbar.InthecaseofN2gas,the50%breakdown eldstrengthforthepositiveimpulsevoltagewaslowerthanthatforthenegativeimpulsevoltage,andthedielectricstrengthofN2gas,evenattwicethepressureofSF6gas,waslessthan50%ofthestrengthofSF6gas,sothattheN2gas-insulatedbusbarmaybeapproximately2.1timesaslargeastheSF6gas-insulatedbusbar.AremarkabledifferencebetweenCO2andN2gaswasfoundfromtheobservationofdischargeformsandpartialdischargeproperties.

1Introduction

TheSF6/N2gasmixtureisconsideredtobepromisingasanalternativetoSF6gaswhichisspeci edasagashavingalargeimpactontheglobalenvironment,andtheresultsofvariousstudiesonthegasmixturehavebeenreported[1–5].FromtheviewpointofeliminatingtheuseofSF6gas,CO2andN2gashavebeenindividuallystudiedaspossiblealternativestoSF6gas[6].

Itisknown,however,thattheinsulationcharacteristicsofCO2andN2gasesaregenerallyinferiortothoseofSF6gas.Forthisreason,forCO2orN2gas-insulatedswitchgear(GIS)tohaveinsulationcharacteristicssubstantiallyequiva-lenttothoseoftheexistingSF6gas-insulatedswitchgeartheratedpressureofCO2orN2gasmustberaisedtoavaluehigherthanthatofSF6gasconventionallyused.

FordesigningGISinpracticevariousstudiesandinvestigationsonpropertiesofthegasusedasadielectricmediumarerequired.Themajorpropertyofthegasforde ningthesizeordimensionofGISincludesthe50%lightningimpulsebreakdowncharacteristic.Moreover,sincetheimpulsebreakdowncharacteristicsofthegashastheso-calledareaeffect,fromtheviewpointofpracticalapplicationthestudiesmustbecarriedoutbyusingtestingdeviceshavingdimensionssimilartothoseofcomponentsofGISactuallyused.However,therehasbeenalmostnoreportofstudiescarriedoutbyusinghigh-voltagetesting

deviceswithelectrodedimensionssimilartothoseofactualGIS.

Inviewofthisbackgroundwehavecarriedoutstudiesonthelightningimpulsebreakdowncharacteristicofhigh-pressureCO2gas,N2gasandSF6/N2gasmixtureincoaxialelectrodearrangementsthathavequasiuniformelectric elddistributionandaresimilartotheGISactuallyused.Inadditiontothelightningimpulsebreakdowncharacteristicswehavealsocarriedoutvariousstudiesonotherinsulationpropertiesofthealternativegasesbyusingotherdevices.Theseresultsarereported.2

CharacteristicsrequiredfordesigningGIS

rIEE,2006

IEEProceedingsonlineno.20050007doi:10.1049/ip-smt:20050007

Paper rstreceived25thFebruaryandin nalrevisedform28thOctober2005Y.Hoshina,M.Sato,M.ShiikiandM.HanaiarewithToshibaCorporation,Japan

E.KanekoiswithUniversityoftheRyukyus,JapanE-mail:yoshikazu.hoshina@toshiba.co.jp

IEEProc.-Sci.Meas.Technol.,Vol.153,No.1,January2006

ForthepracticaldesignofGISitisnecessarytocollectbasicdataonthecharacteristicsofinsulatinggasshowninFig.1onthebasisofwhichdatathedesignstandardelectric eldstrengthsfortheinsideofGISwillbedetermined.InadditiontothecharacteristicsshowninFig.1itisalsonecessarytocollectdataoninsulatorbreakdowncharacter-isticswhicharecloselyassociatedwiththeelectric elddistributiononthesurfaceofelectrodeembeddedintheinsulatorforsupportingandinsulatingthehigh-voltageconductorfromthetankinGIS.Thesecharacteristics,however,donotdependontheinsulatingpropertiesofthegasbutprimarilyontheelectric eldonthesurfaceofembeddedelectrodeaswellasonthestateinthevicinityoftheelectrode.Accordingly,thedesignstandard(electric) eldstrengthfortheembeddedelectrodeisusedintheSF6gas-insulatedswitchgearcanbeusedforthealternativegas-insulatedswitchgear.

2.1Gasgapbreakdowncharacteristicsundercleanconditions

GasgapbreakdowncharacteristicsundercleanconditionsareconsideredtobemostessentialforthedesignofGISwhichincludethecharacteristicsrelatingtolightning

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