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Fig030140RelationshipoftimetpbetweenconsecutivePDpulsesversus
appliedDCvoltageUDCforvariousspecimenconfigurations
DuetothehighermaterialresistivityofXLPEascomparedtoPK,thelongesttp
timeisobservedforthehomogeneousXLPEspecimen.Whilereplacingthecavitylayer
withPKmaterialinthisspecimen,thecurvemoveddownwards,reflectingtheinfluence
ofthecavity-wallmaterialindicatedinthemodelinFigure3.8c.Thelowersurfaceresis-
tivityofPKwithrespecttoXLPEresultsinbypassingthevoidcapacitanceduringthe
chargingperiodandrecoveryvoltagebuild-up.ConsideringthePKspecimen,there-
placementofthemiddlevoidlayerbyXLPEresultsinaslightlylowerDCinception
voltage(i.e.,11.5kVforPKvs.9.0kVforPK-XLPE-PK);thus,ahigherrepetitionrate
ofthePDpulseinthelattercase.
Asmentionedbefore,thePDinceptionvoltagethresholdstronglydependsoncav-
itythicknessinsmallcavities.ThesimulationpresentedinFigure3.15highlightsthis
effectfortwocavities:(1)withathicknessof1.3mm(redline);and(2)withathickness
of2.3mm(blueline).Sincethevoidcapacitanceinthesecondcaseishalfthatofthe
thinnerone(Cc1=2Cc2),thecorrespondingtimeconstantshaveaτ1>τ2relationship.
However,takingtheeffectofthethickness-dependentelectricfieldstrengthinto
account,thebreakdownelectricfieldinthefirstcasewillbe4.2kV/mm(which
correspondstoinceptionvoltageUinc1=5.5kV),whereastheinceptionvoltageis
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