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properties.Infact,therelationshipsofthoseresistorsarecrucialforpartialdischarge
inceptionvoltage(PDIV)calculationsatDC.
Fig03080EquivalentcircuitforPDmodelingatDC
Intheequivalentcircuit(Fig.3.9),thesourceofthedischargesrepresentscapaci-
tanceCcandresistanceRc.Theairresistivityisveryhigh(10
15Ωomatatemperature
of200C);hence,theresistanceRc,representingthegaseousvoidismuchhigherinreal-
itythantheresistanceofthesoliddielectric,whichisrepresentedbyRa2andR′a2.
IndexedresistancesRaandRbreflectpartofahomogeneousdielectricthatisfreefrom
discharges.Inthisrepresentation(Fig.3.9a),branchresistanceRahasbeensplitac-
cordingtothecavityprofile;i.e.,Ra2andR′a2beingparalleltothecavity(markedin
red),andRa1,R′a1,andR′a3mimickingtheremainingparts.Itisimportanttonoticethat
resistanceRcisusuallymuchbiggerthanRa2andR′a2,whichbypasstheresistancerepre-
sentingthegaseousvoid.
Voltagebuild-upUcrequiredforPDinceptioncorrespondstothevoltagedropon
resistanceRAresultantfromthecombinationofcomponentsRa2andR′
a2(RA=Ra2||R′a2).
Thisisactuallyaveryimportantnoveltyinmodelingpartialdischargeoccurrences
atDCvoltage.ThiseffectishighlightedinFigure3.9a,indicatingthetwo-foldeffectof
volumeandsurfaceresistance.
AtDCvoltage,twodistinctivestagescanbedistinguishedthatrepresentthePD
mechanism:
a)inceptionstage–whenPDwilloccur,assumingfulfillmentofallconditions;
b)chargingstage–whenpostdischargerecoveryoccurs.
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