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303020PartialdischargesatDCvoltage
TheuseofDCvoltageinhigh-voltagetransmissionsystemsiscurrentlyundergoing
arenaissance.Infact,thisappliestoallvoltagelevels:ononeside,thehighestvoltage
levelandfutureHVDC(HighVoltageDirectCurrent)networks(notonlypeer-to-peer
butalsomeshedones),andontheother,enhancementsinpowerelectronicstechnolo-
gieshaveresultedinbroadinterestinDC(alsoatmediumandlowvoltagelevels).
Today,DCsystemsareregularlyobservedinapplicationssuchastractionorsolar
convertersandarebeinganalyzedmoreandmoreinpower-distributionsolutions.
Theaboveelementsimplyinterestinthedesignofreliableinsulationforhigh-voltage
powerequipment.Oneofthemostreliablequalityindicatorsofhigh-voltageinsulation
systemsisthemeasurementofpartialdischarges.Themechanismofpartialdischarges
isslightlydifferentatACandDCvoltages.Manyresearchstudiesoverthelastcentury
havebeendedicatedtounderstandingthistopic;however,mostofthemwasfocusedon
alternatingvoltage.ThischapterisfocusedonthedischargesoccurringatDCvoltage
andtheinfluenceofmaterialresistivitythatimpactsPDdynamics.DCinsulationis
acrucialelementofhigh-voltagepowerequipmentsuchascables,transformerbush-
ings,convertertransformers(especiallyvalvewinding),capacitors(includingDCcapac-
itorsinHVDCmodules),andwallbushings[JACO16,FARD19].
Itisknownthatprogressivehigh-voltageinsulationdeteriorationcausedbyPDsin
gas-filledcavities(voids)isoneofthemajorfactorsthatlimitthelifetimeofpower
equipment.Thequantitycharacterizingpartialdischargesininsulationsystemsatdirect
voltageisthenumberofimpulsesofdischargesregisteredinthedetectioncircuitat
apredefinedtimeperiodoratimeintervalbetweensuccessivePDpulses.Similarto
alternatingvoltage,theconditionsforinitiatingdischargesindefectsinthestructureof
insulationsystemsaredeterminedbythevalueoftheelectricfieldstrengthnecessary
forionizationinthegassourcesofdischarges.Withsteady-stateDCvoltage,thedistri-
butionofelectricfieldstrengthininsulatedsystemsisaresistivedistribution.Theabove
factisthebasisforananalysisofthedischargemechanismatdirectvoltage,including
thefollowing:
–conditionsofinitiatingdischargesingasinclusionsinsoliddielectrics;
–theimpactofthepropertiesofinsulationmaterialsonthequantitiesthatcharac-
terizethisphenomenon.
ThemaininfluenceonPDdynamicsatDChasmaterialresistivity,unlikeitsper-
mittivityatAC.ToanalyzethestaticinceptionconditionsatDC,theadopted“a-b-c”
modelisapplied[GEMA32,DENS79,FROM95b,SERI19].Theinvestigations
wereperformedonamodelspecimenreflectingoneofthemosttypicaldefectsinhigh-
-voltageinsulation.InthemodelshowninFigure3.8,theresistorsrepresentthematerial
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