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however,thelowestvalueobtainedfortheXLPEvoidlayerembeddedinPKresults
fromthehigherpolyethyleneresistivityascomparedtoPK(thus,theproportionally
highervoltagedropinthecavity).
ThePDpatternsrecordedatDCvoltageareshowninFigure3.13witha60-second
zoomedview(600-secondviewforXLPE).
a)
c)
b)
d)
Fig030130Partialdischargesat20kVDCvoltageforspecimens:
a)PK;b)XLPE;c)PK-XLPE-PK;d)XLPE-PK-XLPE
Thefourspecimensthathavethesamegeometrybutdifferentcompositionsof
insulatingmaterialsrevealslightlydifferentPDinceptionvoltages.IntheACcase
(determinedmainlybymaterialpermittivity),thespreadislessvisiblethaninthe
DCcase(forcedbymaterialresistivity),wherethedeviationsbetweenthemateri-
alsaremorepronounced.IntheACmeasurement,thelowestvaluewascalculated/
detectedforthePK-XLPE-PKspecimen(8.8kV/9.2kV
,respectively)andthehigh-
estoneforXLPE-PK-XLPE(9.1kV/9.9kV
,respectively).Inthefirstcase,the
higherresistivityofXLPEwithrespecttoPKleadstoahighervoltagedropon
thevoidthanintheoppositecase(i.e.,lowerterminalUDCvoltage).Inthecase
ofDCvoltage,thelowest9kVPDIVwasalsocalculatedforthePK-XLPE-PK
specimen.ItisworthnotingthedifferentapproachinthePDIVdetermination.
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