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areturntothebasicsofDCelectricitygenerationandtransmissionfromtheendofthe
19
thcentury.AsgraphicallyillustratedinFigure1.1a,thetransmissionvoltagelevel
acenturyagowasaround25kV;inthe40s,220kVlineswereintroduced;andtoday,
thehighesttransmissionlevelis1.2MV!Thesametrendcanbeobservedinpower
transformerevolution.Themaximumpowerofatransformeracenturyagowasaround
150kVA;today,thelargestsingleunitshaveapowerof2GVA.ItisalmostlikeMoor,s
lawinelectronicsregardingthedoublingofthenumberoftransistorsinaprocessor;
however,thescaleisnoteveryotheryearinthiscasebutrathereveryotherdecade.
a)
b)
Fig01010Evolutionof:a)transmissionnetworkvoltage;b)transformerpower
Thedevelopmentofelectricalpowerengineeringisperceivedintheareaofgen-
erationthroughrenewableenergy,intransmissionthatstrivestowardsultra-highvolt-
ages,andatthedistributionlevelwithanintroductionofthesmartgridconcept.
Therapiddevelopmentofpowerelectronicsoverthelastdecadehasresultedin
abroadspectrumofapplicationareaswhereHV/MVandLVinsulationisinvolved.
Forexample,thereareHVDC(high-voltagedirectcurrent)systems;ontheMVside,
inverterdrivesareverypopularnowadays;andintheLVarea,solarandwindinverters
arecommon.Researchonmulti-levelconvertertopologies,modulationtechniques,
andcontrolstrategiesisdrivingtowardshighervoltages(includingHVDCapplica-
tions)andrequiresmoreattentiontowardstheelectricalinsulationoftheindividual
buildingblocksaswellasthewholesystem.
Withinelectricalworkingstresses,impulsestimulilikepulse-widthmodulatedsig-
nals(Fig.1.2)createspecialchallengesontherequirementsofinsulationsystems.
Therepetitivecharacterofthepulses(usuallyinthekHzrange)combinedwithrapid
voltagechanges(inthens-usrange)andthemodulationofthewholepatternresultsin
anewclassofinsulation-agingprocesses.Themodulationpatternsinfluencedischarge
formationandspace-chargerelaxation.Themechanismofthedegradationprocesses
ininsulatingsystemsisdifferentfromthatofasinusoidalfrequencyof50/60Hz.
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