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1、InsulationagingandlifemodelsLetusconsiderasolidinsulationsystemsubjectedtoone(ormore)stressthatcausesirreversiblechangesofmaterialpropertieswithtime,thusreducingprogressivelytheattitudeofinsulationinenduringthestressitself.Thisprocessiscalledagingandendswhentheinsulationisnom
2、oreabletowithstandtheappliedstress.Therelevanttimeisthetime-to-failure,alternativelycalledinsulationlife[1].Insulationlifemodelingconsistsoflookingforadequaterelationshipsbetweeninsulationlifeandthevaluesofthestress(es)appliedtoit.Inthecaseofelectricalinsulation,thestressesmo
3、stcommonlyappliedinserviceareelectricfield(duetovoltage)andtemperature,butalsootherstresses,suchasmechanicalstresses(bending,vibration)andenvironmentalstresses(suchaspollution,humidity)canbepresent.ThetheoryofagingandphenomenologicallifemodelsTherateofchangeofmaterialproperti
4、eswithtimecanconstituteameasureoftheseverityofaginginprogress.Then,end-of-lifeoccurrencecanbeassociatedwiththetimerequiredbyachosendiagnosticproperty(e.g.electricortensilestrength,density,andsoon)forreachingaproperlimitvalue.Consideringadielectricsubjectedtoasinglestress,S(co
5、nstantwithtime),themainrequirementsthatagivenproperty,P,mustmatchinordertobeadiagnosticpropertyarethatthepropertyisactuallycorrelatedtoagingandthatitchangesmonotonicallywithagingtime,t.Then,ifasuitablefunctionofP,F(P),canbefound,thatincreaseswithagingtimeanddependsonappliedst
6、ress,thefollowingrelationshipholds:FPKSt()=()(1)whereK(S)isagingrate,constantwithtimeaswellasS.Eventually,asituationisreachedwhenthematerialisnomoreabletowithstandtheappliedstress,i.e.itdoesnotperformanylongerthetaskforwhichithasbeenrealized.Thispoint,namedend-point,correspon
7、dstoalimitvalueofP,PL,andthetimetoitsachievementiscalledtime-to-end-pointofthematerial(orsystem),orlife,L[1,2].Then,fromeq.(1)theLvsSrelationship(i.e.thelifemodel)isachievedinimplicitform:LFPKS=()L/()(2)HypothesesonK(S)areneededinordertoexplaintheLvsSrelationship.Often,therel
8、ationshipbetweenlifeandappliedstressesisderived(neglectingthephysica
