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Chapter3 SolidandLiquidInsula5ngMaterials SolidandLiquidInsula5ngMaterials • Liquidsandsolidshavehighermoleculardensi5esthangases: • Prac5calfactorscauseaworsethanexpectedperformance: • • • IntermsofPaschen’sLawabeAerinsula5ngperformanceisexpected. Impuri5es Non-homogeneity 1 Polariza5on&Dielectricconstant • Dielectricshavethepropertyofpolariza5on: • • E-fieldiszero-dipolesarrangedrandomly. E-fieldnotzero–dipolesalignedinthedirec5onof thefield. • Addi5onalchargeon • • • Moleculesaredipoles electrodes Highercapacitance HigherpermiOvityε C ε r = dielectric C air ! LossesinDielectrics tanδ • AC:Polarizedmoleculesperiodiclyalignwithfield: • • • Inter-molecularfric5on Heatgenera5on Lossespropor5onaltotanδandV2: P = VI R = VIC tan δ = V 2π fCV tan δ = 2π fCV 2 tan δ ! I ωCS tan δ = R = IC RS I IR IR IC C R IC I * V (a) (b) 2 SolidInsula5ngMaterials Typicalmaterials Dielectric Material constant (typical) Typical tan δ Electric (typical) strength ( -4 Mica 5.5 - 7 30.10 Paper - 20 – 50 . 10 Glass 4.5 - 7 10 - 100. 10 Porcelain 6 3 – 30.10 Polythene 2.3 1 - 10. 10 PVC PTFE 5.5 Properties - -4 temperatures windings (up to 20 kV) together with epoxies. Oil-impregnated in HV transformer - - -4 10 -50 Brittle -4 20 - 40 - -4 30 - 40 - 11 - 30 - LV cables High temperature applications. 2. 10 19 - 4 - 18 - EPDM rubber 2-3 - - - Silicone rubber 3-6 - - with silica filler Insulation of rotating machine -4 -4 >100. 10 Stable at high 2 Epoxy resin, Applications kV/mm) winding insulation. Glass cap and pin insulators. Glass fibres together with epoxy resin Insulators, bushings Cross-linked (XLPE) polythene used in hv cables up to 110 kV Encapsulation of MV Ct’s and VT’s Transformer bushings and insulators: cycloaliphatic resin Hydrophobic surface properties Insulators, using a fibreglass core Insulators, using a fibreglass core SolidInsula5ngMaterials FailureMechanishms • • • Thermalbreakdown Treeingduetointernalpar5aldischarges. Trackingduetoexternaldischarges. 3 SolidInsula5ngMaterials ThermalBreakdown • Thermalrunaway: Rateofheatgenera5on>Rateofheatloss Thermalinstability • • • Twodrivers: Increasedvoltage Increasedthermalresistance • • Increased voltage P Heat generated V 1 <V 2 <V 3 Increased thermal resistance P Heat generated: P(T) RT1 RT2 RT3 V3 V2 V1 RT1<RT2 <RT3 Heat lost (conduction) Heat lost (conduction) Temperature rise Temperature rise SolidInsula5ngMaterials Thermalbreakdownincables • Thermalrunawaydueto: Increasedlosses–Overloading Increasedthermalresistance–Dryingoutofsurroundingsoil • • • Appropriatefillingmaterialrequiredforcabletrenches Increased voltage P Heat generated V 1 <V 2 <V 3 Increased thermal resistance P Heat generated: P(T) RT1 RT2 RT3 V3 V2 V1 RT1<RT2 <RT3 Heat lost (conduction) Temperature rise Heat lost (conduction) Temperature rise 4 SolidInsula5ngMaterials InternalPar5alDischarges&Treeing • • • • Electricfieldhigheringas-filledvoids(Lowerε) ε E =ε E ! 1 n1 2 n2 Internalpar5aldischargesinthevoidsgivesriseto treeing DamageslaOcestructureofsolidmaterial Formscarbondeposits SolidInsula5ngMaterialsFailure ExternalDischarges&Tracking • SurfacedischargesduetohighE-field: • • • Conduc5ngcarbonaceoustracksform–Tracking Lossofmaterial-Erosion Aggravatedbydust andpollu5on: • • • • Causedbypoordesign Electronbombardment UVdegrada5on Ozoneforma5on 5 Insula5ngLiquids • Mineraloilusedintransformersforcoolingandinsula5on,togetherwithcellulose (paper) Moleculesmorecloselypacked: • Causesoffailure: • • • • • • Higherflashovervoltageexpected Water(50ppm:50=>23kV/mm) Fibrebridges Gasbubbles Standardoiltest:2.5mmgapwithstands60kV Insula5ngLiquids FlashoverMechanishms • Waterdrops: • Fibrebridgeforma5on: • • • • Cellulosefibredipole(+absorbedwater)arearrangedhead-on-tail DipolesmovetothehighestE-field Airbubbles: • Par5aldischargesduetoE-fieldenhancement Par5aldischargesduetoE-field enhancement ε E =ε E ! 1 n1 2 n2 + polarised particle E1 > E2 _ q E1 + q E2 Oil _ __ 6 Insula5ngLiquids • Prac5calaspects: • • • • Conservator(expansiontank)+silicagelbreathers Barriers(hardpaperinsula5on)betweenwindingspreventfibrebridgeforma5on Regularoilpurifica5onto removefibresand moisture Conservator Chemicaloiltestsand HV gasanalysis Buchholz relay Oil Silica gel breather GasAnalysisofOil • • Transformerinsula5onbasicallyconsistsofoilandcellulose Maindegrada5onprocessesofoilandcellulose: • • • • Corona Pyrolysis-decomposi5onduetohea5ngintheabsenceofoxygen Arcing Oilanalysisplaysanimportantpartintransformerdiagnos5cs&condi5on monitoring: • • Analysesgasquan5tyandcomposi5oninoil Standardflashovertest:2.5mmgapwithstands60kV 7 GasAnalysisofOil • Typicalgasesthatareformedare: • • • • • • • Methane(CH4) Ethane(C2H6) Ethylene(C2H4) Acetylene(C2H2) Hydrogen(H2) Carbonmonoxide(CO) Carbondioxide(CO2) Combina5onsofInsula5onSystems ParallelSystems • • Electricfieldinbothpartsthesame: V E= d ! Example: • Twoelectrodesd=5cmapart. • MaterialAisasolidmaterialwithεR=6anddielectricstrengthof600kV/cm • MaterialBisaninsula5ngoilwith aεR=2.3permiOvityand dielectricstrengthof120kV/cm. • Ignoreendeffects:Atwhat voltagewillthesystemfail? Answer:MaterialB . V = Ed = 120 5 = 600kV ! ( )( ) 8 Combina5onsofinsula5onsystems Seriessystems • Example1: • Uniformairgapof10cm,V=200kV: V 200 E= = = 20kV /cm d 10 ! • • • • E-Fieldbelowflashoverlevel Example2: Airgap(εR=1)of1cminserieswithepoxysheet(εR=3)of9cm ε E Epoxy = Air E Air ε Epoxy V = 200kV = d1E Air + d2E Epoxy = 1E Air + 9E Epoxy ⎛ 9⎞ = E Air ⎜ 1+ ⎟ = 4E Air ⎝ 3⎠ E = 200 = 50kV /cm 4 ! Air Combina5onsofinsula5onsystems Seriessystems(cont.) • Voidindielectric: ε E = ε 2 E2 !1 1 • • Fieldstrenthhighestinmaterialwithlowestε BreakdowninairbubbleifE>30kV/cm epoxy air 9 Prac5calDesign:Insulator • • Notethatthefieldishigherintheair sec5onsoftheseriespathbetweenthe electrodes Furtherthefieldstrengthishighnear theelectrodes SurfaceDischarges • • Fieldlinescrossboundary betweensolidandair Fieldstrengthinairhighest ε E = ε 2 E2 !1 1 • • Airhasalowerbreakdown strengththanglassand breaksdownfirst Aironthesurfaceofthe glassbreaksdown–the glassremainsintact 10 InsulatorPollu5on • Saltylayerformsontheinsulatorsurface: • • • Pollu5onlayerabsorbsmoisture-fog,humidity Pollu5onlayerbecomesconduc5ng Pollu5onlayerdriesoutatthe narrowestparts: • • • Nearthecoastorinindustrialareas Highestcurrentdensity/power dissipa5on Arcsdevelopoverthedry bands: • Completeflashoveroccurs PolymericMaterials • EPDMRubber(EthylenePolytheneDieneMonomerRubber): • • • Hydrophobic Hasbeenshowntoageinseverelypollutedregions Siliconerubber: • • • • Hydrophobic,evenwhenpolluted Hydrophobicproper5esrecover HighTemperatureVulcanised(HTV)SiliconeRubber RoomTemperatureVulcanised(RTV)SiliconeRubber 11 HydrophobicPolymericMaterials • Siliconerubber Hydrophobicity 12 SummaryofInsula5ngMaterials Air SF6 Solids Liquids 1 1 3-6 2-4 30 (at 1 bar) 120 (at 4 bar) 200 - 400 240 Dielectric constant Dielectric strength (kV/cm) - Can flow - Can flow - Self-restoring Advantages - Can flow - High dielectric - Self-restoring strength - Abundant - Good arc quencher in circuit breakers - Can support - Can be conductors cleaned/ - High dielectric recirculated strength and be - Some types replaced can be - Can be moulded used as (epoxies) coolant circulated - Low dielectric strength - Low dielectric constant Disadvantages a problem when used in series with solid or liquid insulating materials - Hot house gas - Breakdown products toxic - Not selfrestoring - Can not fill small spaces - Absorbs moisture - Affected by impurities 13