Download HS 03 Solid and liquid insulating materials

Chapter3
SolidandLiquidInsula5ngMaterials
SolidandLiquidInsula5ngMaterials
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Liquidsandsolidshavehighermoleculardensi5esthangases:
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Prac5calfactorscauseaworsethanexpectedperformance:
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IntermsofPaschen’sLawabeAerinsula5ngperformanceisexpected.
Impuri5es
Non-homogeneity
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Polariza5on&Dielectricconstant
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Dielectricshavethepropertyofpolariza5on:
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E-fieldiszero-dipolesarrangedrandomly.
E-fieldnotzero–dipolesalignedinthedirec5onof
thefield.
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Addi5onalchargeon
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Moleculesaredipoles
electrodes
Highercapacitance
HigherpermiOvityε
C
ε r = dielectric
C air
!
LossesinDielectrics
tanδ
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AC:Polarizedmoleculesperiodiclyalignwithfield:
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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
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Thermalbreakdown
Treeingduetointernalpar5aldischarges.
Trackingduetoexternaldischarges.
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SolidInsula5ngMaterials
ThermalBreakdown
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Thermalrunaway:
Rateofheatgenera5on>Rateofheatloss
Thermalinstability
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Twodrivers:
Increasedvoltage
Increasedthermalresistance
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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
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Thermalrunawaydueto:
Increasedlosses–Overloading
Increasedthermalresistance–Dryingoutofsurroundingsoil
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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
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SolidInsula5ngMaterials
InternalPar5alDischarges&Treeing
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Electricfieldhigheringas-filledvoids(Lowerε)
ε E =ε E
! 1 n1 2 n2
Internalpar5aldischargesinthevoidsgivesriseto
treeing
DamageslaOcestructureofsolidmaterial
Formscarbondeposits
SolidInsula5ngMaterialsFailure
ExternalDischarges&Tracking
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SurfacedischargesduetohighE-field:
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Conduc5ngcarbonaceoustracksform–Tracking
Lossofmaterial-Erosion
Aggravatedbydust
andpollu5on:
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Causedbypoordesign
Electronbombardment
UVdegrada5on
Ozoneforma5on
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Insula5ngLiquids
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Mineraloilusedintransformersforcoolingandinsula5on,togetherwithcellulose
(paper)
Moleculesmorecloselypacked:
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Causesoffailure:
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Higherflashovervoltageexpected
Water(50ppm:50=>23kV/mm)
Fibrebridges
Gasbubbles
Standardoiltest:2.5mmgapwithstands60kV
Insula5ngLiquids
FlashoverMechanishms
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Waterdrops:
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Fibrebridgeforma5on:
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Cellulosefibredipole(+absorbedwater)arearrangedhead-on-tail
DipolesmovetothehighestE-field
Airbubbles:
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Par5aldischargesduetoE-fieldenhancement
Par5aldischargesduetoE-field
enhancement
ε E =ε E
! 1 n1 2 n2
+
polarised
particle
E1 > E2
_ q E1
+ q E2
Oil
_
__
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Insula5ngLiquids
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Prac5calaspects:
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Conservator(expansiontank)+silicagelbreathers
Barriers(hardpaperinsula5on)betweenwindingspreventfibrebridgeforma5on
Regularoilpurifica5onto
removefibresand
moisture
Conservator
Chemicaloiltestsand
HV
gasanalysis
Buchholz relay
Oil
Silica gel
breather
GasAnalysisofOil
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Transformerinsula5onbasicallyconsistsofoilandcellulose
Maindegrada5onprocessesofoilandcellulose:
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Corona
Pyrolysis-decomposi5onduetohea5ngintheabsenceofoxygen
Arcing
Oilanalysisplaysanimportantpartintransformerdiagnos5cs&condi5on
monitoring:
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Analysesgasquan5tyandcomposi5oninoil
Standardflashovertest:2.5mmgapwithstands60kV
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GasAnalysisofOil
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Typicalgasesthatareformedare:
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Methane(CH4)
Ethane(C2H6)
Ethylene(C2H4)
Acetylene(C2H2)
Hydrogen(H2)
Carbonmonoxide(CO)
Carbondioxide(CO2)
Combina5onsofInsula5onSystems
ParallelSystems
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Electricfieldinbothpartsthesame:
V
E=
d
!
Example:
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Twoelectrodesd=5cmapart.
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MaterialAisasolidmaterialwithεR=6anddielectricstrengthof600kV/cm
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MaterialBisaninsula5ngoilwith
aεR=2.3permiOvityand
dielectricstrengthof120kV/cm.
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Ignoreendeffects:Atwhat
voltagewillthesystemfail?
Answer:MaterialB
.
V = Ed = 120 5 = 600kV
!
( )( )
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Combina5onsofinsula5onsystems
Seriessystems
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Example1:
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Uniformairgapof10cm,V=200kV:
V 200
E= =
= 20kV /cm
d 10
!
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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
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! Air
Combina5onsofinsula5onsystems
Seriessystems(cont.)
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Voidindielectric:
ε E = ε 2 E2
!1 1
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Fieldstrenthhighestinmaterialwithlowestε
BreakdowninairbubbleifE>30kV/cm
epoxy
air
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Prac5calDesign:Insulator
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Notethatthefieldishigherintheair
sec5onsoftheseriespathbetweenthe
electrodes
Furtherthefieldstrengthishighnear
theelectrodes
SurfaceDischarges
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Fieldlinescrossboundary
betweensolidandair
Fieldstrengthinairhighest
ε E = ε 2 E2
!1 1
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Airhasalowerbreakdown
strengththanglassand
breaksdownfirst
Aironthesurfaceofthe
glassbreaksdown–the
glassremainsintact
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InsulatorPollu5on
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Saltylayerformsontheinsulatorsurface:
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Pollu5onlayerabsorbsmoisture-fog,humidity
Pollu5onlayerbecomesconduc5ng
Pollu5onlayerdriesoutatthe
narrowestparts:
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Nearthecoastorinindustrialareas
Highestcurrentdensity/power
dissipa5on
Arcsdevelopoverthedry
bands:
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Completeflashoveroccurs
PolymericMaterials
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EPDMRubber(EthylenePolytheneDieneMonomerRubber):
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Hydrophobic
Hasbeenshowntoageinseverelypollutedregions
Siliconerubber:
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Hydrophobic,evenwhenpolluted
Hydrophobicproper5esrecover
HighTemperatureVulcanised(HTV)SiliconeRubber
RoomTemperatureVulcanised(RTV)SiliconeRubber
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HydrophobicPolymericMaterials
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Siliconerubber
Hydrophobicity
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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
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