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High Voltage EngineeringHigh Voltage Engineering
Breakdown in LiquidsBreakdown in Liquids
KE 40503KE 40503
High Voltage EngineeringHigh Voltage Engineering
Breakdown in LiquidsBreakdown in Liquids
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BEE 3243 Electric Power Systems Module 1
Breakdown in Solids and Liquids: IntroductionBreakdown in Solids and Liquids: Introduction
Practical insulation structures usually consist ofcombination ofsolid and liquids and / orgaseousdielectrics.
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mechanisms, formation electron ofavalanches isresponsible for a rapid growth of current associatedwith the electrical failure of gaseous media.
But this not the case for the liquid and solidinsulations.
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BEE 3243 Electric Power Systems Module 1
Breakdown in Solids and Liquids: IntroductionBreakdown in Solids and Liquids: Introduction
Numerous investigatorhave studied the breakdownofliquid and solid dielectrics for decades.
But the findings and theories postulated by such
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acceptable general theories explaining breakdownmechanisms in liquid orsolid media.
The state ofknowledge in this area is still very crudeand inconclusive.
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BEE 3243 Electric Power Systems Module 1
Part 1Part 1
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**Breakdown / Failure in**Breakdown / Failure in LiquidLiquid Dielectrics**Dielectrics**
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BEE 3243 Electric Power Systems Module 1
Liquid Dielectrics: IntroductionLiquid Dielectrics: Introduction
Liquid dielectrics, particularly hydrocarbon oils, are usedin insulating media in high voltage equipment (e.g. oilimmersed transformer, oil circuit breaker, powercapacitor / resistor / inductor etc.)
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,dielectrics have higher densities (more volume).
In comparing with the solid dielectrics, liquid dielectric fill
the space to be insulate more easily (maximum spaceusage).
Moreover, liquids are better coolants and have self-arcquenching capabilities should an arc be set up in them(therefore have higherVb level than that of air).
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BEE 3243 Electric Power Systems Module 1
Oil as the Choice of Dielectric Material in
Power Transformers
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Transformer oilsTransformer oilsOil immersed transformerOil immersed transformer
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Oil as the Choice of Dielectric Material in
Power Transformers
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Outdoor type oil filled circuitOutdoor type oil filled circuitbreakerbreaker
Indoor type oil filled circuitIndoor type oil filled circuitbreakerbreaker
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Oil as the Choice of Dielectric Material in
Power Cables
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Oil filled power cablesOil filled power cables
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BEE 3243 Electric Power Systems Module 1
Oil as the Choice of Dielectric Material in Power CablesOil as the Choice of Dielectric Material in Power Cables
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BEE 3243 Electric Power Systems Module 1
Oil as the Choice of Dielectric Material in
Capacitors
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Oil filled capacitorsOil filled capacitors
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BEE 3243 Electric Power Systems Module 1
Problem with Liquid DielectricsProblem with Liquid Dielectrics
In application, liquid insulators are used less frequentlythan eithergases and solids, hence their insulatingpotentials remain underexploited.
There is less si nificant activit in li uid than solids. This
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is largely because early studies allowed reasonablemodels of behaviourto be established, and there is lotless competition in material development andengineering.
One of active interest at present is the replacement ofmineral oils in transformers with vegetables oils, toreduce environmental impact.
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BEE 3243 Electric Power Systems Module 1
Problem with Liquid DielectricsProblem with Liquid Dielectrics
Solids range from crystalline materials to semi-crystallinepolymer blends with in-organics fillers, and thus verycomplicated, but give a great deal of versatility.
Liquids on the other hand are on a molecular scale more
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, composition)
Perhaps of this molecular freedom, experiment results
from liquids are more diverse (varies) and often theirbehaviour is harderto predict.
It is also harder to keep liquid clean and free frommoisture.
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BEE 3243 Electric Power Systems Module 1
Conduction Process Lead to Failure inConduction Process Lead to Failure in
Liquid DielectricsLiquid Dielectrics
a) Process seen in liquids
Can be categorised as:Can be categorised as:
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ovemen o o s an par cu a escontamination
c) Cavity formation
d) Chemical reaction
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BEE 3243 Electric Power Systems Module 1
a) Conduction Process Seen in Liquidsa) Conduction Process Seen in Liquids
Conduction in liquids is largely by ions. High fieldconduction maybe through electrons injected from thecathode, and thus avalanches mechanism are proposedaround this.
Dissociation of molecular species to form charged ions
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may a so p ay a par .
However, in commercial application it is not generallyheld that such mechanisms applies since the field
required are well above observed critical field values.
Unlike solid, particulate defects can enterat liquid at anytime. This might be metallic particles, carbonisedmaterials from discharges ordegraded insulation, andmaybe, most importantly, water molecules.
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BEE 3243 Electric Power Systems Module 1
b) Conduction due to Movement of Oil andb) Conduction due to Movement of Oil and
Particulate ContaminantParticulate Contaminant
Dielectrophoresis
Can be categorised into four mechanismsCan be categorised into four mechanisms
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Thermal convection
Electroconvection
Static electricfication
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BEE 3243 Electric Power Systems Module 1
Problem: Dielectrophoresis in LiquidsProblem: Dielectrophoresis in Liquids
Dielectrophoresis (force is exerted on dielectric particlewhen it is subjected to a non-uniform field) can lead tocollection ofmetallic (high permittivity) particles at high
stress regions.
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This resulting in enhanced fields and localiseddischarges thereby leading ultimately to insulationfailure.
Such mechanisms can also lead to forced coalescencebetween droplets of waterin oils (thereby forming largedrops from number of smaller ones).
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BEE 3243 Electric Power Systems Module 1
Problem: Electroconvection in LiquidProblem: Electroconvection in Liquid
In electroconvection process, space charge generated
in the liquid is subject to mechanical forces from the
applied electric field.
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s pro uces movemen n e o , an s a
mechanism forspace charge movement and particles
defect mobility.
This also may cause further damage through partial
discharge and hence material degradation.
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BEE 3243 Electric Power Systems Module 1
Problem: Static Electrification in LiquidsProblem: Static Electrification in Liquids
Static electrification (electrostatic charge) may occurs whenliquid dielectric are moving across a surface of material.Usually occurs foroils or water cooled systems.
Water benefit of being able to dissipate charges by ionicconduction, whereas ions tends to have a longer life in oils.
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When oils is force to a metallic structures (e.g. fromwindings, cores, enclosures etc.), molecule picks uppositive and negative charge.
The oils deposits negative charges on solid insulationsurfaces, retaining the positive charge.
This will creates fields within windings which can lead tofailure. Thus the correct choice ofoils and flow conditions isessential in transformer design.
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BEE 3243 Electric Power Systems Module 1
d) Conduction due to Cavity Formationd) Conduction due to Cavity Formation
Because ofviscous nature ofliquids, mechanicalperturbations (e.g. flow changes) may lead tobubbles (cavities) being formed.
The cavities may likely formed/trapped during
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, ,evolution of gasses through discharges at theelectrodes surface orionisation decomposition inthe bulk.
Rapid changes in temperature and pressure mightalso lead to cavities. These will lead to growth of thevoid and increase the discharge intensity, andfailure may follow.
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BEE 3243 Electric Power Systems Module 1
d) Conduction due to Chemical Reactiond) Conduction due to Chemical Reaction
Local partial discharges permeated (spread) in liquidsmay create high permittivity chemical compounds, inwhich will increase / lead to the breakdown process.
Thus, chemical analysis of transformer and cable oils
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.
If this seen to be the case, the greatest strength of liquidscome into play, as the oils now may need to be replaced
with a new one. This is a maintenance standardprocedure in a cable and or transformer.
Clearly the oil which impregnates pressboard or paper isnot replaced, so damage in this material will often lead tofailure.
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Note ReferencesNote References
Subir Ray,An Introduction to High Voltage Engineering,
Prentice Hall India, 2004
Haddad & Warne,Advance in High Voltage Engineering, IET
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Power and Energy Series, 2004
S.M.Rowland, Breakdown in Gaseous, Solid and Liquid
Dielectrics, MSc. Lecture Note, The University of Manchester,
2006
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