Breakdown in Liquids 2011 Update

8/10/2019 Breakdown in Liquids 2011 Update

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Breakdown in LiquidsBreakdown in Liquids Breakdown in LiquidsBreakdown in Liquids

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Dr RieDr Rie


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BEE 3243 Electric Power Systems Module 1

Breakdown in Solids and Li uids: IntroductionBreakdown in Solids and Li uids: Introduction

Practical insulation structures usually consist ofcombination of solid and li uids and / or aseousdielectrics.

mechanisms, formation electron of avalanches isresponsible for a rapid growth of current associatedw e e ec r ca a ure o gaseous me a.

solid insulations.

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Breakdown in Solids and Li uids: IntroductionBreakdown in Solids and Li uids: Introduction

Numerous investigator have studied the breakdownof li uid and solid dielectrics for decades.

But the findings and theories postulated by such

acceptable general theories explaining breakdownmechanisms in liquid or solid media.

The state of knowledge in this area is still very crude .

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**Breakdown / Failure in**Breakdown / Failure in LiquidLiquid Dielectrics**Dielectrics**

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, ,in insulating media in high voltage equipment (e.g. oilimmersed transformer, oil circuit breaker, powercapac or res s or n uc or e c.

,dielectrics have higher densities (more volume).

n compar ng w t t e so e ectr cs, qu e ectr cthe space to be insulate more easily (maximum spaceusage).

Moreover, liquids are better coolants and have self-arc

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(therefore have higher Vb level than that of air).


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Oil as the Choice of Dielectric Material in

ower rans ormers

Transformer oilsTransformer oilsOil immersed transformer Oil immersed transformer

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ower rans ormers

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Outdoor type oil filled circuitOutdoor type oil filled circuitbreaker breaker

Indoor type oil filled circuitIndoor type oil filled circuitbreaker breaker


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ower a es

8Oil filled power cablesOil filled power cables


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Oil as the Choice of Dielectric Material in Power CablesOil as the Choice of Dielectric Material in Power Cables

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apac tors

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Oil filled capacitorsOil filled capacitors


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In application , liquid insulators are used less frequentlythan either gases and solids , hence their insulating.

There is less si nificant activit in li uid than solids. This

is largely because early studies allowed reasonablemodels of behaviour to be established, and there is lotless com etition in material develo ment and engineering.

ne o ac ve n eres a presen s e rep acemen omineral oils in transformers with vegetables oils , toreduce environmental impact.

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- polymer blends with in-organics fillers , and thus very

complicated , but give a great deal of versatility.

Liquids on the other hand are on a molecular scale more,

composition)

Perhaps of this molecular freedom , experiment results

from liquids are more diverse (varies) and often theire av our s ar er o pre c .

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moisture.


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Conduction Process Lead to Failure inConduction Process Lead to Failure in

qu e ec r csqu e ec r cs

Can be categorised as:Can be categorised as:

A. Process seen in liquids

B. Movement of oils andparticulates contamination

C. Cavity formation

D. Chemical reaction

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Conduction in li uids is lar el b ions . Hi h field conduction maybe through electrons injected from the

cathode , and thus avalanches mechanism are proposed

Dissociation of molecular species to form charged ions

may a so p ay a par . ,

held that such mechanisms applies since the fieldrequired are well above observed critical field values.

Unlike solid, particulate defects can enter at liquid at anytime. This mi ht be metallic articles , carbonised

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materials from discharges or degraded insulation , andmaybe, most importantly, water molecules .


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B)B) Conduction due to Movement of Oil andConduction due to Movement of Oil and

ar cu a e on am nanar cu a e on am nanCan be categorised into five mechanismsCan be categorised into five mechanisms

i.i. DielectrophoresisDielectrophoresis

ii.ii. Forced and Natural ConvectionForced and Natural Convection

..

..

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v.v. a c e ec r c ca ona c e ec r c ca on


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..

Dielectrophoresis (force is exerted on dielectric particlewhen it is sub ected to a non-uniform field can lead tocollection of metallic (high permittivity) particles at highstress regions.

This resulting in enhanced fields and localiseddischarges thereby leading ultimately to insulationa ure .

(combination/grouping) between droplets of water in oils(thereby forming large drops from number of smaller

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.


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..

have the benefit of keeping a liquid cool .

However , this has disadvantage (new mechanism)whereby defects can be moved from the benign region

.e. o om o rans ormer an o one o g e area.

dielectric due to the present of metallic / high permittivity

defects .

This may cause further damage through partial

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discharge and hence material degradation .


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..

In electroconvection process, space charge generated

in the liquid is subject to mechanical forces from theapplied electric field .

s pro uces movemen n e o , an s amechanism for space charge movement and particles

.

This also ma cause further dama e throu h artialdischarge and hence material degradation .

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..

Static electrification (electrostatic charge) may occurs whenliquid dielectric are moving across a surface of material.Usually occurs for oils or water cooled systems.

Water benefit of being able to dissipate charges by ionicconduction , whereas ions tends to have a longer life in oils.

When oils is force to a metallic structures (e.g. fromwindings, cores, enclosures etc.), molecule picks up

.

The oils deposits negative charges on solid insulation, .

This will creates fields within windings which can lead tofailure . Thus the correct choice of oils and flow conditions is

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essential in transformer design.


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Because of viscous nature of liquids , mechanical

perturbations (e.g. flow changes) may lead to.

The cavities may likely formed/trapped during, ,

evolution of gasses through discharges at theelectrodes surface or ionisation decomposition in

.

Rapid changes in temperature and pressure mightalso lead to cavities . These will lead to growth of thevoids (large empty spaces) and increase thedischar e intensit , and failure ma follow.

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Local partial discharges permeated (spread) in liquids may create high,

breakdown process.

For instance, the insulated compound in the oil molecules ischemically changed into more carbonise molecules (i.e. conduction)due to discharges . Alteration in the liquid colour is the easiestn cat on o con rm ng c em ca -c ange s tuat on.

Thus chemical anal sis of transformer and cable oils can be used toindentify if discharges are taking place .

,play, as the oils now may need to be replaced with a new one. This isa maintenance standard procedure in a cable and or transformer.

22 Clearly if the oil which impregnates pressboard or paper is not

replaced , so damage in this material will often lead to failure .


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Subir Ray, An Introduction to High Voltage Engineering ,,

Haddad & Warne, Advance in High Voltage Engineering , IET

Power and Energy Series, 2004

. . , ,Dielectrics , MSc. Lecture Note, The University of Manchester,

2006

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Breakdown in Liquids 2011 Update

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