High Voltage - Breakdown in Liquids

8/2/2019 High Voltage - Breakdown in Liquids

1/21

KE 40503KE 40503

High Voltage EngineeringHigh Voltage Engineering

Breakdown in LiquidsBreakdown in Liquids

KE 40503KE 40503

High Voltage EngineeringHigh Voltage Engineering

Breakdown in LiquidsBreakdown in Liquids

1


2/21

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.

2

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.


3/21


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

3

acceptable general theories explaining breakdownmechanisms in liquid orsolid media.

The state ofknowledge in this area is still very crudeand inconclusive.


4/21


Part 1Part 1

4

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


5/21


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.)

5

,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).


6/21


Oil as the Choice of Dielectric Material in

Power Transformers

6

Transformer oilsTransformer oilsOil immersed transformerOil immersed transformer


7/21



Power Transformers

7

Outdoor type oil filled circuitOutdoor type oil filled circuitbreakerbreaker

Indoor type oil filled circuitIndoor type oil filled circuitbreakerbreaker


8/21



Power Cables

8

Oil filled power cablesOil filled power cables


9/21


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

9


10/21



Capacitors

10

Oil filled capacitorsOil filled capacitors


11/21


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

11

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.


12/21


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

12

, 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.


13/21


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:

13

ovemen o o s an par cu a escontamination

c) Cavity formation

d) Chemical reaction


14/21


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

14

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.


15/21


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

15

Thermal convection

Electroconvection

Static electricfication


16/21


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.

16

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).


17/21


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.

17

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.


18/21


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.

18

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.


19/21


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

19

, ,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.


20/21


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

20

.

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.


21/21


Note ReferencesNote References

Subir Ray,An Introduction to High Voltage Engineering,

Prentice Hall India, 2004

Haddad & Warne,Advance in High Voltage Engineering, IET

21

Power and Energy Series, 2004

S.M.Rowland, Breakdown in Gaseous, Solid and Liquid

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

2006

High Voltage - Breakdown in Liquids

Documents

Transcript of High Voltage - Breakdown in Liquids