Purity aspects of liquid dielectricsPurity aspects of liquid dielectrics

Dielectric Breakdown Dielectric Breakdown In LiquidsIn Liquids

Central Mechanical Engineering Research Institute Central Mechanical Engineering Research Institute DurgapurDurgapurIndiaIndia

Dependence LadderDependence Ladder

Dielectric Dielectric breakdownbreakdown

Dielectric Dielectric

strengthstrength

Ion size, Ion size, liquid viscosity, liquid viscosity,

density, density, purity of liquidpurity of liquid

etc.etc.

Classification Based on PurityClassification Based on Purity

Dielectric Strength Vs. PurityDielectric Strength Vs. Purity

Minutes of Exposure of “Degassed Oil” to Air Under Atmospheric Conditions

Die

lect

ric S

treng

th (k

V)

Since the fundamental classification of insulating liquids is based on the degree of gassification, it is of importance to trace the dielectrics strength of an insulating liquid as it passes from the “pure” to

“impure” state.

Semi-permeable MembraneSemi-permeable Membrane

In a pure liquid, the movement of charged particles towards the electrode and their discharge establishes a film of neutral molecules

Film has high potential difference across it Rupture of this film results in ionization and general dielectric

failure

Stress DistributionStress Distribution

With polar materials at low voltage, a sharp rise in field strength from anode to cathode

Current-voltage relation depicts that charged molecules closely follow polar molecules, resulting in unequal field distribution between electrodes and steep potential drop in adjacent region

In the region : a) kinetic motion of charged particles becomes accelerated

b) electro-striction becomes pronounced

c) ionization by collision of liquid molecules reaches its max. importance

Pure Insulating LiquidPure Insulating Liquid

Breakdown remains unaffected by applied pressure Time required for the formation of insulating membrane in pure

liquid is negligible compared to that involved in the breakdown of impure liquid

Impulse breakdown falls in a linear relation with temperature Breakdown is a function only of potential drop across the

insulating membrane, thus breakdown voltage is independent of gap distance

Special Case : Behaviour of Liquid at Special Case : Behaviour of Liquid at Boiling pointBoiling point

At the boiling point , liquid vaporization eliminates dissolved gases and “pure” behaviour results.

As the pressure falls, the temperature remaining constant, the boiling of the liquid ceases, the liquid absorbs air or other gas in proportion to the pressure applied, and enters the classification of “impure” liquids

Breakdown Voltage Vs. PressureBreakdown Voltage Vs. Pressure(at boiling)

pure liquid behaviour

transition to “impure” state

Impure Insulating LiquidsImpure Insulating Liquids

Explanation of the behaviour is based on :Elimination of dissolved gasesIonization under voltage stress

Causes of gas elimination :Hollow space formation resulting from the kinetic motion of charged particlesElectro-striction effects

Behaviour of impure liquid can be modified by changing the solubility of gases in it

Solubility may increase/decrease with increase/decrease in temperature

Breakdown is affected by nature of the gas

Dielectric strength increases with decreased gap distance

Dielectric strength increases, for the same gap, with removal of suspended/secondary impurities

Role of Electrode PositionRole of Electrode Position

The ease of gas escape from the electric field determines the relation of the gap distance to electrical breakdown.

With parallel, plane electrodes faced vertically (gap horizontal), gas bubbles when formed are easily ejected

With horizontally faced electrodes (gap vertical), the ease of gas ejection is reduced

Role of Electrode ShapeRole of Electrode Shape

The radius of electrode curvature affects the breakdown of impure liquids only in the range of relatively short radii.

In this respect, “impure” oils bear close resemblance to the behaviour of air. Dielectric strength shows an increase with decreased radius of electrode curvature.

0 2 4 6 8 10 20 30 40 50

560

240

160

80 Field

stre

ngth

(kv/

cm)

Radius of curvature (mm)

Temperature DependenceTemperature Dependence

pure

impure

Dielectric strength decreases with increased temperature

If solubility increases with temp, dielectric strength increases

If solubility decreases with temp, dielectric strength decreases

The fact that the same gas may increase in solubility in one liquid and decrease in other makes the situation complex for impure liquids

Viscosity EffectViscosity Effect

Frequency EffectFrequency EffectSince electro-striction and gassing effects become less pronounced with decreased stress concentration, an increase in dielectric strength with increased frequency is to be expected.

Experimental data for hexane with copper discs using a gap of 0.5 mm