Dielectric Breakdown In Liquids Purity aspects of liquid dielectrics

1. Purity aspects of liquid dielectricsPurity aspects of liquid dielectrics
Dielectric BreakdownDielectric Breakdown
In LiquidsIn Liquids
Central Mechanical Engineering Research InstituteCentral Mechanical Engineering Research Institute
DurgapurDurgapur
IndiaIndia

2. Dependence LadderDependence Ladder
DielectricDielectric
breakdownbreakdown
DielectricDielectric
strengthstrength
Ion size,Ion size,
liquid viscosity,liquid viscosity,
density,density,
purity of liquidpurity of liquid
etc.etc.

3. Classification Based on PurityClassification Based on Purity

4. Dielectric Strength Vs. PurityDielectric Strength Vs. Purity
Minutes of Exposure of “Degassed Oil” to Air Under Atmospheric Conditions
DielectricStrength(kV)
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.

5. 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

6. 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

7. 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

8. Special Case : Behaviour of Liquid atSpecial 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

9. Breakdown Voltage Vs. PressureBreakdown Voltage Vs. Pressure
(at boiling)
pure liquid behaviour
transition to
“impure”
state

10. Impure Insulating LiquidsImpure Insulating Liquids
Explanation of the behaviour is based on :
Elimination of dissolved gases
Ionization under voltage stress
Causes of gas elimination :
Hollow space formation resulting from the
kinetic motion of charged particles
Electro-striction effects

11.  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

12. 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

13. 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
Fieldstrength(kv/cm)
Radius of curvature (mm)

14. 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

15. Viscosity EffectViscosity Effect

16. Frequency EffectFrequency Effect
Since 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