Post on 02-Jul-2015
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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