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Breakdown in Liquid Dielectrics

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Steve:
The breakdown mechanisme of liquid dielecrics depends on the purity of the dielectrics. To explain the phenomenon of breakdown, the liquid dielectrics are classified as (1) contaminated dielectrics containing emulsified water and solid mechanical particles, (2) technically pure dielectrics practically free from emulsified water and mechanical particles, and (3) highly purified dielectrics which are free from moisture and mechanical particles and are well degassed.
In the contaminated dielectrics the breakdown occurs due to the formation of conducting bridges between the electrodes by droplets of emulsified water and suspended particles especially fibrous particles. The time taken to form the bridge depends on the extent of contamination, the shape of the electrodes and the gap between them. For a short duartion voltage, the example and impulse voltage be applied, no continues bridge will be able to form in the liquid. When breakdown takes place the temperature will rise and the conductive bridges are broken due to the thermal convection flow in the liquid. After some time a second set of bridges will be formed and so on, thus making the breakdown process a rather indeterminate quantity in a highly contaminated liquid.

In technically pure liquid dielectrics the breakdown is initiated by ionization of the gas contained in the liquid. All liquids dissolve a certain quantity of gas especially air. At the spot where the electric field initiates ionization of the gas the intensity of the electric field rises sharply. The gas will act as a conductive medium eventually leading to a final breakdown. The distance between the electrodes, pressure, temperature, frequency of the applied field and to some extent the chemical composition of the liquid are the factors which influence the dielectric strength of technically pure dielectrics.
In degassed high purity liquid dielectrics breakdown is evidently due to the collision ionization initiated by secondairy electrons emitted from the kathode due to the strong electric field.

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