Impulse ionization and breakdown in liquid dielectrics

A photographic technique was used to measure ionization under impulse voltage in transformer mineral oil and in transformer Askarel (a chlorohydrocarbon). Two electric-field conditions were investigated: 1. The liquid under test surrounded disk electrodes between which were placed impregnated pressboard and dried photographic paper. Above a threshold voltage, Lichtenberg figures occurred in the thin film of liquid under the rounded electrode edge. For mineral-oil immersant, the figures were independent of voltage polarity, while for Askarel immersant, the figures depended markedly on voltage polarity. The chlorine atoms in the Askarel and in carbon tetrachloride are shown to affect the liquid-ionization process and the puncture voltage of Askarel-impregnated pressboard. Chopped-wave tests demonstrate how the discharges develop with time. 2. The pressboard was replaced by a polystyrene sheet and the upper electrode was raised above the sheet to permit the liquid to fill the intervening space. Ionization records were obtained on the photographic paper at threshold stresses which, in most cases, were lower than either the breakdown stress with the polystyrene barrier present or the breakdown stress of the oil without the barrier. The threshold impulse-ionization stress was the same when the photographic paper was under the transparent barrier and not in contact with the stressed oil layer. Threshold-ionization stresses ranged from 400 to 800 kv per centimeter in virtually uniform fields.