Evidence for transitions in the prebreakdown mechanism of liquid dielectrics

Studies using both n-hexane and transformer oil stressed with direct voltage have indicated that, for nonuniform electric fields, cavitation may be produced together with associated gas-initiated light emission and acoustical disturbances. Transformer oil is shown to emit light below the cavitation-onset stress by an assumed electronic electroluminescence which is also invoked to explain light emission under uniform field conditions. Cavitation onset is shown to be strongly temperature dependent. Tests at elevated temperatures indicate that the discontinuities which were found in the conduction characteristics are not associated with bubble formation or collapse, and alternative explanations are offered. A distinction is drawn between the integrated conduction characteristics and the current pulse behaviour which accompanies the gas-phase disturbance in the liquid.