Discharges in AC-stressed oil gaps

Rates and phase distribution of discharges in transformer oil in an AC-stressed point-plane gap have been measured using a phase-synchronized multichannel analyzer. Three types of discharges were observed: negative discharges, which occurred at a high rate, and small and large positive discharges, which occurred at lower rates. The large positive discharges resulted in breakdown of the oil gaps. The discharge rates increased with reduced tip curvature and increasing voltage. The discharge activity was found to vary stochastically with time, and the total occurrence rate varied from 0.007 to 25 Hz. The discharges appeared over a wide phase window around the peak voltage.