PREBREAKDOWN PHENOMENA IN MINERAL-OIL UNDER STEP AND AC VOLTAGE IN LARGE-GAP DIVERGENT FIELDS

This paper presents an experimental study of the propagation of prebre akdown phenomena in transformer oil, in large point-plane gaps (5 to 2 0 cm), in positive polarity under step and ac voltages. The prebreakdo wn phenomena are characterized via the simultaneous recordings of tran sient currents, charges, photocurrents, and high-speed photographs of the emitted light. In these experimental conditions, prebreakdown phen omena in oil are basically composed of weakly luminous branched filame nts, most of time not connected to the point electrode, and propagatin g continuously. Bright luminous fast discharges reilluminate periodica lly the main branch of the propagating discharge from the point electr ode to the extremity of these weakly luminous filaments. Below the bre akdown voltage, streamers stop at an average length which increases li nearly with the voltage. Above a critical length, they cannot stop any more and lead to breakdown. The main propagation velocity remains con stant throughout the propagation. These observations, which present a number of similarities with leader phenomena in gases, are then discus sed.