A comparison of prebreakdown phenomena in mineral oil under step and AC voltages in divergent fields at large distances

Current and photocurrent recordings and visualizations of positive discharges in transformer oil were carried out under AC and step voltages in large point-plane geometries (5 to 20 cm). The characteristics of discharges in large gaps (stepped propagation) are presented. Comparison is made betwen streamer propagation under AC and step voltages. The stopping lengths of single streamers are similar under both types of voltage wave. Under AC, cumulative effects may also occur, which tend to make the propagation easier. The streamer lengths increase linearly with the inception voltages at a rate of 20 kV/cm up to a critical distance beyond which the streamers do not stop. The existence of this phenomenon explains the nonlinearity of breakdown curves versus distance