Opposite leaders as a cause of barrier insulation breakdown

Summary

Tests with standard lightning pulse impact are generally required for high voltage constructions. Streamer form of discharge occurs in air under such conditions. Streamer is a channel of low-temperature (non-equilibrium) plasma growing due to intensive ionization at the tip of the channel. Streamers themselves do not lead to breakdown due to their high electric resistance. Breakdown happens when plasma transfers to high-temperature (equilibrium) state. Usually this transfer follows streamers closure of the electrode pair. Solid dielectric barriers are used to stop streamers and increase breakdown voltage without lengthening interelectrode distance. An important issue is how geometric parameters of solid dielectric parts influence breakdown voltage. Two breakdown mechanisms should be distinguished. The simpler one may be called “bypassing breakdown”. In this case streamers turn when reaching the barrier surface. After reaching the edge of the barrier streamers turn again: in the direction to the opposite electrode. Breakdown voltage increases because the air path between the electrodes lengthens. In this case breakdown voltage can be increased by increasing the barrier diameter or creating “labyrinth insulation”. When air path between electrodes is long enough one can observe the second breakdown mechanism. Experimental data are presented which prove that a pair of high temperature plasma channels (“leaders”) forms without streamer closure under such conditions. It becomes possible due to intensive heating of streamer channels by capacitive (non-closed) currents charging barrier surfaces. This mechanism may be called “opposite leaders”. Some types of creeping discharges may be considered as an instance of this case. The experimental investigation is implemented in sphere-sphere and sphere-plane electrodes systems with plane dielectric barriers. Photographs of streamer discharge and breakdown are analyzed as well as current oscillograms. Also surface charge on barrier surface is visualized to get additional information about discharge structure. A character of barrier parameters influence on breakdown voltage is crucially different in these two mechanisms. That is why it may be of interest for high voltage insulation developers.

Additional informations

Authors

GOCSEI

Keywords

streamer, barrier insulation, breakdown, lightning pulse, solid insulation