Effects of back electrode configuration on creepage discharge propagation over an oil-immersed insulation surface under lightning impulse voltage

Using model which consist of an insulation cylinder, grounding electrode and HV electrode on the insulation cylinder surface, and the back electrode inside the insulation cylinder, we investigated the creepage discharge characteristic under lightning impulse voltage. We examined the surface flashover characteristic using several kinds of models whose distance between the back electrode and the insulation surface is constant vertically and whose back electrode configuration is a stair type. We also observed the streamer. propagation length and examined estimation methods of flashover voltage (FOV) by combining the streamer propagation characteristic and FOV characteristic. We concluded that the flashover. voltage is estimated well by the voltage addition method and for this estimation the streamer propagation characteristic is suitable up to the boundary where the distance between back electrode and insulation surface changes and the FOV characteristic or the streamer propagation characteristic is suitable in the area beyond the boundary.