Pre-Breakdown Current and Light Emission in Transformer Oil

Prebreakdown current, photo current, and light emission are measured simultaneously in transformer oil under nonuniform fields. For a negative needle point, the light emission is divided into two patterns. They change from a bush-type to a tree-type as the applied voltage or the oil pressure increase. In the case of the tree-type, they are cau-sed by the streamer only at the initial stage. In the case of the bush-type, the light is emitted intermittently within a few hundred us. The initial one is caused by a tiny streamer but those that follow seem to occur within bubbles created by the initial one. For a positive needle point, positive streamers emanate from the point radially. When they reach the cathode, a few bright spots are formed on the cathode from which negative streamers start.