Pre-breakdown current and light emission in transformer oil

Pre-breakdown light emission, current and photo current are measured, simultaneously. For a negative needle point, figures of light emission are divided into two patterns. They change from a bush-type to a tree-type as the applied voltage and the oil surface pressure increases. In the case of the tree-type, they are caused by streamer only at the initial stage. In the case of the bush-type, the lights are emitted intermittently within a few hundred microseconds. The initial one is caused by a tiny streamer but the followings seem to occur within bubbles created by the initial one. For a positive needle point, positive streamers emanate from positive point radially. When they reach near the cathode, a few bright spots are formed on the cathode from which negative streamers start.