Streamer generation and propagation in transformer oil under AC divergent field conditions

Prebreakdown phenomena in transformer oil are studied under AC voltage in different field configurations, ranging from very divergent point-plane geometry to quasi-uniform fields produced by rods or spheres. A detailed study of phenomena in both polarities is presented, and the minimum conditions required for streamer breakdown voltage measurements under AC are established. In negative polarity, the generation of a microscopic gas bubble ( approximately 10 mu m in diameter) immediately precedes the development of streamers. In the case of bushlike streamers, it is concluded from energetic considerations that the vaporization of the liquid constitutes the main process of streamer growth. In positive polarity, similar conclusions are drawn for slow bushlike streamers, whereas in the case of fast filamentary streamers, it is observed that the conductivity of filaments determines their propagation.<>