Sub-microsecond streamer breakdown in transformer oil-filled short gaps

A two dimensional axis-symmetric fluid model is developed to present the streamer discharge process in transformer oil under positive and negative sub-microsecond impulse voltage, which is based on the hydrodynamic drift-diffusion approximation to simulate the dynamics of discharge inception and propagation. The streamers' spatial evolution with time are obtained, i.e., the distribution of electric field and space charge density under different conditions such as the amplitude of applied voltage and rise time. Simulation results verify the breakdown voltage has a distinct polar effect: negative streamer requires more than 2 times voltage threshold than that of positive one. It is revealed that the negative streamer, which usually has a bushy profile, is much more diffusive than the positive one. The magnitude and rise time of the applied voltage have evident influences on the shape and speed of the streamer in oil. The formation of positive or negative streamers are both characterized by a self-generated field enhancement at the head, and space charges enhance the front field of ionization region to assist the growing discharge channel to propagate longer. In terms of the generation of charged particles, it is considered that field-dependent molecular ionization predominates the charge generation mechanism of streamer discharge processes in transformer oil.