Streamer physics in transformer oil stressed by pulse voltage

Summary form only given. The electrical behavior of dielectric liquids such as transformer oil subjected to high electric-field and the possible breakdown phenomena is of great practical interest, since this liquid has been widely used in high-voltage power apparatus and pulse power systems, accordingly has led to extensive research work. This paper presents streamer discharge process such as initiation and propagation until breakdown in transformer oil under positive and negative nanosecond pulse voltage. Simulation computation via finite element method is undertaken through developing a two dimensional axis-symmetric fluid model. In this fluid model, three charge carriers, such as electron, positive and negative ions are utilized to account for the charge generation, recombination, and transport mechanisms in the discharging process. The streamer discharge profile and distribution of electric field and space charge density are obtained under different conditions such as polarities, the amplitude of applied voltage, rise time. Simulation results show that the discharging phenomena of positive and negative streamers are fundamentally different, such as electric-field profiles, initiation voltage (IV), breakdown Voltage (BDV) and propagation speed of positive and negative streamer are basically distinguished from each other. Generally, positive streamers tend to be filamentary in appearance, while negative streamers form in a bushy shape, which possibly imply that physical mechanisms behind both polarity streamers are different. It can be found that space charges play an key but different role in positive and negative streamers initiation and propagation. During streamer approaches the ground electrode, “ionization wave” which propagates toward ground can be observed. The rise time of applied voltage have evident influences on the average speed of streamer propagation. It is considered that field-dependent molecular ionization predominates charge generation mechanism of streamer discharge process in transformer oil. Field-dependent molecular ionization is firstly introduced by Zener Ionization in solid dielectric. Space charge effect contributes to further developed ionization until the whole gap eventually breakdowns.