Numerical Model of Two-Phase Gas–Liquid Streamer Discharge in Ester-Based Insulating Oil Under Impulse Voltage

Ester-based insulating oil (EO) is a high flame resistance and environment-friendly liquid dielectric, which shows excellent effect on improving the safe operation of transformers. It is easier to obtain the distribution of electric field (EF) and carrier in the discharge process by simulation than by experimental observation. In this study, a two-phase gas–liquid plasma discharge model was established, which corrected the electron velocity and molecular ionization potential (IP) and added the gas phase (GP) process in the original model. The results indicate that by adding GP processes, the improved model better reflects the multi-branches characteristic of the streamer than the original model. Impact ionization plays an important role as one of the key mechanisms for generating streamer branches. Under the action of Joule heat, GP region is generated near the needle electrode, and impact ionization occurs in the GP region to promote the generation of streamer branches and secondary streamer. GP has no effect on the length and velocity of the main streamer. The number of streamer branches increases with the voltage, simultaneously weakening the energy within the main streamer channel and resulting in finer channels. In contrast to the density distribution of charged particles within the main streamer channels, the density of charged particles within the streamer branches decreases with an increasing applied voltage. This phenomenon might be due to a shielding effect among the streamer branches, hindering their ionization efficiency.