Numerical Modeling of Branching-Streamer Propagation in Ester-Based Insulating Oil Under Positive Lightning Impulse Voltage: Effects From Needle Curvature Radius

Ester-based insulating oil provides the possibilities for controlling pollutant emissions and enhancing transformer safeties. A 2-D needle-plate electrode branching-streamer model has been established to numerically investigate the propagation characteristics of initial streamer branches and the effects from needle radius in ester-based insulating oil under lightning impulse (LI) voltage. The results indicate that by adding the source term that affects charge carrier density variation to the current continuity equation, the revised model could better capture the branching-streamer morphology in the experiment than the existing model. The accumulation of positive ions in the streamer branches promotes the electric field distortion, introducing the maximum electric field at the head of the streamer branches, which causes the appearance of the branching streamer due to the localized charge carrier surge (CCS). The electric field at the needle zone is affected by the curvature radius, and the smaller curvature radius enhances the electric field, allowing more space charge to be generated, which leads to easier distortion of the electric field at the head of the streamer branches and promotes forward propagation of the streamer branches. In addition, the smaller curvature radius contributes to the generation of the secondary streamer.