2D simulation of the positive streamer in SF₆: effect of high attachment rate in atmospheric and sub atmospheric pressures.

In high voltage applications, pressurized SF₆, a highly electronegative gas is used. Understanding physical mechanisms leading to breakdown of this insulating medium are of utmost importance for designing fast switching electrical equipment. Breakdown in SF₆ at atmospheric pressure occurs via a streamer-leader transition. This is difficult to achieve numerically with current simulation models limited to streamer studies which are suitable for low pressures. This work presents an attempt to understand streamer discharges in the gas using a plasma fluid model implementation in COMSOL® Multiphysics at 0.1 and 1 bar. The effect of the high electronegativity of SF₆ is evaluated by comparing streamers in SF₆ to that in Air on the axes of electric field distribution, electron and ion densities profiles, streamer velocity and diameter for a cathode directed streamer in 2D axisymmetric point to plane configuration. Depending on pressure, a reduction or loss of the conduction path between the streamer head and the tip of the needle electrode is obtained in SF₆ as opposed to a near consistent electron density profile in the streamer channel for air. This showcases the high influence of attachment in SF₆ at lower reduced electric fields in comparison to air.

Index terms – Streamer Discharges, Numerical Modelling, Electronegative Gases