Surface discharge propagation in C4F7N/CO2 mixture under positive impulse voltages

Abstract

C4F7N/CO2 is a promising mixture to replace SF6 in an electrical insulation field for its environment friendly property. Understanding surface discharge behaviors of the C4F7N/CO2 mixture is essential for designing an effective insulation system. In this work, surface discharge propagation characteristics in the C4F7N/CO2 mixture under positive impulse voltages are investigated. The surface discharges are detected electrically and observed by optical imaging. The residual surface charge distribution is acquired by an electrostatic probe method and a dust figure method after surface discharge. Comparative studies in CO2 and SF6 are further conducted. Results exhibit some significant differences between CO2 and C4F7N/CO2 mixtures in the propagation pattern and morphology of surface discharges. The surface discharge in the C4F7N/CO2 mixture develops in a stepwise expansion pattern, which is a result of alternative formation of streamers and leaders. A leader channel and a streamer development region are clearly distinguished in the optical image. The dust figure shows that no charge is deposited on the leader channel area after discharge, proving that the leader channel is electrically neutral and of high conductivity. The residual surface charge distribution in the C4F7N/CO2 mixture is obtained and resembles pine branches. A little difference is found in the surface discharge propagation characteristics between the C4F7N/CO2 mixture and SF6.