Pre-Breakdown Phenomena in Technical Air with and without C5-Fluoroketone for a Rod-Plane Gap

Abstract

Technical air with a few %vol C5-fluoroketone (C5-FK) is one of several alternatives to the potent greenhouse gas SF6 in medium voltage (MV) gas insulated switchgear. MV products using this gas mixture are already commercially available, but there is still a lack of knowledge about this gas mixture especially regarding pre-breakdown phenomena. In this study, positive and negative streamers are studied in technical air with 7.5 %vol C5-FK at 1.0, 1.3, and 1.5 bar (absolute pressure). Similar tests were done in technical air (80% N2, 20% O2) for comparison with the C5-FK results. The streamers emerged from a grounded needle electrode placed 53mm below a planar electrode. The electrode setup was stressed by a non-standard lightning impulse voltage with voltage peaks varying from 52 to 129 kV. A photomultiplier tube (PMT) was used to register the streamers, and an iCCD camera was used to image the streamers. The pre-breakdown phenomena in C5-FK+air were found to be shorter, less than 1/5 of the length, and to branch less than in pure technical air. The streamer branches in both gas mixtures became wider and less clear when the pressure was decreased. Leader-like channels were observed for C5-FK+air at 1.5 bar (both polarities) and at 1.3 bar (negative streamers).