Influence of Residual Fluoro-Organic Gas Compounds on the Electric Performance of SF₆-Free Gas Insulated Systems in Consecutive Test Series

Abstract

From the technical point of view, SF₆ is an optimum gaseous insulation and switching medium and is, therefore, preferably used in medium- and high-voltage gas insulated systems. Nevertheless, a political demand has emerged for alternative solutions with improved environmental friendly properties, since SF₆ is listed in the Kyoto Protocol (1997) as greenhouse gas, of which emissions need to be reduced. To figure out the best gas candidates for this purpose, the dielectric breakdown strengths of different potential gas mixtures are under investigation. During these dielectric breakdown studies with alternative fluorinated gas mixtures, an influence on the dielectric breakdown strength in subsequent test series in the same gas vessel was observed, despite evacuating the vessel. This influence is potentially caused by the attachment of fluoro-organic gas molecules to the surface of the test vessel and their release over time in subsequent investigations. To estimate and quantify this influence, AC breakdown test series were performed with fluoro-organic compounds (mixed with carrier gas) and natural-origin gases in consecutive test series in the same test vessel. Parameters such as the exposure time and type of the fluoro-organic gas were investigated as well as the influence of evacuation duration and gas flushing. Furthermore, additional typical material samples from gas-insulated systems were placed in the test gas vessel to provide additional attachment surfaces for the fluoro-organic gas molecules. The influence of the different material samples was taken into account individually. The results are presented and discussed with regard to the independence of measurements, and a recommendation on how to avoid interactions is given.