Breakdown characteristics of SF6 and Liquefied SF6 at decreased temperature

Abstract

SF6 gas has been used as arc quenching and insulating medium for high and extra high voltage switching devices due to its high dielectric strength, its excellent arc-quenching capabilities, its high chemical stability and non toxicity. Despite of its significant contributions, the gas was classified as one of the greenhouse gas in the Kyoto Protocol. Thus, many researches are conducted to find out the replacement materials and to develop the SF6 gas useless electrical equipment. This paper describes experiments on the temperature change-related breakdown characteristics of SF6 gas (SF6) and SF6 liquid (LSF6) in a model GIS(Gas-Insulated Switchgear) chamber in order to show the possibility of more stable and safe usages of SF6 gas. The breakdown characteristics are classified into three stages, namely the gas stage of SF6 according to Paschen's law, the coexisting stage of SF6 gas with liquid in considerable deviation at lower temperature, and the stage of LSF6 and remaining air. The result shows that the ability of the LSF6 insulation is higher than the high-pressurized SF6. Moreover, it reveals that the breakdown characteristics of LSF6 are produced by bubble-formed LSF6 evaporation and bubbles caused by high electric emission and the corona. In addition, the property of dielectric breakdown of LSF6 is determined by electrode form, electrode arrangement, bubble formation and movement, arc extinguishing capacity of the media, difficulty in corona formation, and the distance between electrodes. The bubble formation and flow separation phenomena were identified for LSF6. It provides fundamental data not only for SF6 gas useless equipment but also for electric insulation design of hightemperature superconductor and cryogenic equipment machinery, which will be developed in future studies.