Application of a Heptafluoroisobutyronitrile gas (C_4F_7N) mixed with the Background gas of CO_2 in GIS as SF_6 Alternative

摘要

SF_6 has been successfully used for more than 40 years in high voltage gas-insulated switchgears (GIS). However, since SF_6 gas has been specified as one of the greenhouse gas in the Kyoto Protocol, the research and development of the alternative gas have been carried out by manufacturers and universities in order to save tons of CO_2 equivalents. Several types of gas have been investigated in the past years but there was no suitable candidate of SF_6 due to the limitation of the dielectric performance, toxicity and so on. Therefore, the research is extended to new or newly manufactured fluorinated molecules and gas mixtures. In 2016, the KEPCO (Korea Electric Power Corporation) announced a new policy of eco-friendly GIS adaptation using the alternative gas specified below than GWP 500 (100 years) for 170kV 50kA GIS. The mixture gas using C_4F_7N is the strongest candidate for the alternative gas and has been activated in Korea. Some of the main advantages of C_4F_7N is a higher dielectric performance than SF_6 and a low mixing ratio of C_4F_7N yields a performance close to the dielectric performance of SF_6. According to the previous research, only 20% of mixing ratio by volume shows similar performance to that of SF_6. However, in case of outdoor application, the mixing ratio is limited below 10% by volume with normal GIS pressure range due to liquefaction temperature. Therefore, the dielectric performance of alternative gas using C_4F_7N could be slightly lower than SF_6 with same pressure range. Moreover, alternative gas using C_4F_7N mostly consists of background gas so that different gas properties should be considered on breaking and switching performance. This paper presents the feasibility of the application of a heptafluoroisobutyronitrile gas (C_4F_7N) mixed with the background gas of CO_2 in 170kV 50kA GIS as an alternative of SF_6. Especially, breaking and switching for Circuit Breaker, switching for the Disconnector, Earthing Switch, dielectric and temperature rise tests were performed and most results show comparable with SF_6. It is expected that a heptafluoroisobutyronitrile gas (C_4F_7N) mixed with the background gas of CO_2 could be a suitable alternative to SF_6 for GIS.