Dielectric properties of gas mixtures with per-fluorocarbon gas and gas with low liquefaction temperature

Abstract

Thanks to its excellent dielectric strength and arc interruption performance, SF 6 gas has been widely used for power equipment since the 1960s. However, since SF 6 has a high global warming potential (GWP), it was designated at COP3 in 1997 as a greenhouse gas to be reduced; hence the search for an alternative insulating gas to SF6 to be widely used for power equipment. Aiming to reduce the global warming potential (GWP) of existing power equipment using SF6, the present study investigated breakdown characteristics when various alternative gases to SF6 were mixed. Alternative gases to SF6 handled in this report were based on four kinds of gases, namely c-C 4 F 8 , 1-C 3 F 6 , C 3 F 8 and C 2 F 6 , from among per-fluorocarbon gases with relatively high dielectric strength. However, since these base gases have relatively high liquefaction temperature, despite their advantage in terms of GWP and dielectric strength, a gas with a low liquefaction temperature was mixed as an additive. Three kinds of gases were used as additive gases, including CF4, a fluoride gas with a relatively low liquefaction temperature, in addition to N2 and CO2. Accordingly, in this report, various combinations of the four kinds of base gases and three kinds of additive gases were used to obtain the respective insulation characteristics while varying the mixture ratio of the respective gases. Consequently, it emerged that relatively high and positive synergism was likely to be obtainable when the mixture ratio of the base gas was 20%. Furthermore, a comparison of dielectric strength with respect to the GWP revealed that the GWP was likely to be reduced to 10% or less while maintaining about 80% of dielectric strength, under conditions that the upper limit of the liquefaction temperature assuming indoor equipment was 0 °C.