Dielectric properties of gas mixtures with C<sub>3</sub>F<sub>8</sub>/C<sub>2</sub>F<sub>6</sub> and N<sub>2</sub>/CO<sub>2</sub>

Abstract

Thanks to its excellent insulation and cutoff performances, SF6 gas has been applied to power equipment since the 1960s and is now widely used for several kV to 1,000 kV-class GIS, GCB, and GIL. However, since 1997, when SF6 was designated at COP3 as a greenhouse gas to be reduced, there has been a wish to use an alternative insulating gas to pure SF6 gas. In the present study, alternative gases were selected from among mixtures excluding SF6 with the need to reduce GWP (global warming potential) in mind. Gas mixtures containing such substances and with a boiling point of -20°C or less, chemically stable, non-toxic, and not ozone-depleting were prioritized. Furthermore, the availability and environmental performance were taken into consideration when deciding on component gases. Consequently, to launch this series of studies, four types of gas mixtures were used combining a gas in Group A (C2F6, C3F8) – electronegative gases with relatively high dielectric strength – and a gas in Group B (N2, CO2) – gases existing in the natural world. The GWP of SF6 is 22,800 whereas that of C2F6 is 12,200 and that of C3F8 is 8,830, or several times smaller than that of SF6. In the present paper, insulation characteristics were experimentally obtained while varying the mixture ratio under a quasi-uniform electric field assuming GIS. Consequently, compared to the GWP of pure SF6, the GWP was about 12% to 38% for gas mixtures with C3F8/N2 or C3F8/CO2 and 18% to 70% for gas mixtures with C2F6/N2 or C2F6/CO2. Consequently, it emerged that, while assuming breakdown voltage proportional to gas pressure, the GWP was likely to be reduced by 30% to 90% while maintaining dielectric strength. In addition, a study was conducted on the synergism of a gas mixture through analysis using the Boltzmann equation. Consequently, the synergism was confirmed while its degree varied depending on the type of each gas mixture, and the mechanism thereof was clarified.