Dielectric properties of gas mixtures with C3F8/C2F6 and N2/CO2

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.