Possibility of Gas Mixtures Containing c-C₄F₈ as a SF₆ Substitute in Gas Insulation

Abstract

The very high global warming potential (GWP) of SF₆ gas has stimulated worldwide research on finding a new gas or gas mixtures applicable for gas insulation. As it is now considered very difficult to find a suitable pure gas substituting for SF₆, the research principally focuses on applying SF₆/ N₂ mixtures. In a previous paper, we have proposed to use mixtures including low content PFC (perfluorocarbon) mainly for the purpose of reducing latent GWP drastically, among which we have selected mixtures with c-C₄F₈ (perfiiluorocyclobutan)¹⁾ This gas is completely nontoxic and has no ozone-depleting ability. Its GWP is about 40% as low as that of SF₆. Furthermore, liquefaction recovery is much easier in mixtures with c-C₄F₈ than in SF₆ / N₂ due to the higher liquefaction temperature of c-C₄F₈ (about -6°C). Sparkover characteristics of c-C₄F₈ / N₂ mixtures were already reported for limited experimental conditions at a relatively lower gas pressure of 0.66 atm or 1 atm²⁾. In this study, we measured sparkover voltage of gas mixtures including 5–20 vol. % c-C₄F₈ in N₂, CO₂, or air under a quasi-uniform electric field condition. The applied voltage was DC, 60 Hz AC, or a lightning impulse voltage. The pressure ranged from 0.1 to 0.4 MPa. The sparkover voltage so obtained increases more or less nonlinearly with the increase in mixing ratio of c-C₄F₈, irrespective of an applied voltage waveform or a mixing component gas. From the experimental results, we have evaluated the degree of nonlinearity, or synergism, by using an empirical formula that was proposed by one of the authors in 1972. Although the degree of nonlinearity depends on a mixed component, it becomes larger with increasing Pd (P is gas pressure and d electrode separation). In the case of SF₆/ N₂, on the other hand, the degree of nonlinearity is almost independent of Pd. We have also compared the degree of nonlinearity between applied voltage waveforms.