Possibility of Gas Mixtures Containing c-C4F8 as a SF6 Substitute in Gas Insulation
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.
KeywordsGlobal Warming Potential Electrode Separation Limited Experimental Condition High Global Warming Potential Charge Simulation Method
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