Impulse breakdown characteristics and cost/benefit analysis of SF/sub 6//CCl/sub 2/F/sub 2//CO/sub 2/ mixtures

A comprehensive study has been carried out in SF/sub 6//CCl/sub 2/F/sub 2//CO/sub 2/ mixtures to measure 50% breakdown voltages V/sub 50/ using both positive and negative polarity lightning impulse (1.2/50 mu s) voltages under nonuniform fields (5-mm rod, 230-mm plane electrode) over a pressure range of 0.1 to 0.5 MPa for a gap spacing of 20 mm. The sum of SF/sub 6/ and CCl/sub 2/F/sub 2/ concentrations in the mixture was always maintained in the range of 21 to 40%, the rest being CO/sub 2/. Among the different sets of SF/sub 6//CCl/sub 2/F/sub 2//CO/sub 2/ mixtures studied, two ternary mixtures, namely 20% SF/sub 6//20% CCl/sub 2/F/sub 2//60% CO/sub 2/ and 30% SF/sub 6//10% CCl/sub 2/F/sub 2//60% CO/sub 2/, were found superior to SF/sub 6/. From the known gas content in the mixtures and from the measured breakdown voltages, a cost/benefit analysis has been carried out for the various mixtures investigated. A striking feature of this analysis is that the most promising ternary gas mixture, namely 30% SF/sub 6//10% CCl/sub 2/-F/sub 2//60% CO/sub 2/, costs only 33% as much as pure SF/sub 6/, giving 100% to 170% positive breakdown strength, and 90 to 100% negative breakdown strength as compared to that of pure SF/sub 6/.<>