Impulse breakdown characteristics and cost/benefit analysis of SF 6/CCl2F2/CO2 mixtures

A comprehensive study has been carried out in SF₆/CCl ₂F₂/CO₂ mixtures to measure 50% breakdown voltages V₅₀ using both positive and negative polarity lightning impulse (1.2/50 μs) 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 SF₆ and CCl₂F₂ concentrations in the mixture was always maintained in the range of 21 to 40%, the rest being CO₂ . Among the different sets of SF₆/CCl₂F₂/CO₂ mixtures studied, two ternary mixtures, namely 20% SF₆/20% CCl₂ F₂/60% CO₂ and 30% SF₆/10% CCl ₂F₂/60% CO₂, were found superior to SF 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 ₆/10% CCl₂-F₂/60% CO₂, costs only 33% as much as pure SF₆, giving 100% to 170% positive breakdown strength, and 90 to 100% negative breakdown strength as compared to that of pure SF₆