Maximum electric strength and vapor pressure of dielectric gases and mixtures

In general, there is a positive correlation between the electric breakdown strength of strong dielectric gases and their boiling points, and this relationship has been a factor inhibiting the use of alternatives to SF6 in compressed-gas-insulated equipment operating over a wide range of temperature. It was suggested many years ago that it might be advantageous to use some of these gases in mixtures rather than alone, and hence to find a mixture superior to SF6 in cost, electric strength, low-temperature performance or immunity to particle or electrode surface roughness effects. This paper notes that in general the saturated vapor pressure of a mixture of gases is less than the sum of the saturated vapor pressure of the component gases; and the deficiency is large if the range of boiling points of the component gases is large. The physical basis of this effect is discussed, and we calculate the range of compositions of a number of binary and ternary SF6 mixtures at 0.44 MPa and 25°C which also do not exhibit condensation at 0°C, or at −30°C. We have also measured and reported here 60 Hz breakdown strengths of these limiting and other mixtures. This work was performed at the Westinghouse Research and Development Center and the DuPont Experimental Station under a Research Contract 847-1 with the Electric Power Research Institute, "Gases Superior to SF6 for Insulation and Interruption".