Prediction of the dielectric strength for c-C4F8mixtures with CF4, CO2, N2, O2and air by Boltzmann equation analysis

The dielectric strength of c-C 4 F 8 , and mixtures of c-C 4 F 8 with CF 4 , CO 2 , N 2 , O 2 and air, is studied through solution of the Boltzmann equation. The reduced ionization coefficient α / N and reduced attachment coefficient η / N are calculated, allowing the reduced effective ionization coefficient ( α – η )/ N and the critical reduced electric field strength ( E / N ) cr (the reduced electric field for which ( α – η )/ N = 0), to be determined. A high value of ( E / N ) cr for an electronegative gas, such as those considered here, indicates good insulating properties. It is found that c-C 4 F 8 –N 2 and c-C 4 F 8 –air have very similar ( E / N ) cr values, higher than those of the other three mixtures, and superior even to that of pure SF 6 for c-C 4 F 8 concentrations above 80%. Comparison of the results obtained for c-C 4 F 8 and c-C 4 F 8 –N 2 with experimental values from the literature supports the validity of the approach taken here and the parameters used.