Assessment of electron swarm parameters and limiting electric fields in SF6+CF4+Ar gas mixtures

Abstract

The electron swarm parameters, namely electron drift velocity, mean energy, effective ionization coefficient and limiting values of number-density-reduced electric fields, E/N, are calculated and assessed in SF6+CF4+Ar ternary mixtures for SF6 concentrations that vary from 1% to 20% over a range of E/N from 50Td to 600Td by solving Boltzmann's equation. The limiting electric field values for the ternary mixtures are deduced from the graphs of the calculated effective ionization coefficients as a function of E/N. Furthermore, breakdown voltage measurements are conducted for the ternary mixtures. The limiting electric field values obtained from the measured breakdown voltages are compared with those of the Boltzmann analysis and found to be in good agreement. The insulation strength of the ternary mixtures with constant Ar concentrations indicated an increase with increasing contents of SF6 while the share of CF4 in the mixture reduced in the investigated E/N range up to 600 Td with SF6 concentrations varying from 1% to 20%. Moreover, a significant synergy is observed due to the non-linear nature of the mixture, and the maximum synergy appears to be around 10% SF6 in the mixtures of this study with fixed Ar concentration. Furthermore, theoretical calculations of breakdown voltages from Boltzmann analysis are found to be in good correlation with the experimental values.