Investigation on critical breakdown electric field of hot sulfur hexafluoride/carbon tetrafluoride mixtures for high voltage circuit breaker applications

Sulfur hexafluoride (SF6) gas, widely used in high-voltage circuit breakers, has a high global warming potential and hence substitutes are being sought. The use of a mixture of carbon tetrafluoride (CF4) and SF6 is examined here. It is known that this reduces the breakdown voltage at room temperature. However, the electrical breakdown in a circuit breaker after arc interruption occurs in a hot gas environment, with a complicated species composition because of the occurrence of dissociation and other reactions. The likelihood of breakdown depends on the electron interactions with all these species. The critical reduced electric field strength (the field at which breakdown can occur, relative to the number density) of hot SF6/CF4 mixtures corresponding to the dielectric recovery phase of a high voltage circuit breaker is calculated in the temperature range from 300K to 3500 K. The equilibrium compositions of hot SF6/CF4 mixtures under different mixing fractions were determined based on Gibbs free energy minimization. Full sets of improved cross sections for interactions between electrons and the species present are presented. The critical reduced electric field strength of these mixtures was obtained by balancing electron generation and loss mechanisms. These were evaluated using the electron energy distribution function derived from the Boltzmann transport equation under the two-term approximation. The result indicates that critical electric field strength decreases with increasing heavy-particle temperature from 1500 to 3500 K. Good agreement was found between calculations for pure hot SF6 and pure hot CF4 and experimental results and previous calculations. The addition of CF4 to SF6 was found to increase the critical reduced electric field strength for temperatures above 1500 K, indicating the potential of replacing SF6 by SF6/CF4 mixtures in high-voltage circuit breakers. (C) 2013 AIP Publishing LLC.