Influence of PTFE Vapor on Dielectric Breakdown Property of Hot SF₆ Gas

PTFE vapor has a crucial effect on the interruption performance of SF₆ circuit breakers. Therefore, we analyzed the influences of PTFE vapor on the dielectric breakdown properties of hot SF₆ gas at gas pressures of 0.01~1.6 MPa and gas temperatures up to 3 500 K, which represents the conditions prevailing during the dielectric recovery phase in a high voltage circuit breaker. By minimizing the Gibbs free energy under the assumptions of local thermodynamic and chemical equilibrium, we calculated the equilibrium compositions of SF₆ mixtures with PTFE vapor, and analyzed the reduced ionization coefficient α/N, the reduced attachment coefficient η/N, and the reduced effective ionization coefficient (α-η)/N. Moreover, the critical reduced electric field strength (E/N)_cr was determined by balancing the electron generation and loss processes. The results confirm the superior breakdown properties of pure SF₆ at relatively low gas temperatures. However, (E/N)_cr of SF₆ mixtures with PTFE vapor are clearly higher than that of pure SF₆ for high gas temperatures. In addition, it is also noted that the increase of gas pressure shifts the variations of (E/N)_cr to higher gas temperatures.