PTFE vapor has a crucial effect on the interruption performance of SF

6

circuit breakers.Therefore

we analyzed the influences of PTFE vapor on the dielectric breakdown properties of hot SF

6

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 SF

6

mixtures with PTFE vapor

and analyzed the reduced ionization coefficient α/N

the re

duced 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 SF

6

at relatively low gas temperatures.However

（E/N）cr of SF

6

mixtures with PTFE vapor are clearly higher than that of pure SF

6

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.