Breakdown Electric Field Calculation of Hot SF6 and Its Application to High Voltage Circuit Breakers

The calculation of the critical electric field strength Ecr of hot SF6 is important for a compact circuit breaker design. First, the reduced critical electric field strength (E/N)cr is obtained for 2, 4 and 8atm respectively, and for the temperature range of 300 to 3000K, by solving the Boltzmann equation. Then the variation of Ecr with pressure and temperature is derived from the corresponding (E/N)cr data of SF6. It is found that the pressure effect could not be neglected to predict the Ecr by comparison of the calculation results of Ecr at 8atm using the (E/N)cr data of 8atm and 2atm, where the maximum deviation is above 50%. Finally, the calculation results of Ecr are used to evaluate the dielectric breakdown strength of a puffer type high voltage SF6 circuit breaker by comparing the ratio of applied electric field strength Ea and Ecr. It demonstrates that the analysis results can provide an indicator for the probable breakdown areas inside the arc quenching chamber.