Calculation and characteristic analysis of critical breakdown field strength of SF6 and the mixtures

The reduced critical electric breakdown field (E/N)cr of SF6 and its substitute or mixtures gets very crucial for electrical breakdown evalution. The (E/N)cr of SF6 is investigated under 0.4 MPa and 0.8 MPa respectively, and over the temperature range of 300 to 3000 K, by solving the two-term Boltzmann equation. The results show that the (E/N)cr of SF6 decreases with increasing temperature, and the decreasing rate depends on the SF6 decomposition rate; at temperature within 1500 and 2500 K, the (E/N)cr of SF6under 0.8 MPa is evidently higher than that under 0.4 MPa, however almost equal between 0.8 MPa and 0.4 MPa at other temperature. The calculations for (E/N)cr of SF6-CF4, SF6-CO2, SF6-O2, SF6-CH4, SF6-C2H6 and SF6-CHF3 under 300 K indicate, when x(SF6) in mixture is over 10%, the (E/N)cr of all these mixtures changes approximately linearly with x(SF6); among these gases, the (E/N)cr of CF4 and O2 approaches to the highest; the (E/N)cr of SF6-CF4 and SF6-O2 closely appears, and both higher than that of the others. And r, the ratio of the critical breakdown field of SF6 to that of a gas mixture, is introduced to conclude that under proper pressure and mixing ratio, the critical breakdown field of a gas mixture becomes equivalent to SF6.