Impulse Breakdown Behavior of Negative Rod-Plane Gaps in SF6-N2, SF6-Air and SF6-C02 Mixtures

Breakdown behavior of a 50 mm negative rod-plane gap is investigated for N2, air, CO2 and their binary mixtures with SF6 for pressures of up to 0.5 MPa using rod of 0.794 mm radius and impulse voltages of 2/50 ¿sec. It is shown that at high gas pressures, mixtures containing low SF6 content can have breakdown voltages lower than the corresponding values in pure gases. Furthermore, the results indicate that SF6-CO2 mixtures perform somewhat better than SF6-N2 and SF6-air mixtures and can have breakdown voltages slightly higher than those for pure SF6. On the contrary, SF6-air and SF6-N2 mixtures have breakdown voltage levels which are always below their respective values for SF6. Furthermore, it is shown that when gas pressure is equal or higher than 0.1 MPa, leader-type corona exists in almost all of these mixtures. Also when the gas pressure and SF6 content of the mixture are high, leader discharge extends into the gap in a stepped manner. Detailed results of these investigations are described and discussed in this paper.