Leader Step Time and Low Probability Impulse Breakdown Voltage Measured in SF₆ Gas Mixtures

Abstract

SF₆ is used worldwide today owing to its high dielectric strength and good heat transfer properties. But SF₆ is extremely sensitive to non-uniformity of the electrical field. Therefore in some electrical apparatus, such as cubicle type GIS, SF₆ gas mixtures are used¹. Experiments show that when SF₆ is mixed with low electrical strength gases, such as air or CO₂, the low probability impulse breakdown voltage U _i and dc breakdown voltage U _b of the mixtures are higher than that of SF₆ ^2,3. the view of corona stabilization enhancement⁴ is usually used to explain the increase of U _b: when low electrical strength gas is added to SF₆, the corona onset voltage of mixtures decreases, so more ions are produced leading to the enhancement of corona stabilization. But the increase of U _i can not be explained by this viewpoint because there is hardly any corona stabilization under impulse conditions. Moreover, it is reported⁵ that when Freon 113 (C₂C1₃F₃), whose electrical strength is higher than that of SF₆, is mixed with SF₆, not only will U _b and U _i increase, but the corona onset voltage will also increase. Obviously, this phenomenon conflicts with the view of corona stabilization enhancement.