Breakdown behavior of SF/sub 6/ gas-insulated systems at low temperature

The controversy surrounding low-temperature SF/sub 6/ breakdown is addressed in detail. Earlier relevant studies are reviewed, some of the existing data is analyzed in a new light, and further theoretical considerations are presented. These discussions served to outline an experimental approach aimed at confirming or invalidating breakdown invariance at subnormal temperatures. Low-temperature dc breakdown of an SF/sub 6/ gas-insulated system was investigated experimentally for temperatures ranging from -50 to 24/spl deg/C, using associated pressure values that had been selected carefully to avoid phase transition of the gas-insulating medium. The context allowed experimentation under both uniform and nonuniform field conditions; the nonuniformity was due to the active role of the cathode-gas interface at high fields. The experiment was conducted for molecular densities ranging from 2.596 to 16.43/spl times/10/sup 19/ cm/sup -3/ (equivalent to pressures of 105 and 624 kPa, respectively, at 24/spl deg/C) and for gap lengths starting at 0.5 mm and extending to 7 mm. Data sets show consistency, low statistical scattering, and high reproducibility. Data analysis led to several major conclusions. At constant density, the breakdown of the SF/sub 6/-insulated system is temperature dependent, which is responsible for a decrease in the electric strength, by /spl sim/10%. This decrease occurs for uniform field conditions, the effect being small if not negligible for nonuniform field conditions, and is noted to appear at a threshold temperature (-25 to -30/spl deg/C), take a constant value, and be fairly independent of density.<
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