BREAKDOWN BEHAVIOR OF SF6 GAS-INSULATED SYSTEMS AT LOW-TEMPERATURE

The controversy surrounding low-temperature SF6 breakdown is addressed in detail. Earlier relevant studies are reviewed, some of the existin g data is analyzed in a new light, and further theoretical considerati ons are presented. These discussions served to outline an experimental approach aimed at confirming or invalidating breakdown invariance at subnormal temperatures. Low-temperature de breakdown of an SF6 gas-ins ulated system was investigated experimentally for temperatures ranging from -50 to 24 degrees C, using associated pressure values that had b een selected carefully to avoid phase transition of the gas-insulating medium. The context allowed experimentation under both uniform and no nuniform field conditions; the nonuniformity was due to the active rol e of the cathode-gas interface at high fields. The experiment was cond ucted for molecular densities ranging from 2.596 to 16.43x10(19) cm(-3 ) (equivalent to pressures of 105 and 624 kPa, respectively, at 24 deg rees C) and for gap lengths starting at 0.5 mm and extending to 7 mm. Data sets show consistency, low statistical scattering, and high repro ducibility. Data analysis led to several major conclusions. At constan t density, the breakdown of the SF6-insulated system is temperature de pendent, which is responsible for a decrease in the electric strength, by similar to 10%. This decrease occurs for uniform field conditions, the effect being small if not negligible for nonuniform field conditi ons, and is noted to appear at a threshold temperature (-25 to -30 deg rees C), take a constant value, and be fairly independent of density.