Insulating Characteristics of Small SF₆ Gas Gaps with Covered Electrodes for Lightning Impulse Voltages

Abstract

Insulating characteristics of small SF₆ gas gaps have been investigated for lightning impulse voltages. Electrodes, that radius is 20mm, are covered with 20μm thick PET films. At the region of gas gap less than 1mm, under 0.4MPa, the breakdown voltages are at least twice higher than Paschen's curve. And when gas gap is become longer than 1mm, the breakdown voltage of gas gap is getting near Paschen's curve. These phenomena can be explained well by assuming the lack of an initial electron. When gas gap is less than 1mm, the initial electron seems to be emitted from PET films at about 70kV/mm. When gas gap is over 1mm, the initial electron seems to be emitted from ions in gas gap. When the gas gap is Omm and the gas wedge gap is constructed, the flashover of gas occurs at about 3.5kV. With this voltage, the electric fields of covering PET films surface less than 35μm gap region are higher than 70kV/mm. Initial electrons are generated enough in this region, but flashover does not occur. This may be due to the secondary ionization effect (γ effect). PET film has the tendency to catch electrons, and avalanches are stopped effectively on covering PET films.