Impulse breakdown of gas-insulated gaps (10%SF6/90%N2) with thick coated electrodes

The present paper deals with breakdown experiments and streamer inception calculations of gas insulated gaps (10%SF₆/90%N₂) in series to thick coated electrodes. Experiments by means of a rod-plane geometry were carried out. The rod electrode was covered with silica filled epoxy resin of various thicknesses. As insulation gas a mixture of 10%SF₆/90%N₂ was used. The rod diameter, the distance to the plane electrode and the gas pressure were varied. The arrangement was electrically stressed with a step-voltage. In addition to the experiments a streamer inception calculation for thick coated electrode arrangements was carried out. Firstly the electric field along the symmetry axis was calculated and the streamer criterion was applied to the gaseous insulated part. As a first approach the effective ionization coefficients without presence of an insulator surface were used. A correlation between measurements and calculations led to the following main conclusions: The application of the streamer criterion by means of the ionization coefficients without presence of an insulator surface can serve as an excellent method for determining the breakdown voltage of a thick coated electrode arrangement if streamer inception leads to breakdown. Surface conditioning of thick coated electrodes has to be taken into consideration if a streamer discharge has taken place; Calculation of streamer inception voltages can be used to determine thick coated electrode arrangements with optimized dielectric withstand strength