Free Conducting Particles in a Coaxial Compressed-Gas-Insulated System

The effect of free conducting particles (FCPs) on the voltage-insulating performance of a short length of transmission line was quantitatively studied over the pressure range 0 to 300 psig for nitrogen and 0 to 125 psig for SF6. The FCPs were selected aluminum filings deliberately inserted into the concentric cylindrical (3"×10" dia.) electrode system. DC voltages of either polarity were produced by a Van de Graaff generator. The observed forms of interelectrode current included intermittent pulses or \´bursts\´, \´fireflies\´, steady corona, and sparks. Up to five-fold reductions in the current-onset voltage insulating strength resulted from the introduction of FCPs. For pressures up to 75 psig the sparking voltage in SF6 was 4 to 5 times higher than the sparking value in nitrogen. An electrostatic particle trap proved effective in reducing FCP-caused discharges. The dynamic equilibrium mechanism of \´fireflies\´ was studied in detail.