Breakdown and prebreakdown characteristics of SF/sub 6//CF/sub 4/ mixtures in nonuniform electric field

Measured prebreakdown and breakdown voltage characteristics of SF/sub 6/-CF/sub 4/ mixtures are presented. The investigated gas insulated system consisted of point-to-sphere electrode configuration placed in a laboratory gas chamber. The measurements were carried out at the pressure of mixture equal to 300 kPa. The content of SF/sub 6/ in CF/sub 4/ was changed from 0% to 100%. The interelectrode gap was varied from 5 mm to 30 mm. Breakdown characteristics were measured for AC, DC and impulse voltages. Prebreakdown activity was analyzed under AC voltage using a wide-band PD detector and a coupled PRPD analyzer. The quantities characterizing PD intensity were analyzed, including energy spectrum of pulses, phase distributions of PD pulses' charge and the number of pulses as a function of phase. Special attention was paid to 25% SF/sub 6/-75% CF/sub 4/ mixture, because of its superior electric strength observed particularly under AC and DC voltages.