BREAKDOWN OF SF(6) IN NONUNIFORM FIELD GAPS UNDER COMBINED DC - FAST OSCILLATING IMPULSE AND STANDARD LIGHTNING IMPULSE VOLTAGES

Breakdown and volt-time characteristics of nonuniform field gaps in co mpressed SF6 gas were studied using a point-to-plane electrode arrange ment. Combined voltages, consisting of very fast transient (VFT) overv oltage and standard lightning impulse (SLI) voltage superimposed upon dc voltage were applied. The experimental gas pressure ranged from 100 to 500 kPa. The results presented show that the magnitude and the pol arity of the direct voltage, as well as the gas pressure influence the corona discharge and the breakdown characteristics. It was found that the breakdown voltages under the VFT are higher than the correspondin g values obtained under SLI voltages. At the higher gas pressures the volt-time characteristics of VFT fall below these for SLI.