Insulator flashover in SF6 under impulse voltage conditions

The authors describe a study of the impulse-flashover characteristics of insulating spacers in an SF₆-gas environment. Measurements have been made using 0.15/20 mu s and 0.15/1500 mu s waveforms for: plain cylindrical spacers; spacers with a defect at the insulator/gas/electrode triple junction; and spacers with metal inserts which shift the maximum field to the midgap region. The dependence of the surface charge accumulation on the rate of production of initiatory electrons is discussed. Metal inserts have been found to provide effective shielding of the triple junction, provided that the midgap field remains below the inception level. Unlike the case of the triple-junction defect where the region of enhanced field is highly localised, the inserts increase the field over a wide area so that the conditions for discharge propagation are satisfied at the inception level. This has been confirmed by comparison of the breakdown data with calculated values of discharge inception voltage.