INSULATOR FLASHOVER IN SF6 UNDER IMPULSE VOLTAGE CONDITIONS

The paper describes 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: (a) plain cylindrical spacers (b) spacers with a defect at the insulator/gas/electrode triple junction (c) spacers with metal inserts which shift the maximum field to the midgap region. It has been shown that, when the spacer is discharged between pulses, the effect of the triple-junction defect is independent of the polarity of the adjacent electrode, whereas, when charges are allowed to accumulate during the tests, there is a further large reduction in strength only for the case of a defect at the cathode. This has been explained on the basis of the dependence of the surface charge accumulation on the rate of production of initiatory electrons. Metal inserts have been found to provide effective shielding of the triple junction, provided that the midgap field remains below the inception level. However, 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.