EFFECT OF SURFACE-CHARGE ON IMPULSE FLASHOVER OF INSULATORS IN SF6

The effect of surface charge on the impulse flashover of short spacers in SF6 has been investigated. A corona source was used to deposit linearly or uniformly distributed monopolar charge, or bipolar charge with polarity corresponding to the adjacent electrode (homocharge) or of opposite polarity (heterocharge). A split-electrode arrangement allowed scanning of the entire spacer surface by a capacitive probe. With a heterocharge distribution, which facilitates both inception and propagation of the discharge, the flashover level is reduced by as much as 50% for sinusodially distributed charge of peak density 40-mu-C/m2. With monopolar charge, only positive charge near the cathode has any significant effect on breakdown, the reduction in impulse strength being approximately 35% at 40-mu-C/m2. Tests with metal inserts showed that, although the shielding effect is reduced in the presence of heterocharge, the impulse strength of spacers with inserts is still superior to that of plain spacers for the same charge density. The possible effects of surface charge on flashover of longer spacers is discussed in terms of a transition from a streamer to a leader type of breakdown mechanism.