The influence of surface charge on lightning impulse breakdown of spacers in SF6

We have investigated the influence of surface charges on the discharge deve lopment across particle contaminated spacer surfaces under SF6 for lightnin g impulse (LI) voltages (1.2/50 mu s). Surface charges may be generated by de, ac or lightning impulse stress. The discharge characteristic shows a st rong reduction of the insulation strength if the applied voltage and the su rface charge have opposite polarities. The investigations were performed wi th a needle protrusion attached to the bottom electrode to give severe fiel d distortion. The bottom electrode was biased positively. The influence on the discharge process is observed by measuring the surface charge distribut ion and predischarge currents, The results reveal changes in streamer onset voltage, streamer to leader transition, and leader development.