Early stages of discharge development on insulating spacers in gases-a photographic study

In gas insulated substations (GIS) spacers are subjected to a large number of transient overvoltages and the transients associated with switching operations have very small rise-time (≈10 nanoseconds). An understanding of the early stages of discharge development should be useful in the design of GIS insulation system. As a part of this investigation, dedicated high speed streak photography techniques have been developed to study pre-discharge phenomena on spacers made from polytetrafluoroethylene (PTFE), acrylic and nylon. Since the discharge path may not always follow along the electrode axis, thus making streak photographic records difficult to interpret, it was decided to make a very shallow (0.5 mm) and narrow (1 mm) grove along the spacer axis. The arc was observed to propagate along the grove consistently. Although the breakdown voltages for different spacer materials varied significantly, the lowest flashover voltage being for PTFE spacers, the temporal development of the discharge on different materials was substantially the same in all cases. However, the flashover voltage with a spacer was lower than that for a plain air gap. In the present paper results of additional experiments in nitrogen (N ₂) and sulphur hexafluoride (SF₆) at different ambient pressures are described and discussed