Leader-like discharges in a crack running parallel to a plane of an underlying grounded conductor

Partial discharges (PD) developing in a crack or delamination were studied using a simulated crack. The configuration was similar to a rod-plane electrode system, with a rectangular block of insulator inserted between rod and plane (backing) electrodes. In order to provide a crack, a second block of insulator was placed a small distance s 50 mum < s < 1000 mum) above and parallel to the first to form a gap (crack) above the surface of the larger block. The length of the crack g was adjusted by inserting an insulating spacer at a distance g from the rod electrode (20 mm < g < 100 mm). "s" and "g" were the main variables. The tests were performed in air with four kinds of polymeric materials (PTFE, PMMA, PET, and epoxy resin). The experimental results showed interesting features, in particular, that leader-like discharges (LDs) of positive polarity develop through the crack over a long distance with high luminosity for s < 500 mum and g > 50 mm. Outside these values of "s" and "g", positive LDs do not appear. Negative LD appeared over the entire ranges of "s" and "g" studied. These characteristics for LDs were observed for all of the materials used and in the applied voltage range up to 95% of the breakdown voltage. Measurements of the charge distribution on the crack wall revealed a remarkably high-density accumulation of negative charges (ap -1.2 times 10 -8C/cm²) for s < 500mum and g > 50 mm, which also corresponded to the ranges where the positive LDs appear. Positive LDs are considered to be induced by the propagation of a positive streamer extending to the area where the high-density negative charge had accumulated. The accumulation occurs in cracks of width s < 500 mum, because in such cracks there is insufficient space for the neutralization of the negative accumulated charge, which is performed by the drift of positive ions to the area of the accumulated negative charge. It is suggested that high-voltage apparatu- s where the composite insulation is adopted must be designed such that its structure does not allow delamination to occur at the interface longer than 50mm, and running parallel to backing electrode