Partial discharges of small-air-gap in cast-resin insulation system

Partial discharges of small-air-gap for the molded insulation system has been investigated in view of the Paschen curve. It was found that Vs (partial discharge inception voltage: PDIV) in a smaller Pd (pressure times gap distance) region than so-called Paschen's minimum was determined in terms of glow-like discharge with micro-second order of discharge-current probably due to the ionic current for both MGM (Metal-Gap-Metal) and MGI (Metal-Gap-Insulation) electrode systems. In the Paschen's minimum and the larger Pd regions for MGM, the partial discharge behavior showed a streamer-like discharge current waveform with relatively short rise-time of an order of 10 ns or less and short discharge-duration. For the MGI electrode system, on the other hand, the discharge type was found to show glow-like discharge with small discharge current of less than few A. The glow-like discharge, Townsend and/or streamer discharges were also recognized by the high speed CCD camera photographs.