Partial Discharges in Gaseous Cavities in Solid Dielectrics under Impulse Voltage Conditions

An investigation is described into partial discharge phenomena in artificial air-filled cavities of known dimensions in polyethylene. During initial tests, the 50 percent impulse-inception stress is much higher than the discharge stress calculated from Paschen's curve. It is independent of the impulse repetition rate, and is larger for 1/50-μs impulses than for 500/3000-μs surges; for both waveshapes the extinction stress is lower than the inception stress. After repeated discharges, the inception and extinction stresses are both reduced to the Paschen curve value. Initially, the 50 percent impulse-inception stress increases as the cavity diameter is decreased at constant depth and as the depth is decreased at constant diameter. After aging, the stress becomes independent of the cavity diameter. Details are given of the discharge magnitudes and time lags. It has been shown that a "main discharge" occurs usually on or near the crest of the surge, during which all or a substantial part of the cavity is discharged, followed by a number of smaller "reverse discharges" on the wavetail. When a discharge occurs on polarity reversal its magnitude tends to be appreciably greater but it can be reduced by suitable voltage conditioning. Experiments have also been made with the cavity adjacent to an electrode instead of totally enclosed in the dielectric.