A phase-related investigation of AC corona in air

A computer-based partial discharge detector has been used to measure AC corona pulses in air in a point-cup electrode system. The pulse height distribution was measured as a function of phase angle and pulse height for a range of voltages. The onset voltages for both positive and negative half-cycles are the same, and significantly lower than for the DC case, due to space charge effects. In all other respects the patterns for the two half-cycles are different: pulses only occur at the onset voltage (rising) for the positive half-cycle, but continuously for the negative half-cycle, except for the higher applied voltages which exhibit a glow discharge transition near the (negative) peak. Differences between the distributions for avalanches and streamers increase as the breakdown voltage is approached. Qualitative explanations of the patterns and behavior are given, together with calculations showing that space charge from one cycle can linger in the gap between half-cycles.