Using a commercial partial discharge detector to investigate ac corona in air

A computer-based commercial partial discharge detector has been used to measure ac corona pulses in a point/cup electrode system, in air. 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, but slightly 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 from 11 kV (rising) to 14 kV (falling) for the negative half-cycle. Differences between the distributions for avalanches and streamers increase as the breakdown voltage is approached. Qualitative explanations of the patterns and behavior are given