The effect of humidity on the charge/phase-angle patterns of AC corona pulses in air

AC corona pulses have been measured using a computer-based commercial partial discharge detector. The pulse height distribution was measured in air as a function of phase angle and pulse height for a range of voltages using a point/cup electrode system. An earlier paper discussed the effect of space charge on the onset voltages for both positive and negative half-cycles and on the patterns obtained. In the work reported in this paper, the humidity was varied, and large changes were found in the patterns observed in both half-cycles. At high humidities the negative half-cycle corona largely ceased but a second positive corona onset appeared in the second quarter of the cycle. An explanation for these phenomena is proposed in terms of the effect of the production of negatively charged water molecule clusters. Their production results in a lower effective ionization coefficient and thus higher critical field, and their lower mobility results in a more effective negative space charge. The former reduces the positive space charge in the positive half-cycle thus allowing avalanches and streamers to develop during the second quarter of the cycle. The latter results in a stronger negative space charge during the negative half-cycle which suppresses the corona during that half-cycle.