Stability and energy of low current surface discharges on wet surfaces

Low current discharges are of concern in the context of predicting the lifetime of outdoor polymeric insulators. The nature of ac surface discharges changes markedly between peak currents of 0.5 mA and 10 mA: the electrical characteristics typically changing from unstable to stable behavior when the peak current increases above 1 mA. This change is important because of the implications of energy transfer between the discharge and the underlying material surface, and resulting changes to ageing mechanisms. In this paper, tests with artificial wind and rain have been conducted to investigate the processes by which unstable discharges become stable. Results show that under the impact of wind and rain, low current (<; 1 mA) unstable discharges can transform to stable discharges. Discharge length compression was observed for the stable dry-band discharges in the current range between 5 mA and 10 mA under similar experimental conditions. Analysis of electrical energy in the discharge indicates that the energy density per cycle rises significantly during both the unstable to stable discharge transition, and during discharge compression. It is clear therefore that ageing as a result of surface discharges on outdoor insulation must be thought of as a dynamic process, rather than one in equilibrium. The implication is that short-term adventitious events may control insulator life expectancy rather than long-term 'average' conditions.