AC flashover characteristics on trapezoidal contamination models of actual insulators

The fundamental characteristics of an ac contaminated flashover were investigated by using a simplified model which consists of a trapezoidal unit and a series of units in order to simulate the current density distribution on actual insulators. It is found that the lowest flashover voltage decreases with an increase in the voltage build-up rate, and after passing a minimum, tends to increase. Experiments also show that, if the leakage length is equal, a disc insulator string model having a metal plate on the connected area of units tends to flashover at a lower voltage than a long rod insulator model having no metal plate. The results are discussed according to the characteristics of the ac reignition voltage on contaminated surfaces in previous reports. It is deduced that the accumulation of Na ions in the discharge space at flashover dominates the reduction of contaminated flashover voltage.