Experimental Study on the Flashover Characteristics of Polluted Insulators under Short-tail Lightning Impulse Waveform

The pollution flashover accidents of insulator strings do harm to the safe operation of transmission lines. The paper aims to investigate the flashover characteristics of a 2-unit glass suspension string under 0.5/5 mu s short-tail impulses. Firstly, in order to represent the pre-heating effect of operation voltage, a composite test circuit was established. It can generate a combined voltage waveform consists of the power frequency ac voltage and a subsequent 0.5/5 mu s impulse. Secondly, adopting the artificial contamination test method with ESDD in the range of 0.010.1mg/cm(2), in the absence of the ac voltage, the U-50% of the 2-unit glass suspension string decreases with increasing of ESDD, and reached saturation when ESDD is larger than 0.025 mg/cm(2). Comparing to the clean insulator, U-50% was reduced by 25%. Thirdly, the result of composite test indicates that the prestressed ac voltage has little influence on the U-50% of the insulator. According to the observed flashover paths, most of the flashover was happened between surface of two insulators. The potential distortion caused by the pollutant deposits layer was beneficial to streamer inception. This might be the reason rather than the formation of pre-heated dry bands that leads to a lower U-50% when the polluted insulator is subjected to short tail impulses.