Analysis of Breakdown Voltage of Low Pour Point Synthetic Ester Insulating Liquids under Lightning Impulse Voltage of both Polarities

In this article, lightning impulse breakdown behaviour of two low pour point synthetic ester liquids is presented in comparison to a typical synthetic ester at both positive and negative polarities. Traditional mineral insulating oil has been also considered for reference purposes. A detailed breakdown behaviour analysis of the four test liquids under a non-uniform field (medium gap, point-plane electrode system) and quasi-uniform field (smaller gap, U-plane electrode system) is envisaged. The lightning impulse breakdown measurements based on the source voltage waveforms and light activity during the discharge process are presented. The Weibull breakdown failure rates and streamer velocity during the breakdown of different liquids for all the cases (+/- polarities and both electrode configurations) are reported in support of the discussions. In the case of non-uniform fields, the lightning breakdown voltage of the low pour point liquids is found to be higher than typical synthetic esters and is comparable to mineral oil under both polarities. While in the case of quasi-uniform field, the lightning breakdown voltage of the low pour point liquids is found to be lower than mineral oil and comparable to the typical synthetic ester under both polarities. These findings add to limited knowledge on the application of esters in cold countries and allow insulation designers to estimate the behaviour of the low pour point synthetic ester liquids under lightning conditions.