Lightning properties of selected insulating synthetic esters and mineral oil in point-to-sphere electrode system

This article presents the results of the studies on lightning properties of two synthetic ester liquids and mineral oil. The measurements concern the standard lightning impulse voltage at both polarities and a point-to-sphere electrode system representing highly divergent electric field distribution. The studies were based on the IEC 60897 Standard wherein the standard recommendations were extended to include light emission registration using a photomultiplier technique. Comparisons of the lightning impulse breakdown voltages (LIBV), times to breakdown (TtB) and light waveforms were made. The results concerning LIBV and TtB showed that at negative polarity both the esters have almost the same lightning characteristics which are slightly below the characteristics of mineral oil. At positive polarity the results indicated an improvement. Regarding the light emission, the mineral oil tested seemed to be more resistant against the appearance of fast light pulses indicating the intense stepwise propagating discharges.