Motion of conductive particles and the effect on AC breakdown strengths of esters

This paper investigated the motion of a metallic cylinder particle of mm size under uniform AC field in three transformer liquids, including a mineral oil, a synthetic ester and a natural ester. The clean transformer liquids were also artificially contaminated by copper particles of μm size, and the relationship between the AC breakdown strengths and the amount of copper particles was determined. It is found that the breakdown voltages of esters are influenced by metallic particles to a less degree than mineral oil. The slower motion of metallic particles in esters of higher viscosities than mineral oil reduce the chance of breakdown, and this might account for the better performance of esters under AC stresses with the existence of metallic particles.