Breakdown and withstand strengths of ester transformer liquids in a quasi-uniform field under impulse voltages

This paper presents breakdown and withstand strengths of a synthetic ester, a natural ester and a mineral oil in a quasi-uniform sphere-to-sphere electric field under impulse voltages. The influences of impulse waveforms (lightning and switching), voltage polarities (positive and negative) and testing methods (rising-voltage, up-and-down and multiple-level) on the breakdown behaviour were investigated. The results indicated that breakdown voltages of both the ester liquids are comparable with those of the mineral oil under various test conditions. The withstand voltage at 1% breakdown probability was also obtained based on Weibull distribution fitting on the cumulative probability plot, which was built using the ~1000 impulse shots. It was found that the withstand voltages at 1% breakdown probability of the ester liquids are almost the same as that of the mineral oil. The results in this paper were further compared with other available test data in the literature. By comparing the performances between the ester liquids and the mineral oil, recommendations for insulation design were given to aid the application of ester liquids in high voltage large power transformers.