Impulse breakdown voltages of ester-based transformer oils determined by using different test methods

Increasing interests on ester-based oils as potential alternatives to mineral oil, stimulate researches of oil properties and facilitate cross comparison among various studies. Lightning impulse breakdown voltage, one of the most important oil property parameters, is regarded as the criterion for large power transformer insulation design. In this paper, three different testing methods including rising-voltage method (refer to ASTM and IEC Standards), up-and-down method and multiple-level method, were employed to determine the lightning impulse breakdown voltage of two ester-based oils, Midel 7131 as synthetic ester and FR3 as natural ester. Meanwhile, mineral oil Gemini X was also tested as the baseline for benchmarking. The results clearly indicated that test methods have significant influence on breakdown voltage measurement values. Nevertheless, mineral oil showed higher lightning impulse breakdown voltage than ester-based oils irrespective of the testing methods being employed. © 2009 IEEE.