Increase of static electrification was experienced in aged transformers of both shell-form and core-form. The investigation disclosed increase of accumulated charge by aging of insulator (pressboard). For assessment of static electrification (SE) in aged transformers, accumulated charge evaluation by potential measurement becomes necessary instead of conventional electro-static charging tendency (ECT) of oil. As for the increase of SE, recent research disclosed that oxidation of sulfuric contents in oil, such as sulfide and sulfoxide, caused increase of high ECT of oil. Sulfoxide in pressboard also showed good correlation with accumulated charge on the pressboard. Sulfoxide in oil and pressboard can be a good indicator for the increase of SE. Further accumulation of the field data is important for development of diagnostic technology for SE in transformers.
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