Transformer insulation dry out as a result of retrofilling with natural ester fluid

Transformer cellulose insulation generates moisture as it degrades, primarily dependent on temperature. As moisture levels increase in paper/liquid systems, dielectric performance decreases. Moisture in cellulose insulation combined with higher temperatures accelerates the cellulose aging process. Multiple accelerated life tests in the lab demonstrated that the aging rate of cellulose can be significantly slowed when impregnated with natural ester fluid compared to mineral oil. The reduction and suppression of moisture buildup in cellulose is the primary reason for the reduced aging rate. This paper reviews chemical mechanisms involved based on laboratory studies and field data from fourteen power transformers retrofilled with natural ester fluid. The data provides support that insulation dry out can result from moisture migration and hydrolysis after retrofilling transformers with natural ester fluid. The concepts of moisture migration, hydrolysis and dry out are beneficial for new transformers as well and should work to maintain like-new dry insulation conditions.