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Evaluation of Natural Ester Retrofilled Transformers After One Year of Continuous Overload

Despite the efforts of the utilities to designate properly sized transformers for the distribution network, transformers are routinely subjected to significant overloads during peak hours. This is especially common during hot summer days where the hot spot temperature value may exceed the design limits. With the utilization of natural ester liquid in distribution transformers, the rate of paper degradation is expected to be reduced in comparison to a traditional mineral oil unit at same conditions. The acceptable top oil temperature for natural ester is significantly higher than that for mineral oil. The improvements for both solid and liquid insulation enhance the ability to subject the distribution transformer to overload conditions. An experimental protocol was performed in 2017 to further investigate these properties of natural ester oil. In this document, the research work performed by Southern California Edison (SCE) is summarized and described in detail. For a period of one year, 11 transformers were subjected to thermal overload and thus, to an accelerated aging process. Transformers were loaded for cycles of 4 weeks, up to 145% of nameplate rating. After each loading cycle the transformers were cooled down to ambient temperature and tested using dielectric frequency response to document and trend the changes in the dielectric conditions of the complex insulation system. Challenges, lessons learned and recommendations are discussed herein.