Practical Considerations for the Usage of Ester Fluids in Distribution Transformers

In 2018 Southern California Edison began utilizing distribution class transformers with natural ester fluid. In addition, they embarked on a program where units removed from service were retro-filled with ester fluids before reinstallation. It subsequently became clear that the dielectric characteristics of ester fluids significantly affected the conventional parameters used in condition assessment of distribution transformers. In response, SCE embarked on a research program where they conducted a series of controlled experiments including aggressive loading conditions. Two of these studies will be discussed at length. Mineral oil and ester units were subjected to one-year overload test periods, accelerated aged at up to 188% of nameplate rating. The results of the first-year cycle were previously published in [1]. The specimens were evaluated at specified intervals to extract dielectric response and insulation resistance data. This allowed SCE to trend changes in insulation behavior over time in response to varying degrees of overload. DGA and fluid quality tests were performed on each unit to correlate specific test parameters with load level and fluid type. These tests aim both to identify the aging markers and to validate the possibility of safe overload conditions.