In-service ageing comparison study of natural ester and mineral oil filled distribution transformers

In 2011, Shaanxi Power Grid Corporation, a Chinese utility, launched the first batch of natural ester-filled transformers in China to investigate the effectiveness of environmentally friendly insulating liquid in operating transformers. After 7 years continuous operation, two healthy 10kV 200kVA units were selected for study, one is filled with a soybean based natural ester liquid, EnvirotempT FR3T Fluid, the other is filled with mineral oil. These two distribution transformers (DT) were operating in the same city for about the same time period. Each transformer is made by a different manufacturers but have similar designs, however, their loading profiles differed significantly. The transformers were taken off the grid in June of 2018. Routine tests of the transformers and oils show that the transformers are in good condition. In January of 2019, both transformers were disassembled to test the oil and winding paper insulation samples. The oil condition, degree of polymerization and water content of paper samples from both units were compared. The details of the teardown and laboratory analysis are presented in this paper. In view of this in-service 7 year test, the compatibility of NE liquid with transformer construction materials under normal loading rate condition is confirmed. Also, the results offer useful status information about oil-paper insulating system for performance diagnosis in daily operation.