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To evaluate the behavior of paraffinic oils in long term service in transformers, twenty-eight oil-filled 25KV A distribution transformers were subject to accelerated aging tests. Three paraffinic oils, a blend of a fourth paraffinic oil and a traditional naphthenic oil and the naphthenic oil alone were examined . The aging procedure was based on the IEEE Standard 345-72 test procedure. Periodically electrical endpoint tests were performed. When the transformers had exceeded five times {he loading guide life expectancy, they were taken to breakdown electrically and the failure modes were examined. Oil samples were periodically analyzed. In many cases, the transformers were exposed to additional oxygen using a controlled leak in order to increase the rate of degradation of the insulation system .[he behavior of the transformers and the changes in oil quality during 19ing are discussed. The results of these tests are compared with the .hanges found in tube aging and oxidation tests. This work was partofa program sponsored by the Electric Power Research Institute.

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