Accelerated aging studies of distribution transformers containing naphthenic and paraffinic mineral oils

To evaluate the behavior of paraffinic oils in long term service in transformers, twenty-eight oil-filled 25KVA 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 end- point tests were performed. When the transformers had exceeded five times the 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. The behavior of the transformers and the changes in oil quality during aging are discussed. The results of these tests are compared with the changes found in tube aging and oxidation tests. This work was part of a program sponsored by the Electric Power Research Institute.