Inhibited transformer oil

Transformer oils usually serve well for many years, and often for the entire life of the transformer, but trouble still occurs from time to time through oxidation of the oil, which causes sludging and the formation of acids. Oxidation may be retarded by using an oxidationinhibited oil. Available types of anti-oxidant, their properties, mode of action and influence on oil properties are discussed. For the evaluation of inhibited transformer oils, new and more informative oxidation tests are required, and such tests are briefly described in the paper. A description is given of some results obtained in the laboratory, in trials with small transformers under laboratory conditions and with transformers in service. It was possible to distinguish between contamination and deterioration of oil in service, and to assess the rate of decrease of oxidation stability of the oil. Questionswhich arise from the use of inhibited oils are discussed. It is concluded that, at present, the greatest value of inhibited oils will be in distribution transformers (particularly those which have given trouble because of oil oxidation) rather than in large transformers or in switchgear. With more practical experience of the superior oxidation stability of these oils, smaller and cheaper transformers may become possible. Further development work includes attempts to improve the correlation of laboratory test methods with service performance.