High voltage transformers and cables form integral parts of the electrical power distribution network and therefore their reliable operation is crucial to preventing costly outages. A liquid dielectric, typically mineral oil, is used in transformers whilst dodecylbenzene is gradually replacing mineral oil in paper/oil cable systems. As such, the oil serves as a convenient medium for sampling to assess plant health. In the current investigation, samples of dodecylbenzene cable oil were aged at 135 ºC under conditions where the oxygen diffusion into the oil was deliberately limited. The resulting aged samples were subjected to a battery of tests to determine changes in their physical, chemical and dielectric properties. Aging resulted in yellowing, oxidation, increased water content, increased dielectric loss, increased electrical conductivity and reduced electrical breakdown strength. Copper accelerated the aging process resulting in the formation of an insoluble precipitate; however, the inclusion of this material did not affect the electrical properties of the oil indicating that it is non-polar in nature. Instead, the deterioration of the dielectric properties was found to be correlated with the production of small polar molecular compounds, which remain dissolved within the oil, such as water, alcohols, ketones, carboxylic acids and aldehydes.

Abstract-In high voltage transformers a liquid dielectric, such as mineral oil, serves both as an electrical insulator (in conjunction with paper) and as a coolant. Similarly, in paper/oil cables mineral oil or dodecylbenzene in conjunction with paper serves as the electrical insulator. In such systems, the oil serves as a convenient medium for sampling to indicate plant health. In the current study dodecylbenzene was aged at elevated temperatures in the presence of air and copper. A battery of tests was then performed to assess the changes in physical, chemical and electrical properties. After aging the oils were yellowed and oils aged with copper showed oxidation, increased water content and the formation of a precipitate, whilst those aged in the absence of copper showed much less aging. Changes in electrical properties were noted such as increased dielectric loss, increased electrical conductivity and decreased dielectric breakdown strength. It was found that the presence of the precipitate did not affect the electrical properties of the oil.

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