A utility viewpoint on transformer oils

Electric utilities now have an all-time peak interest in the fundamental properties of transformer insulating oils. The history of the choice and development of these oils has been largely empirical and satisfactory, but recent and continuing changes are reducing confidence based on this history. The changes include the use of inhibitors and sealed transformers, higher dielectric stresses and operating temperature, and new sources of crude oils and methods of refining. More knowledge is now needed on the minimum tests necessary to be made on any oil at any time of its service life to indicate its remaining life under its own service conditions.