Evaluation of alternate mineral oils for use in transformers and other electrical apparatus

Since World War I, electrical insulating oils for transformers and allied electrical equipment have been refined from naphthenic petroleum crude oil stocks. The resulting oils have had acceptable dielectric, mechanical and heat transfer properties and little or no wax formation occurs at temperatures down to -40°C. Usable naphthenic crude oils appear to be coming into short supply and there is a real possibility of a short fall in supply of the traditional oils in the 1980´s. This report describes the results of a program directed toward evaluation of potential replacement liquids, particularly other mineral oils. Paraffinic petroleum crude oils occur in much greater quantities and can be refined by much the same techniques as naphthenic oils. They are promising sources of insulating oils - if the resulting fluids meet the necessary technical requirements. Paraffinic oils are complex mixtures of hydrocarbons, just as are naphthenic oils. In general, the hydrocarbons present in paraffinic oils are the same as those present in naphthenic transformer oils. The relative amounts do vary. The similarities and differences in the composition of the oils evaluated are discussed. Electrical insulating oils are often evaluated by standard tests intended to determine their functionality in the actual application. The results of these standard tests on experimental samples supplied by refiners are encouraging. Not all samples meet all of the usual requirements for fresh oils, but it does appear that, by proper refining, fluids can be made which meet the present standard requirements at typical ambient and operating temperatures. Not all standard tests actually evaluate the functionality of oils. Naphthenic and paraffinic oils are compared in additional tests more closely related to actual applications. Paraffinic oils can form wax at temperatures above 0°C. Dewaxing can be done, but becomes progressively more expensive as the temperature at which wax forms is decreased. The samples of paraffinic oils evaluated in this program have been treated to give a variety of low temperature characteristics. The low temperature behavior of these samples is discussed and the consequences to the operation of electric apparatus are considered.