Transformer oils are widely used to surround coils in transformers. The transformer oil provides two major functions. The first is as an insulator and the second is as a heat transfer medium to carry heat from the coils to the cooling surfaces of the transformer.
Desirably, when a transformer is filled with transformer oil, it is operated with little or no maintenance or attention. Accordingly, transformer oils must not only have the right properties initially but they must resist change from aging, oxidation, thermal and electrical breakdown, or from other causes. Tranformer oils must also be useful through wide temperature ranges; specifically a transformer oil must have a low enough viscosity at low temperatures to maintain its ability to transfer heat from the transformer coils to the cooling surfaces of the transformer. The breakdown of transformer oil causes it to change its viscosity characteristics, causes it to deposit sludge, and causes it to create gaseous products which create explosion hazards.
The specific properties required in a transformer oil are difficult to attain in a single fluid. The use of additives and inhibitors to obtain these properties is possible, but it is most desirable to employ uninhibited transformer oils. Uninhibited transformer oils have been made by severly treating suitable charge stocks, first with concentrated sulfuric acid and then with clay. The sulfuric acid and clay-treated oils are adequate as transformer oils, but the process produces environmental problems in disposing of spent acid and spent clay. These problems, in fact, are so severe that ecological restrictions prevent the use of acid and nonregenerable clay.
Transformer oil composition is a compromise among various competing influences. For example, pure hydrocarbons are best to obtain good power factor and dielectric strength. For good impulse strength, pure paraffins and naphthenes would be best. For best gassing tendencey, high aromatic content is required, especially multi-ring aromatics. However, for good oxidation stability, naturally occuring sulfur compounds are needed. To obtain an oil meeting all specifications, sacrifices must be made in some properties to obtain adequate performance in others. Blending pure compounds is expensive, so treatments of naturally occurring materials to obtain the properties required for a useful uninhibited transformer oil are of great value.