Oxidation effects. Transformer oil fills power transformers and other high-voltage devices providing reliable isolation and removing heat from the hot parts of the equipment. In high-voltage circuit breakers the oil also acts as an arc extinguisher.
Generally, insulating oils work at high temperatures (70-80 °C). Temperature and air cause the oil to oxidize, resulting in insoluble precipitation of sludge, acids, water and other products of aging.
One of the products of aging is sludge; it may accumulate on the inner surfaces of the transformer parts, degrading oil circulation and, accordingly, reducing heat removal from hot parts. Also, the sludge reduces the dielectric strength of oil, and its presence is undesirable. If no action is taken, it may cause transformer failure.
Another undesirable component in the transformer oil is acid. It causes corrosion of metal surfaces of the transformer unit and destroys the solid insulation. Water in the oil lowers its dielectric strength.
Accordingly, the most important qualities for transformer oil are: oxidation stability, absence of water and solids, low pour point at low temperatures.
An important parameter of oil is its dielectric strength. It is the ability of the oil to withstand voltage without breaking down. The dielectric strength is determined by the breakdown voltage, which is the minimum voltage at which oil stops resisting and passes electric current, breaking down the insulator. The dielectric strength is affected by the presence of water in transformer oil. Even a small amount of water can significantly reduce the oil’s dielectric strength.
Another parameter is the dissipation factor; it arises when oil is exposed to the alternating magnetic field. In practice, this parameter is measured by the dielectric loss tangent. The smaller is loss, the lower is tangent.
Engineers, of course, are interested in transformer oil working in the same unit without replacement. On average, the oil life is around 5 – 10 years.