Transformer oil is a kind of mineral oil obtained by distillation and refining in natural petroleum. It is a liquid natural hydrocarbon which is pure and stable, has low viscosity, good insulation and good cooling. The mixture of the compounds is mainly composed of a compound such as an alkane, a cycloalkane saturated hydrocarbon or an aromatic unsaturated hydrocarbon. The transformer oil is a light yellow transparent liquid with a relative density of 0.895. Freezing point <-45 °C.
The main role of transformer oil:
(1) Insulation: Transformer oil has a much higher dielectric strength than air. The insulating material is immersed in the oil, which not only improves the insulation strength, but also protects against moisture.
(2) Heat dissipation: Transformer oil has a large specific heat and is often used as a coolant. The heat generated when the transformer is running causes the oil close to the iron core and the winding to be heated and expanded. Through the convection of the oil, the heat is dissipated through the radiator to ensure normal operation of the transformer.
(3) Arc-extinguishing effect: On the on-load tap changer of the oil circuit breaker and the transformer, an arc is generated when the contacts are switched. Because the transformer oil has good thermal conductivity and can contact a large amount of gas under the high temperature of the arc, a large pressure is generated, thereby improving the arc extinguishing performance of the medium, and the arc is quickly extinguished.
The performance of transformer oil usually has the following requirements:
(1) The transformer oil density should be as small as possible to facilitate the precipitation of moisture and impurities in the oil.
(2) The viscosity should be moderate, too large will affect the convection heat dissipation, too small will reduce the flash point.
(3) The flash point should be as high as possible, generally not lower than 136 °C.
(4) The freezing point should be as low as possible.
(5) The lower the content of impurities such as acid, alkali, sulfur and ash, the better, so as to avoid corrosion of insulating materials, wires, fuel tanks, etc. as much as possible.
(6) The degree of oxidation should not be too high. The degree of oxidation is usually expressed in terms of acid value, which refers to the amount of potassium hydroxide (mg) required to absorb the free acid in 1 gram of oil.
(7) The stability should not be too low, and the stability is usually expressed by the precipitate of the acid value test, which represents the oil's ability to resist aging.