Transformer oil has the following main functions:

　　(1)The insulation effect of transformer oil is much higher than that of air in terms of insulation strength.Immersing insulation materials in oil can not only improve insulation strength,but also protect them from moisture erosion.

　　(2)Transformer oil has a high specific heat and is commonly used as a coolant for heat dissipation.The heat generated during the operation of the transformer causes the oil near the iron core and winding to expand and rise,and through the convection of the oil,the heat is dissipated through the radiator,ensuring the normal operation of the transformer.

　　(3)The arc extinguishing effect is applied to the on load tap changer of oil circuit breakers and transformers,and an arc is generated when the contacts switch.Due to the good thermal conductivity of transformer oil and the ability to separate 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 quickly extinguishing the arc.

　　There are usually the following requirements for the performance of transformer oil:

　　(1)Try to keep the density as low as possible to facilitate the precipitation of moisture and impurities in the oil.

　　(2)The viscosity should be moderate.If it is too high,it will affect convective heat dissipation,while if it is too low,it will lower the flash point.

　　(3)The flash point should be as high as possible,generally not lower than 135℃.

　　(4)The freezing point should be as low as possible.

　　(5)The lower the content of impurities such as acid,alkali,sulfur,ash,etc.,the better to avoid their corrosion on insulation materials,wires,oil 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(in milligrams)required to absorb 1 gram of free acid in oil.

　　(7)The stability should not be too low.The stability is usually expressed by the sediment of the acid value test,which represents the anti-aging ability of the oil.

　　Transformer oil is 25#insulating oil,with relatively stable physical and chemical properties,but it has a corrosive effect on rubber and promotes its aging.Due to its high viscosity,it can prevent rust,but it cannot remove rust.

　　Waste oil generally cannot be added to transformers anymore.If there are only a small amount of insoluble impurities or slight moisture,it can be used after filtration,but it must be inspected and qualified.