The main function of transformer oil:
（1） 绝缘作用 变压器油具有比空气高得多的绝缘强度。绝缘材料浸在油中，不仅可提高绝缘强度，而且还可免受潮气的侵蚀。
(1) insulation action transformer oil has much higher insulation strength than air. The insulating material immersed in the oil can not only improve the insulating strength, but also protect against moisture attack.
（2） 散热作用 变压器油的比热大，常用作冷却剂。变压器运行时产生的热量使靠近铁芯和绕组的油受热膨胀上升，通过油的上下对流，热量通过散热器散出，保证变压器正常运行。
(2) heat dissipation. Transformer oil has a large specific heat and is often used as coolant. During the operation of the transformer so that the heat generated near the core and winding up by the thermal expansion of the oil, oil on the convection heat radiator through the shed, to ensure the normal operation of the transformer.
（3） 消弧作用 在油断路器和变压器的有载调压开关上，触头切换时会产生电弧。由于变压器油导热性能好，且在电弧的高温作用下能分触了大量气体，产生较大压力，从而提高了介质的灭弧性能，使电弧很快熄灭。
(3) arc suppression will cause arcing when the contacts are switched on the load voltage regulating switch of oil circuit breaker and transformer. Because the transformer oil has good thermal conductivity, it can touch a large amount of gas under the high temperature of the arc, and produce great pressure. Thus, the arc extinguishing performance of the medium is improved, and the arc is soon extinguished.
(1) density as small as possible to facilitate moisture and impurity precipitation in the oil.
(2) viscosity should be moderate, too much influence convection heat dissipation, too small will reduce flash point.
(3) flash point should be as high as possible, generally should not be lower than 135 degrees centigrade.
(4) the freezing point should be as low as possible.
(5) the lower the content of acid, alkali, sulfur, ash and other impurities, the better to avoid their corrosion on insulating materials, wires, tanks and so on.
(6) the degree of oxidation can not be too high. The degree of oxidation usually expressed in acid value, it refers to the amount of absorption of potassium hydroxide free acid 1 grams oil required (mg).
(7) the degree of stability should not be too low, stability usually test sediment acid value, which represents the ability of anti ageing oil.