The primary function of electrical insulating oil is to cool and insulate, also to eliminate electric arc when switch off.

Under the action of heat and electromagnetic field, once transformer oil contacts with oxygen and gradually oxidized, it would generate alcohol, aldehyde, acid and the like, which finally form and separate out insoluble gelatin and grease sedimentation. These acidic materials would decrease transformer’s insulating performance, so transformer oil must be purified.

Water and gas in oil exist in two forms -- dissolved water and dissolved gas (below saturation point) exist in the majority of oil system and oil equipment; and free water, gas (above saturation point) exist in some oil.

Due to system leakage, bad airproof, oil in long-term heat load, oil storage in open-air and other reasons, water and gas in the air enter into the oil, which causes oil containing water and gas.

Untreated transformer oil usually contains water: 50 ~ 60ppm, gas: 10% ~ 20%. To make the transformer oil reach the application standards, the transformer oil must be treated in rated vacuum degree and temperature, so as to degas, dehydrate and remove mechanical impurities.

The electric property of oil depends largely on water and gas content, impurities and pollution level. Even new oil would suck in gas and water during storage and transport, so it needs to be treated before use. Meanwhile, oil also needs to be degassed regularly as its electric property would be affected by gas and water in different air temperature condition.