In order to ensure the quality of new oil and running oil, purification treatment is a common and important means. Transformer oil purification after purification can remove moisture on one hand, improve the dielectric strength of insulating oil and turbine oil demulsification degree, and reduce the oil degradation rate, on the other hand may also remove mechanical impurities that mix oil and oil after the degradation of the sludge generated by sediment, to avoid the oil system has been blocked due to bad heat dissipation, can prevent the abrasion of mechanical impurities on the equipment.

The pretreatment methods of transformer oil are generally divided into three kinds, sedimentation method, filtration method (pressure type and vacuum type) and centrifugal separation method.

Sedimentation method is also known as gravity sedimentation method, it is the use of turbidity liquid solid and liquid particles by gravity and sedimentation principle to remove impurities and water in the oil. Purification by sedimentation method is simple and easy, and does not need more and more complex equipment. But this method of purification is not complete, can only remove most of the water in the oil and can naturally settle down the mixture. Therefore, the general settlement method can only be used as a pretreatment. The pre-treated oil shortens the time, ensures the purification quality and reduces the purification cost.

Pressure filter paper filter method, pressure filter method using equipment called pressure filter, also known as frame plate oil filter. It is the use of gear oil pump pressure, so that the oil through the oil filter paper with adsorption and filtration, the transformer oil water, fine impurities and water-soluble acid substances removed, so that the transformer oil can be purified. The following points should be paid attention to when using pressure type oil filter.

Filter paper should be dried before use, so as to improve the performance of filter paper to absorb water. At the same time, the filter paper should be especially careful when baking to avoid fire. If the transformer oil contains a lot of water and impurities, the oil should be treated by sedimentation method or centrifugal oil filter, and then filtered by pressure type oil filter. During filtration, the fiber of the filter paper should be avoided from being brought into the oil. Within 5 ~ 10min of the beginning of oil filtration, the oil drawn out by the oil filter should be re-directed to the inlet of the oil pump for circulating filtration. In order to reduce viscosity and improve filtration speed and efficiency, the oil temperature should be increased to 40 ~ 45℃.

Vacuum filtration method is by means of vacuum oil filter, that is, transformer oil atomized in high vacuum and not too high temperature, out of the trace water and gas in the oil. Because the vacuum oil filter also has a screen, it can also remove some solid impurities. It is not only more economical than the pressure filtration method, but also has a high degree of purification and efficiency. It can release the trace water in the oil, maximizes the strength of the insulating oil, and reduces the oil consumption. When the flash point of transformer oil drops and precipitates combustible gas due to failure, the flash point can be restored by vacuum purification treatment.

Centrifugal separation method is the use of oil and water and impurities of the density of three different in the centrifuge rotating centrifugal force produced by different separation and purification. Among them, oil is the lightest and gathers in the center of the rotating drum, while water is more dense and dumped in the outer layer of the oil. The solid impurities in the transformer oil are dumped in the outermost layer, so as to achieve the purpose of separation and purification. Using this method to purify oil efficiency is relatively high, where the oil contains more water and mechanical impurities, can be used in this method. Especially to remove impurities such as slag and carbon particles in oil, the effect will be better, but this method also has limitations, that is, only suitable for removing impurities larger than oil density.