During the operation of transformers, it is often required to add oil after taking certain samples. The oil should be prepared first before being added to the transformers oil system since it can accumulate moisture and gas while in storage.
If the transformer oil contains emulsified water, this water can be removed by a centrifuge. The cetrifuge method separates materials with different densities. When the mixture enters the rotating drum, it separates. Particulate matter forms the outer ring, and the oil forms the inside ring, with water in the middle. Separation is achieved by plates inside the centrifuge.
Operation of the centrifuge depends on the amount of water in the oil. In clarification mode, small amounts of water are separated and collected. If the amount of water in the oil is higher than 0.5% of the total volume, the centrifuge operates in purification mode with continuous removal of water.
The intensity of the processes in the centrifuge increases significantly with higher temperatures. However, temperatures in excess of 70ºС/158ºF may also cause oil oxidation. In many centrifuges the oil is processed in vacuum.
The main disadvantage of this method is the complexity of the equipment, its setup and the relatively low processing rate due to the need to clean out the drum from time to time.
Emulsified and some solved water can also be removed by special filter-presses. Transformer oil passes through a porous material which can trap moisture. One of the possible media is soft cardboard which absorbs water rather well. Such filters however, such can contaminate purified oil with paper fibers. Cellulose in the oil can decrease dielectric strength of the oil and increases dissipation losses.
The most efficient approach to the drying of transformer oil is the thermovacuum processing method. First the oil is subjected to vacuum and intensively emits gases and water vapor from the surface of the oil film. Then emission slows, and to intensify the process the oil surface area is maximized, while the thickness of oil film is minimized.
It is also possible to dry oil by ultrasonic cavitation. Oil under vacuum is subjected to ultrasonic waves. These ways cause formation of gas bubbles in the oil, and water evaporates inside the bubbles, which then rise to the surface to release the vapor. This method is still in development.