Surge tank is a special container connected to the high voltage transformer tank. It is designed to compensate for oil volume changes occurring under the influence of temperature. Reducing the area of contact between insulating liquid and the air is an additional function, which protects insulating liquid from premature aging.
Usually surge tanks are installed in all transformers from 25 kVA and 6 kV voltage.
The volume of such devices is selected based on a simple requirement: a transformer must be filled with transformer oil in all weather conditions. The operating principle of the surge tank is quite simple: oil temperature increase causes its expansion, after which the oil is pushed through a tube from the oil tank into a surge tank. When its temperature decreases, oil returns into the oil tank.
Surge tank has a dehumidifier. The latter is filled with silica gel. This device is necessary to remove moisture from the oil.
Oil indicator (oil sight glass) is located on the outside of the container. It has markings of control temperatures -45°C, +15°C, + 40°C. These figures give an idea of the amount of oil contained in the surge tank when the transformer is disconnected. In fact, the oil indicator helps to control the level of oil when filling the transformer in the factory and during operation. On the opposite side, there is a slot through which all internal surfaces of the surge tank are cleaned and painted.