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Cooling method of three oil-immersed power transformers

The commonly used cooling methods for oil-immersed power transformers are generally divided into three types: oil-immersed self-cooling, oil-immersed air-cooled, and forced oil circulation.
a) Oil-immersed self-cooling: The heat dissipation process of the oil-immersed transformer is such that the iron core and the coil first transfer heat to the oil in the vicinity thereof to raise the temperature of the oil. The oil with a high temperature increases in volume and the specific gravity decreases, moving toward the upper part of the tank. Cold oil adds natural movement to the original location of the hot oil. The hot oil releases heat along the tank wall or the radiator pipe, and is carried away by the surrounding air through the tank wall or the pipe wall. After the temperature is lowered, it returns to the lower part of the fuel tank to participate in the circulation. Thus, due to the difference in oil temperature, the natural circulation of oil occurs. The hot oil flows downward from the upper part of the transformer tank along the inner surface of the radiator (without the radiator along the tank wall), and transfers heat to the air through the wall or tank wall during the downward flow, and the cooled oil The lower part of the radiator enters the fuel tank, and then rises through the oil passages, and the heat of the coil and the iron core is taken away during the ascending process, and the hot oil is again collected in the upper part of the fuel tank, so that the circulation is repeated continuously.

Oil-immersed self-cooling transformers rely on the radiation from the tank wall (or radiator wall) and the natural convection of the air around the transformer to carry heat away from the tank surface. In order to increase the heat dissipation surface, some of the transformer walls are corrugated, some are welded to the tubes, and some are equipped with heat sinks to promote oil convection. The low-voltage variation of distribution transformers and power plants is the cooling method.

b) Oil-immersed air-cooled type: The fan is installed on the radiator, and the air is accelerated by the method of blowing the fan, thereby increasing the heat dissipation capability. The blower can increase the convection heat dissipation by 8.5 times. After using the same transformer, the capacity can be increased by more than 30%.
 oil-immersed power transformers

c) Forced oil circulation cooling method: If the simple idea is to lower the temperature of the oil without increasing the speed of the oil flow, it will not achieve the desired cooling effect. When the oil temperature drops to a certain level, the viscosity increases, and the viscosity assembly deteriorates the heat dissipation effect. Artificially speeding up the flow of oil will speed up heat dissipation. The forced oil circulation cooling method is to add a power-oil pump that accelerates the flow rate of the oil in the oil passage.
Forced oil circulating air-cooled transformers are fitted with air coolers on the transformer tank wall or on separate supports. The oil in the cooler is cooled by a fan. In order to prevent oil leakage and air leakage of the oil pump, submersible pumps and submersible motors are widely used. The submersible pump is installed under the cooler, the suction end of the pump is directly installed in the first oil circuit (the cooler is multi-circuit), and the discharge end is connected to the second circuit through a pipe equipped with a flow relay. The function of the flow relay is to signal and put into the reserve cooler when the submersible pump fails and the oil flow stops.

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