The cooling methods of oil immersed power transformers can be divided into several types based on their capacity: oil immersed self cooling, oil immersed air-cooled, forced oil circulation air-cooled, forced oil circulation water-cooled, etc. The specific method is explained in detail by Qizheng Electric:
1. Oil immersed self cooling type
The oil immersed self cooling system does not have special cooling equipment. Oil circulates naturally inside the transformer, and the heat emitted by the iron core and winding is transferred to the oil tank wall or radiator through the convection of the oil. According to the size of transformer capacity, it can be divided into three different structures:
1) Smooth box wall. Transformers with very small capacities adopt this structure, where the casing is welded with steel plates and the walls are completely smooth;
2) Heat dissipation rib type box wall. Welding some heat dissipation ribs on the smooth box wall expands the area in contact with air, making it suitable for transformers with slightly larger capacity
3) Cooling with heat pipes or radiators. For transformers with larger capacity, in order to increase the cooling surface of the oil tank, several radiators are installed outside the tank. The radiators are a set of heat dissipation pipes with upper and lower headers, which are connected to the oil tank through flanges and are detachable components.
When the transformer is running, the oil in the tank is heated due to the heating of the iron core and winding. The hot oil rises to the top of the tank and then enters the heat dissipation pipe through the upper inlet of the heat dissipation pipe. The outer surface of the heat dissipation pipe comes into contact with the external cold air, cooling the oil. Cold oil descends inside the heat dissipation tube and flows into the lower part of the transformer oil tank from the lower end of the tube, automatically circulating oil flow to effectively cool the transformer core and winding.
The oil immersed self cooling system has a simple structure and high reliability, and is widely used in transformers with a capacity of less than 10000kVA.
Oil flow path of oil immersed self cooling transformer
1 fuel tank; 2 iron cores and windings; 3 heat dissipation pipes
2. Oil immersed air-cooled
Oil immersed air-cooled cooling system, also known as oil natural circulation or forced air-cooled cooling system. It installs one to several fans next to each radiator in the transformer oil tank, changing the natural convection of air into forced convection to enhance the heat dissipation capacity of the radiator. Compared with self cooling systems, it can improve cooling efficiency by 150% to 200%, which is equivalent to a 20% to 40% increase in transformer output capacity.
When the load is small, the fan can be stopped and the transformer can operate in a self cooling mode. When the load exceeds a specified value, such as 70% of the rated load, the fan can be automatically put into operation. This cooling method is widely used in medium capacity transformers above 10000kVA.
3. Forced oil circulation air-cooled
The forced oil circulation air-cooled cooling system is used for large capacity transformers. This cooling system is based on an oil immersed air-cooled system, with a submersible oil pump installed on the connecting pipeline between the main casing of the fuel tank and the radiator (also known as the cooler) with a fan. When the oil pump is running, it forces the oil in the tank to be sucked into the radiator from the top, and then enters the tank from the bottom of the transformer, achieving forced oil circulation. The cooling effect is related to the circulation speed of the oil.
4. Forced oil circulation water cooling
The forced oil circulation water cooling system consists of submersible oil pumps, oil coolers, oil pipelines, cooling water pipelines, etc. During operation, the oil in the upper part of the transformer is sucked in by the oil pump and pressurized, forcing the oil to pass through the oil cooler. Cooling water is used to cool the oil. Therefore, in this cooling system, the heat from the iron core and winding is first transferred to the oil, and the heat from the oil is then transferred to the cooling water. This cooling method has a good effect, but the sealing requirements of the transformer are very high, and the oil pressure during the cooling process must be higher than the pressure of the cooling water.