10 Method of cooling Transformer -Electronicsinfos

What is the importance of cooling in a transformer?

    10 method of cooling transformer 2024

    Transformers operate at high temperatures, which causes energy losses in the transformer. The energy losses in a transformer are due to Copper losses(I²R).

    if transformer temperature is increased up to threshold value their efficiency is disturbed.

    Cooling plays an important role in optimizing transformer performance. Effective cooling ensures that the transformer operates at its Maximum efficiency.

    When temperatures of the transformer are controlled, it increases the lifespan of transformers. we used a different method to cool down the transformer when their temperature are increased. 

    The method of cooling depends on the type of transformer and its working condition. Some other factors play an important role when selecting of cooling method. 
    Different types of cooling methods for transformers including

    Factors to consider when selecting a cooling method for transformers

    1. Regulatory Standards
    2. Transformer Type and Capacity
    3. Maintenance Requirements
    4. Environmental Conditions
    5. Load Variations
    6. Energy Efficiency
    7. Safety and Reliability

    10 Method of cooling Transformer (Dry + Forced)

    Dry Type Transformer

    1. Self-air cooled Transformer
    2. Air blast-cooled Transformer(AB)

    Oil Immersed Type Transformer

    1. Oil-immersed, self-cooled Transformer(ONAN)
    2. Oil-immersed, self-cooled & air-blast Transformer(ONAB)
    3. Oil-immersed, water-cooled Transformer
    4. Oil-immersed, forced oil-cooled Transformer
    5. Oil-immersed, self-cooled & water-cooled Transformer
    6. Oil-forced, air-forced cooled Transformer (ONAF)
    7. Forced oil, water-cooled Transformer
    8. Forced oil, self-cooled Transformer

    10 Method of cooling Transformer -Electronicsinfos

    Self-air cooled Transformer(1.5MVA- 3MVA)

    The capacity of a Self-air-cooled transformer is 1.5MVA to 3MVA. This type of transformer is cooled down by atmospheric air. These transformers are used in schools, hospitals, shops, shopping centres etc. 

    Self-air cooled transformer protects against humidity or resistance. The transformer core or coil is not enclosed in a container. The low-voltage instrument transformer is based on the self-air-cooled transformer.

    10 Method of cooling Transformer -Electronicsinfos

    Air blast-cooled Transformer(15 MVA - 35 MVA)

    To Cool down the transformer, Air is circulated with the help of a blower from bottom to top is called an Air blast-cooled transformer. This type of transformer is enclosed in a metal box with a capacity between 15 MVA to 35 MVA. 

    10 Method of cooling Transformer -Electronicsinfos

    Oil-immersed, self-cooled Transformer( UPto 30 MVA)

    This type of transformer is also called ONAN(Oil Natural Air Natural). It's widely used because of its simple working operation. The manufacturing capacity of this type of transformer is up to 30MVA. 

    This type of transformer is enclosed in a metal tank in which the transformer core and windings are immersed in insulating oil. In this type of transformer oil is circulated in the transformer naturally. 

    The small oil-immersed self-cooled transformer surface is plane but the large transformer surface is corrugated with external tubes. we also used radiators on large-capacity transformers. The main principle of this type of transformer is the convection method.

    10 Method of cooling Transformer -Electronicsinfos

    Oil-immersed(self-cooled & air-blast)Transformer(up to 60 MVA)

    Large transformers use combinations of self-cooled and air blast methods. The construction of this type of transformer is like an oil-immersed self-cooled transformer. 

    The main difference is just we used blowers with the main tank. This circulating air provides very fast cooling for the transformer. 

    The motor of the blowers is controlled with a thermostat automatic method. The thermostat on the motor circuit and operate the blowers. when the temperature reaches its normal state, the thermostat senses this temperature and turns off the motor.

    Oil-immersed, water-cooled Transformer( Upto 500MVA)

    Large self-cooled transformers are expensive, due to this we used an oil-immersed water-cooled method. The core and windings are totally immersed in insulating oil, but we circulate the water in the external tubes. 

    This type of transformer is used in the high voltage transmission lines. we prefer this method where water is available in excess quantity, and we achieve the head of water easily.

    The main disadvantage of this system is we need a water tube system in the transformer. Due to the leakage of water tubes, the insulation of oil deteriorates.

    Oil-immersed forced oil-cooled Transformer

    In this type of method, both the core and winding of the transformer are immersed in oil. A pump is also used to circulate the oil in external tubes, For this purpose, we use a pressure system that circulates the oil in tubes to provide efficient cooling. In this method,  

    we also used heat exchangers like automotive radiator type which consists of oil-to-heat exchanger tubes. The pump rotates the oil quickly up to down which cools down the transformer temperature quickly.

    Oil-immersed(self-cooled & water-cooled)Transformer

    In this method, The core and windings are immersed in the insulating oils. This is a combination of Self-cooled and water-cooled.

    Oil-forced, air-forced cooled Transformer(OFAF)

    In this method, the heat is exhausted from windings with the help of Blowers. we also use an oil pump system that circulates the oil in external tubes. This is a combination of oil-forced and air forces.

    10 Method of cooling Transformer -Electronicsinfos

    Forced oil, water-cooled Transformer

    This system is used where when we use water as a coolant. we know that water has high heat conductivity and specific heat so that's why we used this method widely. in this method, we used an oil pump between the transformer and heat exchangers. 

    when oil enters heat exchangers the pump increases the pressure of oil in the transformer. heat exchangers are directly connected to the transformer tank. 

    The pressure of cooling water is less than the oil pressure in the heat exchangers. we both separate tubes for oil and water to avoid leakage mixing of water and oil.

    Forced oil, self-cooled Transformer

    In this method, The oil is circulated by the force. For this purpose, we used radiators so oil is easily circulated in the tubes.


    The cooling of transformers is critical to ensuring their function efficiently and safely. The most common cooling methods include Oil Natural Air Natural (ONAN), Oil Natural Air Forced (ONAF), and Oil Forced Air Forced (OFAF). 

    ONAN is the simplest, relying on natural convection to circulate oil and dissipate heat, while ONAF and OFAF use forced air to enhance the cooling effect. 

    Proper cooling extends the life of transformers by preventing insulation degradation and maintaining operational integrity.

    FAQS(Frequently Ask Questions)

    What is the cooling of the transformer?

    Transformer cooling is the process of dissipating heat from the transformer to the surrounding environment using air, oil, or both.

    Why do transformers need cooling?

    Transformers generate heat due to energy losses during power transformation. There are different types of losses occur including magnetic losses (eddy current and hysteresis losses) and electrical losses (copper loss). Without cooling, this heat can cause insulation failure and other damage.

    What are the common methods of transformer cooling?

    The common methods include ONAN (Oil Natural Air Natural), ONAF (Oil Natural Air Forced), and OFAF (Oil Forced Air Forced).

    How does the ONAN cooling method work?

    In ONAN, natural convection circulates hot oil within the transformer, which then cools down as it releases heat to the atmosphere through the transformer's surface or attached radiators.

    What is the role of radiators in transformer cooling?

    Radiators increase the effective surface area for heat dissipation allowing for more efficient cooling in methods like ONAN.

    How does forced air to improve transformer cooling?

    In ONAF and OFAF methods, fans or pumps are used to force air or oil over the cooling surfaces.

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