Transformer Cooling Methods

Losses in the transformer are of the order of 1% of its full load kW rating. These losses get converted in the heat thereby the temperature of the windings, core, oil and the tank rises. The heat is dissipated from the transformer tank and the radiator in to the atmosphere. Transformer cooling helps in maintaining the temperature rise of various parts within permissible limits. In case of Transformer, Cooling is provided by the circulation of the oil. Transformer Oil acts as both insulating material and also cooling medium in the transformer. For small rating transformers heat is removed from the transformer by natural thermal convection. For large rating transformers this type of cooling is not sufficient, for such applications forced cooling is used.

As size and rating of the transformer increases, the losses increase at a faster rate. So oil is circulated in the transformer by means of oil pumps. Within the tank the oil is made to flow through the space between the coils of the windings.

Several different combination of natural, forced, air, oil transformer cooling methods are available. The choice of picking the right type of transformer cooling method for particular appilcatiion depends on the factors such as rating, size, and location.

Transformer Cooling Methods

Different Transformer Cooling methods are:

Air Cooling For Dry Type Transformers:

  • Air natural Type (A.N.)
  • Air Forced type (A.F.)

Cooling For Oil Immersed Transformers:

  • Oil Natural Air Natural Type (O.N.A.N.)
  • Oil Natural Air Forced Type (O.N.A.F.)
  • Oil Forced Air Natural Type (O.F.A.N.)
  • Oil Forced Air Forced Type (O.F.A.F.)

Oil immersed Water Cooling:

  • Oil Natural Water Forced (O.N.W.F.)
  • Oil Forced Water Forced (O.F.W.F.)

Air Natural Transformer Cooling:

This type of Transformer Cooling method applies to dry type transformer of small rating. The surrounding air in the vicinity of the transformer is used for cooling. Small transformers below 25kVA can be readily cooled by air natural cooling. However air natural cooling is also used for large dry type transformers. Cooling takes place by convection air currents.

Air Forced Transformer Cooling:

This type of cooling is provided for dry type transformers. The air is forced on to the tank surface to increase the rate of heat dissipation. The fans are switched on when the temperature of the winding increases above permissible level.

Oil Natural Air Natural Transformer Cooling:

This type of Transformer cooling is widely used for oil filled transformers up to about 30MVA. In Natural cooling, the oil in the tans gets heated due to the heat generated in the core and windings. The hot Oil flows upward and the cold Oil comes down according to the principle of convection. The oil flows in the transformer tank by the principle of natural convection hence this type of cooling is called Oil Natural Cooling. Heat is transferred from core and transformer windings to the oil and the heated oil is cooled by the natural air. Cooling area is increased by providing the cooling tubes.

Oil Natural Air Natural Cooling Transformer
Oil Natural Air Natural Cooling Transformer

Oil natural Air Forced Transformer Cooling:

In this method, air fans are mounted near the Transformer and the forced air is directed on to the cooling tubes to increase the rate of cooling. The fans are provided with automatic starting. When the temperature of the oil and windings increases above a permissible value the thermostats switch on cooling fans. This happens during heavy load condition and during higher ambient temperatures.

 In higher rating transformers where the heat dissipation is difficult this type of cooling is used. Fans are used to forced and air blast on radiators. Forced air cooling increases the heat dissipation rate. In this type of cooling oil circulates by natural convection and the blast of air is directed towards the better heat dissipation rate.

Oil Natural and Air Forced Transformer Cooling
Oil Natural and Air Forced Transformer Cooling

Oil Forced Air Forced Transformer Cooling:

Transformers above 60 MVA employ a combination of Forced Oil and Forced Air Cooling. Oil Natural Air Forced type of cooling is not adequate to remove the heat caused by the losses which is approximately equal to 1% of the transformer rating (0.6MW). In case of Forced Oil and Forced air cooling system a separate cooler is mounted away from the transformer tank. This cooler is connected to the transformer with pipes at the bottom and the top. The oil is circulated from the transformer to the cooler through the pump. The cooler is provided with the fans which blast air on the cooling tubes. This type of cooling is provided for the higher rating transformers available at the Substations and Power Stations.

Oil Forced Air Forced Transformer Cooling
Oil Forced Air Forced Transformer Cooling

Oil Forced Water Forced Transformer Cooling:

This type of cooling system needs a heat exchanger in which the heat of the transformer oil is given to the cooling water. The cooling water is taken away and cooled in separate coolers. The oil is forced through the heat exchanger. The oil pump pumps the oil from transformer to the het exchanger though the top pipes. Oil from the heat exchanger is pumped back to the transformer through the bottom pipe.

This type of cooling Is provided for very large transformers which have ratings of some hundreds of MVA (Generating Transformer will have very high rating and rating equal to the rating of the generator). This type of transformers is used in large substations and power plants.

Oil Forced Water Forced Transformer Cooling
Oil Forced Water Forced Transformer Cooling

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Comments 32 comments

stars439 profile image

stars439 6 years ago from Louisiana, The Magnolia and Pelican State.

You know what your talking about. Your knowledge is good. GBY.

S. Tivayki 5 years ago

Thanks again for this very useful hub.

I just want to correct a statement which you may have missed. For oil forced air forced cooling 1% of 60MVA does not contribute to 6kW.

Keep up the good work!

kiran111 profile image

kiran111 5 years ago Author

Thanks S. Tivayki, I corrected the mistake. Iam very Thankful to you...

John 5 years ago

Still wrong, should be 0.6MW.


Shane 5 years ago

Isn't oil which is forced through a cooler given the letter D, so that should be ODAF for the paragraph Oil Forced Air Forced Transformer Cooling, where a separate cooler is used? Not 100% sure, just asking, cheers

papu 5 years ago


prakash 4 years ago

more reference with diagram

Ruskin 4 years ago

I want to power point Present of Cooling system to Transformer.

kumaresh .m 4 years ago

mm k super

Syabonga Blose 4 years ago

I would like to thank you for providing with correct information which it did help me to complete my assignment on transformers.keep it up with good work!!!

No2nunu Amahle 4 years ago

I would like 2 gv a thank 2 d hub pages,it realy hlp m in dung m transfrmers assignment,rise up

Hemanth 4 years ago

If you don't mine can you specify the temperature for different transformer cooling methods like for air dry natural cooling will have certain temp change right that one.......... 4 years ago

why is it not all the topics described ...???

bhagat 4 years ago

hey friend is three phase delta to star supply step down or step up.

jug 4 years ago

it will be more nice if it is with 2D pictures with explanations

jagan DEEE BE 4 years ago

not in the diagram the more reference in the book


very usfull in the information

shiyamu 4 years ago

very useful but diagram is missing.

saravana 4 years ago

thanks for the information

Brett Hunter 4 years ago

If there are two pumps on the system, two heat exchangers, should both be pumping towards the heat exchangers, or could you have a system where the oil circulates at th bottom.

Prasath eee king 4 years ago

It's very useful and nic explanations:-)

devendra sen 4 years ago

thanks it is very usefull information

Raj 4 years ago

Thanx n very nice and easily method to understand


thanks it is very nice & easily method to understand


rifan 4 years ago

thanx man, it is usefull

Moh'd Alsmadi 4 years ago

Excellent Article

mahmoud 4 years ago

like about topic

msdee 4 years ago

usefull for sure. i really grasp so much hey.

jimmy 4 years ago

nice one

suneel 3 years ago

thank you for giving information &it is very use full

sam 3 years ago


what you know abaut cooling KNAN method ?

Abdul Majeed 3 years ago

very nic dear But

what you know abaut cooling KNAN method

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