Transformer Cooling Methods (All You Should Know)

Transformer Cooling Methods (All You Should Know)
distribution transformer

What is transformer cooling mean?

Transformer cooling is the process of transformer heat dissipating to the surrounding atmosphere using air, oil or both of them, to increase the transformer efficiency and prevent it from burning out due to excessive heat and temperature rise.

Why do transformers get hot?

Transformers get hot because, the energy losses that happen due to transforming power from one side to another.

These losses are magnetic losses i.e. Eddy current loss and  hysteresis loss and electrical loss i.e. Cooper loss. These losses appear in the form of heat dissipation.

When current passes through a wire, a cable or a winding it produces heat in normal conditions as losses. Transformers are not an exception, electrical transformers are mainly windings and an iron core.

When current passes through the winding it produces heat. The main source of heat in transformers are windings and core, which cause the temperature of the transformer to raise up. 

So its essential to use cooling methods for transformers to compensate the heat and control the transformer temperature rise.

  • For more information about What causes a transformer to burn read my detailed article here.
  • I have another article about transformer temperature rise, you can find it here.

Why Is Transformer Cooling Important?

If the heat generated in a transformer exceeds the allowable values, its temperature will increase, if no cooling method is used, the temperature rise of the transformer will cause the winding to burn out.

Losses in transformers produce heat, serious problems like insulation failure and short circuit could happen due to excess temperature in transformers.

So it is important for transformers to have cooling methods, to cool down windings through transformer oil. Transformer cooling method depend on its power rating.

Why should You check transformer temperature?

Its necessary to check transformer temperature for any unusual rises to make the right decision if any changes happen.

In my work, we have a transformer of 2500KVA. I made thermal imaging for it to make sure it is not too hot.

Once, I find its temperature higher than normal conditions. So, we regularly checked the transformer by the thermal camera to ensure it still safe to work.

The cooling system of this transformer had an issue. Its oil had water contaminates. We discovered that by making oil analysis. The solution of this issue was filtering the oil.

The below are the lessons from this true story:

  • All transformers has temperature rise.
  • Temperature rise should be within designed limits.
  • The cooling system is the main way to keep the transformer temperature in limits.
  • You should keep an eye on the transformers temperature. Any change should be analyzed.

Transformer Cooling Methods

Air, Oil and water are used in cooling the transformers. Based on the type of air and oil moving system, the transformer cooling system is named. The following are these cooling types full form:

  1. (ONAN) cooling, means : Oil Natural Air Natural
  2. (ONAF) cooling, means : Oil Natural Air Forced
  3.  (OFAF) cooling, means : Oil Forced Air Forced
  4. (OFWF) cooling, means : Oil Forced Water Forced
  5. (ODAF) cooling, means : Oil Directed Air Forced.

Let’s dive into details about these transformer cooling methods.

What does ONAN transformer mean?

ONAN transformer
ONAN Transformer, No Fans No pumps

ONAN transformer cooling stands for the the first letters of Oil Natural Air Natural. No fans or oil pumps are used in this method.

ONAN transformer (or ONAN cooling) is the simplest transformer cooling method at which the nature oil and nature air flow are used in cooling oil immersed transformers.

How does ​ONAN transformer cooling work?

Its clear from the name, ONAN, that the nature flow of the air around the transformer and the oil inside it is the main principle of ONAN cooling.

The heated oil and the heated air flows upward direction naturally. In four simple steps we can understand the working principle of ONAN cooling method of transformers.

  • The heat generated through transformer windings and core transfers to the oil
  • The oil transfers the heat to the transformer body and radiator, and the radiator is cooled by the natural air flow.
  • The heated oil in the transformer flows in the upward direction naturally, the vacant place is filled up by cooled oil which is coming from the radiator, this flow keeps happening continually
  • The heat from the oil will dissipate in the atmosphere due to, the natural air flow around the transformer.

ONAN cooling advantages and disadvantages

ONAN Advantages

This is the simplest cooling method, and has no devices, motors or pumps. So it has some advantages as follows.

  • Reliability, as no equipment are used in this method. No fans and no oil pumps, which in turns means no faults.
  • Simplicity, no special design required. No control circuits, no temperature sensors.
  • Reduced maintenance. Cooling fans and pumps need regular check and preventive maintenance.
  • And low cost. Due to no fans, pumps and maintenance. All these cost are reduced in this method.

ONAN cooling disadvantage

Compared to other transformers cooling methods, ONAN cooling method has a lower overall heat dissipation capacity.

Cooling fans and oil pumps increase the cooling efficiency in the other cooling methods.

ONAN is suitable for lower power transformers.

How does cooling affect transformer power?

If we have two electrical transformers, One is ONAN cooled, and the other is ONAF. Then the ONAF cooling transformer power rating can be increased when cooling fans are working.

If the power of the two transformers are 2500KVA. Then the ONAN cooling transformer power will be 2500KVA as maximum.

While the ONAF transformer power can be increased to be up to 3250KVA depending on the number of cooling fans.

ONAN vs KNAN transformer

ONAN and KNAN are both the same principle of natural cooling using oil and air. However ONAN transformers use mineral oil which is represented as O, while KNAN transformers use non-mineral oil which is represented by the letter K.

The application and the working environment of the transformer determine the oil to be used.

Mineral oil flash point is lower than 300°C, while non-mineral oil fire point is higher than 300°C, This makes non-mineral oil safe to use in hazardous locations where explosive atmosphere exist. Zone 1 and Zone 2  explosive areas.

The heat dissipation of mineral oil is higher than non-mineral oil. So the tank design in case of non-mineral oil, i.e KNAN transformer, is larger in size.

What is ONAF transformer?

Fans are forcing air flow
Fans are forcing air flow

ONAF, Oil Natural Air Forced, transformer cooling represents the cooling method of transformers using forced air flow with fans, and natural transformer oil flow, i.e no pumps, to enhance and accelerate the heat dissipation from the transformer.

ONAF is a cooling method for both oil immersed and dry type transformers.

For larger ratings, the generated heat increases and the cooling using ONAN is not enough.

Heat dissipation can be improved by applying forced air on the transformer surface. Fans are mounted near the radiator and are automatic or manually controlled.

The fans produce forced air on the transformer body and radiator to provide faster heat dissipation than natural flow of the air.

Transformer ONAN and ONAF ! What is the difference?

As mentioned above, ONAN stands for Oil Natural Air Natural. This is a simple transformer cooling method, it has no fans for air and has no pumps for the oil.

While ONAF stands for Oil Natural Air Forced, we use cooling fans to increase the cooling efficiency and to speed up the heat dissipation.

This means that the transformer cooling has fans for air. These fans are installed on the body of the transformer and force air flow to cool down transformer body.

By this way the power rating of the transformer is increased based on the number of fans in the ONAF system.

ONAF cooling increases the power rating of the transformer, on the other hand it needs maintenance and increases noise.




Oil Flow



Air Flow


Forced With Fans

Cooling efficiency



Transformer Power

One Rating Only

More than One Power Rating depending on the Cooling Fans Stages


The lowest cooling method ever


Loading Capacity


133% up to 167%


Not require

Requires Fans Maintenance


Which One to choose, ONAN or ONAF?

ONAF cooling is more efficient and faster than ONAN cooling method. The heat dissipation is faster and the transformer power is higher and more efficient.

You may ask, If ONAF is better than ONAN, Why do we use ONAN the first place? the answer is one word, cost.

For simple applications where the load is fixed and small, then only ONAN cooling is the best choice. No need to add additional cooling fans and pumps with all other costs related to them, such as control circuits and maintenance program.

Sometimes the application needs one transformer with more than one power rating.

In my working location, transformers with 3 or 4 power rating, depending on the cooling stages. i.e the number of fans working in the stage.

This ONAF transformer is the best choice for our application. As our loads are not fixed all the time. So when the loads are high, we operate more cooling stages to raise up its power.

 (OFAF) Oil Forced Air Forced transformer cooling:

OFAF represents, oil forced air forced transformer cooling class. It has oil pumps and air fans to make the transformer cooling faster and more efficient.

This is the most powerful transformer cooling method. It has pumps circulating oil through radiator, also air fans are used to force air flow to cool down the transformer body.

OFAF methods increase the transformer power rating.

(OFWF) Oil Forced Water Forced :

OFWF represents oil forced water forced. OFWF is a transformer cooling method using pumps to circulate oil through heat exchangers and water to dissipate heat from the heat exchangers.

Separate coolers cool down the heated water to be used in a closed cycle. 

(ODAF) Oil Directed Air Forced cooling

ODAF vs OFAF Transformer Cooling
ODAF vs OFAF Transformer Cooling

ODAF, is an improved OFAF electric transformer cooling method, to get the highest cooling efficiency from oil flow, by directing the oil to go through the windings then to the radiator.

The oil in OFAF, has a path inside the transformer, this path doesn’t allow the cooling oil to effectively cool down the windings.

Making a redesigned path to force oil to go through the windings then to the radiator to cool it down more efficiently is called Oil Directed (OD). While AF is using cooling fans as we discussed above.

What is Multistage Transformer Cooling System?

Electric transformer multistage cooling system is, a cooling system with cooling fans and oil pumps, work in stages depending on the transformer temperature.

Multistage cooling system is automatically controlled by temperature sensors based electrical panel.

The cooling stage is depending on the transformer temperature rise. The control system controls the number of fans and pumps that should work.

ONAN cooling is the simplest cooling method. It is also has no maintenance, and low noise level. But it is just for smaller transformers.

In case of larger transformers, fans and oil pumps are used to enhance the cooling. In this case the transformer power rating is enhanced too, and it has more than one power rating.

What does it mean a transformer has more than one power rating, 45/60/75 KVA ?

If we have a transformer has power rating of 45/60/75 KVA, and cooling method of ONAN/OFAF/OFAF, This means the transformer has multistage cooling system and its power rating varies according to the cooling stage. In this case the transformer power rating is:

  • 45 KVA in case of ONAN cooling. No fans or pumps working.
  • 60 KVA in case of operating the first stage of the ONAF cooling. A certain cooling number of units of fans and pumps works in this stage. In this stage no need for operating all fans and pumps.
  • 75 KVA when ONAF cooling is fully operated. i.e all cooling fans and pumps are fully operated. The transformer temperature rise is higher than the rise in the first stage.

Loading capacity for multistage cooling transformer:

Loading Capacity Type Of Cooling
100 % ONAN
100-133% ONAN-ONAF
100-133-167% ONAN-ONAF-ONAF
100-133-167% ONAN-ONAF-OFAF
125% ONWF
 167% OFWF

Dry type transformer cooling methods

The only applied cooling methods of dry type transformers are :

  • Air natural (AN). No cooling fans are used in this method, the natural air flow is enough to cool down the transformer.
  • Air forced (AF). Fans are used to forced air flow on the transformer body. This cooling method increases the transformer capacity up to 50%

There is no oil in dry type transformers so, its cooling depends mainly on air. To make the air cool the transformer winding, its design is larger in size than oil immersed transformer, to make air flow easier between the windings.

If the dry type transformer is located inside a closed enclosure, it should has a good ventilation should to circulate air and dissipate the heated air to the outside.

Dry type transformer cooling fans

Cross flow fans are used in forced air cooling, AF,  of dry type transformer to fast dissipate the heat of the windings of both high and low voltage sides.

Dry type transformer fans are single phase induction motor fans, with overload protection, ambient temperature range of about -25℃ ~+60℃ and, usually, 220v or 110v operating voltage. The fans increases the power rating of the transformer as well as the life span of it.

I have written an article answering important questions about transformer cooling. I highly recommend you read it here.

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