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What does 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 of 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 is windings and core, which cause the temperature of the transformer to raise up. 

So it’s essential to use cooling methods for transformers to compensate for 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, and serious problems like insulation failure and short circuits 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. The Transformer cooling method depends on its power rating.

Why Should You Check Transformer Temperature?

It’s necessary to check the 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 found its temperature higher than normal conditions. So, we regularly checked the transformer with the thermal camera to ensure it was still safe to work.

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

Below are the lessons from this true story:

• All transformers have a temperature rise.
• Temperature rise should be within designed limits.
• The cooling system is the main way to keep the transformer temperature within limits.
• You should keep an eye on the temperature of the transformer. 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 in 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 the ONAN transformer mean?

How does ​ONAN transformer cooling work?

It’s clear from the name, ONAN, that the natural 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 flow upward direction naturally. In four simple steps, we can understand the working principle of the 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 airflow.
• 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 airflow 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 is used in this method. No fans and no oil pumps, which in turn means no faults.
• Simplicity, no special design required. No control circuits, and no temperature sensors.
• Reduced maintenance. Cooling fans and pumps need regular checks and preventive maintenance.
• And low cost. Due to no fans, pumps, or maintenance. All these costs are reduced with this method.
  

ONAN cooling disadvantage

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

Cooling fans and oil pumps increase the cooling efficiency of 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 is 2500KVA. Then the ONAN cooling transformer power will be 2500KVA as the maximum.

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

ONAN vs KNAN transformer

Here’s a comparison of ONAN (Oil Natural Air Natural) and KNAN transformers in table format:

Aspect	ONAN Transformer	KNAN Transformer
Cooling Medium	Mineral oil	Kerosene (or other non-mineral oils)
Environmental Impact	May have a higher environmental impact due to mineral oil.	Often chosen for reduced environmental impact and fire safety.
Fire Point of Cooling Medium	Below 300°C	Above 300°C (higher fire point)
Cooling Process	Natural convection within the oil and natural air circulation around the transformer.	Natural convection within the kerosene (or other non-mineral oil) and natural air circulation around the transformer.
Safety	Generally considered safe but may pose a fire risk due to the lower fire point of mineral oil.	Offers enhanced fire safety due to higher fire point, making it suitable for hazardous areas.
Transformer Size	Typically has a smaller footprint as mineral oil has lower heat dissipation capabilities.	Usually occupies a larger physical footprint because non-mineral oils dissipate heat more effectively.
Temperature Tolerance	Components may have lower temperature tolerance due to the limitations of mineral oil.	Components can be designed for higher-temperature operation due to the superior cooling properties of non-mineral oils.
Applications	Suitable for a wide range of applications and transformer classes.	Commonly chosen for applications with stringent fire safety requirements, such as hazardous locations (Zone 1 and Zone 2).
Cost	Generally cost-effective.	Often more expensive than ONAN transformers due to the higher cost of non-mineral oils.
Standards	Complies with relevant industry standards.	Complies with relevant industry standards.

This table provides a side-by-side comparison of key aspects of ONAN and KNAN

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.

It’s noteworthy that KNAN transformers, typically occupy a larger physical footprint compared to mineral oil-filled transformers.

This is due to the superior heat dissipation capabilities of non-mineral oils, necessitating larger tank designs. Non-mineral oils, thanks to their ability to withstand higher temperatures, enable the creation of active transformer components designed to operate in more demanding conditions.

Non-mineral oils tend to be more expensive than their mineral counterparts. However, they offer distinct advantages tailored to the intended use of the transformers.

Non-mineral oils can be broadly categorized into two types: synthetic and natural. Both categories share the benefit of higher flash points, making them safer for deployment in hazardous areas designated as Zone 1 and Zone 2.

What is an ONAF transformer?

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 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 airflow to cool down the transformer body.

In 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.

	ONAN	ONAF
Oil Flow	Natural	Natural
Air Flow	Natural	Forced With Fans
Cooling efficiency	Lower	Higher
Transformer Power	One Rating Only	More than One Power Rating depending on the Cooling fan stages
Cost	The lowest cooling method ever	Higher
Loading Capacity	100%	133% up to 167%
Maintenance	Not require	Requires Fans Maintenance

Which One to choose, ONAN or ONAF?

ONAF cooling is more efficient and faster than the 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 in the first place? the answer is one word, cost.

For simple applications where the load is fixed and small, 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 programs.

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

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

 (OFAF) Oil Forced Air Forced transformer cooling

“OFAF” stands for “Oil Forced Air Forced” transformer cooling, which is a cooling method used in power transformers. In an OFAF transformer cooling system:

1. Oil: The transformer is immersed in oil, which serves as both an electrical insulator and a cooling medium. The oil helps dissipate heat generated during the transformer’s operation.

2. Forced Air: The “Forced Air” aspect indicates that the cooling process is enhanced by forcing or directing external air over the transformer’s cooling surfaces. Fans or blowers are used to facilitate this airflow, increasing the rate at which heat is dissipated from the oil and transferred to the surrounding atmosphere.

3. Forced Cooling: The combination of directed airflow and oil cooling creates an efficient forced cooling system. The forced air increases the heat transfer rate from the oil to the air, allowing the transformer to handle higher power ratings and heat dissipation requirements effectively.

OFAF cooling is commonly used in medium to large power transformers, where the combination of forced air and oil cooling allows the transformers to operate at their full rated capacity without overheating.

This method provides a more controlled and efficient cooling process compared to natural convection methods like ONAN (Oil Natural Air Natural) cooling.

(OFWF) Oil Forced Water Forced

(ODAF) Oil Directed Air Forced cooling

ODAF (Oil Directed Air Forced) is an improved transformer cooling method derived from OFAF (Oil Forced Air Forced). Its primary goal is to maximize cooling efficiency by optimizing the flow of cooling oil within the transformer.

In traditional OFAF cooling, the oil’s path inside the transformer may not effectively cool down the windings.

To address this limitation, ODAF introduces a redesigned oil flow path. In this method, the cooling oil is directed to flow through the windings first and then to the radiator.

This redirection of the transformer oil enhances its cooling capacity, allowing it to more effectively dissipate heat from the transformer’s core and windings.

Additionally, while ODAF employs directed oil flow to improve cooling, it may also incorporate forced air cooling.

Fans or blowers are used to force air over the transformer’s cooling surfaces, such as radiators or cooling fins.

This combination of produced oil flow and forced air cooling results in a highly efficient cooling process.

ODAF cooling is particularly beneficial for power transformers with higher power ratings and increased heat dissipation requirements.

By optimizing the oil flow path and enhancing the cooling mechanisms, ODAF allows transformers to operate at their full rated capacity without the risk of overheating.

What is a Multistage Transformer Cooling System?

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

The 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 also has no maintenance and a low noise level. But it is just for smaller transformers.

In the 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 if a transformer has more than one power rating, 45/60/75 KVA?

If we have a transformer that has a power rating of 45/60/75 KVA, and a cooling method of ONAN/OFAF/OFAF, This means the transformer has a 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 to operate 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 airflow is enough to cool down the transformer.
• Air forced (AF). Fans are used to force airflow on the transformer body. This cooling method increases the transformer capacity up to 50%

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

If the dry-type transformer is located inside a closed enclosure, it should have good ventilation 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 the dry-type transformers to 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 increase the power rating of the transformer as well as its life span it.

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

 

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