Transformer cooling is the process of transferring heat from the transformer to the surrounding environment, typically using air, oil, or a combination of both. This helps maintain optimal operating temperature, improving efficiency and preventing damage or burnout from overheating.
Table of Contents
Why do transformers get hot?
Transformers get hot primarily due to energy losses that occur during their operation. These losses include:
-
Core Losses (Iron Losses):
These happen in the transformer’s core when it is magnetized and demagnetized repeatedly by the alternating current. They include:- Hysteresis Loss: Caused by the resistance of the core material to changes in magnetization.
- Eddy Current Loss: Small electric currents induced in the core, generating heat.
-
Copper Losses (Winding Losses):
These occur in the windings due to the electrical resistance of the copper (or aluminum) wires. The current flowing through the windings produces heat proportional to the square of the current (I²R losses). -
Stray Losses:
Caused by leakage magnetic fields inducing currents in nearby conductive parts, such as the transformer’s tank or structural components.
These heat-generating losses are why cooling systems are essential to keep transformers within safe operating temperatures.
Why Is Transformer Cooling Important?
Transformer cooling is crucial for several reasons:
-
Prevents Overheating:
Transformers generate heat during operation due to energy losses. Without proper cooling, excessive heat can damage insulation, windings, and other components. -
Improves Efficiency:
Operating at optimal temperatures reduces energy losses, enhancing the transformer’s efficiency and performance. -
Extends Lifespan:
Overheating accelerates insulation degradation, reducing the transformer’s operational life. Cooling systems help maintain longevity by keeping temperatures in check. -
Ensures Safe Operation:
High temperatures can lead to faults, fires, or complete transformer failure, posing safety risks. Effective cooling minimizes these hazards. -
Maintains Load Capacity:
Proper cooling allows transformers to handle higher loads without exceeding safe temperature limits, ensuring reliable power delivery.
Oil-Immersed Transformer Cooling Methods
Transformers are cooled using air, oil, or water. The cooling system is named based on how air and oil circulate. Here are the main types:
- ONAN: Oil Natural, Air Natural
- ONAF: Oil Natural, Air Forced
- OFAF: Oil Forced, Air Forced
- OFWF: Oil Forced, Water Forced
- ODAF: Oil Directed, Air Forced
What does the ONAN transformer mean?
ONAN, short for Oil Natural Air Natural, is a common cooling method in power transformers. Here’s how it works:
-
Oil: The transformer is immersed in oil, which acts as both an insulator and a cooling medium. The oil absorbs heat generated by the transformer’s core and windings.
-
Natural Convection: As the oil heats up, it rises naturally, while cooler oil flows down to replace it. This cycle continuously transfers heat away from the transformer components.
-
Air Cooling: The heat absorbed by the oil is then released into the surrounding air. Transformers equipped with fins or radiators increase the surface area to improve heat dissipation. Warm air rises, carrying the heat away.
ONAN systems are widely used in oil-immersed transformers with low to moderate power ratings. They rely solely on natural convection, requiring no mechanical fans or pumps, making them cost-effective, simple, and reliable. However, for high-power transformers, more advanced methods like ONAF (Oil Natural Air Forced) or OFAF (Oil Forced Air Forced) are preferred to improve cooling efficiency.
How does ONAN transformer cooling work?
The ONAN cooling method relies on the natural flow of oil inside the transformer and air around it, as suggested by its name (Oil Natural Air Natural). Here’s how it works in four simple steps:
-
Heat Transfer from Windings and Core:
Heat generated in the transformer’s windings and core is transferred to the surrounding oil. -
Oil Transfers Heat to Radiator:
The heated oil carries this heat to the transformer body and radiators, where it is cooled by natural airflow. -
Continuous Oil Circulation:
As the heated oil naturally rises, cooler oil from the radiator flows in to take its place, maintaining a continuous circulation. -
Heat Dissipation to Air:
The radiator releases heat into the atmosphere through natural air convection, keeping the transformer temperature under control.
This self-sustaining process ensures efficient cooling without the need for external mechanical devices.
ONAN cooling advantages and disadvantages
Advantages of ONAN Cooling
-
High Reliability:
Since ONAN uses no moving parts like fans or pumps, there’s minimal risk of mechanical failure. -
Simplicity:
The system doesn’t require complex designs, control circuits, or temperature sensors, making it easy to implement. -
Low Maintenance:
Without fans or pumps, routine maintenance is significantly reduced, saving time and effort. -
Cost-Effective:
The absence of additional cooling equipment lowers initial costs, operating expenses, and maintenance costs.
Disadvantages of ONAN Cooling
-
Limited Heat Dissipation:
ONAN has a lower cooling capacity compared to systems that use forced air or oil circulation, such as ONAF or OFAF. -
Restricted Application:
Suitable primarily for lower-power transformers, as it may not efficiently cool high-power units.
For transformers with higher power ratings or operating in hotter environments, enhanced cooling systems like ONAF or OFAF are preferred for better efficiency.
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 transformers operate on the same principle of natural cooling, but they differ in the type of oil used. ONAN transformers use mineral oil, which is represented by “O” and has a fire point below 300°C. In contrast, KNAN transformers use non-mineral oils (represented by “K”), such as synthetic or natural esters, which have fire points above 300°C. These non-mineral oils are often preferred for their higher flash points, making them safer in hazardous environments (e.g., Zones 1 and 2)
KNAN transformers tend to be larger than ONAN models. This is because non-mineral oils, while offering better thermal stability and allowing components to operate at higher temperatures, generally require larger tank designs to achieve equivalent cooling performance. Non-mineral oils are also more expensive than mineral oils, but their enhanced safety and performance in specific conditions often justify the cost
Overall, the choice between ONAN and KNAN depends on the transformer’s application and environmental requirements. KNAN is typically used in areas where fire safety and environmental considerations are crucial.
What is an ONAF transformer?
ONAF stands for Oil Natural Air Forced, a transformer cooling method designed for efficient heat dissipation. Here’s how it works:
-
Oil as a Cooling Medium:
The transformer is immersed in oil, which acts as both an electrical insulator and a heat conductor. The heat generated by the transformer’s core and windings is transferred to the oil. -
Natural Oil Circulation:
As the oil absorbs heat, it rises due to natural convection, while cooler oil moves in to replace it, creating a continuous flow of heat transfer. -
Forced Air Cooling:
To enhance the cooling process, external air is forced over the transformer’s radiators or cooling fins using fans or blowers. This accelerates the heat dissipation from the oil to the surrounding environment.
Key Benefits:
- Enhanced Cooling Efficiency: Suitable for transformers with higher power ratings.
- Flexibility: Combines natural and forced cooling to meet greater heat dissipation demands.
ONAF is commonly used in medium to large power transformers, ensuring reliable performance even under heavy loads.
Transformer ONAN and ONAF! What is the difference?
The main difference between ONAN (Oil Natural Air Natural) and ONAF (Oil Natural Air Forced) cooling systems lies in the method used to dissipate heat.
-
ONAN Cooling:
- Oil Natural: The transformer is immersed in oil, which naturally circulates and absorbs heat.
- Air Natural: The heat from the oil is transferred to the surrounding air through natural convection, without the aid of fans or additional mechanical components.
- This is a simple, reliable cooling method used in smaller to medium power transformers, with no moving parts like fans or pumps.
-
ONAF Cooling:
- Oil Natural: Similar to ONAN, the transformer is immersed in oil, and the heat is transferred from the core to the oil.
- Air Forced: Unlike ONAN, ONAF uses cooling fans to force air over the transformer’s body, enhancing the heat dissipation process.
- The addition of fans speeds up the cooling, allowing the transformer to handle higher power ratings and more demanding cooling requirements.
Key Differences:
- Cooling Efficiency: ONAF uses fans to enhance cooling, which increases its efficiency compared to the passive cooling of ONAN.
- Power Rating: ONAF transformers can handle higher power ratings due to the forced air cooling. The number of fans directly impacts the cooling capacity and transformer rating.
- Maintenance & Noise: ONAF systems require more maintenance due to the mechanical components (fans), and they also produce more noise compared to the simpler ONAN systems
In summary, ONAF is ideal for transformers requiring higher cooling capacity, but ONAN remains the more maintenance-free and quieter option for smaller transformers.
|
|
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 |
(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:
-
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.
-
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.
-
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
“OFWF” stands for “Oil Forced Water Forced,” which is a cooling method used in power transformers. In an OFWF transformer cooling system:
-
Oil: The transformer contains oil as a cooling and insulating medium. The oil helps dissipate heat generated during the transformer’s operation.
-
Forced Water: The “Forced Water” aspect indicates that the cooling process involves actively circulating water through cooling tubes or coils within the transformer tank. Water is used as a coolant to remove heat from the transformer’s components.
-
Forced Cooling: OFWF cooling combines the use of forced water circulation with oil cooling to efficiently dissipate heat from the transformer. Water, as a cooling medium, has high heat-carrying capacity, making it effective at absorbing heat from the oil and the transformer’s core and windings.
OFWF cooling is often employed in large power transformers, especially those with high power ratings and demanding heat dissipation requirements.
The forced water circulation, along with the use of oil as an insulating medium, allows these transformers to operate at their full rated capacity without overheating.
This cooling method offers enhanced control and efficiency in managing the transformer’s temperature during operation.
(ODAF) Oil Directed Air Forced cooling
ODAF (Oil Directed Air Forced) is an advanced transformer cooling method that improves upon the traditional OFAF (Oil Forced Air Forced) system. The main goal of ODAF is to increase cooling efficiency by optimizing the flow of cooling oil within the transformer.
In the conventional OFAF cooling system, the cooling oil may not effectively reach and cool the transformer’s windings. ODAF addresses this by redesigning the oil flow path. In this enhanced method, the oil is directed to flow through the windings first, ensuring better heat dissipation before moving to the radiator.
This redesigned oil flow significantly improves the cooling capacity, allowing it to remove heat more efficiently from the transformer’s core and windings. Additionally, ODAF may incorporate forced air cooling, where fans or blowers push air over cooling surfaces like radiators or cooling fins, further enhancing the cooling process.
The combination of directed oil flow and forced air cooling results in a highly efficient system. ODAF is particularly advantageous for power transformers with higher power ratings and increased heat dissipation needs. By optimizing oil flow and cooling mechanisms, ODAF ensures that transformers can operate at full capacity without the risk of overheating.
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.
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 |
I have written an article answering important questions about transformer cooling. I highly recommend you read it here.
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