Transformer Cooling: All you need to know about transformer cooling methods and classes

Transformer Cooling: All you need to know about transformer cooling methods and classes

No transformer is perfect; each one experiences some inefficiencies that lead to losses, primarily manifesting as heat. If this heat isn’t effectively managed, the resultant high temperatures could lead to severe issues, such as the failure of insulation materials. Therefore, it’s essential for transformers to have a cooling system in place to prevent these problems.

What is a transformer cooling class?

A transformer cooling class defines the method and efficiency with which a transformer dissipates the heat generated during its operation. This classification is crucial as it determines how the transformer handles thermal stress and maintains its performance under varying electrical loads. Each cooling class is designated by a series of letters such as ONAN (Oil Natural Air Natural), OFAF (Oil Forced Air Forced), and others, which detail the type of coolants used (oil, air, water, gas) and whether the circulation is natural or forced. Understanding these classes helps in selecting the right transformer for specific applications, ensuring that it operates within its thermal limits.

What is the significance of transformer cooling class?

By choosing the appropriate cooling class, operators can ensure that transformers do not overheat and degrade prematurely, which is crucial in maintaining system reliability and reducing downtime. Moreover, the cooling class affects the physical size, installation environment, and cost of maintenance of the transformer, making it a key consideration in the design and operational planning of power distribution systems. This careful selection helps in achieving optimal performance and durability, tailored to specific environmental conditions and load requirements.

Types of transformer cooling methods

Dry-type transformers

As the name suggests, dry type transformers lack liquid coolants. Instead, these transformers rely on air or other dry mediums for cooling. Common cooling classes for dry-type transformers are:

  • AN (Air Natural): The AN cooling method operates purely on natural air convection, where warmer air around the transformer rises and cooler air moves in to take its place, creating a self-sustaining cooling cycle. This method is energy efficient, as it requires no additional power for fans or pumps, which also reduces potential failure points and maintenance needs. It’s ideal for environments that are not excessively confined and have moderate ambient temperatures, like outdoor installations or well-ventilated buildings.
  • AF (Air Forced): the AF method involves using fans to force air across the transformer’s surfaces, significantly enhancing heat dissipation compared to natural convection. This active cooling allows the transformer to handle higher power loads and operate effectively in smaller, more confined spaces. The forced air method is particularly suitable for challenging environments like industrial settings or enclosed spaces where natural airflow is insufficient to maintain safe operational temperatures.

Advantages of dry-type transformers

  • Safety and Environmental Considerations: Dry-type transformers do not utilize oil or other flammable liquids as cooling or insulating mediums, significantly reducing the risk of fire and environmental pollution from leaks or spills. They benefit from transformer cooling systems that use air circulation. This feature makes them particularly advantageous for installations within buildings, such as schools, hospitals, and commercial complexes, where safety and environmental sustainability are paramount. The absence of hazardous materials also simplifies compliance with environmental regulations and enhances the transformer’s safety profile, making it a preferred choice in environmentally sensitive areas.
  • Reduced Maintenance: The lack of liquid coolants in dry-type transformers means there are no risks of leakage, which can be a common issue with liquid-filled transformers. This characteristic not only eliminates the need for regular monitoring and maintenance of coolant levels but also reduces the overall maintenance demands. Consequently, dry-type transformers are generally less costly to maintain over their operational life. They provide a reliable power supply solution with minimal downtime, which is particularly valuable in settings where constant power is essential.
  • High Resistance to Pollution: Dry-type transformers are well-suited for operation in environments with high levels of dust and chemical contaminants. Unlike liquid-filled transformers, where pollutants might compromise the insulating fluid, dry types are designed with materials that resist contamination. This resilience makes them ideal for harsh industrial environments, including manufacturing plants, mining operations, and areas with high airborne particulates. Their robust design ensures that external environmental conditions have a minimal impact on performance, thereby maintaining consistent operational efficiency and extending the equipment’s lifespan.

Liquid-filled transformers

Liquid-filled transformers use a fluid—typically oil or a synthetic insulating liquid—as both a coolant and an insulator. This setup allows for more effective heat dissipation and enables these transformers to handle higher power capacities than their dry-type counterparts. The most common cooling classes for liquid-filled transformers are:

  • ONAN (Oil Natural Air Natural): This method utilizes the natural convection of oil, which heats up, becomes lighter, rises, and passes through radiators where it cools by releasing heat to the air. The cooled oil then sinks back to absorb more heat, creating a continuous, energy-efficient cycle. This process is suitable for smaller or medium-sized transformers in environments where energy efficiency is a priority, though it offers less cooling capacity than forced systems.
  • ONAF (Oil Natural Air Forced): In this cooling class, while the oil circulates naturally thanks to the thermal convection within the transformer, air is forced over the cooling surfaces by fans. This combination enhances the heat dissipation capacity compared to entirely natural systems like ONAN. ONAF is particularly useful in medium to large transformers where additional cooling is required to handle increased electrical loads without relying entirely on mechanical pumping systems for the oil.
  • OFAF (Oil Forced Air Forced): OFAF uses pumps to circulate the oil and fans to move air, maximizing the cooling efficiency. This method is suitable for very high-capacity transformers where both the oil’s enhanced circulation and the increased air flow are necessary to effectively remove the large amount of heat generated. This class is often chosen for critical applications where high performance and reliability under heavy loads are essential.
  • OFAN (Oil Forced Air Natural): This method uses a pump to force oil circulation while relying on natural air convection for cooling. The forced circulation of oil ensures better heat transfer from the core and windings to the oil, while the natural air cooling is used to dissipate heat from the oil to the environment. OFAN is a compromise between fully natural systems and those that require extensive mechanical assistance, offering improved cooling effectiveness with somewhat reduced mechanical complexity and energy usage compared to OFAF.
  • OFWF (Oil Forced Water Forced): In this system, oil and water are both circulated mechanically to manage high heat loads. The oil circulates through the transformer, absorbing heat, and transfers it to water in a heat exchanger. The water, carrying the heat, is then cooled externally, often using cooling towers. This method supports extremely high-power transformers, providing excellent cooling efficiency and minimizing thermal stress for enhanced reliability and extended life in heavy-duty applications.

Advantages of liquid-filled transformers

  • Higher Efficiency and Capacity: Liquid-filled transformers offer superior cooling efficiency thanks to the high heat capacity of the liquid used, typically mineral oil or silicone-based fluids. This efficient heat transfer mechanism allows these transformers to handle higher voltages and capacities compared to their dry-type counterparts. The enhanced cooling efficiency ensures that the transformer can operate closer to its theoretical performance limits without the risk of overheating. This capability is particularly valuable in areas where high power output is required, such as in industrial settings or dense urban power grids, enabling reliable and consistent power distribution even under peak load conditions.
  • Compact Design: Liquid-filled transformers can be designed to be more compact for the same power rating than dry-type transformers. The liquid coolant’s ability to absorb and dissipate heat more effectively allows for tighter winding designs and reduced physical spacing requirements within the transformer casing. This space efficiency makes liquid-filled transformers ideal for installation sites with limited space, such as urban substations or inside buildings. The smaller footprint also potentially reduces the costs associated with land acquisition and site preparation for power infrastructure projects.
  • Longevity and Reliability: The robust cooling system of liquid-filled transformers plays a critical role in reducing the thermal stress on internal components, such as the core and windings. By maintaining a lower operational temperature, the degradation of insulation and other sensitive materials is significantly slowed, thereby extending the transformer’s operational life. Furthermore, the consistent and effective management of heat enhances the overall reliability of these transformers, making them a preferred choice for critical infrastructure setups where failures can lead to significant disruptions. This reliability is crucial for hospitals, data centers, and financial institutions, where power stability is essential for operations.


Understanding the various transformer cooling methods and classes is essential for ensuring optimal performance, safety, and longevity of transformers in diverse operational environments. Whether choosing between dry-type or liquid-filled transformers, each cooling class offers distinct advantages tailored to specific needs and conditions. By carefully selecting the appropriate cooling system, operators can enhance efficiency, reduce maintenance costs, and ensure reliable power distribution in both industrial and urban settings. Ultimately, the right cooling strategy is fundamental to maximizing transformer life and maintaining system integrity under all operating circumstances.

At TTES, we have over 100 combined years of experience manufacturing, maintaining, and repairing transformers. If you’re looking for industry-leading lead times of just 20 weeks on average, don’t hesitate and reach out to us for a free quote!

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