What Is the Difference Between ONAN, ONAF, and OFAF Transformer Cooling Systems?

Power and electrical transformers constantly deal with high current and voltage in power distribution networks, leading to iron and copper losses. As a result, they generate heat, demanding effective thermal management for safe and reliable operation throughout their service life. That is where the transformer cooling method comes into the picture.

Transformers, depending on their size, application, and load, use several cooling methodologies — ONAN, ONAF, and OFAF. Each method represents a different air and oil combination. Since these methods directly influence a transformer's efficiency, performance, and reliability, understanding them is essential.

In this blog, we will briefly understand the three transformer cooling methods and the basic differences between them.

ONAN is one of the most common and conventional cooling methods, widely used in oil-immersed transformers. The method is completely passive and does not depend on mechanical pumps or fans. Instead, it relies on thermal radiation and natural convection to dissipate heat from the windings and core. ONAN cooling systems require minimal maintenance and operate silently. These systems are ideal for transformers with a power capacity of up to ~25 MVA.

How Does the Method Work?

The transformer's windings and core heat the oil, which rises to the top of the tank due to the thermosiphon effect. The oil then flows into external radiators and cools by emitting heat into the surroundings — entirely without mechanical pumps or fans. Once cooled, the oil sinks back to the bottom of the tank, and the cycle repeats.

Advantages

• Low maintenance
• Low noise
• Proven and reliable
• Cost-effective
• No dependence on external power

Limitations

• Low cooling efficiency in confined conditions or hot climates
• Limited overload handling capacity
• Power limit of up to 25–30 MVA

ONAF combines active airflow with passive oil circulation, contributing significantly to improved thermal performance. This method enhances heat dissipation without changing or compromising the oil circulation mechanism.

ONAF systems are ideal for medium-to-large oil-immersed transformers with power capacities ranging between 10 and 60 MVA. This method allows transformers to manage higher power ratings or overloads by enhancing heat exchange — without the need for oil pumps. Reputable transformer suppliers typically offer ONAF-rated units as a standard option for industrial and utility applications.

How Does the Method Work?

The ONAF method is based on the ONAN principle, with the addition of electric fans mounted on the external radiators. Oil circulation inside the tank remains natural through convection; however, the fans significantly increase the rate of heat dissipation into the surrounding air. This ultimately allows the transformer to handle higher loads within the same tank size.

Advantages

• Improved cooling capacity
• No requirement for oil pumps
• Thermal margin flexibility
• Ideal for stepwise ratings

Limitations

• Slight increase in operational costs
• Additional maintenance due to fan addition
• Dependence on external power supply

The OFAF cooling method is typically used when conventional methods such as ONAN or ONAF prove insufficient — generally when a transformer's power capacity exceeds 40–60 MVA, or when the equipment is exposed to peak conditions and excessive loads.

This method enhances both external air cooling and internal oil circulation for superior thermal performance. As an active cooling system, OFAF enables transformers to maintain stable temperatures and operate at higher capacities. Leading industrial transformer manufacturers deploy OFAF cooling in large power transformers, generator step-up units, and specialised equipment such as the furnace transformer, which demands continuous high-load thermal management in steel and metals processing plants.

How Does the Method Work?

OFAF improves cooling by leveraging both forced air and forced oil circulation. Specially designed oil pumps drive the oil through internal ducts and external coolers. Simultaneously, fans push air across the cooler surfaces. This dual-forcing significantly improves heat transfer efficiency, offering precise control over thermal management.

Advantages

• Higher load capacity
• Extended insulation life
• Compact transformer size
• Stable operating temperatures

Limitations

• Complex design
• High energy consumption
• High maintenance and installation costs
• Requires continuous monitoring of fans and pumps

The cooling method you choose significantly impacts the transformer's lifespan, noise levels, footprint, thermal performance, and overall cost. All three methods discussed in this blog are designed to meet specific application scenarios.

To select the ideal method, consider the following:

• Load profile (variable or steady)
• Power rating (MVA)
• Space availability
• Installation environment (enclosed or open-air)
• Ambient temperature
• Operational reliability requirements

For instance, an auto transformer used in voltage regulation applications may be adequately served by ONAN or ONAF cooling, depending on its rated capacity and the duty cycle involved. In contrast, large industrial units require OFAF for sustained performance.

The ONAN method is ideal for low-to-medium-load applications. The ONAF method best suits medium-capacity transformers with variable loads. OFAF is preferred for enclosed or high-capacity installations where active thermal management is necessary.

ONAN, ONAF, and OFAF represent a clear progression in high-performance thermal management. Each method offers its own set of benefits and limitations while delivering efficient performance within its intended application.

No cooling system is universal. The right choice depends on the transformer's load, maintenance resources, budget, and operating environment. By carefully considering these factors, you can ensure a cost-effective and reliable transformer lifespan.

When it comes to choosing any of the cooling methods, working with an experienced and reliable transformer supplier who understands your requirements is essential. At TARIL, we can offer engineering expertise for several transformer cooling challenges.

Whether evaluating a cooling method or planning to upgrade to another, our specialists are available to guide you throughout.

Is it possible for transformers to operate above their ratings during emergencies?

Yes. However, transformers can function effectively only for short durations. Overloads of 150–180% are acceptable for periods under 30 minutes, provided the equipment is not already thermally stressed.

Why is cooling essential in power transformers?

Cooling in power transformers is essential to ensure a longer service life, safe operation, and reliable performance. Excessive heat generated during operation can shorten lifespan, cause failure, or degrade insulation capacity.

How is the ONAN cooling method different from ONAF?

The ONAN cooling method relies solely on passive air cooling and natural oil circulation, with no reliance on mechanical parts. The ONAF method, by contrast, includes radiator-mounted fans that improve heat dissipation capacity.

Is it possible for a transformer to switch between the ONAN and ONAF modes automatically?

A lot of transformers are built for dual-rating operations. The thermostatic controls start the cooling fans as soon as the oil temperature crosses a threshold (generally between 75-80 degrees Celsius) to switch to its ONAF capacity.

Similarly, fans turn off as soon as the temperature drops, switching the transformer back to the ONAN mode.

How does ambient temperature affect the selection of cooling systems?

Ambient temperatures have a significant impact on the transformer's ability to disperse heat. In regions with extreme temperatures of above 40 degrees Celsius, ONAN systems might be less effective unless a larger ONAN cooling system is used.

In such conditions, transformer suppliers generally recommend installing ONAF or OFAF systems to ensure operating temperatures stay within the safe limits.