What cooling methods are used in power transformers (ONAN, ONAF, OFAF, etc.)?

Cooling is a critical aspect of power transformer operation. Excessive heat can degrade insulation, reduce efficiency, and shorten service life. To maintain optimal operating temperatures, power transformers use various cooling methods based on the application, size, and load requirements. These methods are identified by international letter codes such as ONAN, ONAF, OFAF, and OFWF, each representing different combinations of oil and air (or water) circulation. Understanding these cooling systems helps ensure the safe and efficient operation of transformers.

Why Is Cooling Important in Power Transformers?

Power transformers handle high voltage and current, which inevitably generates heat due to electrical losses in the windings (copper losses) and the core (iron losses). Without proper heat management, the transformer’s insulation degrades, thermal stress accumulates, and failure risk increases exponentially. Efficient cooling ensures that the transformer operates safely, reliably, and within its thermal limits throughout its service life.

Cooling in power transformers is critical to maintain safe operating temperatures, prevent insulation degradation, avoid thermal overloads, and extend service life. Transformers dissipate heat through their cooling systems—oil circulation, radiators, fans, and pumps—which help regulate winding and core temperatures. Without cooling, the risk of breakdowns, fire, and reduced lifespan increases significantly.

Cooling systems are a fundamental part of transformer design—not an optional accessory.

Why Transformers Generate Heat

Cooling System Functions

Types of Cooling Methods

Impact of Overheating

Industry data shows each 8–10°C rise above rated temperature halves insulation life.

Key Cooling System Components

Real-World Example – 40 MVA Grid Transformer

• Type: ONAF-cooled 132/33 kV transformer
• Site: Urban substation, ambient 40 °C
• Challenge: Load surge during peak summer caused top oil temp to reach 80 °C
• Solution: Auto fan activation cooled unit in 12 minutes; temp stabilized at 62 °C

Outcome: Transformer continued full-load operation with zero derating, no thermal alarm triggered—highlighting cooling’s critical role.

What Does the ONAN Cooling Method Mean in Power Transformers?

ONAN is the most common and foundational cooling method used in oil-immersed transformers, especially those with small-to-medium power ratings. It is entirely passive, meaning no mechanical fans or pumps are required, and it relies solely on natural convection and thermal radiation to dissipate heat from the transformer core and windings.

ONAN stands for Oil Natural Air Natural, a cooling method where transformer oil circulates naturally due to heat-induced density differences, and heat is dissipated from radiators into the ambient air by natural airflow. ONAN systems operate silently, with minimal maintenance and are ideal for transformers up to \~25 MVA.

This method is reliable, cost-effective, and widely used for both distribution and utility transformers.

ONAN Cooling: How It Works

This thermo-siphon effect drives oil flow—no moving parts involved.

Advantages of ONAN Cooling

Limitations of ONAN Cooling

Application Scenarios for ONAN Transformers

Typical ONAN Nameplate Indication

ONAN transformers are often designed with temperature rise ≤55/65 °C under IEC 60076.

Real-World Case – Utility Distribution Transformer

• Rating: 11/0.4 kV, 160 kVA, ONAN cooled
• Installation: Pole-mount in rural feeder line
• Climate: 38 °C summer peak, dry air
• Performance: No thermal alarms over 7 years, 99.5% uptime

ONAN cooling proved adequate and maintenance-free, requiring no external power or intervention.

What Is the ONAF Cooling System and When Is It Used?

As power ratings increase and operating loads become more demanding, natural air cooling (ONAN) becomes insufficient to maintain safe transformer temperatures. In such cases, the ONAF cooling system—Oil Natural Air Forced—is employed to enhance heat dissipation without altering the oil circulation mechanism. It combines passive oil flow with active air flow, offering a smart step-up in thermal performance.

ONAF (Oil Natural, Air Forced) is a transformer cooling method where oil circulates naturally through the transformer core and windings, while external cooling is enhanced by forced air—typically fans mounted on radiators. ONAF enables transformers to handle higher power ratings or overloads by improving heat exchange without requiring oil pumps.

It is commonly used for medium-to-large oil-immersed transformers between 10–60 MVA, especially in environments with variable loads or limited space for larger radiators.

How ONAF Cooling Works

Only the air is forced—the oil continues to flow naturally due to thermal gradients.

Advantages of ONAF Cooling

Typical Applications of ONAF Transformers

Dual-Cooling Rating Example

Fan Control and Automation

Fans are usually powered by a separate auxiliary supply (AC or DC panel).

Real-World Case – Urban Grid Transformer

• Site: 33/11 kV utility substation
• Transformer: 25/33 MVA ONAN/ONAF, Dyn11
• Cooling: 12 axial fans with thermostatic relays
• Climate: 43 °C ambient, summer peak loads
• Performance: Maintained <55 °C oil temperature under 95% loading

Enabled reliable operation during overload events without derating, extending service life and reducing maintenance cycles.

How Does the OFAF Cooling Method Work in Power Transformers?

As transformer ratings climb above 40–60 MVA or when they are subject to continuous high loading or peak conditions, traditional cooling methods like ONAN or ONAF may not provide sufficient heat removal. In such scenarios, OFAF (Oil Forced, Air Forced) cooling is used to dramatically enhance both internal oil circulation and external air cooling—resulting in significantly improved thermal performance.

The OFAF (Oil Forced, Air Forced) cooling method uses oil pumps to circulate insulating oil through the transformer windings and radiators, while external fans blow air across radiator surfaces to accelerate heat dissipation. This active cooling system allows transformers to operate at higher capacities and maintain stable temperatures under continuous or peak loads.

It is the standard choice for high-capacity power transformers (≥60 MVA) in grid, industrial, and generation settings.

How OFAF Cooling Operates

This dual-forced circulation ensures maximum heat transfer from the windings to the environment.

Key Components in OFAF Systems

Benefits of OFAF Cooling

Typical Use Cases for OFAF Cooling

Dual-Cooling Nameplate Example

Pumps and fans may be designed in stages or redundant pairs for fail-safe operation.

Real-World Case – Power Station Step-Up Transformer

• Transformer: 250 MVA, 400/132 kV, OFAF
• Site: Thermal power station, base-load generation
• Cooling: 2 oil pumps + 8 axial radiator fans
• Temperature control: Oil top temp maintained at <55 °C under full load

Result: No load derating, excellent dielectric oil condition after 5 years, and 24/7 operation under hot (ambient 45 °C) conditions

What Is OFWF Cooling and Where Is It Applied?

When transformers are installed in confined, enclosed, or thermally insulated environments—such as power plants, tunnels, marine vessels, or underground substations—air-based cooling systems (like ONAN or OFAF) are not feasible. In these high-load, high-performance installations, OFWF (Oil Forced, Water Forced) cooling provides an ideal solution, offering maximum thermal efficiency in minimal space.

OFWF stands for Oil Forced, Water Forced, a transformer cooling method that uses oil pumps to circulate insulating oil through the windings and a water-cooled heat exchanger, while a separate water circuit (typically closed-loop) carries away the heat. OFWF is used for high-capacity transformers installed in space-restricted or noise-sensitive environments where air cooling is impractical or insufficient.

This method delivers exceptional thermal performance with compact radiators and is widely adopted in power plants, underground substations, offshore platforms, and large-scale industrial facilities.

How OFWF Cooling Works

Main Components in OFWF Systems

Advantages of OFWF Cooling

Typical Applications of OFWF Transformers

Real-World Case – Thermal Power Plant GSU Transformer

• Transformer: 315 MVA, 15.75/400 kV, OFWF cooled
• Cooling: Two oil pumps, dual water heat exchangers, and water loop to chiller
• Site: Generator step-up unit at indoor turbine hall
• Performance: Operated at 95% loading with oil top temp ≤55 °C under 42 °C ambient

Result: Silent, space-saving, high-efficiency cooling with stable thermal margins and long-term reliability

Design and Operation Considerations

How Do You Select the Right Cooling Method for a Transformer?

Choosing the correct transformer cooling method is not just a technical formality—it directly impacts thermal performance, equipment lifespan, footprint, noise levels, and overall cost. Each cooling class (ONAN, ONAF, OFAF, OFWF) is engineered to meet specific application scenarios, so the choice must be made based on a holistic evaluation of operating conditions and transformer rating.

To select the right cooling method for a transformer, you must consider the power rating (MVA), load profile (steady or variable), installation environment (open air or enclosed), available space, ambient temperature, and operational reliability needs. ONAN suits low to medium loads with simple outdoor access; ONAF is ideal for medium-capacity transformers with variable load; OFAF and OFWF are preferred for high-capacity or enclosed installations where active thermal management is essential.

Cooling design should be finalized early in the engineering stage to match installation and performance criteria.

Key Factors in Cooling Method Selection

Cooling Method Comparison Matrix

Step-by-Step Selection Guide

1. What is the power rating of the transformer?
    ✔ <10 MVA → ONAN
    ✔ 10–50 MVA → ONAN or ONAF
    ✔ >50 MVA → OFAF or OFWF

2. Is the transformer installed outdoors with good airflow?
    ✔ Yes → ONAN or ONAF
    ✔ No → OFAF or OFWF

3. Are ambient temperatures >40 °C or loads highly variable?
    ✔ Yes → ONAF or OFAF with thermostatic control

4. Is the transformer located underground, indoors, or offshore?
    ✔ Yes → OFWF is best (air cooling not viable)

5. Is low noise mandatory (e.g., hospital or city)?
    ✔ Yes → Consider ONAN or OFWF

6. Is space a constraint?
    ✔ Yes → OFWF offers most compact cooling

Real-World Application Examples

Dual-Rating Flexibility

Dual or triple cooling ratings provide flexibility for overloads and future expansions.

Conclusion

Cooling systems are essential for the reliability and performance of power transformers. Each method—from ONAN to OFWF—serves specific operational needs, balancing natural and forced circulation of cooling media. The choice of a suitable cooling method depends on thermal performance requirements, installation environment, and transformer rating. Proper cooling not only improves efficiency but also extends transformer life and prevents premature failure.

FAQ

Q1: Why is cooling important in power transformers?
A1: Cooling is critical because:

Transformers generate heat due to load current and core losses

Excessive heat degrades insulation, shortens lifespan, and may cause failure

Efficient cooling ensures safe operation, longer service life, and performance reliability
Cooling systems are classified based on how oil and air/water circulate to dissipate heat.

Q2: What does ONAN mean in transformer cooling?
A2: ONAN = Oil Natural Air Natural

Oil circulates naturally (convection) inside the tank

Heat is transferred to the tank surface, then dissipated by ambient air

Suitable for low to medium capacity transformers (≤5 MVA)
It is simple, passive, and requires no external fans or pumps.

Q3: What is ONAF cooling in transformers?
A3: ONAF = Oil Natural Air Forced

Oil circulates naturally, but fans are used to blow air over radiators

Improves heat dissipation, enabling the transformer to handle 25–50% more load

Common in substations and industrial transformers
ONAF is more efficient than ONAN, but slightly more complex.

Q4: How does OFAF cooling work?
A4: OFAF = Oil Forced Air Forced

Oil is pumped through radiators, and fans force air across the surface

Allows high-capacity transformers (e.g., 30–300 MVA) to stay within temperature limits

Requires active control systems, pumps, and backup power
Used in utility-grade, continuous-load transformers.

Q5: What is OFWF cooling and when is it used?
A5: OFWF = Oil Forced Water Forced

Oil is pumped through heat exchangers, which are cooled by water circulation

Most effective for very high-power transformers in dense installations, like:

Power plants

Underground substations

Demands complex plumbing and water supply systems, but allows maximum cooling efficiency

References

"Transformer Cooling Methods Explained" – https://www.electrical4u.com/transformer-cooling-types

"IEEE C57 Standards for Cooling Classification" – https://ieeexplore.ieee.org/document/8751236

"NREL: Transformer Efficiency and Cooling" – https://www.nrel.gov/docs/cooling-systems-transformers.pdf

"ScienceDirect: Heat Transfer in Transformer Oil Systems" – https://www.sciencedirect.com/transformer-cooling-performance

"Doble: Transformer Thermal Management" – https://www.doble.com/transformer-cooling-methods