ONAN Cooling for Transformers: A Deep Dive into Oil Natural Air Natural Systems

This four-letter designation describes a cooling system of elegant simplicity. It is a completely passive process, relying on nothing more than the fundamental laws of physics to protect a multi-million-dollar asset. While more complex forced-cooling systems exist for larger units, a deep understanding of the ONAN principle is essential for any engineer involved in the specification, operation, or maintenance of power and distribution transformers.

This comprehensive guide from Leistung Energie provides an in-depth engineering exploration of the ONAN cooling method. We will move from the first principles of heat generation to the physics of natural convection, break down the key components, and analyze the applications and limitations of this foundational technology.

1. The Source of the Heat: Why Do Transformers Require Cooling?

A transformer is a highly efficient piece of electrical equipment, with modern designs often exceeding 99% efficiency. However, no machine is perfect.

The small percentage of energy that is not transferred from the primary to the secondary winding is converted into heat. This heat originates from two primary sources, known as transformer losses:

A. No-Load Losses (or Core/Iron Losses)

These losses occur in the transformer's laminated steel core and are present whenever the transformer is energized, regardless of whether it is supplying a load. They are caused by two phenomena:

• Hysteresis Losses: Energy lost as the magnetic domains in the core steel are continuously reoriented by the alternating magnetic field.

• Eddy Current Losses: Small circulating currents induced within the core laminations by the changing magnetic flux.

B. Load Losses (or Winding/Copper Losses)

These losses are generated in the transformer windings and are directly proportional to the square of the current flowing through them (I²R losses). They are caused by the electrical resistance of the copper or aluminum conductors. As the load on the transformer increases, the current increases, and the load losses rise exponentially.

The Consequence of Unmanaged Heat

This combined heat generated by both no-load and load losses causes the temperature of the core and windings to rise. The most vulnerable component to this temperature rise is the solid cellulose (paper) insulation wrapped around the winding conductors.

High temperatures accelerate the chemical degradation of this paper, reducing its mechanical strength and dielectric properties. The rate of this aging process approximately doubles for every 6-8°C increase in operating temperature. Therefore, effective cooling is not just about performance; it is the key to ensuring a long and reliable transformer lifespan.

2. Decoding ONAN: What "Oil Natural Air Natural" Truly Means

The IEC 60076 standard provides a four-letter code to classify transformer cooling methods. ONAN is the simplest of these and can be broken down as follows:

• O - Oil: The first letter indicates the Internal Cooling Medium that is in direct contact with the windings. In this case, it is mineral oil or a similar dielectric insulating fluid (such as natural esters).

• N - Natural: The second letter describes the Internal Circulation Mechanism. 'N' stands for natural convection, meaning the oil circulates due to the natural thermosiphon effect, without the aid of pumps.

• A - Air: The third letter indicates the External Cooling Medium. The heat is ultimately dissipated into the surrounding ambient air.

• N - Natural: The fourth letter describes the External Circulation Mechanism. 'N' again stands for natural convection, meaning the air circulates over the cooling surfaces without the assistance of fans.

In summary, ONAN describes a completely passive cooling system with no active components or moving parts (no pumps, no fans). Its operation relies entirely on the natural physical process of convection.

3. The Physics of ONAN: A Two-Stage Natural Convection Cycle

The magic of ONAN cooling lies in its two distinct but interconnected convection loops that work together to move heat from the transformer's core to the surrounding environment.

Stage 1: The Internal Loop (Oil Natural - The Thermosiphon Effect)

This stage occurs entirely within the sealed transformer tank and its radiators.

1. Heating: The heat generated by the core and windings is transferred to the surrounding dielectric oil.

2. Expansion and Rising: As the oil heats up, it expands and becomes less dense. This lighter, hotter oil naturally rises to the top of the main tank.

3. Flow to Radiators: The hot oil at the top of the tank flows into the upper headers of the externally mounted radiators.

4. Cooling and Sinking: As the oil flows down through the radiator fins, it transfers its heat to the radiator's metal walls. The oil cools, becomes denser, and consequently sinks.

5. Return Flow: The cooler, denser oil exits the bottom header of the radiators and flows back into the bottom of the main tank. This cooler oil is then drawn back towards the bottom of the hot windings, ready to absorb more heat and repeat the cycle.

This continuous, silent, and natural circulation is known as the thermosiphon effect, and it is the engine of the internal cooling process.

Stage 2: The External Loop (Air Natural)

This stage occurs on the outside surfaces of the transformer, primarily the radiators.

1. Conduction: The heat from the oil is conducted through the metal walls of the radiator fins to the outer surface.

2. Air Heating: The layer of ambient air in direct contact with the hot radiator surface heats up.

3. Rising Air Current: As this air heats, it becomes less dense and naturally rises, creating a steady upward draft of air along the radiator fins.

4. Cool Air Intake: This rising hot air draws in cooler, denser ambient air from below the radiator to take its place. This new cool air then heats up, rises, and continues the cycle.

This self-sustaining, natural flow of air constantly carries heat away from the transformer and dissipates it into the atmosphere.

The efficiency of this process is directly related to the surface area of the radiators and the temperature difference between the radiator surface and the ambient air.

4. Key Components of an ONAN Cooling System

While the principle is simple, it relies on several key engineered components:

• The Transformer Tank: The primary steel vessel that contains the core, windings, and the bulk of the dielectric fluid.

• The Dielectric Fluid: The primary medium for both insulation and heat transfer. Its thermal properties, such as specific heat and viscosity, are critical for efficient convection.

• Radiators: These are the most critical components for heat exchange. They are typically fabricated from panels of pressed steel with fins or corrugations designed to create a large surface area. The number, size, and design of the radiators are carefully calculated during the transformer's design phase to ensure sufficient heat dissipation for its full MVA rating under specified ambient conditions.

• Oil and Winding Temperature Indicators (OTI & WTI): These gauges are the "thermometers" of the system. They provide a direct reading of the oil and winding temperatures, allowing operators to monitor the thermal health of the transformer and ensure the cooling system is functioning effectively.

5. ONAN in Context: Comparison with Forced Cooling Methods

The simplicity of ONAN is its greatest strength, but it also defines its limits. To increase the power capacity of a transformer without increasing its physical size, forced cooling methods are introduced.

• ONAN vs. ONAF (Oil Natural Air Forced): This is the most common step-up from ONAN. An ONAF-rated transformer is simply an ONAN transformer with the addition of a bank of electric cooling fans mounted to blow air across the radiators. The "Natural" external air circulation becomes "Forced." This dramatically increases the rate of heat dissipation. As a result, the same transformer can have two power ratings: a lower ONAN rating (fans off) and a higher ONAF rating (fans on), typically providing a 25-33% increase in capacity.

• ONAN vs. OFAF (Oil Forced Air Forced): For very large power transformers, natural oil convection (thermosiphon) is no longer efficient enough to move heat from the windings to the radiators. In an OFAF system, the "Natural" internal oil circulation becomes "Forced" through the use of oil pumps. This, combined with fans, provides a much higher level of cooling for high-MVA units.

The key takeaway is that ONAN is the baseline for simplicity and reliability. Forced cooling methods add active components (fans, pumps, control systems) which increases complexity and introduces potential points of failure, but are necessary to achieve higher power densities.

6. Applications and Limitations of ONAN Cooling

The unique characteristics of ONAN make it the ideal choice for a specific range of applications.

Ideal Applications for ONAN:

• Distribution Transformers: Virtually all pole-mounted and pad-mounted distribution transformers (up to around 2500 kVA) utilize ONAN cooling. Their loads are typically well within the capacity of this simple, passive system.

• Small to Medium Power Transformers: Transformers in utility and industrial substations up to approximately 10-20 MVA are often specified with an ONAN rating. For this size, the design is still efficient and cost-effective.

• Installations Requiring High Reliability: In remote or unattended substations, the absolute reliability of a cooling system with zero moving parts is a major advantage. There are no fans to fail or pumps to service.

• Applications Requiring Silent Operation: As there are no fans or pumps, ONAN transformers are virtually silent, making them suitable for installation in noise-sensitive areas like residential or commercial zones.

Limitations of ONAN:

• Power Density: ONAN cooling is not as compact as forced cooling. To achieve a very high MVA rating with ONAN alone, the required number and size of radiators would make the transformer impractically large.

• Ambient Temperature Sensitivity: The effectiveness of natural convection is directly proportional to the temperature difference between the transformer and the ambient air. In very hot climates, such as those found across much of Australia, the efficiency of ONAN cooling is reduced. This must be carefully factored into the design, often requiring the transformer to be "derated" or built with a larger cooling surface area.

• Requirement for Unobstructed Airflow: The "Natural Air" component is critical. An ONAN transformer must be installed with adequate clearance on all sides to allow for a free and unobstructed flow of air around the radiators. Installing it in a poorly ventilated indoor vault or too close to a wall will severely compromise its cooling ability and lead to overheating.

Conclusion

The ONAN cooling method is a testament to elegant, robust, and efficient engineering. By harnessing the fundamental laws of thermodynamics, it provides a completely passive and effective method of heat dissipation that ensures the health and longevity of the transformer.

Its defining characteristics are its unparalleled reliability—with no moving parts to fail, no auxiliary power required, and no control systems to maintain—and its silent operation. While forced cooling methods are necessary to achieve the higher power densities required for very large power transformers, ONAN remains the foundational and optimal cooling method for the vast majority of distribution transformers and a highly reliable choice for small to medium power transformers across the global grid.

Understanding these principles is key to specifying a transformer that will deliver decades of trouble-free service. At Leistung Energie, our design philosophy is rooted in this deep understanding of fundamental engineering. We engineer ONAN cooling systems that are meticulously optimized for thermal performance and exceptional longevity, even in the challenging and diverse Australian climate.

Contact our transformer specialists today to discuss the optimal cooling solution and design for your specific application.