What Cooling Methods Are Used in 2000 kVA Oil Immersed Transformers?

Understanding the cooling methods used in 2000 kVA oil immersed transformers is essential before you finalize a substation design, because the cooling system directly determines how much real power your transformer can deliver day after day without exceeding its thermal limits.

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At GNEE, we are not a distributor or trading intermediary-we are a specialized manufacturer of oil-immersed power transformers with over 18 years of factory production experience and an ISO 9001-certified quality system. Our engineering teams have configured cooling solutions for 2000 kVA units operating in tropical deserts, arctic mining sites, and everything in between.

 

In this guide, we explain each standard and optional cooling method, show you how the choice affects capacity and footprint, and give you the exact technical data you need to specify the right cooling class for your application.

 

Radiator panels being mounted on a oil immersed transformer tank
 

 

Every 2000 kVA oil immersed transformer relies on insulating oil not only as a dielectric medium but also as the primary heat-transfer fluid. Losses generated in the core and windings raise the temperature of the active part; the oil absorbs this heat, moves it to the tank walls or external radiators, and dissipates it to the surrounding air.

 

The way this heat transfer is managed-whether purely by natural convection or with the assistance of fans and pumps-defines the cooling method, and each method has a direct impact on the transformer's continuous loading capability, physical size, and auxiliary power consumption.

 

The International Electrotechnical Commission standard IEC 60076-2 classifies these cooling modes using a four-letter designation that describes the internal cooling medium, circulation mechanism, external cooling medium, and external circulation mechanism.

 

Basic Cooling Mode Designations for Oil Immersed Transformers

For a 2000 kVA oil immersed transformer, the most common designations you will encounter are ONAN (Oil Natural, Air Natural) and ONAF (Oil Natural, Air Forced). These two methods cover the vast majority of distribution and light industrial substation applications.

 

In more specialized heavy-industry or utility generator-step-up installations, additional modes such as OFAF (Oil Forced, Air Forced) or ODWF (Oil Directed, Water Forced) may be specified, though these are less typical at the 2 MVA rating.

 

GNEE routinely supplies ONAN and ONAF configurations and can engineer forced-oil or water-cooled variants when a project demands them.

 

 

ONAN-Oil Natural, Air Natural-is the baseline cooling method for a 2000 kVA oil immersed transformer and remains the most widely installed configuration worldwide.

 

In this mode, the entire cooling cycle operates without any moving parts: heat from the core and windings naturally reduces the density of the surrounding oil, causing it to rise; the hot oil flows upward into the radiator panels or corrugated tank walls, where it transfers heat to the ambient air through natural convection and radiation; the cooled, denser oil then descends to repeat the cycle. This self-sustaining thermosiphon process requires zero auxiliary power for cooling and introduces no fan-motor maintenance into the operational budget.

 

Benefits and Reliability of ONAN Cooling for 2000 kVA Transformers

Because ONAN cooling employs no fans or pumps, the transformer achieves the highest possible reliability and the lowest sound level for its class.

A standard GNEE 2000 kVA ONAN oil immersed transformer is guaranteed not to exceed a sound power level of 59 dB(A), making it well suited for urban substations, hospital campuses, and residential perimeters.

The absence of auxiliary cooling equipment also reduces the unit's upfront cost, keeps the control cabinet simpler, and eliminates any risk of cooling-system failure tripping the transformer offline.

For applications where the load is relatively stable and does not frequently exceed the rated 2000 kVA, ONAN is the most cost-effective and maintenance-friendly choice.

 

 

ONAF-Oil Natural, Air Forced-builds on the natural oil circulation of the ONAN mode by adding electrically driven fans mounted adjacent to the radiators. These fans increase the velocity of cooling air over the radiator surfaces, sharply raising the rate of convective heat transfer. Critically, the oil circulation remains entirely natural; only the external air side is forced.

 

For a 2000 kVA oil immersed transformer, adding an ONAF stage typically permits a continuous overload capability of up to approximately 125% of the base ONAN rating-meaning the same physical unit can safely deliver 2500 kVA under forced-air conditions-provided the transformer is designed with this dual-rated capability from the outset.

 

How ONAF Works in a 2000 kVA Oil Immersed Transformer

In a GNEE ONAF-rated transformer, the radiators are sized for the ONAN rating, and high-efficiency, low-noise axial fans are mounted on a support frame adjacent to the radiating surfaces.

A temperature-sensing controller in the control cabinet starts the fans automatically when the winding or top-oil temperature reaches a preset threshold, and stops them when the temperature falls below the cut-out setpoint.

The fans can also be operated manually during commissioning and maintenance.

This automatic staging means the transformer runs silently under light load and only activates forced cooling when real thermal conditions call for it.

 

 

 

While ONAN and ONAF cover the majority of 2000 kVA projects, two further cooling classes are occasionally specified in heavy-industry or space-constrained substations.

 

OFAF: Forced Oil and Forced Air Cooling

OFAF adds an oil pump to the cooling circuit, actively circulating oil through the radiators rather than relying on thermosiphon flow alone. This can achieve an additional incremental capacity above the ONAF rating, but at the cost of higher auxiliary power, increased maintenance, and a more complex control and protection scheme.

 

At GNEE, we have built OFAF-rated 2000 kVA units for steel mills and mining draglines where extreme overloading is a regular operating condition.

 

ODWF: Directed Oil and Water Forced Cooling

For installations where ambient air temperatures are extremely high or where physical space precludes large radiator banks, ODWF cooling uses water-to-oil heat exchangers. This is an uncommon configuration at 2 MVA and is generally reserved for underground substations, marine vessels, or power plants where a reliable cooling water source already exists. GNEE can engineer this configuration when project conditions truly justify it.

 

 

Cooling Method	Designation	Typical Capacity Range	Auxiliary Equipment	Auxiliary Power	Sound Level	Best Application
Oil Natural, Air Natural	ONAN	2000 kVA base rating	None	0 W	≤ 59 dB(A)	Steady load, urban substation, low-maintenance sites
Oil Natural, Air Forced	ONAF	2000 kVA base + 2500 kVA forced	Radiator-mounted fans	Approx. 0.5–1.5 kW	≤ 65 dB(A) at full flow	Cyclic load peaks, high ambient temperature
Oil Forced, Air Forced	OFAF	Up to 2800+ kVA (design-dependent)	Oil pump + fans	Approx. 2–4 kW	≤ 70 dB(A)	Severe overload duty, heavy industry
Oil Directed, Water Forced	ODWF	Application-specific	Pumps + heat exchanger	Depends on water circuit	Very low external noise	Underground, marine, space-constrained

 

 

The cooling method you select for your 2000 kVA oil immersed transformer has a measurable knock-on effect on several physical and logistical parameters.

 

An ONAN-only transformer uses the total radiator surface as the sole heat rejection path, which means the radiators must be sized generously.

 

An ONAF transformer can achieve the same heat dissipation with a smaller radiator bank-or hold the radiator size and gain a higher rating-because forced convection is more efficient per unit area.

 

This can reduce the overall footprint of the unit, which is a decisive advantage in compact packaged substations.

 

However, the ONAF variant carries slightly more oil volume to act as a thermal buffer during fan cycling, and the total weight increases modestly due to the fan assemblies and their support steel.

 

GNEE provides a complete dimensional, oil-volume, and weight datasheet for each cooling configuration in our quotation, so you can validate the transformer layout against your site plan before placing an order.

 

Tell us what your 2000 kVA oil immersed transformer needs to do, and we will deliver the exact cooling method-backed by factory test data and a competitive quotation-within one business day. Your next reliable substation starts with a single conversation.

 

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Can a 2000 kVA ONAN transformer be upgraded to ONAF later?

Not reliably. A dual-rated ONAN/ONAF transformer requires specific radiator sizing and winding temperature margins designed in at the manufacturing stage. Retroactively adding fans to an ONAN-only unit may provide marginal relief but will not deliver a guaranteed ONAF rating. GNEE always recommends specifying the intended ONAF capacity from the start.

 

Does ONAF cooling increase the noise level significantly?

The fan noise adds typically 5 to 6 dB(A) to the transformer's sound level when running. GNEE uses low-noise fan blade profiles and speed controls to keep the total sound power within acceptable limits, and the fans only operate when thermally required.

 

What maintenance do ONAN and ONAF cooling systems require?

ONAN cooling requires no dedicated maintenance beyond the normal periodic inspection of the transformer. ONAF fans require an annual check of blade security, motor insulation resistance, and thermostat calibration-a straightforward task that adds minimal cost to the preventive maintenance routine.

 

What is the difference between Onan and Onaf?

ONAN uses natural oil circulation and natural air flow. Heat moves through natural convection loops inside the tank. ONAF uses forced air to expand cooling when load rises. Both methods control temperature, but ONAF supports higher performance output.

 

What is ONAF in transformers?

Oil Natural Air Forced (ONAF): Forced air cooling systems are utilized for larger transformers when the heat generated is too much for natural cooling techniques to keep the transformer at a safe temperature. This type of cooling system is referred to as oil natural air forced (ONAF).

 

What does ONAF stand for?

Oil Natural Air Forced

"Oil Natural Air Forced" – Natural convectional circulates the flow of oil for cooling. Fan forced air is applied to the cooling surface of the enclosure to improve cooling.

 

What is the difference between Onan Onaf and KNAN KNAF?

KNAN and KNAF follow the same cooling logic as ONAN and ONAF, but with one key difference: the fluid inside the tank. K-class fluids like FR3 or other natural esters have a higher fire point than mineral oil, often exceeding 300°C.