What an ONAF Transformer Really Means

ONAF stands for Oil Natural Air Forced, and an ONAF transformer is simply an oil-immersed transformer whose cooling system has been upgraded with motor-driven fans. The two letters that matter most in daily operation are the last two: the oil circulates by natural convection (thermal siphon effect), while air is pushed across the radiator surfaces by fans rather than relying on ambient air alone. That single addition of forced airflow is what allows the same core-and-coil assembly to carry significantly more load than its natural-cooling counterpart.

For anyone specifying or purchasing distribution and power transformers, understanding ONAF cooling is not academic detail — it directly affects how much capacity you pay for, how the unit behaves on peak load, and what your maintenance obligations look like over a 25–30 year service life.

How the ONAF Cooling Circuit Works

The cooling process in an ONAF transformer follows a continuous thermal loop:

• Heat generation: No-load losses in the core and load losses (I²R) in the windings raise the temperature of the active part.
• Natural oil circulation: Heated oil becomes lighter and rises through the transformer tank toward the radiators, while cooler oil returns to the bottom. No oil pump is used, so there is no forced-oil pipework, no pump wear, and no additional auxiliary power consumption for oil flow.
• Forced air cooling: Fan banks mounted on the radiator headers blow ambient air across the cooling fins, increasing the heat transfer coefficient on the air side and accelerating heat rejection to the atmosphere.
• Control logic: Fans are normally switched by contacts on the winding temperature indicator (WTI) or oil temperature indicator (OTI). A typical staged sequence starts the fans at roughly 65–70 °C winding temperature, raises an alarm near 85 °C, and trips the transformer at the manufacturer's specified limit.

Because fan starting is temperature-driven, an ONAF transformer runs quietly and efficiently at light load, then automatically brings additional cooling online as demand rises.

ONAF vs ONAN, OFAF and OFWF: Which Rating Do You Actually Need?

Cooling class designations are frequently confused during tender preparation. A quick comparison:

• ONAN (Oil Natural Air Natural): Convection oil flow and natural air cooling. The simplest and lowest-maintenance design; standard for small and medium distribution transformers.
• ONAF (Oil Natural Air Forced): Natural oil circulation plus fan-assisted air cooling. Very commonly supplied with a dual rating — for example 100% capacity under ONAN and 120–133% capacity under ONAF — giving load flexibility from the same footprint.
• OFAF (Oil Forced Air Forced): Oil pumps plus fans. Used where higher MVA and tighter temperature rise limits justify pumps and their maintenance.
• OFWF / ODAF: Forced water or directed oil cooling for large power transformers and EHV applications with very high loss densities.

In practice, the choice comes down to load profile and space. If peak demand occurs only a few hours per day, an ONAF transformer with a dual ONAN/ONAF rating usually delivers the best cost per kVA. If the unit will run near nameplate for long continuous periods, either a larger ONAN unit or an OFAF design may be the more robust choice.

The Practical Advantages of ONAF Cooling

• More capacity without a bigger tank: Forced air typically unlocks 20–33% additional capacity compared with ONAN on the same active part, reducing weight, oil volume and civil works.
• Lower first cost: Buying an ONAF transformer is often cheaper than buying a physically larger ONAN unit of equivalent peak rating.
• Load-following flexibility: Staged fan control matches cooling to actual load, which suits cyclic industrial loads and renewable energy applications.
• Efficiency at low load: Fans are off or at low duty for much of the time, keeping auxiliary power consumption modest.

The trade-offs are equally clear: fans consume auxiliary power (typically a small fraction of rated output), require periodic inspection of motors, bearings and blade condition, and add a little to sound pressure levels. These are routine items rather than obstacles, but they belong in the total cost of ownership calculation.

Where ONAF Transformers Are Typically Used

• Urban and industrial distribution substations with limited footprint
• 35 kV and below preassembled and compact substations
• Manufacturing plants, steel mills and mining installations with cyclic peak loads
• Wind and solar step-up applications where output fluctuates through the day
• Commercial and data-centre projects needing reserve capacity for future growth
• Utility networks where a dual ONAN/ONAF rating simplifies load planning

Key Parameters to Confirm Before You Order

A precise specification prevents both over-buying and under-buying. Confirm at minimum:

• Rated power (kVA or MVA) and the ONAN and ONAF ratings separately
• Voltage ratio, vector group (for example Dyn11 or YNd11) and frequency
• Impedance voltage and short-circuit withstand requirements
• No-load and load loss values, with any capitalized-loss evaluation formula
• Temperature rise limits — commonly 60 K top-oil and 65 K average winding rise under IEC 60076-2
• Tap changer type: off-circuit tap changer or on-load tap changer
• Cooling stage control setpoints, alarm and trip contacts
• Enclosure/IP rating, sound level limits, altitude and ambient temperature range
• Applicable standards — IEC 60076, GB 1094 or IEEE C57.12.00 — and the required test reports

Procurement Tips That Genuinely Save Money

• Specify to the load profile, not to a round number. Oversizing for a peak that lasts two hours a day is one of the most common causes of avoidable spend.
• Ask for both ratings in writing. A genuine ONAF transformer should be documented with its ONAN rating, ONAF rating and the temperature rise basis, not just a single nameplate figure.
• Evaluate losses, not just purchase price. Over 25 years, a small reduction in no-load loss can outweigh a noticeable difference in capital cost.
• Buy from the source. Factory-direct supply removes distributor margins and makes customisation — vector group, tap range, tank finish, cooling staging — far easier to agree.
• Insist on full documentation: routine test certificates, temperature rise data, oil test reports, drawings and an operation manual.

Standards, Testing and Documentation

Routine factory tests for an ONAF transformer generally include winding resistance, turns ratio, impedance and load loss, no-load loss and current, insulation resistance, power-frequency withstand, oil dielectric strength, and a temperature rise type test where required by the contract. Reputable manufacturers supply these reports with the unit, together with outline drawings, nameplate data and a maintenance schedule so that commissioning and future condition assessment are straightforward.

Why Factory-Direct Sourcing Is Worth the Call

Runsheng Transformer Co., Ltd. manufactures oil-immersed transformers, dry-type transformers and amorphous alloy transformers, including S11, S13, S20 and S22 series units, together with 35 kV and below preassembled substation equipment. Products are built to national standards and shipped with manuals, inspection reports, test certificates and lifelong quality tracking — a practical advantage when ONAF cooling stages, loss values or special site conditions need to be tailored rather than picked from a catalogue.

Whether you need a single 630 kVA distribution unit or a batch of custom special transformers, confirming the cooling class and ratings early is the cheapest decision you will make on the project.

Contact for specifications and factory-direct quotes: phone +86 13406394988, e-mail runsheng@7813transformer.com, WhatsApp 008613508930968.