What Does ONAF Mean on a Transformer Nameplate?

ONAF stands for Oil Natural Air Forced. It is a cooling class designation for oil-immersed transformers, and it describes exactly how heat is removed from the core and windings: the oil circulates by natural convection, while fans force air across the radiator banks to increase the rate at which that heat is rejected to the atmosphere. An ONAF transformer is therefore not a different machine from a conventional oil-immersed unit — it is the same active part, specified and tested with a forced-air cooling stage that unlocks additional capacity.

Cooling class designations follow IEC 60076-2 and IEEE C57.12.00 conventions, where each letter identifies the coolant inside the tank and the circulation method outside it. Reading the nameplate correctly is the first step in any serious procurement exercise, because the cooling class determines the guaranteed rating, the permissible temperature rise, the noise level, and the auxiliary load of the unit.

How the ONAF Cooling Cycle Works

• Heat generation: No-load losses in the core and load losses in the windings are converted into heat inside the active part.
• Natural oil circulation: Heated oil becomes lighter and rises through the winding ducts and into the top of the tank; cooler oil returns from the bottom, creating a thermosiphon loop with no oil pump involved.
• Radiator dissipation: The hot oil flows through external radiator panels or tube banks, where part of the heat is transferred to ambient air by natural convection and radiation — this is the ONAN base stage.
• Forced air stage: When oil or winding temperature reaches a preset threshold, cooling fans mounted on the radiators start and blow air across the cooling surfaces, multiplying the heat transfer coefficient and lowering the oil temperature for the same load.
• Control and staging: Fan groups are switched by winding temperature indicators or oil temperature indicators with adjustable contacts, so the transformer only runs its fans when the load or ambient temperature justifies it.

Because the oil circulation remains natural, an ONAF transformer has no oil pumps, no forced-oil pipework, and no risk of pump cavitation or flow-related static electrification. That simplicity is one reason the design remains the default choice for medium and large distribution and sub-transmission duty.

ONAN, ONAF, OFAF and ODAF Compared

• ONAN (Oil Natural Air Natural): No fans, no pumps. The most reliable and lowest-auxiliary-load arrangement, typical of small and medium distribution transformers.
• ONAF (Oil Natural Air Forced): Fans on the radiators, natural oil flow. The most common way of increasing a transformer rating by roughly 25 to 33 percent over its ONAN rating.
• OFAF (Oil Forced Air Forced): Pumps drive oil through the radiators and fans force air across them; used where compact dimensions and higher ratings are required.
• ODAF (Oil Directed Air Forced): Oil is pumped directly into the winding cooling ducts, giving the most efficient winding cooling for very large units.
• OFWF and ODWF: Water-cooled equivalents, specified where site noise or space limits rule out air cooling.

Why Dual-Rating ONAN/ONAF Units Are So Common

Most ONAF transformer specifications are written as dual ratings, for example 40/50 MVA or 100 percent / 125 percent of base power. In the ONAN stage the unit operates fan-free and quiet; when the fans start, the permissible load rises to the ONAF rating. This gives the buyer two important advantages: an emergency overload capability without exceeding the temperature limits defined in IEC 60076-2, and the ability to defer a costly upgrade when load grows gradually. It also supports N-1 operation in substations where a single transformer must carry the load of a failed companion unit for a limited period.

Temperature rise limits are normally quoted at the standard reference ambient conditions — a maximum of 40 degrees Celsius, a daily average of 30 degrees Celsius, and a yearly average of 20 degrees Celsius — with top-oil rise generally limited to 60 K and average winding rise to 65 K, depending on the insulation system and the standard invoked. Any site with sustained ambient temperatures above 40 degrees Celsius requires a derating calculation, not an assumption.

Technical Parameters to Verify Before You Order

• Rated power in kVA or MVA for each cooling stage (ONAN and ONAF), plus the overload duration permitted.
• Voltage ratio, vector group, and tapping range, including off-circuit or on-load tap changing where voltage regulation is needed.
• Short-circuit impedance, which governs parallel operation and fault level contribution.
• No-load and load losses, evaluated against the applicable efficiency class such as IE2 or IE3 under EN 50588-1 or the equivalent national standard.
• Insulation level (LI/AC withstand values), bushings, cable boxes, and protection accessories.
• Sound pressure level measured with fans running, since fan noise typically adds several dB(A) to the ONAN figure.
• Fan supply voltage, control scheme, and total auxiliary power consumption.

Where ONAF Transformers Are Used

The cooling class suits almost every application where a naturally cooled unit would otherwise be oversized. Typical installations include urban and rural distribution substations, industrial plants with fluctuating load profiles, mining and metallurgical sites, wind and solar collection substations, data centres and commercial complexes, and 35 kV class compact substations where footprint is tightly constrained. In renewable energy projects, the ONAF rating is often what allows a single unit to ride through the short-term peaks of a variable generation profile.

Cost, Efficiency and Whole-Life Value

An ONAF transformer costs more than an equivalent ONAN unit because of the radiators, fans, control cabinet and additional testing, but it is usually far cheaper than buying the next frame size up. The practical economics come from three areas: lower capital cost per MVA of usable capacity, capitalised loss evaluation over a 20 to 30 year service life, and the ability to delay reinforcement of the substation. Buyers who evaluate only the purchase price, and ignore no-load loss, load loss and fan energy, routinely pay more over the asset life.

Procurement Tips: How to Avoid Overpaying

• Ask for both ONAN and ONAF ratings in the quotation, not a single figure, and confirm which rating is type-tested.
• Request routine test reports, a temperature rise test reference, and fan control logic drawings with the offer.
• Compare losses on a capitalisation basis rather than on price alone.
• Confirm the noise guarantee is stated with fans running, and check the distance and measurement method.
• Verify the manufacturer can supply a complete package: transformer, radiators, fans, control cabinet, protection devices and documentation.

Working directly with a manufacturer rather than through layers of trading companies typically removes intermediate margins and shortens the technical clarification loop. Runsheng Transformer Co., Ltd. produces ONAF-capable oil-immersed transformers across the S11, S13, S20 and S22 series, along with dry-type and amorphous alloy units, and ships each set with manuals, inspection reports, test certificates and lifelong quality tracking.

Installation and Maintenance Essentials

• Keep the radiator bank clear of obstructions; a recirculating hot air pocket can cost several percent of the ONAF rating.
• Set fan start and stop contacts with a deliberate differential to prevent short cycling.
• Inspect fan motors, blades and contactors at each scheduled outage, and clean radiator fins of dust and pollen.
• Trend dissolved gas analysis and oil dielectric strength results rather than relying on a single test.
• Use thermal imaging to compare radiator bank temperatures and detect blocked panels or failed fans early.

Final Thoughts

An ONAF transformer is one of the most cost-effective tools available for matching installed capacity to a load that is not constant. Specified correctly — with both ratings stated, losses evaluated honestly, and the auxiliary system properly designed — it delivers years of service with minimal moving parts and predictable maintenance. Contact us for specification support or a factory-direct quotation: phone +86 13406394988, e-mail runsheng@7813transformer.com, WhatsApp 008613508930968.