What an ONAF Transformer Really Means Before You Ask for a Price

ONAF stands for Oil Natural Air Forced. In an ONAF transformer the insulating oil moves by natural convection: it heats up in the windings, rises, travels through the radiator banks, cools, and returns. Only the air side is forced, using fans mounted on the radiators. That single detail is why ONAF transformers dominate medium and large oil-immersed power transformer specifications worldwide.

For a buyer, the practical meaning is money: an ONAF transformer is normally ordered with two ratings on one nameplate. A 40/50 MVA unit, for example, delivers 40 MVA with the fans stopped (ONAN) and 50 MVA once the fans run (ONAF). Same tank, same footprint, same foundation, roughly 25 percent more capacity. Reading that dual rating correctly, and verifying it in the test report, is where procurement teams win or lose value.

How the Cooling Stages Work Inside an ONAF Transformer

Heat generated in the windings passes into the oil, the oil rises through the tank and into the radiators, and the fans push ambient air across the radiator fins. The cooled oil sinks back to the bottom of the tank and the loop repeats. Because the oil path is passive, there are no oil pumps to fail, no forced-oil leaks and no additional oil-side maintenance.

Fan operation is staged by control logic rather than by guesswork. Typical designs start the first fan group when top-oil temperature reaches a setpoint, usually somewhere between 65 and 75 degrees Celsius depending on design and ambient, then bring in further groups as oil or winding temperature climbs. Winding temperature indicators and current-based controls add a second trigger so a sudden overload starts cooling early. When load drops, fans stop and the unit returns quietly to ONAN mode.

Two standards matter here: IEC 60076-2 for temperature rise, which sets the familiar limits of 65 K average winding rise by resistance and 60 K top-oil rise, and IEC 60076-7 for loading guidance. Chinese GB 1094 and IEEE C57.12.00 are the equivalents most buyers reference alongside them.

ONAF vs ONAN, OFAF and OFWF: Choosing the Right Cooling Class

• ONAN: natural oil, natural air. Lowest auxiliary power and lowest cost, but limited heat rejection per square meter of radiator.
• ONAF: natural oil, forced air. Typically 20 to 33 percent more capacity than the same unit in ONAN, at modest additional cost. The best value option for growing substations.
• OFAF: forced oil and forced air, using pumps. Higher capacity density for large power transformers, with added auxiliary systems to maintain.
• OFWF and ODWF: water coolers or directed oil flow, used for very large units, hydro stations and sites where air cooling alone cannot handle the losses.

Specification Checklist for ONAF Transformer Procurement

• Dual rating stated explicitly, for example ONAN/ONAF in kVA or MVA, with the temperature rise basis quoted.
• Rated voltages, insulation level and impulse withstand, such as 35 kV class or 110 kV class with stated LI and SI levels.
• Vector group, percentage impedance, no-load loss and load loss at reference temperature, plus guaranteed tolerances.
• Tap changer type: off-circuit (OCTC) or on-load (OLTC). An ONAF transformer with an OLTC should have fan control interlocked with load current and tap position.
• Fan package details: number of fans, motor protection class, noise contribution, N+1 redundancy, and a control cabinet with alarm and trip contacts.
• Tests: ratio, winding resistance, impedance, losses and insulation resistance as routine tests, plus a temperature rise type test, impulse test and short-circuit withstand test. The temperature rise test is the only real proof of the ONAF rating.

Where ONAF Transformers Earn Their Keep

Typical applications include wind and solar collector substations where output fluctuates, industrial plants with peaking loads, mining and heavy-duty sites, utility distribution substations where the footprint is fixed but demand keeps climbing, and retrofit projects that need extra capacity without pouring a new foundation.

Runsheng Transformer Co., Ltd. manufactures oil-immersed units for exactly these duties, including S11, S13, S20 and S22 series transformers, 35 kV and below preassembled substations, mining and rectifier transformers, and custom special transformers. Every unit ships with a manual, inspection report, test certificate and lifelong quality tracking.

Cost, Efficiency and the Fan Power Trade-off

Fans consume only a small fraction of rated power, but they run for thousands of hours, so control strategy matters. Good designs run the unit in ONAN mode at part load and bring in fans only above setpoints, which cuts auxiliary energy, reduces fan wear and keeps audible noise lower at night. Poorly sized radiators do the opposite: fans run almost continuously, noise rises by several dB(A) and fan motors become a maintenance item instead of a backup.

When comparing quotations, always compare at the same cooling stage. A cheap offer often quotes the ONAF figure while the radiator area is sized for a lower ONAN rating, so the transformer spends its life with fans running flat out.

Common Buyer Mistakes to Avoid

• Comparing an ONAF price against an ONAN unit without normalizing the rating basis.
• Accepting a nameplate rating with no temperature rise type test in the report package.
• Ignoring fan noise limits and control cabinet quality until commissioning.
• Ordering no spare fan or redundancy, then losing the second rating during a fan failure.
• Oversizing the transformer so heavily that it never reaches the ONAF stage, paying for cooling capacity that is never used.

Talk to a Manufacturer That Builds the Whole Cooling Package

Radiator area, fan staging, control logic and test verification are designed together, not bought separately. If you are specifying an ONAF transformer for a substation, renewable energy project or industrial upgrade, ask for the dual rating, the temperature rise test and the fan control scheme in writing before you compare prices.

Contact Runsheng Transformer Co., Ltd. for factory-direct pricing and technical support: phone +86 13406394988, email runsheng@7813transformer.com, WhatsApp 008613508930968.