What an OFAF Transformer Actually Is

OFAF transformer is the standard designation, taken from the IEC 60076-2 cooling-class system, for an oil-immersed transformer that relies on forced oil circulation and forced air circulation. The letters decode logically: O and F describe the internal cooling medium and how it moves (oil, forced), while A and F describe the external medium and how it moves (air, forced). An OFAF transformer therefore combines oil pumps that drive the coolant through the tank, radiators or heat exchangers with motor-driven fans that push air across the cooling surfaces.

That mechanical assistance is what separates an OFAF transformer from the far more common ONAN (oil natural, air natural) distribution unit on a pole or inside a compact substation. Natural cooling depends on buoyancy and radiation alone; forced cooling uses electrical energy to move heat out much faster, so more megavolt-amperes can be pushed through the same core-and-coil assembly without exceeding its temperature limits.

How the Forced Cooling Circuit Works

Inside the tank, one or more oil pumps draw hot oil from the top of the transformer and push it through external coolers — usually radiator banks or plate-fin heat exchangers — before returning it, cooled, to the bottom of the tank. Fans mounted on the cooler banks force ambient air across the fins, raising the heat-transfer coefficient on the air side. Both sides of the thermal path are enhanced at once, which is why OFAF transformer cooling is substantially more effective than either ONAF (forced air only) or forced-oil-only arrangements.

• Oil pumps — typically centrifugal, glandless designs that avoid leakage paths into the oil circuit.
• Cooler banks — radiator panels or compact plate-fin heat exchangers rated for the specified ambient temperature.
• Fans and motors — protected against rain, dust and, where required, salt-laden coastal air.
• Control and monitoring cabinet — selects cooling stages from winding and top-oil temperature signals, with alarm and trip contacts.
• Protective devices — oil-flow indicators, pressure gauges and coordination with the Buchholz relay to detect pump-related faults.

Most large OFAF transformer designs are specified with staged cooling, quoted as ONAN/ONAF/OFAF ratings. The unit runs quietly on natural cooling at light load, brings fans online as temperature rises, and starts the oil pumps only when load or ambient conditions demand the full forced-oil rating. That staged arrangement improves efficiency, reduces auxiliary power consumption and extends fan and pump life.

OFAF vs ONAF vs OFWF vs ODAF: Where It Fits

• ONAN — oil natural, air natural. The baseline; no pumps, no fans. Common for distribution transformers up to a few MVA.
• ONAF — oil natural, air forced. Fans on radiators give a moderate rating increase with a simple, low-maintenance system.
• OFAF transformer — oil forced, air forced. Higher heat rejection per unit of cooler surface; the usual choice for medium and large power transformers where space is limited or load is high.
• OFWF — oil forced, water forced. Used where a reliable water supply exists and audible noise or footprint must be minimised.
• ODAF / ODWF — directed (guided) oil flow through the windings. A further step up in cooling efficiency for very large units.

Selecting between them is a thermal and economic trade-off, not a prestige choice. An OFAF transformer costs more than an ONAF unit of the same rating because of the pumps, controls and more complex cooler assembly, but it can reduce the physical size and mass of the transformer for a given power throughput — which matters when the substation building, transport route or foundation is already fixed.

Typical Applications

OFAF transformer cooling appears across the power system wherever the ratio of load to available space is high:

• Step-up transformers in wind and solar farms, where output fluctuates and the cooling system must track fast load changes.
• Generator step-up and unit auxiliary transformers in thermal, hydro and industrial captive power plants.
• Grid substations at 35 kV, 110 kV and above, including mobile and skid-mounted emergency units.
• Rectifier transformers and other special-duty units in smelters, traction supply and electrochemical plants, where harmonic losses add to the thermal burden.
• Industrial installations — steel, cement, mining and data-centre yards — where the transformer sits close to the load and ambient temperatures are severe.

What Procurement Teams Usually Verify

Because the cooling system defines the guaranteed rating, tender documents for an OFAF transformer should pin down more than the nameplate MVA:

• Rated power and the corresponding cooling stage (for example, ONAN rating versus OFAF rating) — the same unit may carry two or three published figures.
• Voltage ratio, tapping range and impedance, matched to short-circuit levels at the point of connection.
• Guaranteed temperature rises, agreed at tender and verified by a temperature-rise test to IEC 60076-2.
• Design ambient temperature and altitude; cooler sizing changes materially between a 40 °C and a 50 °C site.
• Pump and fan redundancy, auxiliary supply voltage, and whether automatic stage transfer is required on loss of auxiliary power.
• Noise limits measured at the specified distance, since fans and pumps add to core noise.
• Losses (no-load and load) and the capitalised value the buyer applies to them.

Getting these wrong is the classic oversizing trap: buyers pay for forced-cooling capability the load profile never needs, or specify a single-stage OFAF rating and then discover the unit cannot run safely at light load with the pumps out of service.

Operation and Maintenance Realities

An OFAF transformer is not a fit-and-forget asset. Pumps and fans are the only continuously moving parts on the unit, and their condition drives availability. A practical maintenance programme includes routine inspection of fan blades and motor bearings, checks on pump differential pressure and oil-flow indicators, thermographic scanning of cooler connections, and periodic dissolved gas analysis of the oil to catch overheating or arcing long before it becomes a failure. Control cabinets need their temperature sensors and stage-change logic tested, because a fan that fails to start silently converts an OFAF transformer into an under-cooled ONAN unit — and the first symptom is accelerated insulation ageing.

Working With a Manufacturer Instead of a Reseller

Cooling class, rating stages and cooler layout are design decisions made at the factory, so the specification conversation is far more productive with the manufacturer than with a trading company. Runsheng Transformer Co., Ltd. produces oil-immersed transformers from 30 kVA to 6300 kVA and above — including the S11, S13, S20 and S22 series — alongside SCB10, SCB13, SCB14 and SCB18 dry-type units, SH15 and SCBH15 amorphous alloy transformers, mining and rectifier transformers, and 35 kV preassembled and compact substations. Each unit ships with a manual, inspection report and test certificate, backed by lifelong quality tracking.

Buying direct also keeps the price conversation honest: factory-direct pricing means no intermediary margin layered onto the cooler package, the pumps or the control cabinet.

Quick Answers for First-Time Buyers

Is an OFAF transformer better than ONAF? It cools more effectively per unit of surface area, but it also adds auxiliary power, moving parts and maintenance. It is the better choice when load or space constraints justify it.

Can the pumps be switched off? Only within the natural or forced-air rating published by the manufacturer. Running at the OFAF rating with pumps stopped is a thermal risk, not a saving.

Which standard governs the designation? IEC 60076-2 defines the cooling-class letter codes; GB 1094 and IEEE C57.12.00 cover related requirements regionally.

Contact us for a cooling-class review before you tender: phone +86 13406394988, e-mail runsheng@7813transformer.com, WhatsApp 008613508930968.