What an OFAF Transformer Actually Is

An OFAF transformer is a power transformer built with a combined "Oil Forced, Air Forced" cooling system. The four letters describe exactly what happens inside the unit: insulating oil is pumped through the active part and the cooling equipment (Oil Forced), while fans drive ambient air across the radiator or cooler surfaces (Air Forced). Compared with a naturally cooled design, an OFAF transformer dissipates far more heat from the same core-and-winding package, which is why this cooling mode is standard practice on medium and large power transformers in generation, transmission and heavy industry.

Transformer cooling is normally described using four-letter codes, and understanding them is the fastest way to judge whether an OFAF transformer is the right choice for a project:

• ONAN – Oil Natural, Air Natural. No pumps and no fans; the classic distribution transformer.
• ONAF – Oil Natural, Air Forced. Fans on the radiators raise capacity by roughly 20–30 percent.
• OFAF – Oil Forced, Air Forced. Pumps and fans work together, allowing higher ratings and better overload capability.
• OFWF / ODAF – Oil Forced, Water Forced or Oil Directed, Air Forced. Selected where water cooling or directed oil flow is required.

Many large transformers are specified with multiple stages — for example ONAN/ONAF/OFAF — so the cooling capacity can be stepped up or down automatically as load and ambient temperature change.

How the OFAF Cooling Loop Works

The cooling circuit consists of oil pumps, a bank of radiators or separate oil-to-air heat exchangers, axial fans, and a control cabinet with temperature sensors. Hot oil is drawn from the top of the tank by the pump, pushed through the cooler where fans remove heat, and returned to the tank bottom, keeping a continuous, directed flow over the windings and core.

The advantage is efficiency of heat transfer. Forced oil flow breaks down the stagnant oil boundary layer around the conductors, so the winding-to-oil temperature difference drops and hot spots are suppressed. In practice, an OFAF transformer can be built more compactly than an ONAN unit of the same rating, which saves floor space, oil volume and structural steel in the substation.

Key Technical Parameters Buyers Should Confirm

When reviewing a technical proposal or datasheet for an OFAF transformer, these values decide whether the unit will actually perform in service:

• Rated capacity and voltage class – typically 35 kV, 110 kV and 220 kV class equipment for forced-oil cooling, with distribution-level oil-immersed units in the 30–6300 kVA range using ONAN or ONAF.
• Temperature rise limits – usually a top-oil rise of about 60 K and winding average rise of about 65 K, in line with IEC 60076 and GB 1094 requirements.
• No-load and load losses – the single largest driver of lifetime energy cost and of your project's total cost of ownership.
• Impedance and tapping range – impedance affects short-circuit withstand and voltage regulation, while an on-load tap changer or off-circuit tap links must match grid conditions.
• Cooling control logic – automatic stage switching by winding temperature, pump and fan redundancy, alarm and trip contacts.
• Insulation, tank and protection – oil type and dielectric strength, gasket and paint specification, plus Buchholz relay, pressure relief device and IP-rated control cabinets for outdoor installation.

Why Projects Choose OFAF Cooling

• Higher continuous capacity from a smaller, lighter footprint.
• Better short-time overload capability during peak demand or loss of a parallel unit.
• Lower winding hot-spot temperatures, which slows ageing of the insulation paper and extends service life.
• Predictable cooling control: stages switch in and out automatically, so pumps and fans only run when needed, reducing auxiliary power consumption and noise at light load.

Typical Applications

OFAF transformers are common in power plants, high-voltage substations, steel mills, chemical plants, mine sites, and increasingly in renewable energy projects such as wind farms and large solar parks where load fluctuates sharply. Where indoor installation or fire safety rules rule out oil, engineers instead turn to dry-type designs such as the SCB10, SCB13, SCB14 and SCB18 series, or to other oil-immersed workhorses like the S11, S13, S20 and S22 families for distribution duties.

Procurement Checklist: What to Check Before You Buy

• Confirm the cooling class and the guaranteed temperature rise at your design ambient temperature, not just at a nominal 20 °C.
• Ask for routine test reports, type test certificates and a calibrated loss measurement.
• Check pump and fan brands, service life and local availability of spare parts.
• Verify short-circuit withstand capability and the calculation behind it, not only the nameplate value.
• Request drawings for bushing positions, cable boxes, oil conservator and transport dimensions.
• Clarify documentation: manual, inspection report, test certificate and lifetime quality tracking.

Price Factors and the Factory-Direct Advantage

Price differences between two apparently identical OFAF transformers usually come from four places: copper and core grade, oil quality, cooling components (pumps, fans, control cabinet) and the level of testing and documentation. Buying from a manufacturer instead of a chain of trading companies removes the markup that accumulates at each hand-off, and it keeps technical responsibility with the party that actually built the unit.

As a power transformer manufacturer, Runsheng Transformer Co., Ltd. supplies oil-immersed, dry-type and amorphous alloy transformers together with factory-direct pricing and full documentation, including manuals, inspection reports, test certificates and lifelong quality tracking — a practical advantage when a project needs custom-designed special transformers rather than a catalogue item.

Maintenance Basics for Long Service Life

• Check oil level, oil temperature and the desiccant in the breather at regular intervals.
• Test dissolved gas analysis annually to catch incipient faults early.
• Rotate or service oil pumps and fans; confirm automatic stage switching still operates correctly.
• Keep radiators clean — dust and debris sharply reduce air-forced cooling efficiency.

Summary

An OFAF transformer buys you more capacity per unit of space and a smoother thermal profile, provided the cooling class, losses, impedance and control logic are specified correctly and backed by real test data. Compare total cost of ownership, not the nameplate price alone, and source from a manufacturer able to document every stage of production.

Contact us for technical selection support and a factory-direct quotation: phone +86 13406394988, e-mail runsheng@7813transformer.com, WhatsApp 008613508930968.