What an OFAF Transformer Really Is

An OFAF transformer is an oil-immersed power transformer whose cooling system is active on both sides of the heat path. Oil pumps push the insulating fluid through the radiator bank and winding ducts, while fans blow ambient air across the radiator fins. In the standard cooling-class notation used by IEC 60076 and GB 1094, OFAF sits above ONAF and ONAN: once natural oil convection is no longer enough to hold temperature rise within limits, forced oil flow becomes the next step.

The practical consequence is simple. Because heat leaves the windings much faster, an OFAF transformer can be built with a smaller tank and a tighter core-and-coil assembly for the same MVA, or can hold its temperature rise in a high-ambient site where a naturally cooled unit would be derated. For anyone specifying medium and large oil-immersed units, understanding the OFAF stage is the difference between a transformer that runs comfortably for thirty years and one that trips on hot afternoons every summer.

How the Cooling Stages Stack Up

Most large oil-immersed transformers are not locked into a single cooling class. They are delivered with a staged rating, and the control cabinet switches stages according to top-oil temperature and, on better designs, winding hot-spot temperature. A typical progression looks like this:

• ONAN: pumps off, fans off. The base rating, used at low load and low ambient.
• ONAF: pumps off, fans on. Fans alone unlock extra capacity.
• OFAF: pumps on, fans on. The highest continuous rating in this family, with oil velocity in the windings doing the heavy lifting.
• ODAF: directed oil flow, where oil is guided into dedicated winding cooling ducts for even more aggressive hot-spot control.

The exact capacity gained at each stage is not a universal number. It is set by the manufacturer's thermal model and must be confirmed during the routine temperature-rise test, so always ask for the test report rather than accepting a catalogue percentage.

Why Procurement Teams Specify OFAF Cooling

• Higher power density: more MVA from a smaller footprint, which matters when the substation bay or the civil works budget is already fixed.
• Better hot-spot control: forced oil flow reduces winding hot-spot temperature, the single biggest driver of insulation ageing and loss of life.
• Loading flexibility: staged cooling allows short-term overload without pushing the unit past its thermal limits.
• Fewer parallel units: one well-cooled OFAF transformer can often replace two smaller naturally cooled units, cutting switchgear, cabling and civil cost.

The trade-off is auxiliary power, pump and fan maintenance, and a control system that has to be right. That is why the cooling cabinet, redundancy arrangement and protection logic deserve as much attention in the specification as the winding itself.

Typical Applications

OFAF cooling appears wherever load is heavy and continuous or where ambient conditions are punishing: utility substations at 35kV and above, step-up transformers for wind and solar farms, industrial plant main transformers, rectifier and furnace duty, traction supply, and dense urban substations where the footprint is constrained. Renewable energy projects in particular favour OFAF and ODAF designs because output fluctuates and the transformer must ride through peak generation without exceeding its thermal envelope.

Specifications to Lock Down Before Ordering

• Rated power and cooling-stage ratings: state the MVA for each stage (ONAN, ONAF, OFAF), not just the headline figure.
• Voltage ratio and tapping: on-load tap changer or off-circuit tap changer, plus the full tapping range and step.
• Vector group and impedance: both affect fault levels and parallel operation with existing units.
• Temperature rise and design ambient: a unit rated for 40°C ambient will behave differently in a 50°C desert site.
• No-load and load losses: ask for the guaranteed values and the measurement tolerances.
• Cooling hardware: number of pumps and fans, N+1 redundancy, radiator bank configuration, control cabinet IP rating.
• Insulating fluid and testing: fluid type, routine tests, type tests and any special tests required by the project.
• Documentation: outline drawing, manual, inspection report, routine test certificate and loss report.

Where the Money Actually Goes

Buyers who compare only the purchase price usually lose. Two cost elements dominate life-cycle economics:

• Losses. No-load and load losses are paid for continuously for decades. A slightly higher first cost for lower losses is frequently the cheaper decision once losses are capitalised at the project discount rate.
• Cooling equipment quality. Cheap pumps and fans fail first, and a transformer forced onto a lower cooling stage in summer loses capacity permanently. Specifying reputable pumps, adequate redundancy and a well-designed control cabinet protects the rating you paid for.

Other price drivers include winding material, tap changer brand, radiator surface area, transport weight limits and whether site assembly is included. Comparing offers line by line on these items is the fastest way to spot a quotation that has been quietly stripped down.

Factory-Direct Sourcing and Quality Verification

Cutting out trading layers is the most reliable way to reduce cost without touching the design. A manufacturer with its own winding, core stacking, drying, oil processing and test bay can quote the real build cost and stand behind the result. Runsheng Transformer Co., Ltd. produces S11, S13, S20 and S22 oil-immersed transformers, SCB10, SCB13, SCB14 and SCB18 dry-type transformers, SH15 and SCBH15 amorphous alloy transformers, mining and rectifier transformers, and 35kV and below preassembled substations with high and low voltage switchgear, covering roughly 30 to 6300 kVA with factory-direct pricing and full documentation for every unit.

Before releasing a purchase order, verify three things: the factory witness test or a complete routine test report, the guaranteed loss values written into the contract, and the after-sales tracking commitment. Any serious supplier will provide all three without hesitation.

Quick Answers to Common Questions

• Is OFAF the same as ONAF? No. ONAF uses fans only; OFAF adds forced oil circulation through pumps, which delivers a higher capacity from the same tank.
• Can an OFAF transformer run with pumps off? Yes, at a reduced ONAN or ONAF rating. The staged ratings must be stated on the nameplate.
• Does OFAF mean more maintenance? It means planned maintenance on pumps, fans and the control cabinet. Budget for it and the reliability is excellent.

Getting the cooling class, the staged ratings and the loss guarantees right at the enquiry stage is worth more than any discount negotiated later. For specification support, drawings or a factory-direct quotation, contact Runsheng Transformer Co., Ltd. at +86 13406394988, runsheng@7813transformer.com, or WhatsApp 008613508930968.