What an OFAF Transformer Actually Means

Transformer cooling classes are written as short letter codes, and every letter matters. In OFAF transformer, the first pair means Oil Forced and the second pair means Air Forced: a pump pushes the insulating oil through the cooling ducts inside the windings and core, while fans drive air across the radiator or cooler bank outside. Because both sides of the heat path are actively driven, heat rejection is far higher than on a naturally cooled unit of the same tank size. In procurement terms this delivers three benefits: more MVA from the same footprint, lower temperature rise at the same load, or a lighter and more compact unit for the same rating. That matters on space-constrained sites such as urban substations, containerised installations and retrofits where the building cannot be enlarged.

How the Cooling Stages Work: ONAN, ONAF and OFAF in One Unit

Most OFAF transformer designs are not forced-cooled all the time. They are built as staged-cooling units that switch cooling capacity on and off according to oil and winding temperature:

• Stage 1 — ONAN (Oil Natural, Air Natural): pumps and fans off; the unit runs on thermal circulation alone at its base rating.
• Stage 2 — ONAF (Oil Natural, Air Forced): fans start, oil still circulates naturally, rating increases.
• Stage 3 — OFAF (Oil Forced, Air Forced): oil pumps and fans both run, and the transformer reaches its maximum rating.

This is why a nameplate often shows three figures, for example 40/50/63 MVA, and why the cooling class is written as ONAN/ONAF/OFAF. The control cabinet, pump and fan starters, oil flow indicators and winding temperature device are therefore part of the rating, not accessories. If the cooling control system is unreliable, the third rating does not exist in practice.

Where OFAF Transformers Are Specified

• Medium and large power transformers where the required MVA exceeds what natural cooling can deliver in the available space.
• Urban and indoor substations with strict footprint, height or noise limits.
• Renewable energy projects — wind collector substations, solar step-up stations and battery storage interconnections — where cyclic load profiles allow cooling stages to be matched to output.
• Industrial loads with heavy duty cycles, such as furnace, rectifier and mining distribution systems that need overload capability on demand.

Specifications to Lock Down Before You Compare Quotes

• Rated power per cooling stage (ONAN/ONAF/OFAF), not a single MVA figure.
• Voltage ratio, vector group, tap changer type (OLTC or off-circuit) and tapping range.
• Impedance, no-load loss and load loss, with the reference temperature stated.
• Temperature rise limits and insulation class.
• Insulation levels (lightning impulse and applied voltage) plus clearances for site altitude and pollution level.
• Cooler type, number of pumps and fans, redundancy and spare capacity.
• Cooling control logic, alarm and trip settings, and remote signals to SCADA.
• Noise level at the specified measurement distance, and any low-noise requirement.
• Applicable standards such as IEC 60076 or GB 1094, plus local grid code requirements.

Two bids can look identical and still differ by a wide margin because one is quoted at the ONAN rating only, while the other includes the full OFAF rating with pumps, fans and a functionally tested control cabinet.

Why Two OFAF Transformer Prices Differ So Much

Price gaps of 20 to 30 percent between suppliers are normal and usually traceable to a handful of cost drivers:

• Conductor material and current density — copper versus aluminium, and how hard the design is pushed.
• Core material grade and construction — conventional grain-oriented steel versus step-lap or amorphous, which changes no-load loss.
• Cooler surface area and pump or fan brand — undersized coolers are the most common hidden shortcut.
• Loss evaluation — if the buyer capitalises losses over 25 years, a low-loss design costs more upfront and far less in operation.
• Test scope — routine tests only, or routine plus temperature rise and impulse type tests witnessed at the factory.
• Documentation, packing, transport and commissioning support.

Factory Checks Worth Witnessing

For an OFAF transformer, insist on functional testing of the cooling system, not just electrical tests: pump and fan changeover, loss-of-flow alarm, pressure relay behaviour and duplication of control power supplies. Routine tests should cover winding resistance, ratio, vector group, impedance and load loss, no-load loss and current, insulation resistance, applied and induced voltage, and oil dielectric strength. A temperature rise test on a representative design, or a full type test on the ordered unit, is the only reliable evidence that the OFAF rating is real rather than a calculation on paper.

Common Procurement Pitfalls

• Accepting a nameplate with one MVA figure and no staged ratings.
• No spare pump or fan, forcing a full outage for a minor failure.
• Cooling control that cannot run in manual mode or report to the control room.
• Type test reports from a different design being presented as the offered unit.
• Missing oil flow indicators and pressure relays on the cooler piping.

A Manufacturer Worth Talking To

Runsheng Transformer Co., Ltd. builds oil-immersed and dry-type transformers, including S11, S13, S20 and S22 oil-immersed units, SCB10 to SCB18 dry-type units, SH15 amorphous alloy transformers and custom special transformers, alongside 35 kV and below preassembled substations and high and low voltage switchgear. Units ship with manuals, inspection reports and test certificates, backed by lifelong quality tracking. For projects where the OFAF transformer rating, cooling control and delivery schedule all have to line up, that combination of product range and documentation is often what decides the order.

Next Step Before You Request Prices

Prepare a data sheet with the three ratings, losses, impedance, cooling redundancy and test scope before you ask for quotations. It is the fastest way to get comparable offers and to avoid paying for a rating the cooling system cannot actually deliver.

Contact: phone +86 13406394988 | email runsheng@7813transformer.com | WhatsApp 008613508930968.