What an OFAF Transformer Actually Means

An OFAF transformer is an oil-immersed transformer whose cooling system uses Oil Forced, Air Forced circulation, a cooling class defined in IEC 60076-2. Two things happen at the same time: electric oil pumps push the hot insulating oil from the tank through the radiator bank, and fans blow ambient air across the radiator fins. Because both the internal oil flow and the external air flow are mechanically assisted, an OFAF transformer can dissipate far more heat than a naturally cooled unit of the same tank and radiator size.

The four-letter code is worth understanding, because it explains why two transformers with identical cores can carry very different nameplate ratings. The first pair of letters describes the oil circulation (ON = natural, OF = forced). The second pair describes how the external cooling medium moves (AN = natural air, AF = forced air, WF = forced water). This is why ONAN, ONAF, OFAF and OFWF transformers are really four cooling stages of the same oil-immersed design family rather than four unrelated products.

How the OFAF Cooling Circuit Works

In a typical OFAF transformer, heat generated by the windings and core is transferred to the insulating oil, which is then moved along a defined path by pumps and returned through radiators or coolers before re-entering the tank. Operation is usually staged: a base stage with pumps and fans running, and additional stages that switch in extra pumps or fan groups as oil and winding temperatures rise or as load increases.

• Oil pumps: circulate oil through the radiators at a controlled rate, improving the oil-to-surface heat transfer coefficient.
• Cooling fans: force air across the radiator fins, raising the air-side heat transfer rate well above natural convection.
• Radiator or cooler bank: provides the heat exchange surface; its area is matched to the required temperature rise at rated load.
• Control and protection: oil and winding temperature indicators, pump and fan motor protection, flow indicators and alarm/trip contacts.
• Redundancy: larger units often use duty/standby pumps and duplicated fan groups so a single failure does not force an immediate outage.

Because the cooling is staged, an OFAF transformer is often specified with two or three ratings on the same nameplate, for example an ONAN rating for light load and a higher OFAF rating for peak load. This flexibility is one of the main reasons designers choose this cooling class.

OFAF vs ONAF vs ONAN: What Changes in Practice

• ONAN transformer: no pumps or fans, entirely self-cooled, simplest and lowest maintenance, but the largest tank and radiator footprint for a given rating.
• ONAF transformer: fans only; oil moves by natural convection while fans increase air-side cooling. A common, cost-effective step up from ONAN.
• OFAF transformer: pumps plus fans; the highest cooling capability of the air-cooled oil-immersed family, with the smallest footprint per MVA.
• OFWF transformer: pumps plus water coolers; used where water is available and very high ratings or tight space make air cooling impractical.

In practical procurement terms, moving from ONAF to OFAF typically allows a meaningful uprating of the same core and tank, often in the range of 20% to 30% depending on the design, ambient conditions and permitted temperature rise. The trade-off is auxiliary power consumption, added moving parts, more control logic and a slightly higher noise level from pumps and fans.

Where OFAF Transformers Are Used

OFAF cooling is chosen when load density is high, space is limited, or ambient temperatures are severe. Typical applications include:

• Medium and large power transformers in transmission and primary distribution substations.
• Industrial plants with continuous heavy loads, such as steel, cement, chemical and mining operations.
• Rectifier and furnace transformer installations where harmonic losses raise internal temperatures.
• Compact and preassembled substations in urban areas where the transformer footprint must be minimized.
• Renewable energy projects, including wind and solar collector substations with fluctuating load profiles.

Specification Points Buyers Should Confirm Before Ordering

An OFAF transformer is only as good as its specification. Before requesting quotations, confirm the following with your supplier:

• Rated power and cooling stages: the ONAN rating, the OFAF rating, and how the stages are switched.
• Voltage ratio and vector group: for example 35 kV class input with the required secondary voltage and Dyn11 or YNd11 connection.
• Temperature rise and ambient conditions: design ambient temperature, altitude, and whether the site is indoor, coastal or dusty.
• Tap changer: off-circuit tap changer or on-load tap changer (load tap changing transformer), with the required tapping range.
• Losses: no-load and load losses, which directly determine long-term energy cost and should be compared on a capitalized-loss basis, not on purchase price alone.
• Protection and monitoring: Buchholz relay, pressure relief device, oil and winding temperature indicators, pump/fan failure alarms.
• Enclosure and finish: IP rating, anti-corrosion coating class, and cable or busbar termination details.
• Testing and documentation: routine and type test reports, oil test certificate, instruction manual and quality tracking records.

Common Buying Mistakes to Avoid

Most problems with OFAF transformer purchases come from specification gaps rather than manufacturing defects. Oversizing is the classic error: buyers sometimes pay for an unnecessary OFAF rating when a well-selected ONAF unit would run comfortably at the actual load profile. Conversely, undersizing the radiator bank or ignoring site ambient temperature forces pumps and fans to run continuously, which increases auxiliary power consumption, noise and maintenance.

Other frequent issues include comparing quotations on price only while ignoring loss evaluation, accepting vague cooling-stage descriptions, and failing to confirm that pump and fan motors are rated for the site voltage and that spare parts are available. A supplier that provides complete test data, clear cooling-stage definitions and lifelong quality tracking removes most of this risk. Manufacturers such as Runsheng Transformer Co., Ltd. supply OFAF and ONAF oil-immersed units alongside the S11, S13, S20 and S22 series and SCB10, SCB13, SCB14 and SCB18 dry-type transformers, which makes it easier to compare cooling options on a like-for-like basis within one technical proposal.

Operation and Maintenance Basics

• Check oil level, oil temperature and winding temperature regularly; trending is more useful than single readings.
• Test pump and fan changeover logic and confirm standby equipment starts on demand.
• Inspect radiator fins for dust, leaves and insect nests, especially before summer peak load.
• Monitor pump vibration and motor current; bearing wear shows up early in the current signature.
• Carry out dissolved gas analysis on the oil at defined intervals to detect incipient faults.
• Verify that control settings match the actual load profile, so cooling stages do not run unnecessarily.

Why Factory-Direct Sourcing Makes Sense for OFAF Units

OFAF transformers are engineered products, not catalog items. Buying directly from the manufacturer keeps the cooling-stage design, loss evaluation, tap changer choice and protection scheme in one technical conversation, avoids reseller mark-ups, and gives faster access to test reports and spare parts. For overseas projects, factory-direct supply also simplifies documentation for customs, installation and acceptance testing.

If you are comparing OFAF transformer options for a substation, industrial plant or renewable energy project, it is worth requesting a technical proposal that shows both ONAF and OFAF ratings for the same core, together with losses, dimensions and auxiliary load. That single comparison usually reveals which cooling class delivers the lowest total cost of ownership for your load profile.

Contact us for a quotation or technical discussion: phone +86 13406394988, e-mail runsheng@7813transformer.com, WhatsApp 008613508930968.