Cooling class is one of the most consequential and most frequently misunderstood decisions in transformer procurement. An ONAF transformer sits at the practical center of that decision: it uses the simplest, most proven oil-immersed architecture available, then adds forced-air cooling to extract significantly more capacity from the same core-and-coil assembly. For buyers specifying distribution and power transformers from 30 kVA up to several MVA, understanding what ONAF means, how the fans are staged and controlled, and which parameters actually drive price is the difference between an oversized purchase and a right-sized one. This overview explains the fundamentals from a procurement perspective.

What Does ONAF Mean on a Transformer Nameplate?

ONAF is the standard IEC cooling designation that stands for Oil Natural, Air Forced. Each letter describes one link in the heat-removal chain: the oil moves through the tank and radiators by natural convection, while air movement across the radiator surfaces is provided by motor-driven fans. Nothing about the internal oil circuit is pumped, which keeps the design mechanically simple and highly reliable.

Because cooling class directly determines how much loss a unit can safely dissipate, it is written into the rating. The four classes buyers most often compare are:

• ONAN — Oil Natural, Air Natural: convection oil flow, radiators cooled by ambient air only. The fan-free baseline design.
• ONAF — Oil Natural, Air Forced: convection oil flow plus fans blowing air across the radiator banks, which greatly increases heat rejection.
• OFAF — Oil Forced, Air Forced: oil pumps drive the flow through radiators; common on larger power transformers.
• ODAF — Oil Directed, Air Forced: pumped oil is directed into winding cooling ducts; typical of very large transmission and generator units.

Many ONAF transformer units are dual-rated and carry a nameplate reading such as ONAN/ONAF 20/25 MVA — 20 MVA with fans off, 25 MVA with fans running. That dual rating is the single most useful commercial feature of the design.

How the ONAF Cooling System Actually Works

Heat generated in the core and windings is transferred to the insulating oil. The warmed oil becomes less dense, rises through the tank and flows into the radiator bank, where it gives up heat to the surrounding air before returning to the bottom of the tank. Fans mounted on the radiators push ambient air through the fins, and this forced airflow can multiply the convection heat-transfer coefficient several times over compared with natural cooling alone.

Control is straightforward but worth verifying during inspection:

• Thermostats or a winding temperature indicator stage the fans in at set points, typically starting the first group around 65 degrees Celsius top-oil and bringing in all fans by 75 to 80 degrees Celsius.
• Fans are arranged in parallel groups so a single motor failure reduces capacity rather than shutting the unit down.
• A local control cabinet, usually IP54 or IP55, houses contactors, MCBs, thermostats and manual or automatic selector switches.
• Fan motors are normally three-phase, 380 to 415 V, with weatherproof enclosures and low-noise blades.

Because the fans only run when oil temperature demands it, an ONAF transformer operates as a quiet, low-loss ONAN unit at light load and only becomes an ONAF unit during peaks — an efficiency profile that matters on renewable energy and cyclic industrial duty.

Why Buyers Choose ONAF Over Plain ONAN

• Higher capacity in the same footprint. Adding forced-air cooling typically raises usable rating by 15 to 25 percent, and up to roughly one third with two fan stages, using the same tank and core.
• Better overload headroom. Emergency loading capability during summer peaks or motor starting is substantially improved.
• Lower first cost than oversizing. A dual-rated ONAN/ONAF unit is usually cheaper than a larger ONAN-only transformer of equivalent peak capacity.
• Lower losses at partial load. Fans are off below the first set point, so no-load and light-load efficiency stay close to an ONAN design.
• Proven serviceability. Fans and thermostats are low-cost, field-replaceable items with no impact on the sealed oil circuit.

The trade-offs are equally clear: fan noise adds roughly 3 to 6 dB(A) when running, the control cabinet and auxiliary wiring need maintenance, and radiator fins must be kept clean for the rating to hold. In dusty or coastal sites, that maintenance discipline is essential.

Typical ONAF Transformer Applications

ONAF cooling is the mainstream choice for oil-immersed distribution and power transformers in the 30 to 6300 kVA range and beyond, particularly for industrial plants with cyclic loads, mining and mineral processing sites, renewable energy collector substations, rectifier duty, mobile and compact substations, and urban distribution substations where footprint is limited but peak demand is not.

Specifications That Drive Price and Performance

• Rated power and the ONAN/ONAF dual ratings in kVA or MVA
• Voltage ratio and insulation level, including lightning impulse and power-frequency withstand values
• Temperature rise limits, typically 60 K top oil and 65 K winding average for this cooling class
• Impedance percentage and vector group, for example Dyn11
• No-load and load losses, which determine the energy-efficiency tier over the unit life
• Tap changer type: off-circuit or on-load tap changing
• Cooling control philosophy, fan quantity, and auxiliary supply voltage
• Corrosion protection class for coastal or chemical environments, plus noise limit in dB(A)
• Oil type — mineral oil or a fire-point ester — where fire safety regulations demand it

Two identical-looking quotations can differ by a wide margin purely on loss figures, corrosion class and fan control scope. Always compare on those three lines before comparing headline price.

Standards, Tests and Documents to Request

Reputable ONAF transformer manufacturers build and test to IEC 60076, GB 1094 or IEEE C57.12.00 depending on destination market. Routine tests should include winding resistance, ratio, impedance and load loss, no-load loss and current, applied voltage, induced voltage and oil dielectric tests. For higher ratings, request the temperature-rise type test report, lightning impulse report and short-circuit withstand evidence. A complete delivery package should include the routine test certificate, oil test report, nameplate drawing, cooling control schematic and an operation and maintenance manual.

Installation and Maintenance Essentials

• Allow at least 1.5 m clearance in front of radiator banks and avoid enclosed rooms where hot exhaust air can recirculate back into the fans.
• Verify fan rotation direction, thermostat set points and control cabinet wiring during commissioning.
• Confirm all radiator valves are open and the oil level matches the current temperature before energization.
• Clean radiator fins annually and inspect fan bearings and blades each service interval.
• Recalibrate thermostats periodically and perform infrared scans of bushings, tap changer and cable terminations.
• Schedule dissolved gas analysis at defined intervals to catch incipient faults early.

Procurement Tip: Buy at the Right Rating, at the Right Price

Much of the avoidable cost in transformer procurement comes from over-specification: buying an ONAN unit large enough for a peak that occurs a few hours a year, instead of a dual-rated ONAF transformer sized to the same peak. The second error is buying on headline price and discovering later that loss figures, fan control scope or corrosion protection were cut to reach it.

Working with a manufacturer that holds stock and builds to national standards shortens both the specification cycle and the lead time. 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, plus mining, rectifier and other custom special transformers, alongside 35 kV and below preassembled compact substations and high and low voltage switchgear. Units are delivered factory-direct with manuals, inspection reports, test certificates and lifelong quality tracking, so the documentation needed for acceptance testing arrives with the equipment rather than weeks later.

Quick Buyer Checklist

• Confirm the ONAN/ONAF dual rating you actually need, not just the peak figure
• Compare guaranteed losses, not just price
• Check fan quantity, control set points and auxiliary supply voltage
• Match corrosion class to the site environment
• Require temperature-rise and impulse test evidence for critical duty

Get these five items right and the ONAF transformer you receive will deliver its rated capacity quietly for decades, with maintenance limited to fans, thermostats and clean radiator fins.

Contact Runsheng Transformer Co., Ltd. for factory-direct pricing and technical selection support: phone +86 13406394988, e-mail runsheng@7813transformer.com, WhatsApp 008613508930968.