Transformer Cooling Systems: ONAN, ONAF, OFAF and OFWF Explained

A transformer is a remarkably efficient machine, but it is not perfectly efficient. The small percentage of energy lost as heat in the windings and core — load losses in the copper and no-load losses in the steel — must go somewhere, and in a large transformer that somewhere is a great deal of heat. If that heat is not removed, insulation ages faster, oil degrades, and the transformer cannot carry its rated load. The cooling system is therefore not an accessory; it is what makes the rating possible. This guide explains how transformer cooling works, what the familiar letter codes mean, what each piece of equipment does, and how cooling is controlled and checked in service.

As with everything on this site, always work to your project specification, the manufacturer's instructions and your site safety rules; the general guidance here never overrides those documents. Fans, pumps and hot surfaces are worked on only by competent persons under permit, with the equipment isolated where the procedure requires it. If you are new to transformer testing as a whole, start with our power transformer testing and commissioning guide, which puts cooling in context alongside the electrical tests.

Why transformers need cooling, and where the heat comes from

Heat in a transformer comes from two main sources. Load losses, sometimes called copper losses, are produced by current flowing through the winding resistance, and they rise with the square of the load current — double the load and the winding heat roughly quadruples. No-load losses, sometimes called iron losses, come from magnetising the core and from eddy currents in the steel, and they are present whenever the transformer is energised, even at no load. Together they warm the windings, the core, the oil around them, and eventually the tank. The insulation system — paper and oil — ages faster the hotter it runs, roughly doubling its ageing rate for every sustained temperature rise of several degrees, which is why temperature limits and cooling matter so much for transformer life.

Oil does double duty here, and that is why most large transformers are oil-filled. It is both an insulator and a coolant: it picks up heat from the windings and core, carries it to the tank walls or to external radiators, gives it up to the surrounding air or to a water circuit, and returns cooler to do the job again. The cooling system is simply the engineered path for that journey, and its design decides how much load the transformer can carry without exceeding its temperature limits. A transformer with failed cooling is a transformer that must be de-rated or taken out of service, even if its windings and insulation are perfectly healthy.

Decoding the cooling letters: ONAN, ONAF, OFAF, OFWF

Cooling systems are described by a four-letter code, and once you know the pattern it reads like a sentence. The first letter names the internal cooling medium in contact with the windings: O stands for mineral oil, and other letters cover alternatives such as synthetic liquids or air in dry-type units. The second letter says how that internal medium circulates: N means natural circulation, driven by convection as hot oil rises and cool oil sinks, while F means forced circulation, driven by pumps. The third letter names the external cooling medium: A for air, W for water. The fourth letter says how that external medium moves: N for natural draught, F for forced draught by fans or pumped water.

So ONAN means oil, natural circulation, air, natural draught: the classic self-cooled transformer with radiators and no fans or pumps. ONAF adds fans that blow air over the radiators when needed. OFAF adds oil pumps as well, forcing oil through the radiators so heat moves faster on both sides of the heat exchanger. OFWF replaces the air side with water: oil is pumped through a water-cooled heat exchanger, used where air cooling is impractical or where water is readily available, such as hydro stations or indoor installations. Many large transformers carry two ratings on their nameplate, for example ONAN/ONAF, meaning the lower rating applies with fans off and the higher rating with fans running. That dual rating is the clearest possible statement of what cooling is worth: it is directly worth megawatts of capacity.

Radiators, fans and pumps: the equipment itself

Radiators are the workhorse of air-cooled transformers. They are banks of thin panels or tubes attached to the tank, giving the oil a large surface area exposed to air. Hot oil from the top of the tank flows into the top of the radiators, cools as it falls through them, and returns to the bottom of the tank, cooler and denser. In a natural-circulation system this thermosiphon effect drives the whole flow, which is why radiator valves must be fully open in service and why a radiator isolated by a closed valve shows up as a cold bank and a hot transformer. Radiators also collect dust, leaves, bird nests and industrial deposits, and a fouled radiator loses a surprising amount of its cooling ability — cleaning them is one of the highest-value maintenance tasks there is.

Fans are mounted to blow air across the radiator banks, and they are usually arranged in groups that start in stages as temperature or load rises. Pumps, where fitted, are mounted in the oil pipework and force oil circulation; they have their own motors, bearings, seals and power supplies, and a failed pump on a forced-oil transformer is as serious as a failed fan group. Both fans and pumps are checked in service by observation and by sound and vibration: a fan that is not running when its control scheme calls for it, a pump running backwards after maintenance, or a group that trips its supply will all show up first as rising temperature, often hours before any alarm. Cooling equipment checks belong in every transformer maintenance schedule, not just in outage work.

Cooling control: how the system decides what to run

Cooling control is usually automatic, staged by winding temperature, top-oil temperature, or load current, and sometimes by a combination. A typical scheme might start the first fan group at a moderate winding temperature, add a second group at a higher temperature, and start the oil pumps — where fitted — at a still higher threshold or above a set load. Manual and automatic modes are both provided, and the control panel shows which groups are running and which are in auto. Temperature gauges, including winding temperature indicators that simulate winding temperature from load current and top-oil temperature, drive both the control stages and the alarms and trips, so their accuracy matters: a faulty temperature indicator can leave fans off when they are needed, or run them constantly and mask a real problem.

Control schemes also include failure alarms: fan group failure, pump failure, loss of cooling power supply, and high temperature alarms at one level with tripping at a higher level. Commissioning a cooling system means proving all of this, not just switching the fans on once. Each group is run individually, its airflow and direction checked, its control stage proved by simulating the temperature or load signal, its failure alarm proved by tripping its supply, and the whole scheme left in automatic. A cooling system that has never been proved stage by stage is an assumption, and assumptions about cooling are how transformers overheat quietly on the hottest day of the year.

When temperatures rise: a troubleshooting order

When a transformer runs hotter than usual, work through the causes in order of likelihood and ease of checking. First, the cooling system itself: are the fans and pumps running as the scheme intends, are radiator valves open, are radiators clean and unobstructed, is the cooling power supply healthy, and are the temperature instruments reading plausibly? Second, the operating conditions: is the load higher than usual, is the ambient temperature unusually high, and is the transformer carrying a load above the rating for its current cooling stage — for example, above its ONAN rating with fans failed? Third, the transformer itself: blocked oil ducts inside the tank, a failed internal pump, or a developing internal fault will all raise temperature, and these are investigated with oil analysis and electrical tests, including the tests in our oil testing guide.

One trap deserves special mention: comparing temperatures without comparing conditions. A transformer running at a higher temperature on a hot afternoon at full load is not necessarily in trouble; the same temperature at light load on a cool night is a different matter entirely. Always record load, ambient temperature, cooling stage and oil and winding temperatures together, so trends mean something. Temperature records kept this way are also what justify a decision to de-rate a transformer, schedule an outage, or investigate further — and they are what protect an engineer who says a unit needs attention before it fails. Cooling problems are rarely dramatic; they are gradual, and gradual problems are caught by records, not by alarms alone.

Cooling and the rest of the transformer

Cooling touches every other part of transformer life. Hot oil ages faster and produces more gas, which is why a cooling failure often shows up first in oil and gas analysis and in Buchholz relay behaviour. Overheating accelerates paper ageing, which shortens transformer life even if nothing fails outright. Cooling equipment itself needs maintenance — fan motors, pump seals, control contacts, temperature gauges — and that maintenance is planned alongside overhauling and protection work, not as an afterthought. And when a transformer is specified, its cooling system is specified with it: the rating, the ambient temperature, the altitude, the available cooling medium and the redundancy of fan and pump groups are all part of the purchase, because they decide what the transformer can actually do on site, not just on paper.

The practical lesson is simple. Treat the cooling system as a protection system, because in effect it is one: it protects the insulation from the slow damage of heat, and its failure is one of the commonest causes of premature transformer ageing. Keep radiators clean, prove fan and pump stages, record temperatures with load and ambient conditions, and investigate rising temperature early, while the fix is still cleaning, a fan motor or a control contact rather than a rewind.

Frequently asked questions

What do the letters in ONAN and ONAF mean?

Each letter describes one part of the cooling path. The first letter is the internal coolant and how it moves: O means mineral oil, N means natural circulation. The second pair describes the external cooling medium and how it moves: AN means air, natural; AF means air, forced by fans.

Why does a transformer have two ratings, one for ONAN and one for ONAF?

Because forced cooling removes heat faster, the same transformer can carry more load with its fans running than it can on natural cooling alone. The ONAN rating is the load it can carry with fans off; the ONAF rating is the higher load permitted with fans on.

What should be checked first when a transformer runs hotter than usual?

Check the cooling system before suspecting the transformer itself: are the fans and pumps running as the control scheme intends, are radiators clean and unblocked, is oil circulating, and are the temperature gauges and alarms reading correctly. Only then look at load, ambient temperature and internal condition.