AN vs AF Cooling for Dry-Type Transformers: Engineering Selection Guide

A specification stating “Cooling: AN/AF” looks simple, but it can hide a major engineering decision.

One project may use AN natural air cooling for the transformer's full normal operating duty and start the fans only during short load peaks. Another project may depend on AF forced air cooling to reach the required kVA.

Those two designs carry very different operational risks.

Fan-assisted cooling is not simply an accessory added after transformer sizing. Winding thermal design, airflow paths, enclosure ventilation, ambient temperature, altitude, cables, busbars, protection devices and the transformer room all affect the usable rating.

IEC 60076-11:2018 covers relevant dry-type power transformers up to a highest voltage for equipment of 72.5 kV and addresses areas including enclosure performance and altitude-related thermal and dielectric considerations.

For EPC contractors and procurement teams, AN vs AF cooling for dry type transformers should therefore be evaluated against the project load profile rather than treated as a checkbox on the datasheet.

The Real Issue Is Thermal Capacity, Not the Number of Fans

Under AN cooling, heat from the windings and core is removed through natural air circulation around the transformer.

With AF cooling, fans increase airflow through and around the windings so that heat can be transferred away faster. Siemens Energy documentation likewise identifies AN as natural air cooling and AF as forced air cooling.

The engineering question is what electrical load the transformer can carry while keeping its thermal condition within the specified limits.

A transformer that is designed for 1,600 kVA under AN operation does not automatically become a safely rated 2,000 or 2,240 kVA transformer because fans are installed on site.

The purchaser should therefore request two clearly defined figures when AF capacity is relevant:

AN rated capacity

and

AF permissible capacity or overload capability

If the nameplate shows two ratings, such as 1,000 kVA AN and 1,400 kVA AF, the technical offer should also state whether the AF value is intended for continuous duty or a defined overload period.

AN Is Usually the Simpler Reliability Strategy

Where the transformer normally operates well below its rated kVA, natural air cooling often provides the cleaner operating philosophy.

There are fewer moving components.

There is less dependence on auxiliary control power.

Fan maintenance is reduced.

A fan failure does not immediately remove the transformer’s basic AN capability.

Noise is generally easier to control.

This matters in commercial buildings, hospitals, airports, office complexes and critical power systems where predictable operation may be more valuable than extracting the maximum possible kVA from the transformer footprint.

An AN transformer can still be supplied with an AF system as operational reserve.

That configuration gives the project a clear base rating that does not depend on the fans.

AF Is Most Valuable Where the Load Has Predictable Peaks

Forced air cooling is particularly useful where the load profile changes significantly during the day.

Typical examples include large motors starting at the same time, production lines reaching peak output, HVAC demand increasing during hot periods or an industrial plant expecting staged expansion.

In those cases, AF gives the transformer more thermal flexibility.

A winding temperature controller can start the fans when winding temperature reaches a defined level and stop them after the thermal condition falls.

This is very different from deliberately undersizing the transformer and running the fans continuously just to meet the normal plant load.

That distinction should be established during the tender stage.

Do Not Assume AF Always Gives 40% More Capacity

This is one of the most common procurement mistakes in dry type transformer cooling specifications.

Some transformer designs do offer substantial fan-assisted capacity increases.

Schneider Electric, for example, states that certain TRIHAL transformers can obtain a temporary 40% power increase with forced ventilation. Its guidance also requires the project to review cable or busbar cross-sections, circuit-breaker ratings and transformer-room air inlet and outlet sizing.

That is a product-specific engineering statement, not a universal rule for every dry transformer.

Before accepting a quoted dry type transformer AF overload capacity, ask the manufacturer to state:

the AN rating;

the AF rating;

whether the AF rating is continuous or time-limited;

the maximum design ambient temperature;

the design altitude;

the enclosure configuration;

the applicable temperature-rise limits;

the allowable load after fan failure.

A supplier statement such as “fans increase capacity by 40%” is not enough on its own.

Engineering Scenario 1: Commercial Building or Data Center

Consider a 10/0.4 kV, 1,600 kVA cast-resin transformer installed in an indoor electrical room.

Its normal operating load is around 1,000–1,250 kVA, but the load rises periodically when cooling equipment, UPS systems or other large loads operate together.

For this profile, the better design approach may be:

1,600 kVA under AN as the base operating rating, with AF available for peak-load support.

The project retains a defined transformer capacity even if a cooling fan is unavailable.

For a data center, hospital or continuous-process facility, one practical tender question is therefore more useful than simply asking how many fans are installed:

“What continuous load can this transformer carry if all AF fans are unavailable?”

The answer defines the true operating fallback.

Engineering Scenario 2: Industrial Plant with Cyclic Peak Loads

Now consider a 2,500 kVA dry-type transformer in a cement plant, mining facility, steel-processing line or large manufacturing plant.

Normal loading may be around 1,800–2,100 kVA, with higher loads during certain production periods.

An AN/AF dry type transformer can work well under this duty profile if the peak duration and thermal limits are properly defined.

The temperature controller starts the fans during high thermal loading.

When production demand falls and the winding temperature decreases, the fans can stop.

If the plant already knows that future normal demand will remain close to 3,000 kVA, the EPC team should compare two alternatives:

a 2,500 kVA AN/AF transformer;

a higher-rated transformer capable of carrying the required load under AN.

The cheaper purchase price is not automatically the lower-risk project solution.

AF Cannot Compensate for a Poorly Ventilated Transformer Room

Fans move air through the transformer.

They do not remove heat from the electrical room by themselves.

If hot air cannot leave the room effectively, the fans may simply circulate increasingly warm air around the windings.

This is why transformer room ventilation must be evaluated together with the transformer.

Check the air inlet area, outlet location, maximum ambient temperature, heat generated by nearby equipment, enclosure IP rating, clearance around the transformer and any obstruction created by walls, cable trenches or adjacent switchgear.

This becomes even more critical when several transformers are installed in the same room.

IEC 60076-11:2018 includes specific treatment of enclosure performance and altitude-related thermal conditions.

A high-protection enclosure can materially change the cooling condition compared with an open transformer installed in a well-ventilated room.

Reliability and Maintenance: AF Adds More Failure Points

The fan cost is usually small compared with the transformer itself.

The more relevant issue is the extra equipment introduced into the operating system:

dry type transformer cooling fans;

temperature sensors;

temperature controller;

relays;

auxiliary supply;

control wiring.

Every one of these components becomes a maintenance item.

For projects in remote areas, mines, industrial plants or overseas locations where replacement parts may take weeks to obtain, spare fans and a spare temperature controller can be inexpensive insurance.

Procurement teams should also record the fan model, airflow, voltage, frequency and mounting dimensions in the final documentation.

Procurement Risk: Using AF to Hide an Undersized Transformer

Suppose a project requires approximately 2,500 kVA during normal operation.

One supplier proposes:

2,500 kVA AN.

Another offers:

2,000 kVA AN / 2,500 kVA AF.

The second quotation may look cheaper.

It should not be considered technically equivalent until several questions are answered.

Can 2,500 kVA be carried continuously under AF?

What temperature rise is guaranteed?

What happens after a fan failure?

Are the LV busbars and cables sized for the AF current?

Is the main circuit breaker rated and set for that load?

Can the room ventilation remove the higher heat losses?

Does the nameplate clearly show both ratings?

The central procurement principle is simple:

Do not allow AF cooling to convert a predictable continuous load into a permanent dependency on cooling accessories unless the complete system has been engineered for that duty.

Export Projects Need a Separate Check of Fan and Control Voltage

Correct transformer primary and secondary voltages do not automatically mean the AF system is compatible with the site.

A transformer may be specified as:

11 kV / 415 V / 50 Hz;

10 kV / 400 V / 50 Hz;

13.8 kV / 480 V / 60 Hz.

The dry transformer fan system and temperature controller may use a separate auxiliary voltage.

Before production, the factory should confirm:

fan rated voltage;

50 Hz or 60 Hz;

single-phase or three-phase supply;

source of auxiliary power;

temperature-controller voltage;

available site control voltage.

This is particularly relevant in export projects where 110 V, 120 V, 220 V, 230 V and other auxiliary arrangements may be encountered.

Voltage mismatch in a fan circuit is a small specification error that can create a significant commissioning delay.

What Should Be Checked During FAT?

For an AN/AF unit, a dry type transformer FAT test should cover more than the main transformer electrical performance.

The IEC 60076 series provides the technical framework for relevant transformer testing, while IEC 60076-3 addresses insulation levels and dielectric tests.

Depending on the agreed specification, the FAT package may include checks covering winding resistance, ratio and vector group, no-load loss and current, load loss and impedance, dielectric performance, partial-discharge requirements and the temperature monitoring system.

The AF system also needs functional verification.

Simulate an increase in winding temperature.

Confirm the fan start setting.

Verify fan stopping logic.

Check high-temperature alarm contacts.

Check overtemperature trip contacts.

Confirm fan rotation direction.

Verify the airflow direction.

Listen for abnormal vibration or bearing noise.

Check fan and controller nameplate voltages.

Compare terminal numbers against the approved wiring diagram.

A temperature-rise test should be specified according to the contract and agreed test program rather than assumed to be identical for every order.

Packing and International Transportation Need AF-Specific Attention

The cast-resin windings and transformer frame are mechanically robust, but external fans, control boxes and sensor wiring can be more vulnerable during international transportation.

For sea shipment, the export packing plan should check whether the fan assemblies require extra mechanical protection, whether the controller and terminal boxes are adequately protected against moisture, whether sensor cables are securely fixed and whether spare components are clearly identified in the packing list.

After arrival, the commissioning team should inspect the fan impellers before energization.

A bent fan bracket or damaged impeller is a small transportation issue that can quickly become a cooling or vibration problem after installation.

A Practical AN or AF Selection Rule

Use AN cooling as the primary operating mode when the load is stable and remains comfortably within the transformer rating.

Consider AN/AF cooling when the project has predictable short-duration load peaks.

If normal production requires AF operation for long periods, compare the proposal against the next higher AN transformer rating.

For critical infrastructure, evaluate the transformer load capability after fan failure.

For high ambient temperature, high altitude or restrictive enclosures, request a manufacturer review of dry type transformer temperature rise rather than applying a standard catalogue rating without correction.

The objective is not to decide whether AN or AF is universally superior.

The objective is to establish a clear operating limit with the fans running and another clear operating limit with the fans unavailable.

FAQ

1. Which is more reliable, AN or AF cooling?

AN has fewer moving components and less dependence on auxiliary systems. AN/AF can provide greater load flexibility when peak duty is part of the project design.

2. Can AF increase dry-type transformer capacity by 40%?

Some designs can provide an increase in this range, but it is not a universal rating rule. Use the manufacturer's thermal design, nameplate rating and approved technical data.

3. Can fans be added to an AN transformer later?

Not automatically. Future AF operation should be considered during transformer design because winding thermal capability, airflow paths, fan installation, cables, busbars and protection equipment may all be affected.

4. Should AF fans operate continuously?

They are often controlled automatically from winding temperature rather than operated continuously. The control philosophy should follow the project load profile.

5. What happens if an AF fan fails?

The transformer may need to return to its allowable AN load. The exact limit must come from the manufacturer and project documentation.

6. What should be tested during FAT for an AN/AF transformer?

Check the agreed electrical routine tests plus temperature sensors, controller logic, fan start/stop operation, alarms, trip contacts, rotation direction, airflow, auxiliary voltage and control wiring.

7. Is AF recommended for hot climates?

AF improves airflow through the transformer but cannot replace room ventilation. High ambient temperature must still be included in the transformer thermal design.

8. What information should be sent when requesting an AN/AF quotation?

Provide rated kVA, HV/LV voltage, frequency, normal load, peak load and duration, ambient temperature, altitude, enclosure requirement, installation environment, required AN/AF ratings, vector group, project country, IEC requirements and FAT scope.

Call To Action

If your project is evaluating AN vs AF cooling for dry type transformers, send more than the transformer kVA and voltage.

Provide the required capacity, HV/LV voltage, 50 or 60 Hz frequency, normal load, maximum peak load and duration, ambient temperature, altitude, indoor or outdoor installation, enclosure IP level, required AN/AF duty, project country, applicable IEC specification and FAT requirements.

Huamei Electric can review the AN base rating, AF load capability, temperature-rise design, fan-control arrangement, auxiliary voltage, transformer-room ventilation interface and export packing requirements before the transformer enters production.

This gives EPC contractors, electrical engineers and procurement teams a clearer basis for comparing technically different quotations rather than comparing kVA and price alone.