Transformer Cooling Methods: 6 Codes That Prevent Failure

Transformer cooling methods describe how heat moves from the windings to the oil and from the oil to the outside air or water. The four letter code on the nameplate tells you exactly how that happens.

What Are Transformer Cooling Methods?

Transformer cooling methods are the ways a transformer removes the heat produced by its core and winding losses. Every watt lost in the efficiency calculation ends up as heat that must leave the tank.

If that heat is not removed, winding temperature rises and insulation ages quickly. Paper insulation life roughly halves for every few degrees above its design hotspot temperature.

Small dry type units rely on air alone, while most power transformers use mineral oil as both insulation and coolant. The oil carries heat from the windings to radiators, where air or water takes it away.

Many transformers carry two or three ratings on one nameplate, such as ONAN/ONAF. Each rating matches a different cooling stage.

4 Letters of a Cooling Code

So ONAN means oil natural, air natural, with no pumps or fans at all. ONAF means oil natural, air forced, where fans blow across the radiators.

The coding follows IEC 60076 Part 2 and IEEE C57.12.00. Older American nameplates may still show codes such as OA and FA.

6 Common Transformer Cooling Methods

Cooling moves from passive to active as size grows. A small distribution transformer from our kVA rating guide is almost always ONAN.

K class ester fluids have a fire point of at least 300 °C, compared with roughly 160 to 180 °C for mineral oil. That makes KNAN popular for buildings, schools and vaults.

How Cooling Stages Raise the Rating

These typical increases come from Giga Energy guidance based on IEEE practice. A 10 MVA ONAN unit might therefore carry about 12.5 MVA with fans running.

Fans usually start automatically from the winding or oil temperature indicator. A temperature switch with two set points can switch stage 1 and stage 2 fans in sequence.

Heat Flow Inside an Oil Filled Transformer

This natural loop works because hot oil is lighter than cool oil, and oil has useful thermal conductivity. Radiators are mounted high enough to keep the thermal head strong.

Forced and directed oil methods add pumps to speed up this loop. Directed flow sends oil straight through the winding ducts, lowering the hotspot compared with the average winding temperature.

Transformer Loss and Heat Formula

A 25 percent higher load produces about 50 percent more heat. That is why the fan stage must be working whenever the transformer runs above its ONAN rating.

Monitor oil and winding temperatures with suitable sensors. Many utilities fit RTD sensors for remote temperature indication.

Choosing the Right Cooling Class

Check cooling equipment during routine and type tests. The temperature rise test proves each rating on the nameplate.

Remember that fans and pumps are auxiliaries that can fail. Their supply and alarms should be monitored just like the main protection.

Transformer Loss Calculator

Use the test report values for P0 and Pk. Compare the result with what each cooling stage can remove.

ONAN and ONAF Radiator Brochure

ONAN vs ONAF Video Explanation

Transformer Cooling Methods FAQ

Related Articles

• Transformer Efficiency Calculation
• Transformer Routine and Type Tests
• Transformer kVA Rating Calculation
• Transformer Vector Group Explained
• Protective Relays Explained

External References

• Understanding Transformer Cooling Methods, Giga Energy
• Transformer Cooling System and Methods, Electrical4U
• Guide to Transformer Cooling Classes, Maddox
• T REX ONAN ONAF Radiators, Thermofin
• Transformer Oil, Wikipedia