Understanding Cooling Classes in Substation Transformers

A large transformer with three fans on top, used for cooling in substation transformers.

Understanding Cooling Classes in Substation Transformers

Substation transformers play a vital role in the power distribution network by converting high-voltage electricity into lower voltages suitable for local usage. These Substation transformers are subjected to varying loads and environmental conditions, making efficient cooling mechanisms essential to their proper functioning and longevity.

Cooling classes categorize the different methods used to dissipate heat generated during transformer operation. In this blog, we’ll explore the various cooling classes in substation transformers.

The Importance of Cooling

As electricity flows through a transformer’s core and windings, heat is generated due to resistive losses. If not managed effectively, excessive heat can lead to transformer failure and reduced operational efficiency. Cooling systems are designed to maintain the transformer’s temperature within acceptable limits, thereby ensuring its reliability and preventing premature aging. If you want to learn more about why transformers fail read our blog about that topic, Why do Electrical Transformers Fail

Cooling Classes Explained

There are several cooling classes defined by international standards, with each class representing a specific cooling method. These classes are designated by a combination of letters and numbers, and they indicate how the transformer dissipates heat. There are 4 letters that are indicators of the type of cooling in new transformers before we delve into the different cooling classes, let’s explain how these four-letter codes are designated:

First LetterCooling medium (Internal)
OLiquid with a flash point less than or equal to 300°C
KLiquid with a flash point greater than 300°C, also means a biodegradable fluid, like FR3.
LLiquid with no measurable flashpoint
Second LetterCooling mechanism
NNatural convection through cooling equipment and windings
FForced circulation through cooling equipment, natural convection in windings
DForced circulation through cooling equipment, directed flow in man windings
Third LetterCooling medium (External)
Fourth LetterCooling mechanism
NNatural convection
FForced circulation

Some of the most common cooling classes are as follows: ONAN, ONAF, KNAN, and KNAF. There will beall sorts of different cooling classes depending on the location and other specifics about the project.

There can also be two different cooling classes on a nameplate for a substation, this is called a second stage of cooling. If you look at the nameplate on the left, you can see it says 2500/2800/3500 kVA then ONAN/ONAN/ONAF. What this means is the first two are naturally cooled via oil and air. Then last kVA rating is with air forced via a fan attached to the radiators. With better cooling attached it can reach a higher capacity before getting too hot.

Understanding Cooling Classes in Substation Transformers


Cooling classes in substation transformers provide a standardized way to categorize and understand the cooling mechanisms employed to maintain optimal operating temperatures. The right cooling class is selected based on factors such as ambient conditions, load requirements, and transformer design.

Proper cooling ensures not only the efficient functioning of the transformer but also its longevity and reliability within the power distribution network. As technology continues to advance, transformer cooling methods will likely evolve, further enhancing the efficiency and sustainability of power transmission and distribution systems.

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