transformer-fansSince no transformer is truly an “ideal” transformer, each will incur a certain amount of energy loss, mainly that which is converted to heat. Methods of removing this heat can depend on the application, the size of the unit, and the amount of heat that needs to be dissipated. The insulating medium inside a transformer, usually oil, serves multiple purposes, first to act as an insulator, and second to provide a good medium through which to remove the heat. The windings and core are the primary sources of heat, although internal metallic structures can act as a heat source as well. It is imperative to have proper cooling ducts and passages in the proximity of the heat sources through which the cooling medium can flow so that the heat can be effectively removed from the transformer.

The natural circulation of oil through a transformer through convection has been referred to as a “thermosiphon” effect. The heat is carried by the insulating medium until it is transferred through the transformer tank wall to the external environment. Radiators, typically detachable, provide an increase in the surface area available for heat transfer by convection without increasing the size of the tank. In smaller transformers, integral tubular sides or fins are used to provide this increase in surface area. Fans can be installed to increase the volume of air moving across the cooling surfaces, thus increasing the rate of heat dissipation. Larger transformers that cannot be effectively cooled using radiators and fans rely on pumps that circulate oil through the transformer and through external heat exchangers, or coolers, which can use air or water as a secondary cooling medium.

transformer-MIDAllowing liquid to flow through the transformer windings by natural convection is identified as “nondirected flow.” In cases where pumps are used, and even some instances where only fans and radiators are being used, the liquid is often guided into and through some or all of the windings. This is called “directed flow” in that there is some degree of control of the flow of the liquid through the windings.

The difference between directed and nondirected flow through the winding in regard to winding arrangement will be further discussed with the description of winding types. The use of auxiliary equipment such as fans and pumps with coolers, called forced circulation, increases the cooling and thereby the rating of the transformer without increasing the unit’s physical size. Ratings are determined based on the temperature of the unit as it coordinates with the cooling equipment that is operating.

Usually, a transformer will have multiple ratings corresponding to multiple stages of cooling, as the supplemental cooling equipment can be set to run only at increased loads. Methods of cooling for liquid-immersed transformers have been arranged into cooling classes identified by a four-letter designation as follows:

Four letter cooling classes


Table 1 – Cooling Class Letter Description


First Letter
(Cooling medium)

OLiquid with flash point less than or equal to 300°C
KLiquid with flash point greater than 300°C
LLiquid with no measurable flash point
Second Letter
(Cooling 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
ExternalThird letter
(Cooling medium)
Fourth letter
(Cooling medium)
NNatural convection
FForced circulation

Table 1 lists the code letters that are used to make up the four-letter designation. This system of identification has come about through standardization between different international standards organizations and represents a change from what has traditionally been used in the U.S. Where OA classified a transformer as liquid-immersed self-cooled in the past, it is now designated by the new system as ONAN. Similarly, the previous FA classification is now identified as ONAF.

FOA could be OFAF or ODAF, depending on whether directed oil flow is employed or not. In some cases, there are transformers with directed flow in windings without forced circulation through cooling equipment.

An example of multiple ratings would be ONAN/ONAF/ONAF, where the transformer has a base rating where it is cooled by natural convection and two supplemental ratings where groups of fans are turned on to provide additional cooling so that the transformer will be capable of supplying additional kVA. This rating would have been designated OA/FA/FA per past standards.

SOURCE: Electric Power Transformer Engineering





  1. Prem Mishra
    Jun 19, 2014

    Great article! Gratitude for explaining a few very complex issues in a very simple manner. Would you mind explaining the followings:-
    1 Information/data about the behaviors of synthetic ester (IEC 61099) based dielectric fluid with various solid insulation’s including papers.
    2-Heat dissipation /duct size adjustment for using synthetic ester- as the perception is that low viscosity oil works better and IEC 61099 fluids are viscous- require to increase the duct size or the gap between the conductors.
    3-Is it possible to use synthetic ester in Capacitors.


  2. chathu
    Oct 07, 2012

    Great article..thanks alot for this wonderful service,,

  3. [...] Parallel transformers – SiemensMost transformers installed in parallel have the same kVA, turn ratios, and impedances, which can make it difficult [...]

  4. [...] mineral oil filled, naturally cooled (ONAN) and can be used in indoor and outdoor operations.Unlike cooling-fan or pump noise, the sound radiated from a transformer is tonal in nature, consisting of even [...]

  5. qutaibah marafi
    Jan 31, 2011

    GOOD info thanks

  6. [...] This post was mentioned on Twitter by Electric Engineering and Electric Engineering, Electric Engineering. Electric Engineering said: Transformer Cooling Classes [...]

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