Since 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.

Allowing 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 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:
Cooling Class Letter Description        Code Letter Description
Internal First Letter
(Cooling medium)
O                                                         Liquid with flash point less than or equal to 300°C
K                                                         Liquid with flash point greater than 300°C
L                                                          Liquid with no measurable flash point
Second Letter
(Cooling mechanism)
N                                                         Natural convection through cooling equipment and windings
F                                                          Forced circulation through cooling equipment, natural convection                                                                in windings
D                                                         Forced circulation through cooling equipment, directed flow in                                                                   man windings
External Third letter
(Cooling medium)
A                                                         Air
W                                                        Water
Fourth letter
(Cooling medium)
N                                                         Natural convection
F                                                          Forced circulation

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.

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