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.
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