ELECTRICAL SYSTEM 403
12.4.1 Simple Cooling
AN: Natural cooling by atmospheric circulation, without any special devices. The transformer
core and coils are open all round to the air. This method is confined to very small units at a few kV at low
voltages.
AB: In this case the cooling is improved by an air blast, directed by suitable trunking and pro-
duced by a fan.
ON: The great majority of transformers are oil-immersed with natural cooling, i.e. the heat de-
veloped in the cores and coils is passed to the oil and thence to the tank walls, from which it is dissi-
pated. The advantages over air-cooling include freedom from the possibility of dust clogging the cool-
ing ducts, or of moisture affecting the insulation, and the design for higher voltages is greatly improved.
OB: In this method the cooling of an ON-type transformer is improved by air blast over the
outside of the tank.
OFN: The oil is circulated by pump to natural air coolers.
OFB: For large transformers artificial cooling may be used. The OFB method comprises a forced
circulation of the oil to a refrigerator, where it is cooled by air-blast.
OW: An oil-immersed transformer of this type is cooled by the circulation of water in cooling
tubes situated at the top of the tank but below oil-level.
OFW: Similar to OFB, except that the refrigerator employs water instead of air blast for cooling
the oil, which is circulated by pump from the transformer to the cooler.
12.4.2 Mixed Cooling
ON/OB: As ON, but with alternative additional air-blast cooling. ON/OFN, ON/OFB, ON/OFW,
ON/OB/OFB, ON/OW/OFW: Alternative cooling conditions in accordance with the methods indicated.
A transformer may have two or three ratings when more than one method of cooling is provided.
For an ON/OB arrangement these ratings are approximately in the ratio 1/1.5; for ON/OB/OFB in the
ratio 1/1.5/2.
12.4.3 Natural Oil Cooling
The diagram in Fig. 12.24 is drawn to indicate on the left the thermal flow of oil in a transformer
tank. The oil in the ducts, and at the surface; of the coils and cores, takes tip heat by conduction, and
rises cool oil from the bottom of the tank rising to take its place. A continuous circulation of oil is com-
pleted by the heated oil flowing to the tank sides (where cooling to the ambient air occurs) and falling
again to the bottom of the tank. Oil has a large coefficient of volume expansion with increase of tem-
perature, and a substantial circulation is readily obtained so long as the cooling ducts in the cores and
coils are not unduly restricted.
Fig. 12.24 also shows on the right a curve typical of approximate temperature distribution, the
figures quoted being rises in degrees centigrade. On full load with continuous operation, the greatest
temperature-rise wills probably he in the coils. The maximum oil temperature may be about 10° less
than the coil figure, and the mean oil temperature another 15° less.
The best dissipater of external heat is a plain blacked tank. But to dissipate the loss in a large
transformer a plain tank would have an excessively large surface area and cubic capacity, and would
require a great quantity of oil. Both space and oil are expensive.