The cooling of transformers differs from that of rotary machinery in that there is no inherent relative rotation to assist in the circulation of ventilating air. The losses are comparatively small, and the problem of cooling can in most cases be solved by reliance on natural self-ventilation.
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: The cooling is improved by an air blast, directed by trunking and produced by a fan.
OB: The cooling of an ON-type is improved by air blast over the outside of the tank.
ON: The great majority of transformers are oil-immersed with natural cooling, the heat developed in the cores and coils is passed to the oil and hence to the tank walls.
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.
OFN: The oil is circulated by pump to natural air coolers .
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
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.
Natural Oil Cooling
The oil in the ducts, and at the surface of the coils and cores, takes tip heat by conduction,
and raises cool oil from the bottom of the tank rising to take its place. A continuous circulation of oil is completed 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 temperature, and a substantial circulation is readily obtained so long as the cooling ducts in the cores and coils are not unduly
Forced Oil Cooling
When forced cooling becomes necessary in large high-voltage oil-immersed transformers.
The choice of the method of cooling will depend largely upon the conditions obtaining at
the site. Air-blast cooling can be used, a hollow-wailed tank being provided for the transformer and oil, the cooling air being blown through the hollow space. The heat removed from the inner walls of the tank can be raised to five or six times that dissipated naturally. A cheap method of forced cooling where a natural head of water is obtainable is the use of a cooling coil, consisting of tubes through which cold water is circulated, inserted in the top of the tank. This method has, however, the disadvantage that it introduces into the tank, system containing water under a head greater than that of the oil. Any leakage will be from the water to the oil, so that there is a risk of contaminating the oil and reducing its insulating value.
The heating of the coils depends on their thermal conductivity, which is itself a function of
• Thickness of the winding
• External insulation
A coil design, which allows the copper heat to flow radially outwards with little cross insulation in the path of the flow, leads to economical rating in that a high current density can be employed for a given temperature rise without sacrifice of efficiency.