Power Plant Engineering

Fig. 12.24. Oil-Circulation and Temperature
Distribution.

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ELECTRICAL SYSTEM
405
Conservator
Radiator
For O.B.
Oil pump
for O,F,B,
( ) O.B. and O.F.B. types
e
Fan
motor
Oil
pump
Water inlet
Water outlet
Radiator
( ) O.F.W. type
f
Fig. 12.25. Cooling Methods.
For the largest sizes the radiator type of cooling is used, where separate radiator tanks with fins or
corrugations, connected at top and bottom to the main tank, dissipate the heat by oil circulation. One
such arrangement is shown diagrammatically in Fig. 12.25(d).
The limit of output with oil-insulated, self-cooled transformers is reached when the tank be-
comes too large and costly. Another limit, which obtains in some cases, is the railway-loading gauge,
which pre-cludes the transport of transformer F and tanks exceeding a certain size. In carriage by road
the available routes with their grades, bridges, etc., may decide the type of transformer and tank that can
be used. For larger units, transport in parts must be resorted to, with erection on site. A tank dissipating
about 50 kW is regarded by some manufacturers as the limit for self-cooling.
12.4.4 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, so that very large transformers can be cooled in this way.
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 a, system containing water
under a head greater than that of the oil. Any leakage will, therefore, be from the water to the oil, so that
there is a risk of contaminating the oil and reducing its insulating value. Fins are placed on the copper
cooling tubes to assist in the conduction of heat from the oil, since heat passes three times as rapidly
from the copper to the cooling water we from the oil to the copper tubing. The inlet and outlet pipes are
lagged to avoid water from the ambient air condensing on them and getting into the oil.
For large installations the best cooling system appears to be that in which oil is circulated by
pump from the top of the transformer tank to a cooling plant, returning when cold to the bottom of the
tank. When the cooling medium is water, this has the advantage that the oil can be arranged to work at a

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406
POWER PLANT ENGINEERING
higher static head than the water, so that any leakage will be in the direction of oil to water. The system
is suitable for application to banks of transformers, but for reliability not more than, say, three tanks
should be con-nected in one cooling pump circuit.
Fig. 12.25(e) and (f) shows diagrammatically the usual methods of cooling employed where
separate radiators are necessary. The oil circulation pump in (e) is incorporated only if the natural ther-
mal head is insufficient to generate an adequate oil flow.
Until recently all large units employed oil-circulating systems, but considerable advances have
been made towards increasing the size of self-cooled units by special radiators. It is possible to build
entirely self-cooled units up to 40000 kVA, with the advantage of eliminating breakdown risks due to
auxiliary pumping equipment. The addition of an air-blast system to circulate cooling air over the radia-
tors permits the increase of size to about 75000 kVA. Although an auxiliary fan is involved, the trans-
former is still capable of half- load operation should the air blast fail. A temperature device can be used
to bring the fan into action when the oil temperature reaches a desired limit; this improves the overall
efficiency at small loads. An arrangement of this type is illustrated in Fig. 12.26.
Tank-less, air-insulated. transformers have been built up to 1500 kVA, but larger sizes require
forced air circulation.