Power Plant Engineering

Fig. 12.23
The oil-immersed ends of h.v. bushings may be of re-entrant form, reducing the immersed length
and permitting a more uniform distribution of the axial and radial electric stress components.
12.4 COOLING OF TRANSFORMERS
Consider a transformer with k times the linear dimensions of another smaller but otherwise simi-
lar unit. Its core and conductor areas are k
2
times greater and its rating (with the same flux and current
densities) increases k
4
times, The lasses increase by the factor k
3
but the surface area is multiplied only
by the factor V. Thus the loss per unit area to be dissipated is increased k times. Large transformers are
therefore more difficult to coal than small ones, and require more elaborate methods.
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. Luckily the losses are comparatively small,
and the problem of cooling (which is essentially a problem of preserving the insulation-solid and liquid-
from deterioration) can in most cases be solved by reliance on natural self-ventilation. The various
methods are.

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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.