Power Plant Engineering

402 POWER PLANT ENGINEERING

Voltage

Ta p P l u g

Current

Transformer

Arc

shield

Te r m i n a l B o x

for current

Transformer

SecondaryLeads

Insulation

spaces

for Tubes

Bakekite

tube

Bushing

Transformer

Te r m i n a l B o x

for Current

Transformer

Secondary

LEads

Capacitor Type Oil-Filled Type

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.