ELECTRICAL SYSTEM 407A coil design, which allows the copper heat to flow radially out-wards 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. The strip-on-edge winding, consisting of a single
layer of copper of rect-angular section wound on edge on a bakelite cylinder with one edge bare and in
contact with oil, dissipates heat most effectively. In some designs the flow of heat can be so much
improved that the transformer output entails a larger size of tank.
With cores, ageing is not to be feared when modern steels are used and correctly handled, but
heating and cooling, with the accom-panying expansion and contraction, lead sometimes to a loosening
of the core construction. Owing to the laminated nature of the cores, and the presence (on the surfaces of
the plates) of oxide films and paper, varnish, etc., the flow of heat in cores is almost wholly towards the
edges. On account of the rather greater exposure of iron to the oil in shell-type transformers, these are
better than the core-type as regards the cooling of the iron. On the other hand, the exposed coils of the
core type will cool more readily than those of the shell type.- Tanks. Small tanks are constructed from welded sheet steel, and larger ones from plain boiler-
plate. The lids may be of cast iron, a waterproof gasket being used at the joint. The fittings include
thermometer pockets; drain cock, rollers or wheels for moving the transformer into position, eyebolts
for lifting, conservators and breathers. Cooling tubes are welded in, but separate radiators are individu-
ally welded and afterwards bolted on.
Conservators are require to take up, the expansion
and contraction of the oil with changes of temperature in
service without allowing the oil to come in contact with
the air, from which it is liable to take up moisture. The
conservator may consist of an airtight cylindrical metal
drum supported on the transformer lid or on a neighbour-
ing wall, or of a flexible flat corrugated disc drum. The
tank is filled when cold, and the expansion is taken up in
the conservator. The displacement of air due to change of
oil volume takes place through a breather containing cal-
cium chloride or silica gel, which extracts the moisture
from the air. Some tank details are illustrated in Fig. 12.27.
- Transformer Oil. Oil in transformer construction serves the double purpose of cooling and
insulating. For use in transformer tanks, oil has to fulfill certain specifications and must be carefully
selected. All oils are good insulators, but animal oils are either too viscous or tend to form fatty acids,
which attack fibrous materials (e.g. cotton) and so are unsuitable for transformers. Vegetable oils (chiefly
resinous) are apt to be inconsistent in quality and, like animal oils, tend to form destructive, fatty acids.
Of the mineral oils, which alone are suitable for electrical purposes, some have a bituminous and others
a paraffin base. The crude oil, as tapped, is distilled, producing a range of volatile spirits and oils rang-
ing from the very light to the heavy, and ending with semi-solids like petro-leum jelly, paraffin wax, or
bitumen.
In the choice of oil for transformer use the following char-acteristics have to be considered.
Viscosity. This determines the rate of cooling, and varies with the temperature. A high viscosity
is an obvious disadvantage because of the sluggish flow through small apertures which it entails.
Fig. 12.27. 2000 kVA, 20/6.6 kV, 50 cycle
Transformer.