Power Plant Engineering

Fig. 12.22. Transformer Insulation Details.
Unimpregnated paper insulation readily absorbs oil when the transformer is inserted in its tank.
Small thin-wire coils may be varnished for adhesion. A difficulty, however, is introduced by the shrink-
ing of insulation after a few months of service, resulting in a loosening of the windings. This predisposes
to movement of the coils and breakdown on sudden short-circuit. The coils can be pre-shrunk under
pressure before assembly to simulate service conditions. They may then be assembled and tightened up
without danger of undue further shrinkage.
The permittivity of transformer oil is about 2-2; of bakelite cylinders about 4-4. The electric
stress is therefore twice as great on the oil in the annular ducts as on the bakelite cylinders.
In transformers for very high line voltages e.g. 150-220 kV. The radial width of the oil ducts is
determined by the electric stress and has to be made much wider than would be necessary from con-
siderations of cooling. On such transformers the ducts may be entirely filled with insulating paper, bent
over at the coil ends to form an earth barrier. The paper, oil-saturated, has a high dielectric strength, so
that the h.v.—l.v. spacing can be reduced. The greater freedom of choice of winding separation permits
of better control over the leakage flux. See Fig. 12.22(b).
The multi-layer h.v. barrel winding in Fig. 12.22(c) is a special helix arrangement for transform-
ers of exceptionally high line voltage. The innermost layer is wound over a neutral shield. Succeeding
layers are shortened, giving the additional clearance to the yokes appropriate to the voltage of the layer
to earth. The outermost layer, enclosed by a static shield, is connected to the line terminal, and the
successive layers joined in series to give the electric stress distribution effect of a capacitor bushing to
surge voltages applied to the line terminal. The line and neutral shields may comprise close-wound
helices of conducting strip material applied over the appropriate insulated paper layers: the strips are
connected electrically, but arranged so that they do not form a short-circuited turn. Tappings are usually
required on modern transformers.

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ELECTRICAL SYSTEM
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