Power Plant Engineering

4. Cross-over of stranded conductors. Conductors of large cross- section are not employed, as
being too stiff to handle, and leading to excessive I2R loss. The leakage flux pulsates over the cross-
section of the windings, and may induce eddy e.m.f.’s which produce circulating currents and additional
losses (often referred to as stray losses). The conductor must consequently be subdivided for the same
reason as cores are laminated. A 7.5 mm. square conductor might be approaching the upper limit of size
for a 50 c/s transformer. If a larger section is needed, insulated strands in parallel must be used, and
balance between all strands attained by transposing their relative positions within the coil.

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ELECTRICAL SYSTEM
399
5. Insulation. The insulation between the h.v. and l.v. windings, and between l.v. winding and
core, comprises bakelite-paper cylinders or elephantide wrap. The insulation of the conductors may be
of paper, cotton or glass cape, glass tape being used for air-insulated transformers. The paper is wrapped
round the conductor in a suitable machine, preferably without overlap of adjacent turns. Paper is not
flexible, and a ‘‘half-lap’’ wrapping would cause it to buckle. The wrapped conductor is lashed with
cotton strands wound openly, to give some mechanical protection.
Fig. 12.21. Core and Windings of three-phase 200 HVA, 50 cycle
6000/440 V Core-type Transformer (Bruce Peebles).
Paper insulation usually necessitates the use of round coils, while the crossover of the several
strands in a conductor must be properly shaped, and not merely twisted.
The high-voltage winding is separated from the low-voltage winding by a series of, ducts and
Bakelite cylinders or barrels. Details of the high-voltage winding, which is sectionalized, are shown. It
will be seen that the end turns, i.e. those turns in coils 3, 4, and 6, are more heavily insulated than Nos.
6 and 7 to 23. The reason for this is connected with phenomena occurring during switching operations,
or with line disturbances. Owing to strain on the insulation between turns at the line end of the high-
voltage winding, about 5 percent of the turns are reinforced with extra insulating material.
For large h.v. Transformers the end-turn reinforcement is a matter of careful design. Merely
increasing the insulation thickness may result in markedly raising the impact thereon of impulse voltages.
The winding insulation has to be coordinated with the means adopted for controlling the distribution of
impulse electric stresses, Fig. 12.22(a).

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400
POWER PLANT ENGINEERING
10
20
30
50
70
90
90
H.V.
Axial
duct
Cylinder
barrier
Normal
coil
Radial
duct
Intermediate
reinforced
coil
Line coil
End shield
Flanged
collar
Neutral shield
L.V
.
End ring
Line shield
Line shield
Concentric barrel h.v. wolgs
( )
a
( )
b
( )
c