Техническое чтение для энергетиков. Бухарова Г.П. - 66 стр.

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68
the transformer must be kept down to a minimum. Clearances between conductors or
other live parts, which in power transformer design are regarded as minimum values,
cannot always be provided in voltage transformers, and as reliability is the first
consideration it is only by skilful design and care in manufacture that safety can be
assured.
A voltage transformer comprises a magnetic circuit, usually built, up with iron strips
assembled together to form a core on which the primary and secondary windings are
mounted. The primary winding which is connected to the high-voltage supply
consists of a large number of turns of a fine-gauge wire* and is usually divided into a
number of separate sections. The object of dividing the primary** in this manner is to
limit the voltage across each section to a comparatively low value. In practice, the
voltage per section does not usually exceed 1,000 volts, and frequently is much less
than this figure. Each section consists of layers of wire, 3/8 in. to 3/4 in. wide, with a
strip of paper or other insulating material to separate the layers. For mechanical
reasons and in order to minimize the risk of breakage and open circuits, wire smaller
than 36 W. G. (0.0076 in. dia.), is seldom used in the primary winding.
In voltage transformers for operation at 6,600 volts or less, it is common practice
for the sections of the primary to be assembled on a tube of insulating material
adjacent to the core, a second tube surrounding the sections and carrying the
secondary winding. This disposition of the windings is advantageous in the case of
open-type transformers since the high-voltage winding is shielded from
mechanical damage by the two tubes and only the more robust low-voltage winding
is exposed. For voltages in excess of 6,600 volts this arrangement is undesirable,
partly because the joints between sections which are increasing in number are
inaccessible, and partly because of the increasing cost of the two tubes, both of
which must be capable of withstanding the full working voltage
continuously. The alternative disposition in which the secondary winding is
adjacent to the core and is surrounded by the primary, is more usual, a heavy tube
separating the windings. Only a light tube separates the secondary from the core and
no mechanical protection is provided over the high-voltage windings. This,
however, is unnecessary since transformers for the higher voltages are protected by a
tank or other enclosure containing oil or some other insulating medium.
Voltage transformers are made up in single units for connection to single-phase, two-
phase or three-phase systems. The magnetic circuit of a single-phase voltage
transformer may be of the core type or the shell type, somewhat similar in shape to
the cores of current transformers. The windings are usually disposed on both limbs of
the core-type carcase and on the middle limb of the shell-type. The shell-type
construction is seldom employed where the system voltage exceeds 3,300 volts. Two-
phase voltage transformers are required occasionally and if made up as single units, a
three-limbed core is used, similar in shape to the shell-type current transformer core.
The windings are disposed on the two outer limbs and as the middle limb carries the
common flux for both phases It is of greater sectional area than the outer limbs. The
more usual practice however is to use two separate single-phase transformers on a
two-phase system. Three-phase voltage transformers are built up on three-limbed

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