Why Voltage Transformers are Used. When an instrument or meter having a voltage winding is connected to a high-voltage alternating-current circuit, the use of a voltage transformer (sometimes called a potential transformer) is necessary. It is not practicable to
wind the voltage coil of a meter for direct connection to, say, an 11,000-volt supply, because the space available on the voltage electromagnet is not sufficient to accommodate the number of turns of wire which would be necessary. Moreover it would be quite impossible to insulate adequately the winding and the terminals in such a manner as to render the meter safe to handle when the circuit to which it was connected was alive. Accordingly, a voltage transformer is always used when a meter is installed for use on a high-voltage system. In this connection potentials in excess of 660 volts are regarded as high voltage.
A voltage transformer may be defined as an instrument transformer for the transformation of voltage from one value to another, usually a lower one. The primary winding of a voltage transformer is the winding to which is applied the voltage to be measured or controlled, as the case may be. The secondary winding is the winding the terminals of which are connected to the meter or instrument. The standard voltage at the terminals of the secondary winding is 110 volts.
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. In practice, the voltage per section does not usually exceed 1,000 volts, and frequently is much less than this figure. 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.
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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 cores, all the limbs being of equal cross-sectional area. Each limb carries the primary and secondary windings for one phase of the supply, and when used for connection to a meter, the connections are usually arranged star/star.
When a transformer is switched on to a live line, the voltage between the turns of the high-voltage winding adjacent to the line terminal may be raised momentarily to a value many times the normal. A similar condition may arise as a result of switching operations elsewhere on the system and in an extreme case the voltage between successive end turns may reach a very high value; such a condition persists only for a minute fraction of a second but it imposes an additional stress on the inter-turn insulation which in the absence of precautionary measures may result in failure. The stress on the insulation is greatest between the first and second turns counting from the end of the high-voltage winding and diminishes turn by turn until, at some distance from the end, the abnormal stress disappears entirely.
In power transformer practice it is customary to reinforce the insulation between the end turns of the high-voltage winding and about ten percent of the winding may be dealt with in this manner. In a voltage transformer of comparable voltage, the number of turns in the high-voltage winding is very much greater than in a power transformer and reinforcement of the insulation on ten percent of the turns would be impracticable. It is customary, however, to reinforce the insulation of the whole of the turns in the first section of the winding. As an additional precaution, a reactance coil consisting of a few turns of heavily-insulated wire is sometimes connected between the high-voltage terminal and the end of the high-voltage winding. This reactance acts like a cushion between the line and the high-voltage winding and reduces the severity of the transient stresses.
Notes
voltage [Voultid3] -
to accommodate [a'kmadeit] - ; i
to render [renda] - eiaamu; ; amu
value ['vaslju:] - ; ,
strip [strip] -
adjacent - cyc,
robust [ra'bAst] - ,
inaccessible - ,
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limb - ( )
flux - ;
precaution - ;
reactance -
to reduce -
transient - ;
to impair - ();
Comprehension questions:
1. Why voltage transformers are used?
2. How is the voltage transformer may be defined?
3. What are the voltage transformers made up?
4. What is the behavior of the voltage when a transformer is switched on to a live line?
