an instrument transformer for stepdown of voltage in measurement and monitoring circuits. If a voltage transformer is used, the circuits of voltmeters, frequency meters, electric meters, and automatic control and monitoring devices can be isolated from high-voltage circuits; this makes possible the standardization of the rated voltage of monitoring and measurement apparatus, which is most commonly 100 volts (V). A distinction is made between AC voltage transformers (usually known simply as transformers) and DC transformers.
The primary winding of an AC voltage transformer (see, Figure 1) consists of a large number w1 of turns and is connected in parallel with the circuit whose voltage U1 is to be measured or monitored. Measuring or monitoring instruments are connected to the terminals of the secondary winding, which has w2 turns (w2 ≪ w1). Since the internal resistance of such instruments is relatively high, the voltage transformer operates under conditions close to the no-load conditions, which means that, if voltage losses in the windings are disregarded, U1 and U2 may be regarded as approximately equal to the corresponding electromotive forces and proportional to w1 and w2—that is, U1w2 ≈ U2w1. If the ratio w1/w2—the transformation ratio—is known, the high primary voltage may be determined from measurements of the low voltage on the secondary winding. The approximate nature of the ratio between Ul and U2 is due to the presence of voltage and angular errors in the measured value of U1. These errors are canceled out in compensated voltage transformers.
Voltage transformers are used mainly in high-voltage distribution equipment. Single-phase and three-phase types are manufactured. Most transformers for voltages above 6 kV are oil-filled. For voltages above 100 kV, the cascade transformers are usually used. Laboratory voltage transformers usually are of the multi-range type.
(For a discussion of DC transformers, see.)
REFERENCESVavin, V. N. Transformatory napriazheniia i ikh vtorichnye tsepi. Leningrad, 1967.
Elektricheskie izmereniia. Edited by E. G. Shramkov. Moscow, 1972.
G. M. VOTCHITSEV