Voltage Transformer

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 U_l 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.)