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The instrument transformer is divided into two classes of voltage transformer and current transformer, and the main functions are as follows: accurately transfer voltage and current information of the primary system to related equipment at the secondary side; convert the high voltage and high current of the primary system to low voltage (standard value) and low current (standard value) at the secondary side, conduct standardization and miniaturization of the measuring instrument, relay and other device, and lower requirements for isolation of secondary equipment; well isolate the secondary side equipment and secondary system from the high-voltage equipment of the primary system electrically, so as to guarantee secondary equipment and personnel safety.

The working principle is same with the transformer, and the basic structure is also iron core and primary and secondary winding. The feature is very small and constant capacity, and it is close to no-load state during normal operation. Impedance of the voltage transformer is very small. In case of any short circuit at the secondary side, the current will be increased sharply, leading to coil burnout. For this reason, the primary side of the voltage transformer is connected with fuse, and the secondary side is reliably grounded, so as to avoid personal and equipment accident caused due to high potential against ground at the secondary side when insulation at the primary and secondary sides is damaged. The voltage transformer for measurement is generally has the single-phase double-coil structure, the primary voltage is the measured voltage (such as line voltage of the power system), which can be used as single phase, and can also be used as three-phase when two sets form V-V shape. The voltage transformer for the laboratory has multiple taps at the primary side, so as to adapt to demands for different voltages. The voltage transformer for protective grounding is also provided with a tertiary coil, which is called as the tertiary coil voltage transformer. Three-phase tertiary coil is connected to form open - delta, and two leading-out terminals of the open - delta are connected with the voltage coil of the grounding protection relay. During normal operation, three-phase voltage of the power system is symmetrical, and the sum of three-phase induced electromotive force on tertiary coil is zero. In case of single-phase grounding, there is displacement of the neutral point, and there will be zero-sequence voltage between terminals of open delta, so as to protect the power system. In case of zero-sequence voltage of the coil, there will be zero sequence magnetic flux in the corresponding iron core. For this reason, the side yoke type iron core or three single-phase voltage transformers are adopted for the three-phase voltage transformer (10kV and below). For such mutual inductor, requirements for accuracy of the tertiary coil are not high. However, there is a certain overexcitation characteristic (i.e., when the primary voltage is increased, magnetic flux density in the iron core will also be increased by the corresponding multiple, and damage will not be caused).

Equivalent circuit of the electromagnetic inductive voltage transformer is the same with that of the transformer.