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The instrument transformer can be divided into two types, which are voltage transformer and current transformer. The main functions are as follows: accurately transfer voltage and current information of the primary system to the 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 devices, 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 the same with the transformer, and the basic structure is also iron core and primary and secondary windings. The features of the instrument transformer are: small in capacity and very constant , 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 sharply increased, 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 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 generally has a single-phase double-coil structure, the primary voltage is the measured voltage (similar to the 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 a V-V shape. The voltage transformer that is used in 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 triangle, and two leading-out terminals of the open triangle 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 a displacement of the neutral point, and there will be zero-sequence voltage between terminals of open triangle, 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. Therefore the side yoke type iron core or three single-phase voltage transformer is adopted for the three-phase voltage transformer (10kV and below). For such instrument transformer, the requirements for the accuracy of the tertiary coil are not high. However, it requires 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.