A voltage transformer is an instrument used to transform voltage. The purpose of voltage transformers to convert voltage is mainly to supply power to measuring instruments and relay protection devices, to measure the voltage, power, and electric energy of lines, or to protect valuable equipment, motors, and other equipment in the line when the line fails. Transformers, so the capacity of voltage transformers is very small, generally, only a few volts, tens of volts, and the maximum does not exceed one thousand volts.

　　The impedance of the voltage transformer itself is very small. Once the secondary side is short-circuited, the current will increase sharply and the coil will be burned. For this reason, the primary side of the voltage transformer is connected with a fuse, and the secondary side is reliably grounded, so as to avoid personal and equipment accidents caused by the high potential to the ground on the secondary side when the insulation of the primary and secondary sides is damaged.

　　During normal operation, the three-phase voltages of the power system are symmetrical, and the sum of the three-phase induced electromotive forces on the third coil is zero. Once the single-phase grounding occurs, the neutral point will be displaced, and the zero-sequence voltage will appear between the terminals of the open triangle to make the relay act, thus protecting the power system.

　　If zero-sequence voltage appears in the coil, zero-sequence magnetic flux will appear in the corresponding iron core. To this end, this three-phase voltage transformer adopts a side yoke core (when 10kV and below) or adopts three single-phase voltage transformers. For this kind of transformer, the accuracy of the third coil is not high, but it requires certain overexcitation characteristics (that is, when the primary voltage increases, the magnetic flux density in the iron core also increases by a corresponding multiple without damage).