The impedance of the voltage transformer itself is very small. Once the secondary side is short circuited, the current will increase rapidly and burn the coil. Therefore, 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 high potential to the ground at the secondary side when the insulation of the primary and secondary sides is damaged.

Voltage transformers for measurement are generally made into single-phase double coil structure, and the primary voltage is the measured voltage (such as the line voltage of power system), which can be used in single-phase or two connected into V-V shape for three-phase. The voltage transformers used in the laboratory are often multi tapped at the primary side to meet the needs of measuring different voltages. The voltage transformer for protective grounding is also equipped with a third coil, which is called three coil voltage transformer. The three-phase third coil is connected into an open triangle, and the two leading out ends of the open triangle are connected with the voltage coil of the grounding protection relay.

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

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

Voltage transformer is an indispensable electrical appliance in power transmission and power supply systems such as power plants and substations. Precision voltage transformer is a kind of instrument used to expand the measurement limit and measure voltage, power and electric energy in the electrical testing laboratory. Voltage transformers are very similar to transformers. They are used to change the voltage on the line.