The working principle of voltage transformer: 
Its working principle is the same as the transformer, and its basic structure is iron core, original winding and secondary winding. 
Its characteristic is that the capacity is small and relatively constant, and it is close to no-load state in normal operation. 
The impedance of the voltage transformer itself is very small. 
Once the secondary edge is short-circuited, the current will increase sharply and the coil will be burned out. 
Therefore, the original edge of the voltage transformer is connected with a fuse, and the secondary edge is reliably grounded to avoid personal and equipment accidents caused by the high potential of the secondary edge to the ground when the insulation of the original edge and the secondary edge is damaged. 
The voltage transformer for measurement is generally a single-phase double-coil structure, and its primary voltage is the measured voltage (such as the line voltage of the power system), which can be used in a single phase or two Vmi Vs to form a three-phase. 
The voltage transformer used in the experiment is usually multi-tap on the original side to meet the needs of measuring different voltages. 
The voltage transformer used to protect grounding also has a third coil, called a three-coil voltage transformer. 
The third coil of the three-phase is connected with the open triangle, and the two leading ends of the open triangle are connected with the voltage coil of the grounding protection relay. 
In normal operation, the three-phase voltage of the power system is symmetrical and the three-phase induced potential of the third coil is zero. 
After the occurrence of single-phase grounding, the neutral point shifts, zero sequence voltage appears between the open triangle terminals, and the relay acts to protect the power system. 
If the coil has a zero-sequence voltage, the corresponding core will have a zero-sequence flux. 
Therefore, the three-phase voltage transformer adopts side yoke core (10kV and below) or three-phase voltage transformer. 
For this kind of transformer, the precision of the third coil is not high, but it needs certain overexcitation characteristics (that is, when the primary voltage increases, the flux density in the core also increases by a corresponding multiple and will not be damaged). 
Voltage transformer is an indispensable electrical equipment in power plants, substations and other transmission and power supply systems. 
Precision voltage transformer is a kind of instrument used to expand the measurement of voltage, power and electric energy. 
Voltage transformers are very similar to transformers and are used to change the voltage on the circuit. 
Why does the line need to change the voltage? 
This is because the voltage on the line varies greatly according to the situation of power generation, transmission and power consumption. 
Some are low pressure 220V and 380V, some are high voltage tens of thousands of volts or even hundreds of thousands of volts. 
In order to measure these low-voltage and high-voltage voltages directly, it is necessary to make corresponding low-voltage and high-voltage voltmeters and other instruments and relays according to the line voltage. 
This will not only bring great difficulties to the production of instruments, but more importantly, it is impossible and absolutely forbidden to directly make high-voltage instruments and measure voltage directly on high-voltage lines.