A potential transformer (PT for short) and a transformer voltage (TV for short) are similar in that they are both used to transform the voltage on a power line. However, the purpose of transformer voltage conversion is to transmit electrical energy, so the capacity is large, usually calculated in kilovolt ampere or megavolt ampere units; The purpose of voltage transformers in converting voltage is mainly to supply power to measuring instruments and relay protection devices, to measure the voltage, power, and electrical energy of lines, or to protect valuable equipment, motors, and transformers in the event of line faults. Therefore, the capacity of voltage transformers is very small, generally only a few volt amperes, tens of volt amperes, and not exceeding one thousand volt amperes.

Why is it necessary to change the voltage on the line? This is because the voltage on the transmission line varies greatly depending on the different situations of power generation, transmission, and consumption. Some are low voltage 220V and 380V, while others are high voltage tens of thousands or even hundreds of thousands of volts. To directly measure these low and high voltage, it is necessary to make corresponding low and high voltage meters, other instruments, and relays based on the magnitude of the line voltage. This not only brings great difficulties to the production of instruments, but more importantly, it is impossible and absolutely not allowed to directly produce high-voltage instruments and measure voltage on high-voltage lines.

The basic structure of a voltage transformer is very similar to that of a transformer. It also has two windings, one called the primary winding and the other called the secondary winding. Both windings are installed or wound on the iron core. There is insulation between the two windings and between the winding and the iron core, which provides electrical isolation between the two windings and between the winding and the iron core. When the voltage transformer is in operation, the primary winding N1 is connected in parallel to the line, and the secondary winding N2 is connected in parallel to the instrument or relay. Therefore, when measuring the voltage on high-voltage lines, although the primary voltage is high, the secondary voltage is low, which can ensure the safety of operators and instruments.