What is a voltage transformer?

Voltage transformer is also called instrument transformer (PT). It is an instrument that changes high voltage into low voltage and keeps a certain phase relationship with the original voltage. Its working principle, structure and wiring method are the same as those of transformer, but its capacity is smaller, usually only tens or hundreds of volt-amperes. Its purpose is to reduce the high voltage by a certain proportion so that the low-voltage coil can accurately reflect the change of the high voltage value to solve the difficulty of high voltage measurement. At the same time, because it reliably isolates the high voltage, it ensures the safety of the measurement personnel, instruments and protective devices. In addition, the secondary voltage of the voltage transformer is 100V, which can standardize the instruments and relays.

Voltage transformer is an indispensable electrical appliance in power transmission and power supply systems such as power plants and substations.

Precision voltage transformer is an instrument used in electrical testing laboratories to expand the measurement limit and measure voltage, power and electric energy.

Voltage transformers are very similar to transformers, both of which are used to transform the voltage on the line. However, the purpose of transformers to transform voltage is to transmit electrical energy, so the capacity is very large, generally calculated in kilovolt-amperes or megavolt-amperes; while the purpose of voltage transformers to transform voltage is mainly to power measuring instruments and relay protection devices, to measure the voltage, power and electrical energy of the line, or to protect valuable equipment, motors and transformers in the line when a line fault occurs. Therefore, the capacity of voltage transformers is very small, generally only a few volt-amperes or tens of volt-amperes, and the maximum does not exceed one thousand volt-amperes.

Why do we need to transform the voltage on the line? This is because the voltage on the line varies greatly according to the different situations of power generation, transmission and consumption. Some are low voltage 220V and 380V, and some are high voltage tens of thousands of volts or even hundreds of thousands of volts. To directly measure these low and high voltages, it is necessary to make corresponding low and high voltage voltmeters and other instruments and relays according to the size of the line voltage. This will not only bring great difficulties to the production of instruments, but more importantly, it is impossible to directly make high-voltage instruments and measure voltage directly on high-voltage lines, and it is absolutely not allowed.

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 mounted on or wound around the iron core. There is insulation between the two windings and between the windings and the iron core, so that there is electrical isolation between the two windings and between the windings 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 the high-voltage line, although the primary voltage is very high, the secondary voltage is low, which can ensure the safety of the operator and the instrument.

Working Principle of Voltage Transformer - Animation Tutorial