Voltage transformers are very similar to transformers and are used to change the voltage on the line. However, the purpose of the transformer to convert the voltage is to deliver electrical energy, so the capacity is very large, generally in kilovolt-amperes or megavolt-amperes as the unit of calculation; and the purpose of the voltage transformer to change the voltage is mainly to the measuring instrument and the relay protection device. Power supply, used to measure the voltage, power and power of the line, or to protect valuable equipment, motors and transformers in the line when the line fails, so the capacity of the voltage transformer is very small, generally only a few volts, dozens Volt, no more than a thousand volts.
Why do you need to change the voltage on the line? This is because the voltage on the line varies according to the different conditions of power generation, transmission, and power consumption, and the difference is very different. Some are low voltage 220V and 380V, and some are high voltage tens of thousands or even tens. Ten thousand volts. To directly measure these low voltage and high voltage voltages, 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 meter production, but more importantly, it is necessary to directly make a high-voltage meter and measure the voltage directly on the high-voltage line. That is impossible, and it is absolutely not allowed.
If the voltage transformer is connected to the voltage on the line, then the low voltage and high voltage on the line can be uniformly converted into one or several low voltages according to the corresponding ratio, as long as one or several voltage specifications are used. And the relay, such as a universal 100V meter, can measure and monitor the voltage on the line through a voltage transformer. In the power system, the voltage transformer (PT) is the communication component of the primary and secondary systems, which can correctly reflect the normal operation and fault conditions of the electrical equipment. The primary coil of the PT is connected in parallel in the high voltage circuit. Its function is to convert the high voltage into a rated low voltage of 100V. It is used as a secondary loop power supply for measurement and protection. In normal operation, the secondary winding is approximated to an open circuit state. Therefore, normal The secondary side of the PT in operation is not allowed to short circuit.