The working principle of a voltage transformer is the same as that of a transformer, with its basic structure consisting of an iron core and primary and secondary windings. It is characterized by small and relatively constant capacity, operating close to no-load condition during normal operation.
A voltage transformer has very low internal impedance. Once a short circuit occurs on the secondary side, the current will increase sharply and burn the windings. For this reason, a fuse is installed on the primary side of the voltage transformer, and the secondary side is reliably grounded. This prevents high potential to ground from appearing on the secondary side when the insulation between the primary and secondary windings is damaged, thus avoiding personal injury and equipment accidents.
Voltage transformers for measurement are generally of single-phase double-winding structure. Their primary voltage is the measured voltage (such as the line voltage of a power system), and they can be used singly or in a V-V connection with two units for three-phase applications. Laboratory voltage transformers often have multiple taps on the primary side to meet the needs of measuring different voltages. Voltage transformers for protective grounding are also equipped with a third winding, known as three-winding voltage transformers. The third windings of the three-phase units are connected in an open delta configuration, and the two terminals of the open delta are connected to the voltage coil of the ground protection relay.
During normal operation, the three-phase voltages of the power system are balanced, and the sum of the three-phase induced electromotive forces on the third winding is zero. Once a single-phase grounding fault occurs, the neutral point shifts, and a zero-sequence voltage will appear between the terminals of the open delta, causing the relay to operate and thus protecting the power system.
The occurrence of zero-sequence voltage in the winding corresponds to the appearance of zero-sequence magnetic flux in the iron core. For this reason, such three-phase voltage transformers adopt a side-yoke iron core (for 10kV and below) or three single-phase voltage transformers. For these transformers, the accuracy requirement for the third winding is not high, but it must have a certain over-excitation characteristic (that is, when the primary voltage increases, the magnetic flux density in the iron core increases by a corresponding multiple without being damaged).
Voltage transformers are indispensable electrical equipment in power transmission and supply systems such as power plants and substations. Precision voltage transformers are instruments used in electrical test laboratories to extend measurement ranges and measure voltage, power, and electrical energy. Voltage transformers are very similar to transformers, both used to convert voltage in power lines.
Why is voltage conversion needed in power lines? This is because, according to the different conditions of power generation, transmission, and consumption, the voltage in the lines varies greatly. Some are low voltages of 220V and 380V, while others are high voltages of tens of thousands or even hundreds of thousands of volts. To directly measure these low and high voltages, it is necessary to manufacture corresponding low-voltage and high-voltage voltmeters and other instruments and relays according to the line voltage. This not only brings great difficulties to instrument manufacturing but also, more importantly, it is impossible and absolutely not allowed to directly manufacture high-voltage instruments and measure voltage directly on high-voltage lines.
