Potential Transformer ，Voltage Transformer
The basic structure of a voltage transformer is very similar to that of a transformer. It also has two windings, one is called the primary winding and the other is called the secondary winding. Both windings are installed or wound on the core. There is insulation between the two windings and between the winding and the core, so that there is electrical isolation between the two windings and between the winding and the 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 high, the secondary voltage is low-voltage, which can ensure the safety of operators and instruments.
The impedance of the voltage transformer itself is very small. Once the secondary side is short-circuited, the current will increase sharply and burn the coil. For this reason, the primary side of the voltage transformer is connected with a fuse, and the secondary side is reliably grounded, so that when the primary and secondary sides are damaged, the secondary side will have a high potential to the ground and cause personal and equipment accidents.
Voltage transformers for measurement are generally made of single-phase dual-coil structure, and their primary voltage is the voltage to be measured (such as the line voltage of the power system), which can be used in single-phase, or two units can be connected into a VV shape for three-phase use. The voltage transformer used in the laboratory is often multi-tap on the primary side to meet the needs of measuring different voltages. The voltage transformer for protective grounding also has a third coil called a three-coil voltage transformer. The three-phase third coil is connected into an open triangle, and the two leading ends of the open triangle are connected with the voltage coil of the ground protection relay.
During normal operation, the three-phase voltage of the power system is symmetrical, and the sum of the three-phase induced electromotive force on the third coil is zero. Once the single-phase grounding occurs, the neutral point will be displaced, and a zero sequence voltage will appear between the terminals of the open triangle to make the relay operate, thereby protecting the power system.
Zero sequence voltage appears in the coil, and zero sequence magnetic flux appears in the corresponding core. For this reason, this three-phase voltage transformer adopts a side yoke core (when 10KV and below) or three single-phase voltage transformers. For this kind of transformer, the accuracy of the third coil is not high, but a certain overexcitation characteristic is required (that is, when the primary voltage increases, the magnetic flux density in the core increases by a corresponding multiple without damage).
Voltage transformer is an indispensable electrical appliance in power transmission and power supply systems such as power plants and substations. The precision voltage transformer is an instrument used in the electrical test laboratory to expand the measurement limit and measure voltage, power and electrical energy. Voltage transformers are very similar to transformers and are used to transform the voltage on the line.