The working principle is the same as that of the transformer, and the basic structure is also the iron core and the primary and secondary windings. The characteristic is that the capacity is small and relatively constant, and it is close to the no-load state during normal operation.
The impedance of the voltage transformer itself is very small. Once the secondary side is short-circuited, the current will increase sharply and the coil will be burned. For this reason, the primary side of the voltage transformer is connected with a fuse, and the secondary side is reliably grounded to prevent personal and equipment accidents from occurring when the primary and secondary side insulation is damaged and the secondary side has a high potential to the ground.
Voltage transformers for measurement are generally made of single-phase double-coil structure, and the 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 can be connected in V-V shape for three-phase. use. The voltage transformers used in the laboratory are 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, which is 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 grounding protection relay.
During normal operation, the three-phase voltages of the power system are symmetrical, and the sum of the three-phase induced electromotive forces on the third coil is zero. Once the single-phase grounding occurs, the neutral point will be displaced, and the zero-sequence voltage will appear between the terminals of the open triangle to make the relay act, thus protecting the power system.
When zero-sequence voltage appears in the coil, zero-sequence magnetic flux will appear in the corresponding iron core. To this end, 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 it requires certain overexcitation characteristics (that is, when the primary voltage increases, the magnetic flux density in the iron core also increases by a corresponding multiple without damage).
The function of the voltage transformer: to convert the high voltage into a standard secondary voltage of 100V or lower in proportion to the use of protection, metering and instrumentation devices. At the same time, the use of voltage transformers can isolate high voltages from electrical workers. Although the voltage transformer is also a device that works according to the principle of electromagnetic induction, its electromagnetic structure relationship is exactly opposite to that of the current transformer. The secondary circuit of the voltage transformer is a high-impedance circuit, and the magnitude of the secondary current is determined by the impedance of the circuit.
When the secondary load impedance decreases, the secondary current increases, so that the primary current automatically increases by a component to satisfy the electromagnetic balance relationship between the primary and secondary sides. It can be said that the voltage transformer is a special transformer with limited structure and use form. Simply put, it is the “detection element”.
Post time: May-04-2022