Introduction To Voltage Transformers
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Potential Transformer (PT for short, Voltage Transformer for short VT) is similar to a transformer and is an instrument used to transform voltage.
Its working principle is the same as that of the transformer [2], 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 a 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 a 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 to 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 adopts 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).
A voltage transformer is an indispensable electrical appliance in power transmissions and power supply systems such as power plants and substations. A precision voltage transformer is an instrument used in the electrical testing laboratory to expand the quantity limit and measure voltage, power, and electric energy. Voltage transformers are very similar to transformers in that they are used to transform the voltage on the line.
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