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What is a potential transformer? Introduction about PT.

Publish Time: 2022-06-13 09:43:16     Author: DELIXI

Potential transformer is similar to the transformer, a device to change the voltage. But the purpose of transformer voltage conversion is to facilitate the transmission of electric energy, so the capacity is very large, generally in kilovolt-ampere or mega volt-ampere as the unit of calculation; Purpose and voltage transformer voltage transformation is used to measuring instrument and supply power relay protection device used to measure the line voltage, power and energy, or used in the circuit failure protection circuit of expensive equipment, motor and transformer, voltage transformer capacity is small, therefore, generally only a few volt-ampere, dozens of volt-ampere, biggest and no more than one kva.

Basic Structure

The basic structure of a voltage transformer is similar to that of a transformer. It also has two windings, one is called primary winding and the other is called secondary winding. Both windings are mounted on or wound around an iron core. There is insulation between the two windings and between the windings and the iron core, so that there is electrical isolation between the two windings and between the windings and the iron core. When the voltage transformer is in operation, the primary winding N1 is connected to the line and the secondary winding N2 is connected to the instrument or relay. Therefore, when measuring the voltage on the high voltage line, although the primary voltage is very high, but the secondary voltage is low, which can ensure the safety of operators and instruments.

Working Principles

Its working principle is the same as transformer, and its basic structure is also iron core and primary and secondary windings. Characteristic is the capacity is small and relatively constant, normal operation close to the empty state.

The impedance of the voltage transformer itself is very small. If the secondary side is short-circuited, the current will increase rapidly and burn the coil. Therefore, the primary side of the voltage transformer is connected with a fuse, and the secondary side is reliably grounded, so as to avoid personal and equipment accidents caused by the high potential of the secondary side to the ground when the insulation of the primary and secondary sides is damaged.

Voltage transformers used for measurement are generally made of single-phase double-coil structure. The primary side voltage is the voltage to be measured (such as the line voltage of power system). They can be used in single-phase or two connected in v-V shape as three-phase. The voltage transformer used in the laboratory usually has multiple taps on the original 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 third coil of the three phase is connected to an open triangle, and the two leading ends of the open triangle are connected to the voltage coil of the grounding 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 occurrence of single-phase grounding, neutral point displacement, opening triangle terminals will appear between the zero sequence voltage to make the relay action, so as to protect the power system.

Zero sequence flux occurs in the core when zero sequence voltage is present in the coil. For this purpose, the three-phase voltage transformer uses side-yoke iron core (10kV and below) or three single-phase voltage transformers. For this type of transformer, the accuracy of the third coil is not high, but it is required to have a certain over-excitation characteristics (that is, when the voltage of the primary side increases, the magnetic flux density in the core also increases by corresponding times without damage). 

Voltage transformer is an indispensable electrical appliance for power transmission and power supply system, such as power plant and substation. Precision voltage transformer is a kind of instrument which is used to expand the voltage limit and measure voltage, power and electric energy in electric measuring laboratory. A voltage transformer is similar to a transformer in that it is used to change the voltage on a circuit.

Why do you need to change the voltage on the line? This is because the basis produces electricity, transmission and use different circumstance, the voltage size on the line is different, and differ greatly, have a plenty of low voltage 220V and 380V, have a plenty of high voltage tens of thousands of volts even hundreds of thousands of volts. To directly measure these low voltage and high voltage, it is necessary to make the corresponding low voltage and high voltage voltmeter and other instruments and relays according to the size of the line voltage. This will not only bring great difficulties to the production of instruments, and more importantly, to directly produce high-voltage instruments, directly in the high-voltage line to measure the voltage, that is impossible, and is absolutely not allowed.

Main Types

1. According to installation location, it can be divided into indoor type and outdoor type

35kV and below are mostly made into indoor type; Above 35kV is made into outdoor type.

2, according to the number of phases

Can be divided into single-phase and three-phase 35kV and above can not be made into three-phase.

3. Press the number of windings

It can be divided into two-winding and three-winding voltage transformer. In addition to the primary side and the basic secondary side, the three-winding voltage transformer also has a set of auxiliary secondary side for grounding protection.

4, according to the insulation

It can be divided into dry type, pouring type, oil-immersed type and inflatable type.

Dry type voltage transformer has simple structure, no danger of fire and explosion, but low insulation strength, only suitable for indoor devices below 6kV; Castable voltage transformer has compact structure and easy maintenance, suitable for 3kV ~ 35kV indoor power distribution device; Oil-immersed voltage transformer has good insulation performance and can be used for outdoor power distribution devices above 10kV. Inflatable voltage transformers are used in SF6 fully enclosed appliances.

5. According to the working principle

Electromagnetic voltage transformer: it is a device that uses electromagnetic induction principle to transform voltage or current proportionally.

Capacitive voltage transformer, capacitor voltage transformer (CVT) is composed of series capacitor voltage, then through electromagnetic transformer step-down and isolation, as the meter, relay protection, such as a voltage transformer, capacitor voltage transformer can also be coupled to the power line carrier frequency used in long distance communication, distance measurement, selective circuit high frequency protection, remote control, teletype, etc.

Electronic voltage transformer : A device consisting of one or more voltage or current sensors connected to the transmission system and secondary converter, used to transmit a quantity proportional to the measured, feeding measuring instruments, meters and relay protection or control devices.

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