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# Function of current transformer

2021-08-26

Current transformer is a kind of instrument with mutual influence of two currents. Then the problem comes. What is the function of current transformer? Measurement? Protection? Next, let's take a look~~

1、 Function of current transformer -- Introduction

Current transformer, with English name of current transformer and symbol of TA, is an instrument that can convert primary large current into secondary small current, which is composed of closed core and winding according to the principle of electromagnetic induction. Its structure is shown in the figure below. The primary winding has fewer turns and is connected in series in the circuit requiring current measurement, while the secondary winding has more turns and is connected in series in the measurement surface or protection circuit. When it is in operation, the secondary circuit of the current transformer is closed (otherwise there will be safety hidden trouble), and the impedance of the series coil of the measurement surface or protection circuit is very small, Make its working condition close to short circuit.

2、 Function I of current transformer -- for measurement

One of the functions of current transformer is to measure, which is often used for billing or measuring the current of equipment in operation. When measuring large alternating current, in order to facilitate surface measurement and reduce the risk of direct measurement of high-voltage power, it is often necessary to use current transformer to convert it into a more unified current. Here, current transformer plays the role of current conversion and electrical barrier. The current transformer converts the high current into low current in proportion as required. When used for measurement, the primary side of the current transformer is connected to the primary system, and the secondary side is connected to the measuring surface or relay protection equipment.

3、 Function II of current transformer -- for protection

The second function of current transformer is for protection, which is often used in cooperation with relay equipment. When the line has shortcomings such as short circuit or overload, current transformer sends a signal to relay equipment to block the defective circuit, and then achieve the purpose of protecting the safety of power supply system. The current transformer for protection is different from the current transformer for measurement. It can be used only when the current is several times or dozens of times greater than the normal current, and it requires reliable insulation, satisfactory large accurate limit coefficient, satisfactory thermal stability and dynamic stability.

4、 Function of current transformer -- precautions

1. During the operation of the current transformer, the secondary side is not allowed to be open and must be closed. Once the secondary side is open, the magnetic flux and secondary side voltage will far exceed the normal value (up to thousands or even tens of thousands of volts), which is very harmful to the safety of operators and equipment.

2. The wiring method of current transformer shall comply with the principle of series connection, that is, the primary winding is connected in series with the tested circuit, and the secondary winding is connected in series with the measuring surface or relay equipment.

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