What is the working principle of a current transformer?

Update:11-06-2021
A current transformer is an instrument that converts a large current on the primary side into a small current on the secondary side based on the principle of electromagnetic induction. The current transformer is composed of a closed core and windings. Its primary winding has a few turns and is stringed in the line of the current that needs to be measured.
Therefore, it often has all the current of the line flowing, and the number of turns of the secondary winding is relatively large. It is connected in series in the measuring instrument and the protection circuit. When the current transformer is working, its secondary circuit is always closed, so the measurement The impedance of the series coil of the instrument and the protection circuit is very small, and the working state of the current transformer is close to a short circuit. The current transformer converts the large current on the primary side into a small current on the secondary side for measurement, and the secondary side cannot be opened.
In the lines of power generation, transformation, transmission, distribution and power consumption, the currents vary greatly, ranging from a few amperes to tens of thousands of amperes. In order to facilitate measurement, protection and control, it needs to be converted into a relatively uniform current. In addition, the voltage on the line is generally relatively high, such as direct measurement is very dangerous. The current transformer plays the role of current conversion and electrical isolation.
For pointer-type ammeters, the secondary current of the current transformer is mostly ampere-level (such as 5A, etc.). For digital instruments, the signal sampled is generally milliampere (0-5V, 4-20mA, etc.). The secondary current of the miniature current transformer is milliampere, and it mainly serves as a bridge between the large transformer and the sampling.
Miniature current transformers are also called "instrument current transformers". (The meaning of "instrument current transformer" is that the multiple current ratio precision current transformer used in the laboratory is generally used to expand the range of the instrument.
zero-phase-current-transformer03 is similar to the transformer and also works according to the principle of electromagnetic induction. The transformer transforms the voltage and the current transformer transforms the current. The winding of the current transformer connected to the measured current (the number of turns is N1) is called the primary winding (or primary winding, primary winding); the winding (the number of turns is N2) connected to the measuring instrument is called the secondary winding (or the secondary winding) Winding, secondary winding).
The current ratio between the primary winding current I1 of the current transformer and the secondary winding I2 is called the actual current ratio K. The current ratio of the current transformer when it is working at the rated current is called the current transformer rated current ratio, which is represented by Kn.
Kn=I1n/I2n
The function of the current transformer (CT) is to convert a primary current with a larger value into a secondary current with a smaller value through a certain transformation ratio, which is used for protection, measurement and other purposes. For example, a current transformer with a transformation ratio of 400/5 can convert an actual current of 400A into a current of 5A.
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