Working principle of current transformer

In the lines of power generation, substation, transmission, distribution and consumption, the current varies 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, which is very dangerous. The current transformer plays the role of current transformation and electrical isolation.

For pointer-type ammeters, most of the secondary currents of the current transformers are in the ampere level (such as 5A, etc.). For digital instruments, the sampled signal is generally at the milliamp level (0-5V, 4-20mA, etc.). The secondary current of the miniature current transformer is milliampere, which mainly acts as a bridge between the large transformer and the sampling.

Miniature current transformers are also called "instrument current transformers". ("Instrument current transformer" has a meaning that it is a multi-current ratio precision current transformer used in the laboratory, which is generally used to expand the instrument range.)

Similar to the transformer, the current transformer 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 the primary winding, the primary winding); the winding connected to the measuring instrument (the number of turns is N2) is called the secondary winding (or secondary winding). winding, secondary winding).

The current ratio of the primary winding current I1 to the secondary winding I2 of the current transformer is called the actual current ratio K. The current ratio of the current transformer when it works under the rated current is called the rated current ratio of the current transformer, expressed by Kn.

Kn=I1n/I2n

The function of the current transformer is to convert the 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 ratio of 400/5 can convert an actual current of 400A into a current of 5A.