The Principle Of Operation Of The Current Transformer

In the lines of power generation, transformation, transmission, distribution and consumption, the current varies widely, from a few amperes to tens of thousands of amperes. In order to facilitate measurement, protection and control, it is necessary to convert to a relatively uniform current, and the voltage on the line is generally relatively high, such as direct measurement is very dangerous. Current transformers act as galvanic conversion and galvanic isolation.

For pointer ammeters, most of the secondary current of the current transformer is amper-rated (e.g., 5A, etc.). For digital instruments, the sampled signal is generally in the milliampere level (0-5V, 4-20mA, etc.). The secondary current of the miniature current transformer is in the milliampere level, which mainly acts as a bridge between the large transformer and sampling.

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

Similar to transformers, current transformers also work according to the principle of electromagnetic induction, transformers convert voltage, and current transformers convert current. The current transformer is connected to the winding of the measured current (the number of turns is N1), which 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 secondary 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 when the current transformer works at the rated current is called the rated current ratio of the current transformer, which is expressed by Kn.
Kn=I1n/I2n
The function of the current transformer (CT) is to convert the primary current with a large value into a secondary current with a small value through a certain transformation ratio, which is used for protection, measurement and other purposes. For example, a current transformer with a conversion ratio of 400/5 can convert the actual current of 400A into a current of 5A.