The principle of current transformer is 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, which is stringed in the line that needs to be measured. Therefore, it often has all the current flowing through the line. The secondary winding has a large number of turns. It is connected in series in the measuring instrument and the protection circuit, and the current mutual inductance When the transformer is working, its secondary circuit is always closed, so the impedance of the series coil of the measuring instrument and the protection circuit is very small, and the working state of the current transformer is close to short-circuit. 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.