Current transformer is based on the principle of electromagnetic induction will be the primary side of the large current into the secondary side of the small current to measure the instrument. The current transformer is composed of a closed core and a winding. 

Its primary winding has a small number of turns and is connected to the circuit where the current to be measured is required. Therefore, it often has all the current flow of the line, secondary side winding turns more, series connected in the measuring instrument and Protection Circuit, current transformer in the work, its secondary side loop is always closed, therefore, the impedance of the series winding of the measuring instrument and the protection circuit is very small, and the working state of the current transformer is close to the short circuit. Current transformer is the primary side of the large current into the secondary side of the small current to measure, the secondary side can not open circuit. The article introduces its working principle, parameter description, classification and application.