There is a wide range of currents in power generation, transformation, transmission, distribution and consumption lines, ranging from several to tens of thousands of amperes. In order to facilitate measurement, protection and control, it is necessary to convert to a relatively uniform current. In addition, the voltage on the line is generally high. Direct measurement is very dangerous. Current transformer plays the role of current transformation and electrical isolation.

For pointer type ammeters, the secondary current of current transformers is mostly of ampere class (such as 5a, etc.). For digital instruments, the sampled signal is generally of milliampere level (0-5V, 4-20mA, etc.). The secondary current of micro current transformer is Ma level, which mainly acts as a bridge between large current transformer and sampling.

Similar to the transformer, the current transformer works according to the principle of electromagnetic induction. The transformer changes the voltage while the current transformer changes 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 connected to the measuring instrument (the number of turns is N2) is called the secondary winding (or secondary side winding, secondary winding).

The function of current transformer (CT) is to convert the primary current with larger value into the secondary current with smaller value through a certain transformation ratio for protection, measurement and other purposes. For example, a current transformer with a transformation ratio of 400/5 can convert the actual current of 400A to 5A.