Current transformer is a power transformer transforming high voltage and current into low voltage and current. Its primary winding and primary circuit of electric system are tandem connection. Its secondary winding and measuring meter, relay of secondary are also tandem connection. Its main structure is made up of primary, secondary winding and magnetism iron core.

Current transformer conforms with IEC60044-1, 1996 standard. There are two sorts for measuring and protection.

Measuring current transformers are construct to measure current and electric power. These type of current transformers are mainly used as class 0.2, 0.5, 1.

Protection current transformers are used for relay protection and automatic controlling device. These type of current transformer are mainly used as class 3.