The basic concept of zero-sequence current protection is based on Kirchhoff's current law: the algebraic sum of the complex currents flowing into any node in the circuit is equal to zero. When the line and electrical equipment are normal, the vector sum of the currents of each phase is equal to zero. Therefore, the secondary winding of the zero-sequence current transformer has no signal output, and the actuator does not act. When a fault occurs, the vector sum of the currents of each phase is not zero.

The amount of fault current causes a magnetic flux to form in the annular iron core of the zero-sequence current transformer. The tripping device switches the power supply network to achieve the purpose of ground fault protection.

Function of zero-sequence current transformer: When there is an electric shock or leakage fault in the line, the protection action will cut off the power supply. During operation, install a current transformer on each of the three-phase lines, or the three-phase wires pass through the zero-sequence current transformer together, or install a zero-sequence current transformer on the neutral line N, and use it to detect the three-phase current transformer. The current vector sum.

The current transformer and the leakage protection relay are composed of leakage and ground fault protection devices. According to the zero-sequence current transformer, the zero-sequence running current exceeding the set value is detected, and the leakage protection relay is higher than the rated operating current, and transmits the action signal, and disconnects or connects the alarm circuit.