The basic principle of the zero-sequence current transformer 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.
In the case of normal line and electrical equipment, the vector sum of the currents of each phase is equal to zero, and Ia + Ib + Ic = 0.
Therefore, no signal is output from the secondary winding of the zero-sequence current transformer, and the actuator does not operate. When a ground fault occurs, the vector sum of the phases' currents is not zero. Ia + Ib + Ic = Io ＞ 0, Io is the zero-sequence current or leakage current and fault current. The fault current causes the zero-sequence current transformer. A magnetic flux is generated in the toroidal core, and the secondary side induced voltage of the zero-sequence current transformer causes the actuator to actuate, trip the trip device, and switch the power supply network to achieve the purpose of ground fault protection.
When the system has a ground fault, zero sequence current will appear. The zero sequence current transformer is used to measure the zero sequence current of the component.
It is mainly used for:
1, Zero sequence protection (neutral grounding system)
2, small current grounding inspection instrument (neutral point ungrounded system)
When an electric shock or leakage fault occurs in the circuit, the secondary side of the transformer outputs a zero-sequence current to make the equipment on the secondary circuit connected to protect the action (cut off the power, alarm, etc.)