The residual (zero sequence) current transformer is used to detect zero sequence current. Its structure is similar to ordinary DC current transformers, but its primary winding is the three-phase conductor of the protected system. (Transformer ring core), the secondary winding reacts to the zero sequence current of the main system.
In a system where the neutral point is not directly grounded, it forms a single-phase grounding protection device with the grounding relay. When the system is in normal operation, the vector sum through the three-phase current is zero. When a single-phase ground fault occurs, zero-sequence magnetic flux appears in the magnetic core, and the magnetic flux induces an electromotive force in the secondary winding. The secondary current flows through the grounding relay to make it work.
When the power system generates a zero-sequence ground current, the residual (zero-sequence) current transformer is used with a relay protection device or a signaling device. Actuate equipment components to implement protection or monitoring functions.
There is no set value for the low current grounding wire selection device. Zero-sequence current is just one of the standards of the device. When the primary ground current is small and non-metal ground is used, it is required that the residual (zero-sequence) current transformer should have a certain output to meet the equipment start-up, ground selection or corresponding line trip. The load impedance of the device itself is not large, but each remaining (zero-sequence) current transformer needs to be connected to the device through a cable, so the impedance of the cable is the main load impedance, and the load impedance of the transformer is generally about 2.5Ω.