Single phase grounding fault occurs in small current grounding system. If line selection is not carried out to eliminate the fault, the impact will be very serious. If ferromagnetic resonance occurs, it will bring disaster. Therefore, it is very necessary to use small current grounding line selection device.

        The grounding line selection device is divided into centralized line selection device and distributed line selection device. Compared with the previous centralized line selection device, the characteristics of the distributed small current grounding line selection device are mainly reflected in that the system divides the layers according to the functional needs, maintains the relative independence of each layer, and improves the reliability and operation efficiency; The analog signal processing is completed locally, and the analog signal does not need to be transmitted remotely, which avoids electromagnetic interference and signal attenuation and improves the reliability of data.

        One of the reasons why it is difficult to select the line for small current grounding fault is that the fault signal is weak and mixed in the interference signal, which is difficult to extract. Because the signal is weak, the sensor must have high precision and strong anti-interference ability. The zero sequence current transformer used for small current grounding line selection is a through core zero sequence current transformer, which overcomes the measurement error caused by the imbalance of three-phase voltage transformer, and the range is effectively utilized with higher accuracy.