## 4 Simulation Analysis
### Analysis of Simulation Results
#### 4.2.2 TN Configuration Ground Fault Analysis  
The three-phase symmetry and asymmetry are set on the load side of the TN configuration of the simulation model. The fault is set in the middle phase of line 1, and the fault resistance is 1 k\(\Omega\). The magnitude of the residual current at the outlet of the line and its increase with the load asymmetry are recorded and compared. In the case of a symmetrical load, the three-phase current, neutral line current, and residual current waveform at point A in the middle of outgoing feeder one and the residual current of the closed section are shown in Figure 9. In (a), the three-phase currents are equal, and it can be seen that the load of line 1 is symmetrical. However, it can be seen in (b) that even in the case of a symmetrical load, there is still a current of 1 A on the neutral line, which is caused by the asymmetry of line parameters and power supply (three-phase power supply), the line is a symmetrical four-wire system in physical parameters, but the mutual impedance between the lines is not the same, so the current will also be generated on the neutral line under the condition of the symmetrical operation of the load. It can be seen from (c) that the load is symmetrical, and the residual current will also be generated when the impedance to the ground is symmetrical, which is the reason why the TN-C configuration cannot install the residual current. The load symmetry of branch 3 is changed. The three-phase current at point A, the current flowing through the neutral line, and the residual current at point A are shown in Figure 10. After the phase A current increases, the amplitude of the current flowing on the neutral line increases to nearly 30 mA, and the inherent residual current amplitude at point A also increases to nearly 500 mA. Therefore, the inherent residual current in the TN-C configuration changes with the load symmetry, and the traditional method cannot achieve protection. The calculated residual current of the closed section under the two faults is shown in Figures 9 and 10, respectively, and the waveform is shown in Figure 11, where (a) is the residual current of the closed protective section when the load is symmetrical, and (b) is the load asymmetry. The closed-section residual current at this time shows phase asymmetry. Figure 8: Residual current waveforms upstream of the earth fault closure section and its interior: (a) residual current waveform at the head end of line 1 of the TT configuration after the fault; (b) residual current waveform of the closed section. Figure 9: _Cont._Figure 11: The closed\(-\)section residual current of TN\(-\)C configuration: (**a**) the case where the load is symmetrical; (**b**) the case of load asymmetry. Figure 10: Earth fault under asymmetrical load in TN configuration: (**a**) three-phase current waveform at point Al; (**b**) neutral current at point A; (**c**) residual current at point A.  
Figure 9: Earth fault under symmetrical load in TN configuration: (**a**) three-phase current waveform at point A; (**b**) neutral current at point A; (**c**) residual current at point A. It can be seen from Figure 11 that the load symmetry of the two faults is inconsistent. The inherent residual current of the system is also different because the repeated earthing point on the load side is excluded from the closed section when constructing the closed section. The residual current of the closed section is almost only related to the transition resistance when the earth fault occurs in the closed section. The transition resistances corresponding to the two faults in the figure are both 1 k\(\Omega\), so the calculated residual current of the closed section is the same. The residual current has obvious variation characteristics, not affected by the residual current before the fault.