# AC Fault Ride-Through Strategy of a Transformerless Soft Normally Open Point
## 6 Simulation Analysis
### System Response Characteristics after Optimization  
In steady-state operation, the rated AC voltage of the distribution network is 10 kV, and the DC voltage of SNOP is \(\pm\)10 kV. Therefore, the voltage modulation ratio \(k\) is 0.82. According to the analysis above, each arm of the hybrid MMC contains twenty-two HBSMs  
Figure 10: Fault response characteristics of MMC2 before optimization. (**a**) Three-phase AC voltage (blue is A-phase, black is B-phase, red is C-phase). (**b**) AC zero-sequence voltage. (**c**) AC zero-sequence current. (**d**) SM capacitor voltage. (**e**) Inserted SM number of the upper arm in phase A on the MMC2.  
Figure 9: Fault response characteristics of MMC1 before optimization. (**a**) Three-phase AC voltage (blue is A-phase, black is B-phase, red is C-phase). (**b**) Positive and negative DC voltage (black is positive DC voltage, red is negative DC voltage). (**c**) AC zero-sequence voltage. (**d**) AC zero-sequence current. (**e**) SM capacitor voltage. (**f**) Inserted SM number of the upper arm in phase A on the MMC1.  
and five FBSMs, and the capacitance is the same. At \(t=0.8\) s, an A-phase grounding fault happens on the AC side of MMC1. At \(t=0.9\) s, the zero-sequence control loop is put into operation.  
Figure 11 shows the response waveforms of three-phase AC voltage, positive and negative DC voltage, AC zero-sequence current, SM capacitor voltage and the inserted SM number of the upper arm on the MMC1 side. It can be seen from Figure 11 that the DC voltage fluctuation of the positive and negative poles is suppressed immediately after the zero-sequence control loop is put into operation. The zero-sequence current is suppressed to near zero, and the SM capacitor voltage is stabilized near the rated value. The inserted number of phase A remains around 11, the maximum inserted number of phase B and C is 27, and the minimum is \(-5\).  
Figure 12 shows the response waveforms of the three-phase AC voltage, the AC zero-sequence voltage and the AC zero-sequence current on the MMC2 side. It can be seen from Figure 12 that the zero-sequence voltage component and zero-sequence current component are effectively suppressed after the zero-sequence control loop is put into operation. The AC voltage of the supply area connected to MMC2 recovers to the three-phase symmetrical stable state, which indicates that the fault in the supply area connected to MMC1 will not affect the supply area connected to MMC2, and the fault range is effectively limited.  
At \(t=0.8\) s, a two-phase grounding fault happens at phase B and C on the AC side of MMC1. At \(t=0.9\) s, the zero-sequence control loop is put into operation. Figure 13 shows the response waveforms of the three-phase AC voltage of MMC1, the three-phase AC voltage of MMC2, the zero-sequence voltage of MMC1 and MMC2, the zero-sequence current of MMC1 and MMC2, and the positive and negative DC voltage. It can be seen that the DC voltage fluctuation is suppressed immediately after the zero-sequence control loop is put into operation. The zero-sequence voltage of MMC2 and the zero-sequence current are suppressed to near zero.  
Figure 11: Fault response characteristics of MMC1 after optimization. (**a**) Three-phase AC voltage (black is A-phase, red is B-phase, blue is C-phase). (**b**) Positive and negative DC voltage (black is positive DC voltage, red is negative DC voltage). (**c**) AC zero-sequence current. (**d**) SM capacitor voltage. (**e**) Inserted SM number of the upper arm on the MMC1 (black is A-phase, red is B-phase, blue is C-phase).