## 4 Simulation Verification and Analysis
### Steady-State Operational Validation
#### 4.2.3 Protection Control Validation Under the AC Fault of the GSMMC  
When a fault occurs on the grid side, the decoupling effect of the MMC-HVDC system prevents the wind farm from perceiving the fault, causing it to continue generating at its rated power. This may lead to DC overvoltage, converter shutdown, and wind farm power output limitations, severely affecting system stability. Therefore, it is crucial to study fault mitigation measures for MMC-HVDC systems under AC-side faults.  
Braking resistors (BRs), due to their simple structure, fast response, and lack of complex control requirements, are considered an effective method for DC overvoltage suppression. Previous studies have demonstrated that braking resistors can help mitigate DC overvoltage in MMC-HVDC systems [25; 26]. However, their specific impact on system stability, particularly when the wind farm control strategy remains unchanged, still requires further investigation.  
This section conducts a simulation analysis to examine whether installing braking resistors only on the DC side of the MMC-HVDC system, without modifying the wind farm control strategy, can maintain DC voltage stability during AC-side faults. The system parameters were the same as those in Table 1. According to the transient analysis of Section 4.2.2, when a two-phase ground fault occurred and the wind farm power was  
Figure 9: Simulation waveform of two-phase ground short circuit when the transmission power of wind power is 280 MW (**a**) voltage on the AC side of the GSMMC; (**b**) current on the AC side of the GSMMC; (**c**) the active power output of the GSMMC; (**d**) the power input of the GSMMC and the output power of the wind turbines; (**e**) DC voltage of the MMC-HVDC system.  
280 MW, the direct DC line voltage of the MMC-DC system reached 420 kV, and the peak DC voltage reached 445 KV, exceeding the rated 1.05 p.u. Therefore, we took a two-phase ground fault as an example to verify the feasibility of the protection strategy.  
The B-C-phase grounding fault occurred at 4.5 s, and the fault duration was 0.5 s. As the DC voltage rose, the DC energy dissipation triggered the condition to be reached. The resistor started working, and then the DC voltage decreased. Until the voltage was lower than the input condition of the energy dissipation resistor, the energy dissipation circuit was cut off. This is carried out to keep the DC voltage relatively stable during the fault.  
From Figure 10c, it can be observed that after the BR was activated during the fault period, the DC voltage remained within 1.05 p.u., staying within the safe voltage range. This demonstrates the effectiveness of the BR in suppressing DC overvoltage. Meanwhile, from Figure 10d, it is evident that the output power of the GSMMC reduced to about 150 MW due to the activation of the BR, which dissipated part of the excess energy, thereby reducing the power transmitted to the grid. However, the output power of the wind turbines remained unaffected by the fault, maintaining close to the rated power. This indicates that during a fault, the wind farm continued to generate power as originally set, while the MMC-HVDC system regulated the DC-side energy through the BR, ensuring the safe and stable operation of the system.  
The simulation results demonstrate that braking resistors can effectively mitigate DC overvoltage during AC-side faults, preventing converter shutdown and excessive power curtailment of the wind farm. The results confirm that BRs provide a simple and fast fault mitigation solution without requiring complex control modifications.