## 5 Overcurrent Suppression Strategy and Verification  
Based on the development process of overcurrent in the MMC valve shown in Figure 3, the magnitude of the overcurrent in the MMC valve is primarily determined by the fault current during the two stages S1 and S2. The rate of current rise and the delay in protection action determine the peak value Ipeak. Due to hardware limitations, optimization of the valve control link delay (t2-t3) is limited.  
As shown in Figure 3, the overcurrent suppression can theoretically be approached from two directions.  
1. Reducing the rate of current rise during the fault keeps \(I_{\text{peak}}\) within a reasonable range during the time interval when the control and protection system is enabled. To achieve this, it is necessary to optimize the loop parameters or structure of the MMC-HVDC system to improve the fault loop characteristics.
2. Enhancing the sensitivity of overcurrent protection. Reducing the duration of time interval S1 and enabling the rapid activation of overcurrent protection. Effective suppression of Ipeak can be achieved.  
In summary, the suppression strategy for overcurrent of the MMC valve is shown in Figure 15.  
As shown in Figure 15, once the MMC valve design is finalized, the only options for overcurrent suppression are to modify the fault detection time, t1-t2, and the setting \(I_{\text{set}}\).  
Figure 15: Diagram of MMC overcurrent suppression strategy.  
More sensitive protection methods, such as rate-of-rise protection and unbalanced current protection, can be employed to reduce t\({}_{2}\). Alternatively, without changing the original protection strategy, the setting \(I_{\text{set}}\) can be optimized within a reasonable range, such as by adopting a two-stage overcurrent protection strategy for the bridge arm.  
If the offshore wind power MMC-HVDC transmission system is still in the design phase and the system parameters have not yet been finalized, then the optimal solution for suppressing overcurrent is to optimize the parameters of the primary system.  
By employing the field-circuit coupled analysis method demonstrated in this paper, a comprehensive investigation of overcurrent in MMC valves can be conducted.  
This approach enables the identification of factors that lead to high overcurrent levels in advance and, based on these findings, proposes effective strategies for overcurrent suppression.  
The \(\pm\)500 kV MMC-HVDC system shown in Figure 9 is still in the validation process. Drawing on the simulation waveforms from Figures 11-13, the arrester scheme in the CF model is optimized by reasonably increasing the reference voltage of the arresters.  
After re-evaluating and verifying the insulation coordination design, the MMC valve current under the same fault condition shown in Figure 9 is indicated in Figure 16.  
As shown in Figure 16, the CF model within the system was optimized through a refined design approach.  
Consequently, the system simulation no longer exhibits any additional discharge paths. During the fault process, the current through the MMC valve does not display any spikes. The peak current of the converter valve is approximately 2800 A, which is less than 2 IN (4000 A). Thus, the MMC valve current complies with the safety requirements for system operation during the fault process.