## 5 Conclusions and Future Trend Exploration  
This paper gave a comprehensive review of the existing DC short-circuit current analysis and suppression methods that were used in MMC-based MTDC power grids, including the latest theoretical achievements and techniques applied in engineering practice. The existing MMC modeling, DC short circuit current calculation methods, and suppression methods were classified and the advantages and disadvantages were summarized in this paper. The models of MMC-based MTDC power grids have been studied and investigated for different scenarios. The DSM is feasible for SM-level dynamic analysis and control design. The AVM and SAVM are the preferred models for system-level analysis and control design. The DEM can be applied to design the balancing control of the capacitor voltages and represent the blocked mode. The EDM is the most suitable model for DC fault analysis and protection design. The mainstream of DC short-circuit current calculation in MMC-based MTDC power grids is the equivalent circuit method, where MMCs are formulated as capacitor-inductor series circuits. However, the equivalent circuit method ignores the internal dynamics of MMCs and cannot display the inherent characteristics of the MMC-based MTDC power grids. It is necessary to establish the fault current analysis MMC model when considering the internal dynamics, and propose the short circuit calculation method considering the dynamic characteristics of MMC-based MTDC power grids. Figure 19: Comparison of DC fault current with/without current limiting measure. Figure 18: Block diagram of MMC based on partial bypass of SMs. The existing fault current suppression methods in MMC-based MTDC power grids usually include changing SM topologies, adding current limiting devices, and controlling the MMC. Table 4 presents the above-discussed DC short circuit current suppression methods to make a comparison of their features. In the table, \(\surd\) indicates that the chosen technique is strongly related to the corresponding item; \(\times\) indicates that the chosen technique is not related to the corresponding item; 'B' indicates 'Big'; 'M' indicates 'Moderate'; and, 'S' indicates 'Small'. Most of the existing MMC-HVDC projects adopt HB-SMs and they cannot cut off the fault current by themselves. The cost of replacing the SM topologies with fault ride through capability is very large, and this disadvantage limits its practical application. Although installing fault current limiting devices can achieve good fault current suppression performance, it also has the disadvantages of high cost and impact on normal operational conditions. The future research direction of the fault current limiting devices is to reduce the cost and impact on the normal operational conditions. Suppressing the DC fault current through controlling MMC has the advantages of low cost, small impact on normal operational conditions, and flexibility in modification. Thus, it is an ideal DC fault current suppression method for MMC-based MTDC power grid and it has broad prospects for future development. \begin{table}
\begin{tabular}{c c c c c c c c} \hline
**Method** & **SM Change** & **Add Devices** & **Increase Impedance** & **Iron Core** & **Superconducting Materials** & **Extra Cost** & **Impact on Normal Condition** \\ \hline FB-SM & \(\surd\) & \(\times\) & \(\times\) & \(\times\) & \(\times\) & B & S \\ DB-SM & \(\surd\) & \(\times\) & \(\times\) & \(\times\) & \(\times\) & B & S \\ SLCC-SM & \(\surd\) & \(\times\) & \(\times\) & \(\times\) & \(\times\) & B & S \\ DC-Inductor & \(\times\) & \(\surd\) & \(\surd\) & \(\times\) & S & B \\ Solid FCL & \(\times\) & \(\surd\) & \(\surd\) & \(\times\) & M & S \\ SPCL & \(\times\) & \(\surd\) & \(\surd\) & \(\surd\) & B & S \\ CBPCL & \(\times\) & \(\surd\) & \(\surd\) & \(\times\) & B & S \\ Hybrid MMC & \(\surd\) & \(\times\) & \(\times\) & \(\times\) & \(\times\) & B & S \\ MMC control & \(\times\) & \(\times\) & \(\times\) & \(\times\) & \(\times\) & S \\ \hline \end{tabular}
\end{table}