## 1 Introduction  
With the development of global power interconnection, the requirements of long-distance power transmission and asynchronous power grids' interconnection are increasing. In order to meet these requirements, high-voltage direct current (HVDC) technology is playing a significant role in future global power interconnection [1]. Among all of the converter topologies, the modular multilevel converter (MMC), which was first introduced in 2001, is considered to be the most promising converter topology for HVDC transmission systems [2] due to its advantages of no commutation failure, flexible active and reactive power control, low switching operational frequency, and high output waveform quality, etc. [3]. MMC-HVDC is widely applied in renewable energy integration [4], the construction of DC grids, connection to weak power systems, large city power supply, asynchronous power grids' interconnection [5], and so on [6]. Presently, there are more than thirty MMC-HVDC projects in operation or under construction throughout the world, some of which commissioned in recent years are listed in Table 1. With the ability of changing power flow direction without reversing the voltage polarity, MMC-HVDC is gradually developing towards a multi-terminal direct current (MTDC) power grid [7]. The increasing terminal number of MMC-HVDC systems in China has verified this trend. By providing the possibility of interconnections between asynchronous power systems and various renewable energy resources [8], MMC-based MTDC power grids can potentially enhance the reliability of the AC and DC systems and improve the flexibility and economy of power dispatching [7]. It is playing an important role in future development of smart grids and the global power interconnection [9; 10; 11; 12; 13; 14]. However, the application of MMC-based MTDC power grids brings several technical challenges. One major challenge is the massive DC short-circuit fault currents. When compared with traditional AC systems, the impedances of DC lines and MMCs are much smaller. Once DC short-circuit faults occur, the capacitors in the submodules of MMCs discharge immediately and large short circuit currents are fed to the fault point through the DC transmission lines and the power electronic devices [13]. A typical DC pole-to-pole short circuit of an MMC-based MTDC power grid is shown in Figure 1. It can be seen from Figure 1 that the short-circuit current rapidly rises within a few milliseconds with a peak value of more than 10 kA after the DC short circuit fault occurs. DC current breakers are not commonly available for interrupting short-circuit currents due to the absence of a natural current zero crossing in DC systems, and they have very limited ratings for HVDC applications [15]. It is necessary to analyze and suppress the short-circuit current in order to ensure that the DC current breakers can isolate the fault line. Modelling, short-circuit current calculation methods, and suppression methods are essential in the investigation of countermeasures to the DC short-circuit current in MMC-based MTDC power grids. This paper gives a comprehensive review and evaluation of short-circuit current analysis and countermeasures in MMC-based MTDC power grids. The main contributions of this paper are: \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Commissioning Year** & **Name** & **Location** & **Terminals** & **Capacity, MW** & **DC Voltage, kV** \\ \hline
2020 & Zhangbei & China & 4 & 3000 & \(\pm\)500 \\
2019 & BorWin3 & Germany & 2 & 900 & \(\pm\)320 \\
2016 & Chongqing-Hubei & China & 2 & 2500 & \(\pm\)420 \\
2016 & Luxi & China & 2 & 1000 & \(\pm\)350 \\
2015 & Helwin2 & German & 2 & 690 & \(\pm\)320 \\
2015 & Xiamen & China & 2 & 1000 & \(\pm\)320 \\
2014 & Zhouushan & China & 5 & 1000 & \(\pm\)200 \\
2013 & Nan’ao & China & 3 & 200 & \(\pm\)160 \\
2011 & Nanhui & China & 2 & 18 & \(\pm\)30 \\
2010 & Trans Bay Cable & America & 2 & 400 & \(\pm\)200 \\ \hline \hline \end{tabular}
\end{table}