## 4 Analysis and Suppression Method of Short Circuit Current in MMC-Based MTDC Power Grid
### DC Short Circuit Current Suppression Method in MMC-Based MTDC Power Grid
#### 4.2.2 Fault Current Limiting Devices  
Installing current limiting devices in DC systems and cutting off the fault line with DC CBs is another preferable suppression method for short circuit current. DC inductors and DC fault current limiters (FCLs) are two widely applied fault current limiting devices in MMC-based MTDC grids [47]. The current-limiting DC inductors are usually installed at both ends of the transmission lines in series in order to achieve better fault current suppression performance [48; 49]. Figure 11 shows the short circuit current with different inductive fault current limiters in a four-terminal MMC-based power gird. The detailed structure and parameters of the four-terminal MMC-based power gird can be found in [50]. The inductive fault current limiters have a good performance in limiting the rising rate of the short circuit current.However, the series inductors can decrease the robustness and response speed of the DC system [51]. Another widely applied current limiting device is the DC FCL. FCL has many different topologies, such as resistive FCL, superconducting FCL (SFCL), solid state FCL, and magnetic FCL. * Solid FCLs: solid FCLs are the most popular topologies due to its advantages of minimum power loss under normal condition, quick response, fast recovery capacity, good reliability, and low cost [52]. A resistive based solid-state FCL is proposed in [53]. The resistive based solid-state FCL consists of a current-limiting resistor module \(R\) and a control module in parallel, as shown in Figure 12. The resistive based solid-state FCL has two operational states. In the normal operating state, reverse parallel gate turns off thyristors (GTOS) in a conducting state, which ensures that the current-limiting resistor \(R\) is bypassed and the operational current can flow through the GTOs. Figure 10: Fault current discharge path of various SM topologies. Under DC fault conditions, T\({}_{1}\) and T\({}_{2}\) are turned off according to the operation signal issued by the fault detection system, and \(R\) is rapidly put into use, which increases the system damping and inhibits the fault current's rising. * SFCL: the SFCL operates under a superconducting state in normal operation state, which offers no resistance to the load current. Meanwhile, it can achieve high resistance under fault conditions [50; 54; 55; 56]. Figure 13 shows the short circuit current with resistive FCLs in a four-terminal MMC-based power grid, whose structure and parameters can be found in [50]. It can be found that, although the amplitude of the DC short circuit current can be effectively decreased, the resistive SFCL cannot limit the rising rate. To remedy this defect, reference [50] proposes an improved structure for inductive SFCL, as shown in Figure 14.  
The inductive SFCL consists of several diodes and a saturated iron core SFCL. The saturated iron core SFCL is placed between the bridges composed with four diodes (D\({}_{1}\sim\) D\({}_{4}\)). The proposed SFCL is connected in series in DC transmission lines via connectors \(A\) and \(B\). Under normal  
Figure 11: Short circuit current with different inductive fault current limiters. Figure 12: Operating states of resistive based solid-state fault current limiter (FCL). Figure 13: Short circuit current with the resistive FCLs. conditions, the DC operational current can flow through the diodes groups D\({}_{1}\) and D\({}_{4}\), or D\({}_{2}\) and D\({}_{3}\). Wherever the short circuit fault occurs, the bi-directional SFCL allows for the fault current flow through. The copper made primary coils of the iron core SFCL are connected with the DC system. The secondary coils are made of superconductive materials and powered by a DC source. The external feature of the proposed inductive SFCL is equivalent to a small inductance and it has little impact on the system normal operational dynamics. The improved inductive SFCLs are applied to the four-terminal power grid that is shown in [50].