## 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
#### 4.2.3 MMC Fault Current Suppression Control Strategy  
Although many researches have investigated the topologies and application of current limiting devices, it is still not a perfect method for suppressing the DC short-circuit current. Due to the difficulty in manufacturing and high cost, there is no suitable DC fault current limiting devices for widely application [58]. The full use of converters' own capability through properly designed control strategies has become a prioritized choice, and it has been widely adopted in scenarios, such as DFIGs [59]. Therefore, making full use of the MMCs' self-controlling ability will be an ideal and feasible method of short circuit current suppression for MMC-based MTDC power girds. Several researches have investigated the fault suppression method by designing new MMC control strategies [43; 60]. Reference [60] proposed a comprehensive DC short-circuit fault ride through a strategy for a hybrid MMC with HB-SMs and FB-SMs. Under normal conditions, since the output voltage of the FB-SM is not required, the switches of the FB-SMs of both upper and lower arms are turned on. The circuit of the hybrid MMC is topologically equivalent to a traditional HB-MMC. Under a  
Figure 16: Performance of the proposed CBFCL: (**a**) under normal condition, (**b**) under normal condition considering charge of the capacitor bank, (**c**) under fault condition, and (**d**) after fault clearance. pole-to-pole fault condition, a bipolar output voltage of the FBSM is required in the upper and lower arm. The HB-SMs are bypassed and the circuit of the hybrid MMC is topologically equivalent to the double star-connected cascade H-bridge (CHB) converter that reduces the number of SM in operation and restrains the rise of short circuit current. The voltage injection method is applied to balance bridge arm capacitor energies, which ensures that the AC grid is not affected during the fault. A source-side DC fault current clearance scheme for hybrid MMC-based power grids is proposed in [43]. The hybrid MMC consists of the HB-SMs and FB-SMs and it adopts converter DC current control and DC fault line current control during the fault. Figure 17 shows the flowchart of the protection scheme. After the DC short circuit fault is detected, the nearest MMC switches into fault line current control mode and the fault is isolated by a mechanical circuit breaker. In the proposed scheme, there are no DC CBs required, which significantly benefits the construction of MMC-based DC power grids. However, the above two methods need hybrid MMC and it is still not available for the most widely applied HB-MMC. A DC short circuit current suppression method that is based on bridge arm voltage control is designed in [58]. It is found that the rising rate of the short circuit current is proportional to the number of sub-modules participating in the discharge that can be reduced through reducing the DC component in the reference voltage in the bridge arm after fault. Figure 18 shows the fault current limiting control strategy structure. Under normal operation conditions, the output voltage of the system is larger than the voltage. The DC component compensation of the bridge arm voltage is 0, and the AC component in the bridge arm is not adjusted. Therefore, it has little impact on the normal operation control characteristics of the system. Under DC fault conditions, the system output voltage rapidly drops once the short circuit fault occurs, which will be smaller than the voltage threshold. The control system switches the operation mode to compensate the DC component of the bridge arm voltage. With the DC and AC component of the compensated bridge arm reduced simultaneously, the output DC voltage is further reduced and the fault current is suppressed. The proposed current limiting measure is applied to a double-terminal MMC-HVDC system in [58]. Figure 19 shows the comparison of DC short circuit current with/without a current limiting measure.