## 4 Interruption Performance of HB- vs FB-MMC DC Grids
### Comparison of Fault Interruption Performance  
In this section, we compared the interruption performance of Case-1 and Case-2 according to the location of the occurrence of fault on the DC line. The peak magnitude of the fault current and interruption time are considered key parameters for evaluating the performance of the two cases.  
For a pole-to-pole fault at 5 km from the rectifier, the peak magnitude of the fault current in Case-2 is 15.5 kA; it is only 3.5 kA in Case-1, as shown in Figure 8. The lower peak fault current magnitude in Case-1 is due to the presence of the current-limiting reactor and the fast-operating HCB.  
Figure 6: Simulation of a DC pole-to-pole fault with MMCs: (**a**) current waveform in the transient state; (**b**) voltage waveform in the transient state.  
Figure 7: Simulation of a DC pole-to-ground fault with MMCs: (**a**) current waveform in the transient state; (**b**) voltage measured at converter terminals.  
The total interruption time as well as the peak fault current magnitude in the two cases is compared in Figure 9.  
It can be seen in Figure 9a that the interruption time in Case-1 does not change considerably with the variation in fault location. However, in Case-2 the interruption time increases considerably for faults farther away from the rectifier. The quick interruption in Case-1 can be attributed to the presence of the fast-acting HCB. However, the inherent blocking characteristics of the FB-MMC in Case-2 are dependent on the natural elimination of the fault current. Therefore, if the fault location moves away from the rectifier, the interruption time increases due to the increase in inductive and capacitive components of the transmission line. It can be seen in Figure 9a that for faults occurring beyond 8 km, Case-1 offers a smaller interruption time.  
Figure 9b shows that in both cases the peak magnitude of the fault current decreases upon increasing the distance of the fault from the rectifier. However, due to presence of an HCB in Case-1, the fault is interrupted before its natural peak and therefore the peak magnitude is quite low regardless of the fault location. However, in Case-2 the peak magnitude of the fault current is dependent on the inductive reactance of the line. Therefore, the peak magnitude is quite high for faults occurring on the converter terminal.  
Figure 8: Fault handling interruption performances: (**a**) fault current interruption characteristic of Case-1 with an HCB; (**b**) fault current interruption characteristic of Case-2 with an RCB.  
Figure 9: Comparison analysis of interruption characteristics according to the location of the fault on the DC line: (**a**) interruption time; (**b**) maximum fault current magnitude.  
In conclusion, we found that Case-1 has a lower fault current magnitude and lower interruption time for most fault locations.  
Figure 10 shows the comparison of the energy dissipation of the circuit breaker according to the fault location.  
In Case-1, the circuit breaker must fully cope with the DC fault current, which has high di/dt. Therefore, we confirmed that the energy dissipation in the HCB is more than 0.32 MJ, even if the fault location is far away from the converter side.  
In Case-2, the peak fault current is higher than that in Case-1 and the interruption time is also larger. Therefore, the energy dissipation in the RCB was found to be about 2 MJ when the fault occurs at 20 km.