Protection schemes using resistive-type superconducting fault current limiters with mechanical DC circuit breakers in MMC-MTDC grids

The cooperative characteristics of resistive superconducting fault current limiters (R-SFCLs) and DC circuit breakers (DCCBs) have been investigated with regard to the fault clearing in modular multilevel converter (MMC)-based multiterminal direct current (MTDC) grids. However, research is lacking in complete protection solutions that combine R-SFCLs and mechanical DCCBs while considering the backup protection and the effect of R-SFCLs on the fault characteristics of MMCs. This study investigates a fault current breaking scheme that includes primary and backup protection. Moreover, the fault characteristics of MMCs with R-SFCLs are analysed, and the fault transient process is divided into three stages. Then two fault current limiting schemes with R-SFCLs located at the converter output or the line are proposed. These two protection schemes are applied to the DC test system, verified and compared. The results show that in both schemes, the size of the designed RSFCL should be generally a few tens of Ω. These R-SFCLs maintain the converters unblocked until the primary breakers or backup breakers interrupt the fault currents, and the M-DCCBs are only required to break several kA currents. After a fault occurs, the system will restore within 200 ms for primary protection or 250 ms for backup protection.