Valve-Side Single-Phase-to-Ground Fault Analysis in Bipolar MMC-HVDC Systems With Hybrid SMs

This letter presents a comprehensive fault analysis for a modular multilevel converter high-voltage direct current (MMC-HVDC) system with hybrid half-bridge submodules (HBSMs) and full-bridge SMs (FBSMs) under valve-side single-phase-to-ground (SPG) faults. The analysis shows protection for upper arms is necessary due to the inevitable upper arm overvoltage. It further specifies that the proportion of FBSMs utilized in each arm should be at least 86.6% of the available voltage to safely clear valve-side SPG faults without overvoltage in the lower arms. The accuracy of the theoretical analysis is validated in the positive pole of a bipolar MMC-HVDC system with different ratios of FBSMs modelled in PSCAD/EMTDC.