Strategy of Voltage Stress Supression and Dynamic Simulation Experiments of MMC Sub-Module on Hybrid Cascaded Multi-Terminal UHVDC Transmisson System

When the metallic ground fault occurs at the valve side of modular multilevel converter (MMC) on hybrid cascaded multi-terminal ultra-high voltage direct current (UHVDC) transmission system, the non-fault phase and the fault phase will constitute the fault charging circuit and the rectifier side continuously injects energy into the MMC after blocking, which will cause over-voltage of the faulty MMC valve group sub module. More seriously, the converter valve equipment will be damaged. To reduce the voltage stress of MMC sub module and improve the reliability and safety of converter valve equipment, aiming at the single-phase ground fault characteristics of MMC valve side, the voltage stress suppression strategy based on controllable arrester and MMC step-by-step blocking are proposed from the perspective of consumption of surplus power and optimization of control scheme. Finally, the strategy is verified by the platform of dynamic simulation experiments. The test results demonstrate the proposed strategy is validity, and the voltage stress suppression strategy based on MMC step-by-step blocking is better.