The Analysis of DC Fault Mode Effects on MMC-HVDC System

DC fault is the main fault type of MMC-HVDC (modular multilevel converter based HVDC), the research of DC fault of MMC-HVDC is mainly focused on pseudo bipolar system, while the research of DC fault in bipolar system is still in its infancy. For the above, in this paper, the topology structure and working principle of MMC in bipolar system is introduced. The corresponding equivalent circuits of converter under DC pole-to-ground fault are built according to the AC/DC electrical parameters, the bridge arm sub-module capacitance discharge mechanism and the corresponding circulation paths of fault current on bridge arms. The effects of fault current on bridge arm valve group are analyzed and corresponding controls and protection strategies are proposed. Finally, a 51-level bipolar HVDC model is set up in RT-LAB and the simulation results validate the analysis method of fault mechanism.