DC fault characteristic simulation analysis of MMCHVDC 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 and the effects of different types of DC faults to running state in AC/DC hybrid power system is analyzed. The equivalent circuit of converter under fault condition is built according to the change of the AC/DC electrical parameters and the bridge arm sub-module capacitance discharging mechanism and different DC fault mechanisms of bridge arm valve group is revealed, furthermore, the propagation path of the fault disturbance energy in the hybrid system is given. Finally, a 51-level two ends bipolar HVDC model is set up based in the software RT-LAB and the simulation results validate the accuracy of the theoretical analysis of fault mechanism.