DC Fault Control for an HVDC Transmission System Protection Based on Hybrid-MMC and High-Speed Switch

As a DC fault occurred, it is very important to quickly and reliably clear and isolate the fault for hybrid modular multilevel converter (MMC). Firstly, the comprehensive influence of the proportion of FBSM and modulation ratio on the fault clearing capability is theoretically calculated in the process of bipolar short-circuit for hybridMMC. Increasing the modulation ratio and the proportion of FBSM accelerates the fault clearing speed. Secondly a fault isolation scheme is proposed based on coordination of highspeed switch (HSS) and hybrid-MMC. The proposed strategy does not block MMC, but controls the FBSMs to output negative voltages to attenuate the DC fault current, then the metal oxide varistor (MOV) absorbs the residual energy to accelerate the fault current attenuation, and the Ultra-fast disconnector (UFD) acts to isolate the fault, which improves the reliability and flexibility of the power grid. Finally, simulation on Matlab/Simulink verify the effectiveness and adaptability of the proposed scheme.