A protection scheme for DC-side fault based on a new MMC sub-module topology

Concerning the defects of double-thyristor method in clearing fault on the DC side of MMC-HVDC system, a new MMC sub-module topology is designed, and a protection scheme for DC-side fault based on the new topology is proposed. First, the characteristic of short circuit current when fault occurs on the DC side of MMC-HVDC system after converter blocking is analyzed, as well as the attenuation pattern and influencing factors. And then, concerning the defects of double-thyristor method, an RC absorber is added to the sub-module to avoid the mistriggering of thyristor in normal operation state by utilizing the characteristic that the capacitor voltage cannot jump. At the same time, a group of inverse-series IGBT and diode is added to the sub-module, thus the unidirectional conduction characteristic of diode can be used to block the flow of short circuit current in the diode. On this basis, a new MMC sub-module topology is designed, and a DC-side protection scheme based on fault self-clearing is proposed, which combined with reclosing measures can quickly restore the system to normal operation. RTDS simulation results demonstrate that, the protection method based on the new sub-module can rapidly reduce the fault current on DC side, effectively protect the sub-module devices, and quickly restore the system to normal operation in the case of instantaneous fault.