Active-grounded Modular Multilevel Converter with DC Fault Clearing Capability

In view of the fact that the conventional semi-bridge modular multilevel converter (MMC) cannot block the fault current, and the existing MMC with the fault clearing capability takes too long to clear the fault, an active-grounded MMC (AG-MMC) topology with DC fault clearing capability is proposed. On the basis of conventional semi-bridge MMC, a current transfer branch is added outside the inductors on the upper and lower bridge arms of each phase of the converter, and a cut-off branch and an energy absorption branch are added at the DC outlet of the converter. When a bipolar short circuit occurs on the DC side of the MMC, the cut-off branch can effectively isolate the circuit breaker and the DC line. The current transfer branch can consume AC current and inductor current, and the energy absorption branch can quickly clear the fault current. The topology of AG-MMC and the process of fault isolation and clearing are analyzed in detail, and the design and calculation method of key parameters are given. Finally, the two-terminal MMC and four-terminal MMC simulation systems are built by using RT-LAB OP5607 software. Comparative analysis shows that the proposed AG-MMC can clear faults within ten milliseconds, which has great advantages in terms of economy and practicality. © 2020 Automation of Electric Power Systems Press.