Dual-Arm Modular Multilevel Converter With a Compact Footprint for VSC-HVDC Applications

In this paper, a novel dual-arm modular multilevel converter (DAC) with inherent DC-fault-blocking capability is proposed that needs only two stacks of submodules (SMs), as well as director switches (DSs), for multilevel AC/DC conversion. Compared to the state-of-the-art modular multilevel converter (MMC) technologies with six SM-based arms, the DAC's dual multiplexed converter arm structure, combined with needing approximately 75% less capacitive energy storage than the MMC, facilitate a smaller converter station footprint. This paper presents the operating principle of the DAC and benchmarks it against other voltage source converter technologies based on converter functionalities, component count, capacitive energy storage requirements, and semiconductor losses for high voltage direct current transmission (HVDC) application. For a wider output voltage range, overlap operation is proposed to enable circulating currents to balance the SM capacitors’ energy cycle-to-cycle. A 600 kV HVDC system simulation study is presented that demonstrates the DAC's ability to (1) transmit power bidirectionally under nominal conditions, and (2) extinguish fault currents during DC-side short-circuit fault. A
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scaled-down converter hardware implementation further verifies the DAC's operating principle.