Multiplexed-Stack Converter With DC-Fault Ride-Through Capability and High Compactness

This article presents a new multiplexed-stack converter (MSC) for high-voltage direct-current (HVDC) power conversion. The MSC reduces the number of stacks of submodules (SMs) needed for three-phase alternating-current (AC) systems from six, as in conventional modular multilevel converters (MMCs), to four, which can substantially decrease the size of converter stations. The MSC comprises two full-bridge SM (FBSM)-based Outer stacks, two half-bridge SM (HBSM)-based Inner SM stacks, and director-switch (DS) valves to generate sinusoidal three-phase AC voltages. The MSC offers several benefits over existing converters: it can handle and block DC faults due to its FBSM-based Outer stacks, it can attain high efficiency due to its HBSM-based Inner stacks, and it can ease valve design due to fundamental-frequency and zero-voltage-switching (ZVS) operation of its DSs. A sweet-spot operating voltage is derived where the Outer and Inner stack energies are naturally balanced without the need of DC pole capacitors (PCs). For deviations from the sweet-spot voltage, DC-side PCs enable energy balancing for the stacks. This article presents HVDC-scale simulations and reduced-scale hardware experiments that confirm the MSC’s performance in real and reactive power conversion. This article also contrasts the MSC with other state-of-the-art converters, indicating its competitive efficiency and compactness.