Alternating Submodule Configuration Based MMCs With Carrier-Phase-Shift Modulation in HVdc Systems for DC-Fault Ride-Through Capability

DC short-circuit fault ride through is one of the most important characteristics for the modular-multilevel converters (MMC) employed in high-voltage direct-current (HVdc) transmission systems. During the faults, for providing a fault-tolerant control with reactive power compensation to the grid, the MMC normally changes its structure, which requires a remarkable modification and burden computation for implementing the modulation technique in the MMC. This paper proposes an alternative submodule configuration of the MMC based on a carrier-phase-shift PWM scheme, which is easily implemented for switching the operation modes of the MMC from normal condition to fault-tolerant control. The arms of the proposed MMC are configured by typical half-bridge submodules (HBSM) and suggested series-connected triple SMs (SCTSM) in interleaving series, where the SCTSM is composed of three HBSMs connected in series through an additional IGBT and a clamp diode. With the additional IGBTs and diodes, the SCTSMs can produce bipolar output voltages and the MMC can be restructured to operate as three-phase cascaded multilevel converter during pole-to-pole short circuits to control the converter currents. In addition, the cost and power loss of the proposed MMC are lower than those of the existing MMCs based on full-bridge SMs (FBSM), a hybrid of HBSMs and FBSMs, and clamp-double SMs. PSIM simulation results for the 300 MW–300 kV HVdc system with the proposed MMCs are shown to verify the effectiveness of the scheme.