Low-Capacitance Three-Phase Series-Connected Modular Multilevel Converter With Second Harmonic Voltage Injection and High Capacitor Voltage Ripples for HVDC Application

To significantly reduce energy storage requirements, this article proposes a low-capacitance three-phase series-connected modular multilevel converter with second harmonic voltage injection and high capacitor voltage ripples (HR SC-MMC). The lower limit of the capacitor voltage is reduced by the second harmonic voltage injection. Based on this new limit, the SM capacitance can be redesigned, achieving a considerable reduction through high capacitor voltage ripple operation. The dc value of the capacitor voltage is slightly reduced to keep the peak capacitor voltage unchanged. The control and design methods are introduced to keep the SM capacitor voltage larger than the lower limit within the full operation range. The comparison results show that the HR SC-MMC effectively reduces the valve volume and cost, avoids a significant increase in the total power loss, and keeps the switch installation capacity and P-Q capability unchanged. Although the rms current and power loss of SM capacitors are increased, this may be acceptable, as the capacitor ripple current capability is underutilized and the increase in total power loss is small compared to existing methods. The correctness of the theoretical analysis and the feasibility of the HR SC-MMC are verified via 800-kV 733.3-MW simulations and 300-V 1.6-kW experiments.