Capacitor voltage ripple suppression mechanism and control strategy for hexverter

At low-frequency operating conditions, the submodule capacitor voltage ripple in Hexverter increases significantly, leading to higher capacitance requirements and resulting in increased converter volume and cost. To alleviate the design constraints on DC support capacitors, this paper proposes a capacitor voltage ripple suppression mechanism and control strategy. Based on the AC positive-sequence network model of the Hexverter, the charging and discharging power characteristics of the submodule capacitors are analyzed, and an analytical model of the capacitor voltage ripple is established. Furthermore, through the zero-sequence network model of the Hexverter, the quantitative relationship between the zero-sequence AC circulating current and the zero-sequence transferred power is derived, clarifying the voltage ripple suppression mechanism based on the injection of specific-order zero-sequence AC circulating currents. On this basis, combined with the conventional Hexverter control strategy, a compound control strategy for the Hexverter incorporating a ripple suppression loop is proposed to effectively suppress the submodule capacitor voltage ripple. Simulation results demonstrate that the proposed control strategy can significantly reduce the capacitor voltage ripple, verifying its feasibility and effectiveness.