Suppressing DC Voltage Ripples of MMC-HVDC Under Unbalanced Grid Conditions

There are second-order harmonics in the dc voltage and current when the MMC–HVDC system is under unbalanced grid conditions, even if the negative-sequence current controller is employed. This paper presents a supplementary dc voltage ripple suppressing controller (DCVRSC) to eliminate the second-order harmonic in the dc voltage of the MMC-HVDC system. The instantaneous power of the converter arm and phase unit indicates that there are zero-sequence double-line frequency components in the three-phase unit voltages when the ac system is under an unbalanced fault. Since the zero-sequence components cannot be offset by each other, they lead to the second-order harmonic in the dc voltage and dc current. The DCVRSC is developed to compensate the zero-sequence components in three-phase unit voltages. Simulation results based on a detailed PSCAD/EMTDC model prove that the DCVRSC can eliminate the second-order harmonic in the dc voltage. Meanwhile, the ac currents are kept balanced under the unbalanced fault conditions.