Capacitor-Voltage Ripple Control and Capacitance Reduction Potential of the Auxiliary MMC in a Series-Connected DRU–MMC Offshore HVDC System

This paper develops a capacitor-voltage ripple-control scheme that combines negative-sequence second-harmonic capacitor-voltage feedback with third-harmonic zero-sequence voltage injection. An arm-energy model incorporates the common dc current, auxiliary-MMC voltage allocation, and ac support duty. The dc-series connection of a diode rectifier unit (DRU) and a reduced-rating modular multilevel converter (MMC) reduces the voltage rating of the offshore auxiliary converter, while submodule capacitance remains constrained by internal energy fluctuations. The rated-point comparison at 9000 μF gives maximum single-sided ripple indices of 7.21%, 5.51%, and 3.53% for conventional circulating-current suppression, additional third-harmonic injection, and the combined strategy, respectively. In the corresponding capacitance comparison, the combined-strategy case uses 4460 μF, equivalent to a 50.44% reduction in capacitance and nominal stored energy relative to the 9000 μF baseline. Waveform-based estimates reveal a trade-off: third-harmonic injection alone gives the lowest arm-current root mean square (RMS) value, whereas the combined strategy gives the lowest ripple and the largest negative current extreme. The two configurations recover dc voltage and transferred power following the investigated balanced fault, with larger capacitor-voltage excursions in the reduced-capacitance combined case. These results demonstrate rated-point capacitance reduction potential and identify the current and transient-voltage constraints that must be considered in practical sizing.