Submodule Voltage Ripple Reduction for Back-to-back Modular Multilevel Converters Under Unbalanced Grid Conditions

The Back-to-back Modular Multilevel Converter (BTB-MMC) consists of two Modular Multilevel Converters (MMC) with directly connected dc-link, and ac-sides tied to two grids to transfer active power. This paper focuses on the ride-through control of BTB-MMC when one grid experiences unbalanced conditions, which causes a zero-sequence double-line frequency power ripple within the three phase legs of the single-side MMC connected to the unbalanced grid voltage. To reduce the additional phase leg energy ripple and average submodule (SM) voltage ripple, this paper first summarizes the system control method of the BTB-MMC system, and then proposes a high-frequency dc-link voltage ripple and zero-sequence circulating current injection method to distribute the SM voltage ripple between both-side MMCs, decreasing the required SM capacitance and improving the power density of the BTB-MMC. The injection method is validated through a case study with simulation results.