The Modular Multilevel Converter (MMC) with cascaded full bridge cells is available for AC/AC transmission system without transformers in high voltage line. This research applied a Hexagonal MMC (H-MMC) to 3-phase AC-AC back-to-back (BTB) system. The H-MMC has six arm modules with floating capacitors and is required to regulate the input-output power and capacitor voltages by controlling currents in arm modules. Theory of the power flow of H-MMC is analyzed and the control scheme of the power flow and capacitor voltage regulation is proposed. Currents in the arm modules are controlled by the internal model principle and H-MMC can operate appropriately. Effectiveness of the proposed control method is presented in simulation and experiment.
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