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The Modular Multilevel Converter (MMC) is an emerging and highly attractive multilevel converter topology for medium and high voltage applications. In this paper, the equivalent circuit and power transmission model of MMC is proposed, based on which, the idea that control the current of positive leg and negative leg independently is brought up and a new multi-hierarchy method to balance the capacitor voltages is deduced. First, the average voltage of all submodules in a phase leg is got as a feedback to regulate the dc component of current command, which controls the average voltage of capacitors in a phase leg. Second, by adding active current to the positive leg current command and reverse phase active current to the negative leg can balance the average voltage of the capacitors in positive leg between negative leg. Third, a compensate signal which is added into the PWM control signal of each submodule makes each capacitor voltage in a leg balanced. The feasibility of independent leg current control, as well as the effectiveness of voltage-balancing control is verified by simulation.
Date of Conference: 6-9 July 2011