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Decoupled Control of the Arms of a Modular Multilevel Converter with Orthogonal Reference Signals

The technology of modular multilevel converters has been proposed for high-power dc-ac applications. The modular structure of the converter, which is composed of floating capacitor converters connected in series, is one of its most appreciated advantages. However, the control of the energy stored in the floating capacitors is not simple, because it depends on the circulating current of each leg, i.e., the current that runs through the leg without affecting the power transferred to the load. The harmonic components of the circulating current can be employed to balance the arm energies, and several techniques have already been developed to address this control problem. This paper finds a solution to the control problem of the arm voltages by expressing the circulating current as a combination of orthogonal functions synthesized from the measured voltages and currents. As a result, the voltages of the upper and lower arms of each leg can be directly controlled, independently of one another. Experimental results confirm the feasibility of the developed approach.