Abstract-This paper studies different circulating current references for the modular multilevel converter (MMC). The circulating current references are obtained from the instantaneous values of the output current and modulation signal of the phase-leg. Therefore, determination of the amplitude and phase of the output current is not needed, which is a significant improvement compared to other methods such as those based on injecting specific harmonics in the circulating currents. Among the different methods studied in this paper, a new method is introduced, which is able to reduce the capacitor voltage ripples compared to the other methods. A closed-loop control is also proposed which is able to track the circulating current references. With the discussed methods the average values of the capacitor voltages are maintained at their reference while the voltage ripples are kept low. Experimental results are presented to demonstrate the effectiveness of the proposed and discussed methods.

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Abstract-This paper presents a novel modulation strategy for n-phase neutral-point-clamped (NPC) converters. The proposed modulation strategy is able to control and completely remove the low frequency neutral-point (NP) voltage oscillations for any operation point and load types. Even when unbalanced and/or nonlinear loads are considered, the NP voltage remains under total control. Consequently, the strategy is very attractive for nphase active filters. In addition, it enables the use of low capacity film capacitors in NPC converters.The proposed modulation takes the carrier-based modulation strategy as a basis. It is formulated following a generalized approach that makes it expandable to n-phase NPC converters. In addition, the NP voltage is controlled directly using a closedloop algorithm that does not rely on the use of the linear control regulators or the additional compensators used in other modulation algorithms. Therefore, no tuning of parameters is required and it performs optimally for any operating conditions and kind of loads, including unbalanced and nonlinear loads. Although the high frequency harmonic content of the output voltages may increase, the weighted total harmonic distortion generated by the proposed strategy is similar to that of a standard sinusoidal pulsewidth modulated strategy. The proposed modulation algorithm has been tested in a four-leg NPC converter prototype performing as a three-and four-phase system and operating with balanced and unbalanced loads.

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