Using Both the Circulating Currents and the Common-Mode Voltage for the Branch Energy Control of Modular Multilevel Converters

Many degrees of freedom are available for the branch energy control of modular multilevel converters. These degrees of freedom comprise different components of the common-mode voltage and the internal circulating currents. If all degrees of freedom are used simultaneously, undesired cross-couplings occur in the branch energy control. In the conventional branch energy control, either the internal circulating currents or the common-mode voltage are used to avoid the coupling terms. The paper presents a novel solution that uses both the circulating currents and the common-mode voltage while omitting the couplings. Thereby, the branch currents and thus the losses in the branches can be reduced without affecting the control dynamics. The proposed control approach is validated through simulations and experimentally on a downscaled converter prototype. Furthermore, the improved performance is demonstrated by means of a comparison to the conventional control. In general, the proposed control approach is expected to be especially beneficial for drive systems operated below rated speed and for a low-voltage ride-through of grid-tied converters.