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Multilevel converters and battery energy storage systems are key components in present and future medium voltage networks, where an important integration of renewable energy sources takes place. The modular multilevel converter offers the capability of embedding such energy storage elements in a split manner, given the existence of several submodules operating at significantly lower voltages. This paper analyzes such a converter structure under different operating modes. In order to eliminate the low-frequency components of the submodule output currents, the latter are interfaced to the batteries by means of nonisolated dc/dc converters. Control algorithms are developed for the balancing of the battery state of charges and the respective gain limitations are established. Unbalanced grid conditions are also taken into account through the theory of symmetrical components and solutions are proposed. Finally, the development of a down-scaled prototype is described and experimental results are presented. [ABSTRACT FROM AUTHOR]

In this paper, the analytical solution for the submodule voltage ripple equations of a modular multilevel converter (MMC) is derived, based on the knowledge of the external voltage/current magnitudes, and enhancing a concept previously presented in the literature. In order to achieve high accuracy, all passive elements of the converter, common-mode voltage injection as well as intentionally imposed circulating current harmonics are taken into consideration. The natural charge level mechanism of the capacitor voltages is also explained. As application examples, the three- as well as the two-phase grid-connected MMC cases are chosen. The control of line and circulating currents is also discussed and two respective independent feedback loops are formed. The concept of fictive-axis emulation is tailored for the two-phase MMC case, in order to achieve vector control of the line current and therefore straightforward desired injection of active and reactive power. Finally, the development of a reduced-scale laboratory prototype is presented and a full set of experimental results are provided, verifying the aforementioned concepts. [ABSTRACT FROM AUTHOR]

The internal control of a modular multilevel converter aims to equalize and stabilize the submodule capacitor voltages independent of the loading conditions. It has been shown that a submodule selection mechanism, included in the modulator, can provide voltage sharing inside the converter arm. Several procedures for controlling the total stored energy in each converter arm exist. A new approach is described in this paper. It is based on estimation of the stored energy in the arms by combining the converter electromotive force reference, the measured alternating output current, and the known direct voltage. No feedback controllers are used. Experimental verification on a three-phase 10 kVA prototype is presented along with the description of the new procedure. [ABSTRACT FROM AUTHOR]