Modular Multilevel Converters (MMCs) are a new circuit topology to realize multilevel converters. Major advantages of MMCs in comparison to other multilevel converters or standard Voltage Source Converters (VSCs) are lower costs and a modular design for high voltage applications, higher reliability and longer maintenance intervals. However, modular multilevel topologies generally require advanced strategies for controlling and balancing energies since many energy storages are distributed all over the converter.
This paper presents a novel integrated strategy for current control, energy control and energy balancing of MMCs. The MMC is modeled as a periodic bilinear time-varying system respecting all currents and energies. Furthermore, a control structure is proposed combining a periodic linear quadratic regulator (PLQR) with an extended and least squares (LS) estimator to determine the currents and energies. Finally, simulation results for converter energizing and transmission startup as well as a phase-to-ground fault are given to illustrate the effectiveness of the strategy.