Energy Balancing Improvement of Modular Multilevel Converters Under Unbalanced Grid Conditions

This paper presents a feedback/feed-forward control strategy to improve the voltage balancing of modular multilevel converters (MMCs) under unbalanced grid conditions. The inclusion of a feed-forward compensation improves the disturbance rejection capability against asymmetrical faults and sudden voltage imbalances in the ac grid. The floating capacitor voltages of the MMC are controlled in two stages. The first one equally distributes the voltages in each arm, and the second one balances the energy among the six arms. This last control stage is also divided into the called horizontal and vertical balancing. Based on a detailed analysis of the power terms disturbing the MMC arm energies, the control strategy is tailored to improve the voltage transient response by adding feed-forward terms to both horizontal and vertical balancing controls. In addition, various approaches to manage the MMC under unbalanced grid conditions are compared regarding several aspects, such as capacitor voltage ripple, balancing performance, and negative-sequence current injection. Advantages and disadvantages of both the proposed and the conventional energy-based control schemes are also studied, showing that the voltage control performance can be improved by enhancing the existing MMC control systems.