Arm current balancing control strategy for modular multilevel converters under grid fault conditions

Symmetrical ac current control is generally employed in grid-connected modular multilevel converters (MMCs) under grid fault conditions. However, it can result in unbalanced arm currents and consequently asymmetrical electrical stress and thermal distributions for semiconductor devices as well as unbalanced state of charge (SOC), heating, aging and even overcharge for batteries in MMC based energy storage systems. This paper analyses the reason of the unbalanced arm currents and proposes an effective method to solve this issue. Firstly, the mathematical expressions of the arm currents are derived under different fault severities and operation conditions to show the “side-effect” of the symmetrical ac current control. Then, a zero-sequence voltage injection method is proposed. The injected voltage is directly obtained from the negative sequence ac current controller and ac current without extra controllers. The effect of the injected voltage on the modulation index of MMC is quantitative analysed. The effectiveness and performance of the proposed analysis and control method are verified by simulation results.