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Characteristics analyses and rebalancing method of modular multilevel converter under submodule fault conditions

Modular multilevel converter (MMC) is a promising circuit topology in the field of high-voltage direct current transmission (HVDC) systems. To enhance the reliability of the MMC in this field, the popular method is the addition of redundant submodules (SMs). However, the MMC will become unbalanced when some SMs fails and the performance will be deteriorated, especially when the number of the failed SMs is high. This paper analyzes the impact of the SM fault on the performance of the MMC. Theoretical analyses show that fundamental frequency component appears in the dc side current when some SMs fails. Furthermore, the approximate expression of the fundamental frequency component is derived. Based on the analyses, the number of the redundant SM design criterion is proposed. In addition, a rebalancing method is proposed to eliminate the harmonic component in the dc side and arm currents under SM faults scenario. Simulation results verified the theoretical analyses and the proposed rebalancing method.