Submodule Cyclic Control and Its Auxiliary Methods for MMC Capacitor Voltage Balancing

Submodule capacitor voltage balancing in modular multilevel converters (MMCs) is a key issue for ensuring stable converter operation and reducing device voltage stress. To address the high computational burden and high switching frequency of the conventional sorting-based voltage balancing method under nearest-level modulation (NLM), this paper analyzes the mechanism of capacitor voltage imbalance from the perspective of periodic submodule energy distribution. The effects of submodule insertion time, arm-current integration, and the phase difference of the AC current on capacitor voltage imbalance are investigated. On this basis, a submodule cyclic control method for MMC capacitor voltage balancing is proposed. By periodically circulating the control signals of submodules, each submodule sequentially experiences different insertion patterns within one cyclic control period, thereby achieving balanced energy distribution among submodule capacitors. Furthermore, to address the prolonged cyclic control period caused by an increased number of submodules, a grouping-based auxiliary control method is designed to shorten the cyclic control period while maintaining the capacitor voltage fluctuation within the required range. To provide direct correction of out-of-margin capacitor voltage deviations during the post-fault balancing process, an over-margin auxiliary control method is further proposed. Finally, a three-phase nine-level MMC simulation model is established in PSCAD/EMTDC to verify the proposed method under steady-state and transient operating conditions. The simulation results show that the proposed method can effectively achieve submodule capacitor voltage balancing, shorten the cyclic control period, enhance post-fault voltage-balancing capability, and reduce both the switching frequency and the computational burden of real-time sorting while maintaining satisfactory voltage balancing performance.