Cell Capacitor Voltage Switching-Cycle Balancing Control for Modular Multilevel Converters

Modular multilevel converters (MMC) with the conventional control are subjected to large capacitor voltage ripples, especially at low-line frequencies. The existing attenuation approaches, such as the second-order or high-frequency harmonic injections, are designed based on average models limiting the achievable performance. This letter aims to greatly increase the control time resolution, investigating the possibility to form the control in the timescale of a switching cycle. The circulating current is allowed to have multiple changes in one switching period. This critical alternation equips the MMC with the capability to achieve voltage balancing in a single switching cycle, shifting the MMC from a long-deemed “line-cycle balancing” converter to a “switching-cycle balancing” converter. The proposed approach is experimentally verified under dc–dc operation, which is the most challenging scenario for conventional controls. Test results under 6 kV dc-link voltage are provided performed with the custom-built 10-kV SiC
mosfet
-based MMC prototype.