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-build 10 kV SiC MOSFET based MMC prototype.