Adaptive DC Voltage Component Modulation of Sub-modules in Modular Multilevel Converters (MMCs) for Efficiency Optimization during Wide AC Voltage Range

Realisation of high-efficiency modular multilevel converters (MMCs) during wide AC grid voltage variation is a significant challenge for the voltage-source-converter based high-voltage direct-current system. In this study, the DC component of the sub-module output voltage is investigated to be a new control freedom by building the inherent relationship between the DC voltage component and active sub-module number. Then, the effect of AC voltage on DC voltage component is explored. The adaptive DC voltage component modulation is proposed to dynamically regulate the active sub-module numbers during wide AC voltage variation, resulting in the improved MMC conversion efficiency. Moreover, the detailed performance analysis shows that the adoption of the proposed modulation brings little effect on the sub-module capacitor voltage/current ripple, circulating current suppression as well as the AC and DC terminal voltages. Finally, the simulation and experiment results show that the adaptive DC voltage component modulation is effective to reduce the power losses during a wide AC voltage range.