Optimal Circulant Modulation for Submodule Capacitor Ripple Minimization and Inherent Voltage Balancing in Modular Multilevel DC Converters

Modular multilevel dc-dc converters (MMDCs) have attracted much interests in medium voltage dc applications, but it has to face the main issue of balancing submodule (SM) capacitor voltages. Conventionally, the SM balancing demands monitoring and sorting the real-time SM capacitor voltages. It highly relies on the high-speed bidirectional communication between submodules and central controller, which increases the implementation costs and decreases the system reliability. This paper proposes the optimal circulant modulation with inherent balancing capability as well as the minimized the SM capacitor voltage ripple. Firstly, the optimal SM switching pattern is derived according to the SM ripple expression cycle-by-cycle. Then the full-rank feature of the optimal switching matrix is proven, which promises the inherent balancing of SM capacitor voltages. Finally, the ripple optimization and inherent balancing capability of the proposed optimal circulant modulation are validated by experiments on an LLC-based MMDC prototype.