Hierarchical System Design and Control of an MMC-Based Power-Electronic Transformer

Modular multilevel converter (MMC) is an emerging and highly attractive multilevel topology for medium-and high-voltage applications. This paper proposes an MMC-based power electronic transformer (PET) for dc distribution grid. The high-voltage side is an H-bridge MMC to generate a high-frequency adjustable-magnitude sinusoidal voltage, and the low-voltage side is paralleled multiwindings and rectifiers to allow high-load currents. As the number of MMC voltage levels increases, the complexity of the control system as well as the control algorithm increases largely. In order to simplify the control system and reduce the computation time, a hierarchical control system is designed for the MMC-based PET, and the relevant hierarchical control algorithm is also presented in this paper. The modulation method, capacitor voltage balancing method, and communication solution under the hierarchical control system are all discussed in detail. Simulation and experimental results are presented to demonstrate this system and control method.