Design of control structure for modular multilevel converter

Considering the control complexity of modular multilevel converter(MMC) under high-voltage high-power application with too many sub-modules, an improved hierarchical control structure consisted of three-level controller is proposed, to solve the synchronization and low reliability problems of the traditional control system in this paper. Framework structure and implementation of the hierarchical control system is discussed in detail in up-down order, including the operating staff, the upper scheduling interface, the main controller, the sub-module controller, and the communication mode and content between layers. Experiment of a 3-phase 9- level MMC adopting the proposed control structure verified the system´s feasibility and flexibility.