A Unified Direct Control Strategy for MMC Based on Multi-terminal Flexible Interconnection System

Multi-terminal flexible interconnection technology can effectively improve the power quality of distribution network, strengthen the reliability of power supply and improve the level of new energy consumption. Currently, the modular multilevel converter (MMC) in the flexible interconnection system widely uses the voltage and current decoupling control method based on the rotating coordinate system, but this control method can hardly meet the requirements of the MMC converter performance in the flexible interconnection system under three-phase asymmetry, line fault and transient conditions. In this paper, therefore, a unified direct control strategy for MMC based on multi-terminal flexible interconnection system is suggested for the operational characteristics of the multi-terminal flexible interconnection system. The strategy simplifies the control strategy by unifying the generation of current loop reference; speeds up the switching of MMC converter operation mode under multiple operating conditions by switching the current loop reference; redefines the generation of bridge arm voltage reference to optimize the dynamic performance of the MMC converter. The reliability and effectiveness of the proposed control strategy are also verified by simulation.