Control strategy for modular multilevel converters in sub-module fault state

By analyzing the topology and mathematical model of modular multilevel converters (MMCs), a control scheme is proposed to control the MMC in sub-module fault state based on phase energy controller. An AC-DC decoupled model is built to analyze and design the phase energy controller. In addition, by analyzing the mechanism of circulating currents for MMC in sub-module fault state, a new circulating currents suppressing controller is presented to minimize the circulating currents. Taking the gird-connected MMC rectifier as an example, a comprehensive control scheme is proposed to perform the DC voltage control as well as the reactive power control. Simulation results show that the proposed comprehensive control scheme has excellent stead-state and dynamic characteristics for the MMC in sub-module fault state; and it can eliminate the three phase circulating currents effectively.