A Dual-Ring Control Structure for Enhancing the Fault-Tolerance Ability Under Communication Interruption in MMC

The control structure of a modular multilevel converter (MMC) consists of a central controller and numerous local controllers with complex communication links. However, the communication interruption may lead to MMC operation failure or frequent bypassing of submodules (SMs). Although the fault-tolerance can be improved by adding extra communication link for each local controller in a centralized star control structure, it will greatly increase the cost considering the large number of SMs. In this article, a novel dual-ring control structure based on a consensus algorithm is proposed to enhance the fault-tolerance ability of MMC to the communication interruption. Compared with the star control structure, control tasks in central controller, such as capacitor voltage control and circulating current control, can be reasonably allocated to all local controllers. Each local controller only needs to communicate with adjacent local controllers to obtain the average value of specified information. Two types of communication interruptions, occurring between central controller and local controller or among local controllers, are analyzed, respectively, with the corresponding redundancy design implemented. Simulated and experimental results show that the proposed control structure greatly enhances the fault-tolerance ability to communication interruption as well as effectively reduces the computational burden of the central controller compared with the star control structure.