Performance Comparison of a Modular Multilevel Converter under Centralized and Decentralized Control Structures

A Modular Multilevel Converter (MMC) consists of series-connected bi-directional chopper cells with a floating capacitor on each leg. Despite the inherent modularity of the chopper cells, most MMC controllers have been designed based on a Central Controller Unit (CCU) until recently. Using classical linear controllers, the CCU executes the operator-defined setpoints based on measured signals in the outer-level control. Even though a CCU is relatively fast and has fewer communication routes, the CCU retains the scalability and modularity features of an MMC as modification of the controller may be challenging. For this reason, the decentralized control structure has been considered for the recent MMC applications to utilize the scalability and modularity features of an MMC effectively. The decentralized controllers introduce delays, especially for the circulating current, due to the local controllers, so the delays may drastically affect the MMC currents. In this paper, the centralized and the decentralized control structures are implemented and their effects in the arm and the circulating currents are investigated for an MMC operation.