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The existing power decoupling control method of modular multilevel converter (MMC) was developed from that of conventional two-level voltage source converters. Therefore, it cannot fully realize power decoupling due to neglecting the coupling effects from submodule capacitor voltage, dc current, circulating current as well as control strategies of circulating current, which inevitably results in inaccurate regulation of active and reactive power, deteriorated system dynamic performance, possible overload operation and eventually threatens the safety and stability of MMC. This paper analyzes the intercoupling among all electrical quantities and corresponding control strategies in MMC. An accurate control model of active and reactive power/current with four coupling paths and influence factors in the rotating frame is also established. A full power decoupling control method has been proposed based on the developed model. The proposed control method can realize accurate and completely decoupled active and reactive power regulation, avoid overload operation, and significantly improve the dynamic performance of MMC. Moreover, it facilitates the parameters selection of MMC controller with no extra cost, which is of great significance for practical projects. The effectiveness and accuracy of the proposed analysis and control methods were verified by both simulation and experimental results. [ABSTRACT FROM AUTHOR]

Conventional centralized control strategies may reduce the flexibility and expandability of a modular multilevel converter (MMC) system. To tackle this issue, this paper proposes a distributed control architecture that is capable of assigning certain control tasks to distributed local controllers and improves the modularity of an MMC system. A central controller dealing with the output current regulation based on sensed arm currents is adopted. The control of MMC internal dynamics and the pulsewidth modulation (PWM) generation are distributed into local controllers. Unlike the conventional MMC control that needs all submodule capacitor voltages for capacitor voltage averaging, the proposed capacitor voltage control only relies on local submodule voltage measurement. Consequently, communication-intensive capacitor voltage transmission in each control cycle is not required and the communication burden of the control system can be significantly reduced. The control loops and possible control conflicts among submodules are presented and considered for system stability analysis. The effectiveness of the proposed distributed control architecture and capacitor voltage control for an MMC are confirmed by the start-up, steady state, and transient experimental results. [ABSTRACT FROM PUBLISHER]