Research on double-loop circulating current suppressing strategy based on nonideal PR controller for modular mulitlevel converter

The main inner unbalanced current constituents of the modular multilevel converter (MMC) are the direct current (DC) components and the even circuiting components. Among of them, the inner circulating current will increase the power loss of converters, and reduce the utilization of bridge arm capacities inevitably. Even worse, the current distortion of the bridge arm will be intensified, which will endanger the safe operation of the system. Therefore, it is necessary to take effective measures to suppress the inner circulating current. This paper proposes a novel double-loop circulating current suppressing strategy based on the nonideal proportional resonant (PR) controller, which separately controls the DC components and the circulating components of the inner unbalanced current. Additionally, the circulating components are attached to the feedback signal of the DC component control loop. Meanwhile, by tuning the controller parameters, the closed-loop zeroes and poles can be relocated, and the circulating current suppressing ability as well as the system stability can be enhanced significantly. Furthermore, the double-loop circulating current suppressing controller is designed, followed by the principles of controller parameter tuning. A detailed three-phase MMC model is established in PSCAD/EMTDC simulation environment. The correctness and effectiveness of the proposed strategy get verified.