Optimized Circulating Current Control Method based on Proportional Resonant and Proportional Integral Controllers for Modular Multi-level Converter Applications

MMC has drawn broad interest in high voltage applications due to its superior features such as scalability, modularity, inherent redundancy, and low harmonic generation. The control algorithm of a high voltage MMC application needs a meticulous design, as the number of floating capacitors can be quite a large number. Any voltage mismatch between these capacitors causes a circulating current, which increases the RMS and the peak value of the arm currents, component ratings, and the ripple in the capacitor voltages. In this paper, an optimized closed-loop circulating current control method is proposed based on Proportional Resonant (PR) and Proportional Integral (PI) controllers in stationary abc reference frame to prevent high circulating current inside an MMC. The proposed method is simple and can be applied to single-phase or three-phase MMC applications. More importantly, it suppresses the magnitude of circulating current while reducing the ripple in capacitor voltages. Furthermore, it reduces the ripple in the DC link voltage without any supplementary controller. The verification of the method is verified on a 161- level MMC HVDC system, which is modeled on a Real-Time Digital Simulator (RTDS) and Xilinx Virtex-7 Field Gate Programmable Array (FPGA) units.