Research On Modular Multilevel Converter Sub-Module Capacitor Voltage Ripple Suppression

With the wide application of high power high voltage power converters, multilevel converters are rapidly developed. Compared with the traditional multilevel converters, the Modular Multilevel Converter(MMC) has its unique advantages, it has been extensively studied by domestic and foreign scholars, and first applied in HVDC field. This paper focuses on the sub-module(SM) capacitor voltage fluctuation suppression at power frequency operation, and explores the use of Active Power Decoupling technique for capacitor voltage fluctuation suppression.Firstly, the principle of MMC is analyzed, and the mathematical model is built. A multi-loop control strategy is proposed, including the control of total energy, differential energy, circulation current and AC output current, which is the foundation of the whole research. Simulation validates the model and control strategy.The arm power and sub-module capacitor voltage are analyzed, for capacitor voltages in upper and lower arms, the fundamental wave phases are the same, while the second harmonic phases are opposite. Second circulating current injection can transfer the second harmonic power to the DC-link side, which then can suppress the second harmonic in capacitor voltages. Based on negative sequence, double frequency coordinate, the second harmonics can be transformed as direct component for control, this method can suppress the second harmonics, but is inconvenient for single phase MMC. The proportional resonant(PR) controller can trace and control second harmonics, so it has wider application. The two methods are both validated by Simulation, the results indicate that they are both good at second harmonic voltage suppression, but circulating currents injection will increase the arm currents, moreover, they are helpless for fundamental wave.Active Power Decoupling technique is developed from second harmonic voltage suppression in single phase rectifier, the application in MMC is rare in research literature. This paper employs an active power decoupling topology suit for high voltage and high power applications, by using switching devices and energy storage elements(inductor and capacitor), the ripple energy in the sub-module capacitor is absorbed by energy storage elements, thus the sub-module capacitor voltage ripple can be suppressed. Active damping is used to eliminate the resonant peak of LC circuit, a Proportional-Integral(PI) plus Repetitive Control scheme is proposed. Simulation is based on Matlab/Simulink software, experiment is based on single phase MMC platform, both simulation and experiment results prove that this scheme can suppress fundamental wave and second harmonic ripple at the same time without increasing the arm currents or circulating currents, the system has good static and dynamic performance. Compared with passive filtering method, this method can effectively improve the power density.