Research On The Circulating Current Suppression And Modulation Strategy Of Modular Multilevel Converter

With its advantages of the high modularization and flexibility, modular multilevel converter(MMC) is widely used in the field of medium/ high-voltage and high-power applications. Its circulating current suppressing strategies, modulation strategies and sub module voltage balancing strategy have been carried out in-depth study, but there are still some technical problems to be solved. So this thesis focuses on the circulating current suppressing strategies and modulation strategies of MMC, including the circulating current mechanism, an universal circulating current suppressing strategy and the characteristics of different modulation strategies, i.e. the quality of output power, control complexity and suitable applications. A new modulation strategy, which is suitable for a wide range of output levels, has been proposed.After a detail analysis of the topology of MMC, the basic working principles of sub modules and main circuit, the four-quadrant operation is deeply studied. Then the selection principles of MMC main elements is introduced, as well as the circuit and the parameters of the MATLAB/Simulink simulation system of this thesis.The control architecture for MMC is introduced, which includes the double closed-loop control strategy and the sub module capacitor voltage balancing strategy. Moreover, the circulating current mechanism of MMC is studied in the aspect of phase unit energy fluctuation, based on which an universal circulating current suppressing strategy is analyzed. The strategy can be used in both single-phase and three-phase MMCs, and need not to conduct a coordinate transformation. Finally, a comparison of the circulating current, phase arm current, AC side output voltages and currents, and fluctuation of sub module capacitor voltages and phase unit energy before and after the strategy is applied in the simulation system. The strategy is effective for all kinds of multi-carrier modulation strategies and nearest level modulatiao strategy.In the end of this thesis, the modulation strategies of MMC are deeply studied. Firstly, the principles and characteristics of the carrier phase shift modulation strategy and the nearest level modulation strategy are analyzed in detail. Secondly, a modified predictive modulation strategy based on virtual flux model is proposed, which can be applied in both low and high output level applications, and its output voltage and current characteristics are investigated in the simulation system to verify the correctness and validity of the strategy. Finally, a contrast is conducted among the three different modulation strategies on steady-state and the dynamic characteristics. The latter includes three cases, load step-change, load resistor gradients and modulation index step-change. These comparisons reveal the advantages and disadvantages of each strategy, which provides references for choosing suitable modulation strategies for different applications.