Research On Circulating Current Suppressing Strategy Of Modular Multilevel Converter(MMC)

With the development of power electronics technology and electric power system, powerelectronics technology has been widely used in the fields of new energy power generation andhigh voltage direct current transmission in recent years. Based on the modular topology,modular multilevel converter (MMC) has unique features and benefits such as high qualityoutput wave, low system losses, strong unbalanced operation ability, better fault protectionand recovery capabilities, and easy to extend compared with the traditional voltage sourceconverters, which make it widely used in the field of high voltage direct current transmission.On the basis of the research and analysis of MMC such as the topology, working principle,mathematical model and modulation methods, this dissertation focuses on the analysis andresearch of the circulating current suppressing strategy of MMC. Besides, the effectiveness ofeach control strategy presented in this dissertation is verified on the basis of simulation.Two control strategies for balancing capacitor voltage of sub-module of MMC are putforward, namely the capacitor voltage balancing strategy under the carrier phase shiftmodulation and the ladder wave modulation. Among them, the capacitor voltage balancingstrategy based on carrier phase shift modulation achieves good control effect by combiningmodule voltage balancing control with bridge arm voltage balancing control. The capacitorvoltage balancing strategy under the ladder wave modulation focuses on the introduction ofthe traditional capacitor voltage sorting algorithm and the optimized algorithm of it whichalso named module hold algorithm, having advantages such as strong voltage follow, fastdynamic response, good balance effect and so on.Three control strategies for suppressing the circulating current of MMC are put forward,namely the circulating current suppressing strategy under αβ coordinate and dq coordinate aswell as the circulating current suppressing strategy based on improved ladder wavemodulation. Among them, a corresponding controller for suppressing the circulating current isdesigned by using proportional integral resonance controller based on the circulating currentsuppressing strategy under αβ coordinate. On the basis of the establishment of thecorresponding mathematical model, another corresponding controller for suppressing thecirculating current is designed by combining with capacitor voltage sorting algorithm basedon the circulating current suppressing strategy under dq coordinate. By analyzing the essenceof circulating current, the circulating current suppressing strategy based on improved ladderwave modulation can suppress the circulating current essentially by adopting module changeselection algorithm without increasing the external circuit, while decreasing the THD of output voltage and achieving good effect.Finally, on the basis of the previous theoretical analysis, the simulation models of MMCunder each control strategy above are built and simulated in MATLAB/Simulink. Thecharacteristics and effectiveness of each control strategy are verified according to the analysisand contrast of the simulation results above.