Research On Harmonic Voltages Injection And Capacitance Optimization In Modular Multilevel Converter

Modular Multilevel Converter(MMC)is widely used in voltage source converter based high voltage direct current(VSC-HVDC)transmission systems due to its high modularity,low switching frequency,low output harmonics and capacity scalability,especially in longdistance transmission,asynchronous interconnection,multi-terminal transmission network field.Capacitor is the core energy storage component of the MMC sub-module,and its capacitance optimization problem has received much attention.Arm circulating current control and third-order harmonic voltage injection are the basic effective optimization method,but the current research mainly focuses on a single method,and does not combine the two methods for comprehensive system research.According to the fluctuation characteristics of the capacitor voltage of the MMC sub-module,the optimization strategy of the coordinated harmonic voltages injection is studied.Firstly,this paper introduces the basic operation principle of MMC,establishes the equivalent mathematical model.The MMC modulation techniques and elementary control strategy are introduced.In addition,it briefly describes the principle and design method of inner-loop current control and outer-loop voltage control.Subsequently,according to the common mode equivalent circuit of circulating current,the main components of the circulation are clarified.And the three circulating current control strategies of natural circulating current,circulating current suppression and circulating current injection are analyzed.Thus,their effects on RMS of arm circulating current and capacitance voltage fluctuations are studied.The mathematical model of the second-order harmonic voltage and the double frequency circulating current is derived.And the capacitor optimization strategy based on the second-order harmonic voltage injection is obtained.According to the arm circulating current and switching function,the sub-module capacitor current expression is obtained,and the charging and discharging processes of sub-module capacitor are analyzed.On this basis,the third-order harmonic voltage injection technique is proposed in this paper.The amplitudes and phase angles of the second-and third-order voltage harmonics are designed in optimal coordinated.Based on genetic algorithm,collaborative injection strategy is implemented.The capacitor voltage fluctuation of the sub-module is reduced,and the theoretical analysis is verified by simulation.Finally,this paper introduces the design of the main circuit of the three-phase MMC experimental platform.The design of the control circuit and sub-module circuit are built.With the three-phase MMC experimental platform,the effectiveness of the sub-module capacitor optimization strategy is verified.