Control Strategy Of Modular Multilevel Converter Under Unbalanced Grid Conditions

In recent years,with the increasing application of power electronics,high-voltage power electronics converters have been widely used in FACTS,HVDC,STATCOM,it can improve system power quality effectively. The multilevel converter has become an effective alternative for high-voltage power electronics converters due to its reduced line harmonics and low voltage stress.Among the numerous multilevel topologies,the modular multilevel converter(MMC) is considered as the most competitive voltage source converter(VSC) operating in the range of high power and high voltage levels with high efficiency due to its modular nature,very low output harmonics,simple redundancy and flexible front-end configuration.Firstly,the topological structure and operation principles of MMC are given,analysis and inference the upper and lower leg MMC normal working voltage,output AC voltage and current of the upper and lower leg circulation and AC output current. The common modulation schemes and voltage balance strategies are discussed, modulation strategies of MMC like PD-PWM,PSC-PWM,NLM are analysed in detail. Furthermore,total energy controller and balance energy controller are structured to control the voltage balance of the capacitor.Next, control strategy of Modular Multilevel Converter under unbalanced grid conditions is studied. Equivalent circuit model of one-phase of the three-phase MMC is established and analysised.Then,the operating characteristic and control strategy of MMC under unbalanced grid conditions are studied. The control methods like the control method based on dq coordinate system, the control method based on the PR regulator and the control method based on the nonideal PR regulator are mainly compared and analysed. The simulation model of the control strategy is created by MATLAB/Simulink.Finally, three-phase Modular Multilevel Converter experimental platform based on the hierarchy strategy is built,these include MMC hardware system and MMC software control system.The hardware system consist of a main circuit design of three-phase MMC and DSP controller,FPGA controller, CPLD controller. The validity of proposed controller architecture is verified by experimental results.