Precharge And Interphase Power Imbalance Of Modular Multilevel Converters

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 dissertation first analyzes the topology and working principle of MMC.The relationship between the upper and lower leg voltage,the output AC voltage and the upper and lower leg currents,circulation current and AC output current during normal operation of the MMC is deduced.The current commonly used modulation strategies and basic control strategies are fully explained.The MMC modulation strategies such as Carrier Stack PWM modulation,carrier phase shift PWM modulation and NLM modulation are analyzed in detail.A control system based on current inner loop,voltage and power outer loop was constructed.Next,the dissertation studied the pre-charging scheme for MMC.Divided into DC side pre-charging and AC side pre-charging,detailed analysis of the charging circuit and the establishment of a charging equivalent circuit model,analysis and derivation of the current limiting resistor expression.Combined with the existing pre-charge control scheme,the optimization control method was given for the problem of unbalanced charging and longer charging time,and a MATLAB/Simulink simulation model was established for verification.For the sub-modules with photovoltaic panels,the imbalance of the output power of the photovoltaic panels resulted in the power imbalance between MMC phases.The influence of power imbalance on the converter and output waveform was analyzed,and the solution of energy balance control was given.A MATLAB/Simulink simulation model was built for verification.Finally,a three-phase MMC experimental platform based on layered thinking was designed and built,which includes:MMC hardware system design and software control system design.The hardware system design includes the main circuit design and the design of the main controller(DSP),bridge arm controller(FPGA),and sub-module controller(CPLD).The DSP,FPGA,and CPLD constitute the MMC three-tier control system.The software control system design includes the software design of the above three-layer controller.On the basis of this experimental platform,the current closed-loop control and pre-charge experiments were conducted.The experimental results show the feasibility of the scheme.