Research On Modulation Strategy And Circulating Current Harmonics Control Of Modular Multilevel Converter

With the development of the economy,our nation's present power distribution and structure cannot meet the demand.To solve the problem,the project named “West-to-East energy transmission” has been put forward.HVDC flexible technology is important to make the project come true.Modular Multilevel Converter(MMC)is a noble topology to implement HVDC flexible technology.Modular Multilevel Converter has the following features: modularization,capability of extending Voltages/Power,lower output harmonics,simple redundancy configuration and so on.Therefore,MMC has a big application value and prospect for development.First of all,the paper illustrates the current research status and hotspots of MMC and the meaning of the research.Before the start of the specific research,MMC topology has been analyzed.With appropriate hypotheses,MMC model has been built for the subsequent modulation strategies and circular current harmonics analyses.On the base of the MMC model in chapter 2,the chapter 3 introduces modulation strategies: specific harmonics elimination PWM(SHE-PWM),space vector modulation PWM(SVM-PWM),carrier phase shift PWM(CPS-PWM)and nearest level modulation(NLM).NLM and CPS-PWM have been analyzed in detail.At the end of the chapter 3,a table about the merits and drawbacks and the situations of the above modulations has been given.The chapter 4 focuses on suppressing the circular current harmonics.The harm of harmonics has been illustrated.Then,the mechanism of the circular current harmonics has been revealed with the mathematic model of the circular current.Before designing the controller,the chapter gives the average requirements of the controller.And then,the principle of the proportional resonance controller has been discussed.Besides,the paper has proposed the compound control based on feedforward and feedback.The compound control has the features: simple parameter tuning,quick accommodation time and good control effects.Finally,corresponding simulations and experiments have been conducted and the results indicate that the compound control is a good method.