Research On Operation Optimization Of Modular Multilevel Converter

Modular multilevel converter has many advantages,such as high modularity,strong expansibility,good output characteristics,excellent fault tolerance,etc.It has been highly concerned and widely used in flexible HVDC transmission.At present some progress has been made in the research of normal operation and fault response of MMC,but the research on operation optimization under normal and fault conditions isn't deep enough.In this paper,the operation optimization of modular multilevel converter is studied.Based on the basic principle of optimization strategy,mathematical model,optimization analysis,implementation method with simulation and experiment verification,the following studies are carried out for two working conditions:grid voltage balance and unbalance.(1)The basic principle and control mode of MMC are introduced.The mathematical model of MMC under unbalanced grid voltage is established,and the expressions of MMC output current,circulating current,arm current and module voltage ripple are given.Three control objectives including active power balance,current balance and reactive power balance of MMC under unbalanced grid voltage are analyzed.Combined with the output model of MMC,the influence of different control objectives on the output and internal performance of MMC is analyzed.The MMC grid voltage unbalance control system based on positive and negative sequence separator and proportional resonance controller in static coordinate system is designed.The simulation and experiment result verify the correctness of the grid voltage unbalance analysis and the effectiveness of the control system.Three objectives including active power balance,current balance and reactive power balance under grid voltage unbalance are achieved.(2)The circulating current injection optimization strategy of MMC is studied.And the mathematical model of MMC circulating current injection under unbalanced grid voltage is established.The expressions of arm current and SM voltage after circulating current injection are given.Based on the mathematical model,the effects of circulating current injection on the peak value and RMS value of arm current and module voltage ripple of MMC are comprehensively analyzed.The reference values of injection circulating current for reducing the peak value of arm current and module voltage ripple are given by means of three-dimensional graph analysis and function fitting.The MMC circulating current injection control system based on PR controller and second-order generalized integrator is designed.The circulating current injection strategy is varified by simulation and experiment platform.The effect of circulating current injection on reducing peak value of arm current and module voltage ripple under different control objectives under unbalanced grid voltage is realized.(3)The MMC discontinuous modulation strategy is studied.The basic principle of discontinuous modulation strategy is expounded,and a realization method applied to MMC is proposed.A mathematical model of MMC discontinuous modulation based on piecewise function is established by utilizing the piecewise characteristics of MMC modulation wave under discontinuous modulation.Based on the mathematical model,the effects of discontinuous modulation on MMC switching loss,conduction loss,device loss,loss in SM devices,loss unbalance in SM and SM voltage ripple are analyzed.In the simulation and experiment result,the effect of discontinuous modulation on reducing SM voltage ripple,reducing MMC loss and optimizing sub-module loss distribution is realized in high power factor.(4)The circulating current injection method of MMC under discontinuous modulation is studied.Based on the mathematical model of MMC discontinuous modulation and the model of circulating current injection,the effects of circulating current injection on the switching loss,conduction loss,device loss,SM loss distribution and SM voltage ripple of MMC under discontinuous modulation are obtained.The reference of circulating current injection under loss reduction mode and voltage ripple reduction mode are obtained,by means of three-dimensional graph analysis and function fitting.The circulating current injection strategy under discontinuous modulation is verified by simulation and experiment.The effect of circulating current injection on reducing MMC loss,optimizing sub-module loss distribution and reducing module voltage ripple is realized.(5)The virtual impedance strategy paralleling inductor-resistance on the DC side of MMC is studied.The basic principle of virtual impedance is analyzed.The parameters of virtual impedance is determined by means of Bode plot analysis.The realization method of virtual impedance in MMC is designed.The effectiveness of virtual impedance control is verified by simulation and experiment,and the goal of reducing the ripple of MMC DC side is achieved.