Research On Optimal Control Strategy Of MMC-HVDC

At present,modular multilevel converter is considered to be the best application prospect of voltage source converter.These are due to the advantages of easy expansion,good quality of waveform and high reliability.The flexible DC transmission of MMC has become the focus of experts' attention and research,not only because it is suitable for the rapid development of urban power supply in the future,but also because it plays an important role in the fields of back-to-back asynchronous interconnection and new energy grid connection.In order to ensure high stability,high efficiency and high power quality,the control strategy of MMC is improved and optimized from the aspects of voltage imbalance and circulation.The control strategy is analyzed in theory and simulation.First,on the basis of introducing the topology and working principle of MMC,the mathematical model is established and the basic control strategy is completed,and the strategy of grouping and setting the upper and lower limit voltage balancing control strategy is proposed.Even if the level is constant,the sub modules beyond the limiting voltage should be switched for balancing capacitance voltage.In order to reduce switching loss,the ability to maintain the original state of the submodule is increased by grouping.To make the output DC voltage approach the reference value,the compensation module BC is introduced.It can support DC voltage and suppress circulation effectively,and improve the operation efficiency effectively.Because modular multilevel converter(MMC)has the advantages of easy expansion,good waveform and high operation reliability,it is considered to be the best application prospect of voltage source converter at present.Second,the principle of capacitor voltage balancing and the characteristics of capacitor charge and discharge in MMC are analyzed.On this basis,grouping direct insertion sorting is proposed,that can not only improve the capacitor voltage consistency,but also reduce the switching loss and sorting time.According to the analysis of the circulation principle,the circulation suppression algorithm is proposed.In order to obtain the total number of input modules,the circulation module number and the upper and lower bridge arm sub-modules are calculated.The optimal control of sub-module switching is based on the final calculated number of modules,which not only reduces the circulation amplitude,but also reduces the input and design of the controller.Finally,the simulation results of the proposed control strategy in matlab/simulink show that the control strategy of grouping and setting the upper and lower limit voltage balancing control strategy and compensation module is feasible,and the control strategy of grouping direct insertion sorting and circulation suppression algorithm is correct and effective.