Research On Current Hysteresis Control Of Modular Multilevel Converter

Modular multilevel converters(MMC)are widely used in the field of medium and high voltage distribution and multi-terminal flexible direct current transmission due to their outstanding advantages such as modular structure which is easy to expand,high output voltage level,high conversion efficiency and flexible control structure.Although domestic and foreign researchers have made some achievements in MMC research,with the gradual expansion of MMC in various fields,some problems need to be solved urgently.The modulation strategy and key control technologies of MMC will affect the output harmonic characteristics,switching loss and capacitor voltage fluctuation of MMC,and even affect the safe operation of MMC to a certain extent.Therefore,this paper carries out related research on the modulation strategy and key control technologies of MMC.The main work of this paper are as follows:(1)Complete the analysis and simulation verification of the traditional modulation strategy and control mode of MMC under medium and high voltage occasions.The working mode and control mode of the topology model and the half bridge sub-module are analyzed.In this paper,two traditional modulation strategies and a new hybrid modulation strategy are expatiated and compared.By comparing the different parameters and output characteristics of the three modulation strategies,the advantages and disadvantages of the three modulation strategies are obtained,and the applicable application occasions are pointed out.(2)Based on the bridge arm current hysteresis control method,mathematical derivation is made,and the nearest level approximation modulation based on the bridge arm current hysteresis principle is proposed.Because traditional NLM in output level number is less,the modulation strategy applied in high voltage grade is low output harmonic characteristic and low waveform quality,the modulation strategy based on bridge arm current hysteresis is proposed.By introducing additional controller and finishing the adjustment of the reference voltage of bridge arm,The accurate tracking reference for bridge arm current and better dynamic performance can be achieved.The problem of waveform quality in medium and high voltage applications with nearest level approach modulation is improved.Based on the current hysteresis control principle of the bridge arm,the circulation suppression controller is proposed by establishing the circulation mathematical model,which can significantly suppress the double frequency component of the circulation and improve the waveform quality of the bridge arm current.(3)The bridge arm current hysteresis control is proposed to be applied to the DC ice-melting situation,and the mathematical modeling and controller design are carried out for the DC ice-melting mode and reactive power compensation mode.Aiming at the conversion of the MMC working in the DC melting ice mode and the reactive power compensation mode,the wiring mode of the MMC is described,the controller of the two is designed,and the effectiveness of the control strategy of the whole system is verified by the built simulation model.