Researce And Application Of New Modular Multilevel Converter

The MMC converter has the advantages of simple sub-module and circuit topology,and can be adjusted and expanded based on actual needs,and has good robustness.While with the development of society,the voltage level in power transmission is increasingly higher.The number of sub-modules in the bridge arm is increasing and the floor space is becoming bigger and bigger as well,which leads to the continuous growth of hardware cost of the system.As the difficulty increases,the above-mentioned problems have restricted the development of modular multilevel converters.Thus,in response to the above problems,by studying the existing modular multilevel converter and gradationally controlled modular multi-level converter(GC-MMC),a multi-modular gradationally controlled modular multi-level converter(MGC-MMC)is proposed.This thesis carries out the following research:(1)Aiming at the respective problems of high cost of modular multilevel converters and gradationally controlled modular multilevel converters,a new multi-module gradationally controlled modular multilevel converter has been put forward.(2)The voltage among the modules is proportional which caused the traditional voltage stabilization algorithm could not be applied.So a capacitor voltage balance algorithm suitable for MGC-MMC is presented.According to the simulation results of the single-phase inverter,the model proposed in this paper could operate normally.The performance superiority of MGC-MMC was verified by comparing with ordinary MMC.(3)By studying the valve-level control strategy and station-level control strategy of MMC,the dual closed-loop control principle of MMC-based DC transmission is analyzed.The dq decoupling control method is adopted to build MGC-MMC based flexible DC transmission in the matlab environment.The output waveform is of good quality and the number of sub-modules is greatly reduced,which met the requirements of DC transmission.Then an economical verification is carried out,that is,the verification result showed that the cost of MGC-MMC sub-module is greatly reduced under the premise of outputting the same number of levels.(4)The MGC-MMC has been further studied,which researched the problem of sub-module redundancy.The simulation results demonstrate that the MGC-MCC puts forward in this thesis herited the redundancy characteristics of the ordinary MMC.When the sub-module failed,it could work normally after replacing the failed sub-module with the hot-standby redundant sub-module.Finally,MGC-MMC is able to realize DC fault ride-through in the case of DC short-circuit as well as the function of protecting the power system.