Mode Identification And Analysis On Modular Multilevel Converter

As a composition of high-voltage direct-current transmission system(HVDC),the modular multilevel converter(MMC)plays an important role in the process of long-distance,high-power electric energy transmission,whose operation reliability has always been a hot topic for scholars.MMC is composed of multiple submodules(SM)with the same structure,and each SM contains several switching devices.By controlling the on-off state of switching device,the operation states of SM will be changed,and the output voltage of MMC will also be adjusted.SM usually contains several switching devices,and the combination of on-off states of switching devices makes SM operate in different operating modes.Therefore,SM containing multiple switching devices can achieve multiple operating modes,and unexpected modes may exist in addition to the normal operating modes.Due to the large number of SMs in MMC,the possibility of containing unexpected modes is greatly increased.The appearance of unexpected modes will reduce the operation reliability of MMC,and the output performance will be affected or even result in serious consequence.Thus,it is beneficial to improve the operation reliability of MMC by identifying all the operating modes of SM,discovering and analyzing the trigger conditions as well as effect of unexpected modes.Based on the component tree diagram,a mode identification method is proposed in this thesis which can effectively identify all the operating modes of converter.Then,the proposed method is applied to CDSM-MMC as well as SFB-MMC.Finally,PLECS RT Box is used for experimental verification,which is to prove the effectiveness of the proposed mode identification method and verify the impact of unexpected modes on MMC.The main content of this thesis is as follow:1)Based on the component tree diagram of power electronic converter,a new mode identification method is proposed,which can intuitively and quickly identify all the effective operating modes of converter.2)The proposed mode identification method is applied to CDSM-MMC and SFB-MMC respectively to identify the unexpected modes contained in each SM.Then,the trigger conditions of the unexpected modes and their impact on the performance of MMC are further analized.3)MATLAB/Simulink and PLECS RT Box are used to verify the effectiveness of the proposed mode identification method,and test the impact of unexpected modes on MMC.