Fault Blocking Charateristic Reserch Of MMC-HVDC Based On Mew Sub-modile

Modular multilevel converter developed very rapidly with all its advantages in the field of flexible HVDC,DC short fault is the inevitable question when it was used in the long distance HVDC.Although the traditional MMC composed by HBSM could shut all IGBT to protect the device when a failure in dc side.Short-circuit current cannot be closed,which is caused by the fly-wheel diode parallel connected of IGBT,so that the traditional MMC does not have dc side fault clearing ability.In view of the problems that MMC can not clearance the DC side fault,many of the new sub-module topologies were amended basis of the traditional HBSM,these new sub-modules add the additional blocking devices,which increased the losses of inverter in the case of a more levels.At first,this paper made a brief introduction to the basic principle of MMC,then,after researched the existed new MMC sub-modules possessed of DC fault automatic removal ability,this paper proposed a new sub-module which have three level output ability and a strong fault block function,and designed a hybrid inverter composed by new sub-module and the traditional half bridge sub-module,in order to reduce the additional loss problem of inverter brought by the new sub-modules.Furthermore,the modulation strategies and AC voltage harmonic characteristics of the hybrid inverter was analyzed by the PSCAD/EMTDC.Secondly,based on the superiority of NLMMC,the article puts forward another hybrid sub-modules,which have DC fault block ability and a lower losses,after applied to NLMMC,it makes the inverter not only has the ability of dc fault blocking,but also has a high level output capacity,which reduced the wastage of the converter indirectly.This paper start with the working principle of the topology structure,analyzed the modulation strategy,and proposed a hierarchical voltage balance control strategy and explained the mechanism of fault blocking.Finally,the capacity of level output,loss features and ability of fault blocking are simulated through the PSCAD/EMTDC simulation platform.