Modular multilevel converter(MMC)has been widely applied in flexible high voltage direct current(HVDC)fields. In view of the problems that traditional half bridge sub-module(HBSM)cannot clear the DC side fault

many new sub-modules with the ability of DC fault blocking have been proposed. Compared with half bridge sub-modules

these new modules have additional blocking devices

which increase the losses of converters in the case of huge output levels. The topology of nested-loop mechanism based MMC(NLMMC)has high level output ability

and combined with the characteristics of topology structure

a hybrid sub-module structure is put forward. Application to NLMMC shows that it gives the inverter not only the ability of DC fault blocking

but also a high level output capacity

which reduces the wastage of the converter indirectly. This paper starts with the working principle of the topology structure

the modulation strategy is analyzed

and a hierarchical voltage balance control strategy and the mechanism of fault blocking are discussed. Finally

the capacity of level output

loss features and ability of fault blocking are simulated through the PSCAD/EMTDC simulation platform. This work is supported by State Grid Corporation of China(No. 2014GW-05).