In hybrid AC/DC distribution grid, MMC-based SST is required to replace power electronic transformer for the capability of AC & DC grid interconnection, power flow control and voltage adjustment. In this paper, the existed two basic MMC-based SST topologies are compared in detail and some drawbacks are clarified. One basic topology is selected and for the purpose of reducing the number of modules and providing more LVDC ports, several optimized topologies are derived and presented. Thus the family of these MMC-based SST could come into being, and the control strategies of SST are designed. Simulation results on a typical MMC-based SST in this family are presented to verify the feasibility of these topologies.