Firstly, for the large number of sub module (SM) caused by high voltage applications and high level output of modular multilevel converter (MMC), cascaded two levels (CTL) and multi-level output SM topologies are studied. Secondly, the characteristics of different types of improved SM topologies are summarized and qualitative comparison of their different DC link fault blocking capability is conducted. Then a hybrid self-startup strategy based on full controlled and half controlled rectifier is designed for the serial connected double sub module topology. Lastly, MMC-HVDC with two active terminals in the PSCAD/EMTDC software is built based on these improved topologies, and DC link fault suppressing characteristics of different SM topologies and the startup strategy are analyzed and verified.