This chapter explores MMC-HVDC systems for overhead line power transmission. It covers several key topics: fault clearing methods for DC-side faults using AC circuit breakers, operating principles of hybrid DC transmission systems (including LCC-Diode-MMC and LCC-FHMMC configurations), FHMMC control strategies for both sending-end and receiving-end AC grid faults, comprehensive FHMMC control strategies in LCC-FHMMC hybrid systems, and the principles and control strategies of LCC-MMC series hybrid DC transmission systems.