This chapter examines the control principles and fault analysis of MMC-based DC grids. It covers several key topics: three fundamental types of DC grid voltage control; master–slave control strategy; the principles and design of DC voltage margin controllers; the implementation and design of voltage droop controllers with voltage dead-band; secondary voltage regulation principles; coordinated control methods for primary and secondary voltage regulation; power flow distribution characteristics; modular multilevel power flow controllers; the superposition principle for calculating short-circuit currents; and two approaches to handling faults in MMC-based DC grids.