The converter valve is the key equipment in a voltage source converter based high-voltage direct current (VSC-HVDC) system. This paper is concerned with the structure and principles of the modular multilevel converter (MMC) valve

an analysis of the characteristics and effects of the internal and external faults

and a complete multi-level protection strategy and protection system for the valve with sub-module control circuits

the valve control system and high-voltage direct current (HVDC) control and protection system. A criterion for systemic over-voltage is proposed

effectively improving the over voltage ride through capability of the valve. The strategy as well as function of the valve protection system has been validated through an experimental ±6 kV point-to-point VSC-HVDC system

and used in Zhoushan and Nan’ao multi-terminal VSC-HVDC projects. This work is supported by National High Technology Research and Development Program of China (863 Program) （No. 2013AA050106).