This paper describes the current situation of a conventional HVDC transmission project, looks forward to the future of flexible DC transmission, and introduces the basic principle of DC transmission based on VSC modular multilevel converter technology. A Real Time Digital Simulator (RTDS) simulation model is built according to the flexible DC primary topology of the Y-E back to back DC transmission project. Based on the simulation results, the fault transient state and steady state characteristics are analyzed before charging soft open resistor removal and after unlocking soft open resistor removal. Through the study of the fault characteristics of the flexible DC valve side and the overpressure overflow of valve body in case of valve side failure, a protection and cooperation strategy for flexible DC is designed. This work is supported by Science and Technology Project of the Headquarter of State Grid Corporation of China (No. 52170218000M):Research on Fault Protection and Adaptive Coincidence Method of Hybrid Multi-terminal HVDC Transmission Lines.