Abstract: Compared with the previous projects, the main difficulties are raised in the system design of Chongqing-Hubei project of China are the higher engineering DC voltage, larger capacity and raised requirements for fault ride-through and AC/DC protection etc., but the endurable transient stress level of fully-controlled power electronic devices is limited and the extreme operation mode of AC power grid deteriorates. In view of the limited endurable transient stress level of the device, this paper proposes the accurate calculation method of rising rate for the arm fault current in the converter, the independent arm blocking strategy of the overcurrent protection for the valve controller, and the dynamic setting strategy of the valve over-voltage protection. These measures solve the problem that the device safety and fault ride-through requirement cannot be both satisfied under the weak component capability.Aiming at the problems of high-frequency instability and weak-system instability of the AC grid and flexible DC system in the extreme operation modes, the instability mechanism and the practical engineering methods solving the instability are proposed. To meet the special requirements of AC breaker failure protection and DC differential protection under the access of 500 kV AC power grid, the corresponding solutions and suggestions are proposed.