The high proportion of volatile renewable energy sources connected to the power grid, the long-distance transmission of AC and DC ultra-high voltage and the application of bulk power electronic devices bring serious challenges to existing power grid relay protection and safety control. They also put forward higher requirements for the "three defense lines" layout which has been established in power systems for decades. First, the challenges faced by the power system relay protection and security control system in the new grid environment and their causes are analyzed in detail. Then, this paper summarizes the favorable environmental basis and technical conditions that can meet these challenges, and points out the direction for technological development and scientific research. Finally, the working mode of the new protection, the new protection principle, the protection information sharing, the cooperation technology between the protection and the power grid security control, and the preventive control functions of the power grid are discussed and studied. A new power system security defense system of "regional cooperation+distributed autonomy" is established, one which realizes information sharing and can well apply the new technology.