Frequency and voltage collapse caused by serious failure is a tremendous risk in density urban power network. Trajectory-based time-domain dynamic simulation method is used to analyze the problems of the configuration of under frequency load shedding and under voltage tripping of Shenzhen power system while islanded from the interconnected power system. A scheme for under frequency generator tripping is proposed based on frequency control coordination and optimization to meet both the frequency characteristic requirements of the whole network and regional networks. Simulation results show that the propesed coordination principle and configuration scheme are reasonable and feasible.