1. China Electric Power Research Institute, Beijing 100192, China; 2. NARI Group Corporation(State Grid Electric Power Research Institute), Nanjing 211106, China; 3. State Key Laboratory of Smart Grid Protection and Control, Nanjing 211106, China; 4. Northwest China Branch of State Grid Corporation of China, Xi'an 710000, China

In concern of the instability of the complex transient power angle during the developing process of power grid, a concept of weak stability mode of transient power angle and its identification method are proposed to analyze and control the safety and stability of power system. The mechanism of weak stability mode evolving into dominant instability mode which is caused by the emergency control and its approximate necessary conditions are analyzed. Based on the theoretical analysis, a mathematical model of emergency control based on the transformation of system transient stability constraints is presented. Then a heuristic coordination emergency control strategy based on the dynamic sensitivity index is designed. The adaptability of emergency control algorithm to deal with the complex transient instability is improved and the automatic coordination control method to balance the multiple weak stability modes of power system transient power angle is realized. An example based on the actual grid is used to demonstrate the effectiveness of the presented mechanism of negative effects caused by the emergency control and the presented strategy search algorithm.