1.NARI Group Corporation(State Grid Electric Power Research Institute)，Nanjing211106，China;2.Central China Electric Power Dispatching and Control Center，Wuhan430077，China

With the expansion of power grids，some unusual dynamic behaviors are emerging.In this paper，based on the extended equal area criterion（EEAC），the first swing and the second swing of angle instability phenomena in the interconnected power grid are demonstrated.The evolution phenomena of unstable modes due to the variation of system models，parameters and control strategies are analyzed.The mechanisms of negative control effects of generator tripping in the non-coherent critical generators cluster are revealed.It is also pointed out that the rapid decrease of the DC power and load delivered during the accelerating process of local area generators close to the short circuit point deteriorates the interconnected power system transient stability.The results are verified by large-scale power systems simulation.