SUMMARYIn this paper, a method, which is essential in initiating emergency control measures, is proposed for fast and accurate online detection of transient instability based on real-time measurement. Under a certain disturbance, the candidate critical cluster (CC) of power system is predicted and refreshed at each sampling step. The method identifies the loss of synchronism at each time step by observing the geometric characteristic of the post-fault trajectory of the generalized one-machine equivalent system. Unlike the previous extended equal-area criterion method, it is unnecessary to calculate the unstable equilibrium point (UEP) during instability detection in this method and some limitations caused by the time-invariant assumption are also eliminated. Simulations are carried out on the IEEE 50 generator test system and a real power system. The simulation results indicate that the proposed method has the features of reliability and effectiveness in identifying the loss of synchronism with inter-area mode.