This paper extends the application of the method of modal series to analyze the structural characteristics of power systems following different faults. Modal series method is relatively new established technique in the study of power system dynamics which in contrast of normal form method need not to nonlinear transformation and don’t suffer from parametric resonances. The paper has developed two analytically based indexes in modal series frame which identify onset of inter-area separation. The proposed approach has been applied to the IEEE 50-generator test system. The results obtained indicate that the proposed procedure effectively captures the structural characteristics of the system dynamic behavior and accurately predicts the nature of the system dynamic performance following large disturbances.
