Highly stressed power systems exhibit complex dynamic behaviors such as inter-area oscillations when subjected to large disturbances. In such conditions, nonlinear effects have dominant role in determining dynamic response of the system. In this paper by using of modal series method, dynamic behaviors of the stressed power system in severe and critical conditions have been studied. Also two measures, mode dominance measure (MDM) and most perturbed machine factor (MPF) have been introduced. They determine the most dominant modes and identify the most perturbed generators. Contribution factors have been used to show the links between identified modes and machines from the analysis. Time domain simulation has been helped for validation of the results. The studies carried out on the IEEE 50-generator test system which demonstrates a wide range of dynamic characteristics at different loading levels and fault scenarios.
