Abstract: Heavily loaded, stressed power systems exhibit complex nonlinear dynamic behaviors which can not be analyzed and described correctly by conventional linear methods such as eigen-analysis. Hence, it is needed to use approximate methods to include and take into account system nonlinearities. Normal form of vector fields, a well established mathematical method, and modal series technique, a relatively new established approach are efficient tools to analyze, characterize and quantify some of these sophisticated behaviors such as low frequency inter-area oscillations in stressed power systems. Normal form method has been used extensively in recent years for analysis of nonlinear modal interaction and its role in emerging of the inter-area oscillations after occurrence of the large disturbances. However, normal form has some problems which must be further highlighted. In this paper some of these shortages are addressed by use of simple examples. The IEEE 50-generator test system is simulated and based on the obtained results from linear modal, normal form and modal series methods performance and accuracy of the methods will be investigated. It is shown that normal form technique can not simulate stressed power system well in some regions of its operating space. Also superiority of modal series method and its wider validity region in the state space of the systems is illustrated.
