Naser Pariz

Heavily loaded, stressed power systems exhibit complex nonlinear dynamic behaviors which can not be analyzed and described accurately by conventional linear methods such as eigen-analysis. Normal form of vector fields, a well established mathematical method, and Modal Series technique, a relatively newly established approach have been used as 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 this interaction’s role in causing inter-area oscillations after occurrence of the large disturbances. However, Normal Form has some shortcomings which must be further highlighted. In this paper some of these shortcomings 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 these methods are 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 larger region of validity in the state space of the systems is illustrated.