The novel studies have shown that Flexible AC Transmission System (FACTS) devices can greatly improve the steady state and dynamic performance of power systems. A Unified Power Flow Controller (UPFC), is the most promising device in the FACTS concept. This paper is concerned with the application of the Modal Series Method to analyze the nonlinear behavior of power systems equipped with UPFC. To do so, a general approach is developed upon which the formulation of the close loop state matrix of a power system with a UPFC, is obtained. The proposed method has been applied to a typical single machine infinite bus power system (SMIB). The non linear simulation results are also presented for the example power system to validate the effectiveness of the proposed method. The results of MS method are in good agreement with those obtained from non linear simulation. Furthermore, the effect of various damping coefficients of generator on the proximity measure is examined.

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