In this paper, the effect of Static VAr Compensator (SVC) parameters on the nonlinear interaction of steam power plant turbine-generator set is studied using the Modal Series (MS) and Normal Form (NF) methods. A second order representation of a power system equipped with SVC is developed. Then by MS and NF methods, the nonlinear interaction of torsional modes is assessed under various conditions and the most influencing factors are determined. Furthermore, to better analysis the nonlinear interaction between torsional modes and SVC controllers new nonlinear interaction index is defined. The results show that the stress conditions and some SVC control parameters will adversely affect the dynamic performance of a power system by increasing the nonlinear interaction of torsional modes. In this situation, the nonlinear modal analysis techniques especially MS method can precisely provide a reliable prediction of the torsional oscillations amplitudes and the frequency content of the output system response. As the angle and speed of turbinegenerator segments are used as input signals in several controllers, the frequency content of these signals are quite important in designing such controllers. This analysis is performed on a 4-areas WSCC system, which is equipped with a SVC.

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