This paper deals with the use of Modal Series (MS) method to study the nonlinear interaction between the torsional modes of a turbine-generator set and Static VAr Compensator (SVC) in the stressed power systems. The observations resulting from nonlinear simulations made incentive for suggesting the need for nonlinear analysis of torsional interaction phenomenon in stressed conditions. The obtained results from linear modal analysis were not consistent with those of nonlinear simulations and could not quantify the problem. While the results of nonlinear analysis MS method reveal that, the nonlinear interactions exist between shaft sections and SVC controllers. In the stressed conditions, these nonlinear interactions become more pronounced and it may be a warning for severe damage of shaft segments. By using the indices correspond to the MS method, the nonlinear interactions of torsional modes are evaluated for different conditions. To demonstrate the physical effects of nonlinear interaction of torsional modes with SVC controllers, the torsional torques imposed on shaft sections are provided using the nonlinear simulations. Finally, based on MS method a supplementary controller is designed for SVC to remedy the problem. The proposed approach is implemented on the IEEE 4-machine 11-bus test system.

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