Abstract Due to the importance of nonlinear interactions in the complex power systems, in this paper the subsynchronous torsional interactions (SSTI) between turbine-generator shafts and flexible AC transmission system (FACTS) are studied using the nonlinear modal analysis method. Normal Form method is applied to study the nonlinear effects on the SSTI phenomenon in the presence of Static VAr Compensator (SVC) as a commonly used FACTS device. The results show that under stressed conditions the SVC voltage controller gains have an adverse effect on the nonlinear interactions of torsional modes. These effects can lead to severe excitation of torsional oscillations. The study also illustrates that the type of SVC control system model will play an important role in the nonlinear analysis of SSTI, and using a simple PI model is not suitable for nonlinear analysis of SSTI phenomenon. Moreover, in a multi-area power system, the location of SVC and its capacity have significant effects on the severity of nonlinear interactions of torsional modes.

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