Sara Khazaee - Department of engineering, Urmia University,Iran
Hamid Shayestekhah - Department of engineering, Urmia University,Iran
B Tousi - Department of engineering, Urmia University,Iran

The series capacitors in long transmission lines are used for achieving many goals, such as increasing the amount of power transfer and stability margin or distributing the active and reactive power between parallel lines. In addition to such benefits, series capacitors cause a serious condition in power systems that are called subsynchronous resonance (SSR).In this paper, at first, the situation of system in which there is a potential of SSR accuracy is detected through a detailed mathematical analysis on study system, and after that, an auxiliary controller is designed and performed to demonstrate the efficiency of the proposed controller for mitigating the oscillations caused by this phenomena. The IEEE first benchmark model for SSR is used as a study system. The eigenvalues of system are carried out using MATLAB software. The transient simulation study of system is performed through PSCAD/EMTDC. The FFT analysis is used on the results of transient simulation in MATLAB to detect the unstable modes of oscillation

subsynchronous resonance, series compensation, STATCOM, torsional interaction (TI) effect, PSCAD ، EMTDC,