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This paper presents a novel controller for damping unstable torsional oscillations caused by Subsynchronous Resonance (SSR) phenomenon. The controller is based on the delayed feedback control theory. The first system of the IEEE Second Benchmark Model is used to evaluate the effectiveness of the controller which uses the synchronous generator rotor angular speed as the sole input signal. The output of the controller is combined into the automatic voltage regulator as the stabilizing signal. The time delay parameter of the delayed feedback controller is evaluated by employing a direct method based on the Cluster Treatment of Characteristic Roots (CTCR), which assesses the stability in the parametric domain of time delay in linear time invariant systems. The effectiveness of the proposed delayed feedback controller in damping the unstable torsional oscillations is demonstrated via time-domain simulations on the nonlinear model.