A new method for damping the Subsynchronous frequency Oscillations (SSO) in power systems is suggested. The supplementary damping is achieved based on the active power modulation characteristics of the static phase-shifters, using the rotor speed deviation from the synchronous speed as the control signal. The complex torque coefficient method and an eigenvalue analysis technique are used for small signal analysis and optimization of the parameters of the control system. The analytical results are verified by a detailed digital time simulation study on the first IEEE benchmark model for SSO, using the Electro-Magnetic Transients Program (EMTP) developed by the Bonneville Power Administration (BPA).

The results from the analytical and the digital computer studies reveal the technical feasibility of using a static phase-shifter for damping the shaft torsional stresses of a turbine-generator, as a result of small signal perturbations and severe disturbances.