Design of controllers for the damping of torsional oscillations using time-scale decomposition

A new technique is presented for the design of an excitation controller and a static VAR compensator which are used to improved the damping characteristics of torsional oscillations in a capacitor-compensated power system. To facilitate the controller design, the system matrix is reduced by means of multi-time-scale decomposition while preserving the mechanical shift modes, which are the ones to be damped by the proposed controller. The parameters of the controller are then determined on the basis of the reduced system by using modal control theory. It is found that model reduction can save the effort in controller design without any significant drift in the eigenvalues of the closed-loop system. To demonstrate the effectiveness of the proposed damping scheme under disturbance conditions, computer-simulated dynamic response tests based on a non-linear system model are also performed. It is concluded that all subsynchronous oscillation modes can be effectively damped by the proposed controller.