This paper presents the results of a comparative study on the application of two countermeasures, i.e., the excitation controller and the static VAR compensator (SVC), for the damping of subsynchronous resonance (SSR). In order to stabilize all the SSR modes, a unified approach based on modal control theory is proposed for the design of the excitation controller and the SVC, which are essentially dynamic output feedback compensators. The two damping schemes differ from each other in the way they modulate the reactive power flow in the system in order to damp out the subsynchronous oscillations. To demonstrate the effectiveness of the proposed damping schemes under disturbance conditions, time domain simulations based on nonlinear system model are also performed. The relative merits of the two countermeasures are compared with respect to their validities under various loading conditions and different degrees of series compensations and their capabilities to expand the stable region on the Re-Xc plane.
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