How does a power system stabilizer (PSS) improve generator damping?

A PSS adds supplementary control signal to generator excitation system to damp electromechanical oscillations (0.2-2 Hz). It uses input signals (speed, power, frequency) processed through lead-lag filters to produce a damping torque component in phase with rotor speed deviation. PSS tuning involves: identifying dominant oscillation modes through eigenvalue analysis, designing transfer function for appropriate phase lead at mode frequencies, and field testing. Properly tuned PSS can increase damping from near-zero to 5-10% or more.