Power System Stabilizer (PSS)

A power system stabilizer, usually called a PSS, is a supplementary control added to a synchronous generator excitation system to improve damping of electromechanical oscillations. It works alongside the automatic voltage regulator and modifies excitation in response to speed, frequency, or power deviations.

The key idea is to produce an electrical torque component that counteracts the rotor-speed swing. Without that supplementary damping action, a fast high-gain AVR can improve voltage control while unintentionally reducing oscillation damping.

Key Aspects of Power System Stabilizers:

• Supplementary Excitation Control: A PSS does not replace the AVR, it adds a stabilizing signal on top of the normal voltage-regulation loop. The combined behavior must be tuned carefully so voltage performance and damping support each other.
• Input Signals: Common input choices include shaft speed deviation, electrical power, frequency, or combinations of these signals. The selected input depends on generator behavior and the type of oscillation the stabilizer is meant to damp.
• Phase Compensation: The stabilizer includes washout and lead-lag blocks so its output is applied with the right timing relative to the oscillation. Proper phase compensation is what turns measurement of a swing into useful damping torque.
• Need for Tuning: A poorly tuned PSS can be ineffective or even harmful. Engineers usually tune it using modal studies, frequency-response analysis, and field tests under representative operating conditions.
• System-Level Benefit: PSS deployment often allows higher secure transfers and better dynamic performance across long corridors. It is one of the standard tools for managing poorly damped local and inter-area modes.