What is power system stabilizer?

Power system stabilizer (PSS) control provides a positive contribution by damping generator rotor angle swings, which are in a broad range of frequencies in the power system. These range from low frequency intertie modes (typically 0.1 - 1.0 Hz), to local modes (typically 1 - 2Hz), to intra-plant modes (about 2 -3 Hz). The low frequency modes, commonly called intertie or interarea modes, are caused by coherent groups of generators swinging against other groups in the interconnected system. These modes are present in all interconnected systems and the damping is a function of tie line strength and unit loading factors. Weak ties due to line outages and heavy system loads can lead to poorly damped intertie modes. PSS control can generally provide significant improvements in intertie mode damping, by applying stabilizers to most units that participate in power swing modes.