Electromechanical oscillations in power systems can be classified into four main types:

Local Oscillations
Occur between a single generating unit and the rest of the station, or between the station and the larger system.
Typical frequency range: 0.8–2.0 Hz

Inter-Plant Oscillations
Occur between two electrically close generation plants.
Typical frequency range: 1–2 Hz

Inter-Area Oscillations
Involve two major groups of generation plants swinging against each other.
Typical frequency range: 0.2–0.8 Hz

Global Oscillations
Represent systemwide in-phase oscillation of all generators, typically in isolated or weak systems.
Typical frequency: below 0.2 Hz

Modeling Reference

The PSS is functionally represented by the IEEE Std 421.5-1995 PSS2A/PSS2B/PSS2C model types. These models reflect the dynamic structure and stabilizing components used in practical excitation systems. (Refer to the corresponding figure for the functional block diagram.)

Functional Purpose

The objective of the PSS is to enhance the stabilizing effect of the generator’s excitation system, thereby improving system stability across the widest possible operating range. The PSS achieves this by:

Deriving supplementary stabilizing signals from generator internal frequency or speed perturbations

Modulating the excitation input to counteract oscillatory power swings

Increasing the generator’s damping contribution during system disturbances

Improving overall power transmission stability in both local and interconnected grid conditions

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