A power system stabilizer (PSS) is a control system installed on a generation unit that monitors variables such as current, voltage, and shaft speed. When necessary, it then sends the appropriate control signals to the voltage regulator to damp system oscillations so that frequency does not stray beyond tolerances.
Rapid small positive and negative changes in frequency in the range of 0.2 to 3 Hertz, called frequency oscillations, occur in bulk electric systems due to small variations in load. These oscillations must be damped (reduced) to avoid the potential for growing oscillations that could ultimately push frequencies outside of system tolerances. If this happens, the system will begin shutting down to protect equipment, thus resulting in wide-scale outages.
When small oscillations occur, the generator’s rotational speed increases or decreases due to the frequency fluctuation. However, the turbine control that manages the speed by injecting slightly more or less fuel is not always fast enough to respond to such oscillations. And given that a variety of generators are interconnected in a bulk electric system, each may respond differently, potentially resulting in generators no longer spinning in synch. Thus, it is important to respond to these oscillations quickly. A PSS does this by sending a control signal to the generator voltage regulator when such oscillations are observed. This rapidly changes the generator operation to counteract the frequency oscillations.