Power System Stabilizer (PSS)

Power system stabilizer (PSS) is a sophisticated control system installed on a generation unit that monitors variables such as current, voltage, and shaft speed. While excitation systems with high gain and rapid response times enhance transient stability, they can also diminish small signal stability. PSS control plays a crucial role by damping generator rotor angle oscillations across a wide range of frequencies in the power system.

When necessary, the PSS sends precise control signals to the voltage regulator to suppress system oscillations, ensuring the frequency remains within acceptable tolerances.

Rapid, minor fluctuations in frequency, known as frequency oscillations, occur in bulk electric systems due to small variations in load. These oscillations must be damped to prevent them from escalating and pushing frequencies beyond system tolerances, which could lead to system shutdowns and extensive outages. During these small oscillations, the generator’s rotational speed varies due to frequency changes. However, the turbine control, which manages speed by adjusting fuel input, is not always fast enough to respond to such oscillations.

Given the interconnected nature of multiple generators in a bulk electric system, each generator may respond differently, potentially causing them to fall out of sync. Therefore, it is vital to address these oscillations promptly. A PSS achieves this by sending control signals to the generator voltage regulator upon detecting oscillations, quickly adjusting generator operation to counteract the frequency oscillations.

Difference between AVR and PSS

Automatic Voltage Regulator (AVR) controls the generator’s terminal voltage to a specified setpoint, while the PSS modulates the AVR input to mitigate both low-frequency and local-mode power oscillations. The AVR regulates terminal voltage by controlling the current supplied to the generator field winding through the exciter. The PSS, integrated with the AVR, dampens low-frequency oscillations in the power system by providing a supplementary signal to the excitation system.