12.1 PSS principles and design concepts

Power System Stabilizers (PSS) are crucial for enhancing power system stability. They work by damping electromechanical oscillations through supplementary control signals to generator excitation systems, improving both small-signal and transient stability.

PSS design involves key components like gain blocks, washout filters, and phase compensation. Proper tuning and input signal selection are vital for effectiveness. PSS modulate excitation voltage, adding a damping torque to counter rotor speed deviations and improve overall system stability.

Power System Stabilizers: Principles and Objectives

Enhancing Power System Stability

• Power system stabilizers (PSS) are control devices used to enhance the stability of power systems by providing supplementary control signals to the excitation system of synchronous generators
• The primary objective of PSS is to damp electromechanical oscillations in the power system, which can arise due to disturbances or changes in operating conditions
• PSS improve the dynamic stability of power systems by modulating the generator excitation to provide damping torque in phase with the rotor speed deviations

Improving Small-Signal and Transient Stability

• The use of PSS enhances the small-signal stability of power systems, which refers to the ability of the system to maintain synchronism under small disturbances
• PSS also contribute to the improvement of transient stability, which is the ability of the power system to maintain synchronism following large disturbances such as faults or sudden load changes
• The effectiveness of PSS depends on the proper tuning of their parameters and the selection of appropriate input signals that reflect the system's dynamic behavior (rotor speed deviation, accelerating power)

Design Concepts of Power System Stabilizers

Main Components of a PSS

• A typical PSS consists of three main components: a gain block, a washout filter, and a phase compensation block
• The gain block determines the amount of damping provided by the PSS and is adjusted to achieve the desired level of oscillation damping
• The washout filter is a high-pass filter that removes the steady-state component of the input signal and allows only the dynamic variations to pass through
  • The washout time constant is selected to be long enough to allow the PSS to respond to oscillations in the desired frequency range (0.1 to 2 Hz)

Phase Compensation and Input Signal Selection

• The phase compensation block provides the necessary phase lead or lag to ensure that the PSS output signal is in phase with the rotor speed deviations
  • The phase compensation is achieved using lead-lag compensators, which are designed based on the frequency response characteristics of the generator and the power system
• The input signals commonly used for PSS include rotor speed deviation, accelerating power, and frequency deviation
• The selection of input signals depends on the specific characteristics of the power system and the desired damping performance (local or inter-area oscillations)

Impact on Generator Excitation Control

Modulation of Excitation Voltage

• PSS act as an additional control loop in the generator excitation system, modulating the excitation voltage to provide damping to the rotor oscillations
• The PSS output signal is added to the automatic voltage regulator (AVR) reference voltage, causing the excitation voltage to vary in response to the detected oscillations
• By modulating the excitation voltage, the PSS introduces a damping torque component that opposes the rotor speed deviations, effectively damping the electromechanical oscillations

Coordination with Other Control Devices

• The damping provided by the PSS helps to reduce the amplitude and duration of oscillations following disturbances, improving the overall stability of the power system
• The effectiveness of PSS in damping oscillations depends on the proper coordination with other control devices in the power system, such as AVRs and governors
• Poorly tuned or improperly coordinated PSS can lead to adverse effects, such as reduced damping or even instability in certain operating conditions (high power transfer, weak transmission links)

Input Signals and Parameters for Design

Commonly Used Input Signals

• Rotor speed deviation is the most commonly used input signal for PSS, as it directly reflects the oscillations in the generator rotor
  • Rotor speed deviation is measured using a tachometer or derived from the terminal frequency and voltage measurements
• Accelerating power, which is the difference between the mechanical input power and the electrical output power of the generator, can also be used as an input signal for PSS
  • Accelerating power provides a measure of the imbalance between the mechanical and electrical torques acting on the rotor
• Frequency deviation, obtained from the terminal voltage measurements, can be used as an input signal to the PSS, especially in cases where the rotor speed measurement is not available

Critical Design Parameters

• The gain of the PSS is a critical parameter that determines the amount of damping provided by the stabilizer
  • The gain is adjusted to achieve the desired damping performance while ensuring stability under various operating conditions (different loading levels, system configurations)
• The time constants of the washout filter and the phase compensation blocks are other important parameters in PSS design
  • These time constants are selected based on the frequency response characteristics of the generator and the power system to provide effective damping over the desired frequency range (typically 0.1 to 2 Hz)
• The limiter settings in the PSS, such as the maximum and minimum output limits, are also important parameters that prevent the PSS from adversely affecting the generator excitation under extreme conditions (severe faults, large disturbances)