Supplementary Discontinuous Excitation Control
Power system stabilizers are used to enhance the damping of power system electromechanical oscillations by modulating the generator excitation through supplementary control signals. These stabilizers play a critical role in improving the small-signal stability of the power system, especially during disturbances such as faults, load changes, or switching events.
Commonly used inputs to the stabilizers are:
Shaft speed
Terminal frequency
Power
Power System Stabilizers (PSS) are typically represented in simulation tools like PSCAD as dynamic transfer function blocks. They can be interfaced directly with synchronous machine excitation systems. PSCAD provides several predefined PSS models, each representing a specific control strategy and complexity. If none of the predefined stabilizers meet the modelling requirements, users can design custom stabilizers using the Continuous System Modelling Functions (CSMF) section of the PSCAD Master Library.
PSS1A: Single-input stabilizer using shaft speed deviation as the primary input. A basic model widely used for its simplicity and effectiveness.
PSS2C: Dual-input stabilizer using both shaft speed and electrical power inputs. Includes flexible filtering and phase compensation features to adapt to system dynamics.
PSS3C: Designed for high-performance applications, it includes multiple stages of signal conditioning, gain scheduling, and more advanced filtering. Suitable for complex grid conditions.
PSS4C: Multiband stabilizer that provides damping over a broad frequency range, suitable for inter-area, local, and intraplant modes.
PSS5C: Features advanced logic and signal selection capabilities, allowing adaptive behaviour based on system operating conditions.
PSS6C: Incorporates voltage control integration with stabilizing signals, designed for systems requiring strong voltage and frequency interaction damping.
PSS7C: A model that supports multiple input sources and advanced structure for fine-tuned oscillation damping in large interconnected systems.
These standardised models promote consistent implementation and facilitate the use of field test data for tuning and validation. Selection of a particular PSS model depends on the system’s dynamic characteristics and the specific control objectives.
Supplementary Discontinuous Excitation Control
In some particular system configurations, continuous excitation control with terminal voltage and power system stabilizing regulator input signals does not ensure that the potential of the excitation system for improving system stability is fully exploited. For these situations, discontinuous excitation control signals may be employed to enhance stability following large transient disturbances.
If desired, the user may choose to include one of three types of discontinuous excitation controllers to be included within one the above chosen stabilizers.
DEC1A: Transient Excitation Booster with Dual Action Terminal Voltage Limiter.
DEC2A: Open Loop Transient Excitation Booster.
DEC3A: Temporary Interrupter of Stabilizing Signal.
For further details on background theory for these stabilizer models, see PSCAD References [39].