⚡Power System Stability and Control Unit 12 – Power System Stabilizers and Control

Fundamentals of Power System Stability

• Power system stability refers to the ability of an electric power system to regain a state of operating equilibrium after being subjected to a disturbance
• Classified into three main categories: rotor angle stability, frequency stability, and voltage stability
• Rotor angle stability relates to the ability of synchronous machines to remain in synchronism after a disturbance (generator synchronism)
• Frequency stability concerns the ability of a power system to maintain steady frequency following a severe system upset resulting in a significant imbalance between generation and load
• Voltage stability is the ability of a power system to maintain steady voltages at all buses in the system after being subjected to a disturbance
  • Instability occurs in the form of a progressive fall or rise of voltages of some buses (voltage collapse)
• Transient stability deals with the ability of the power system to maintain synchronism when subjected to a severe transient disturbance (short circuits, loss of generation)
• Small-signal stability is concerned with the ability of the power system to maintain synchronism under small disturbances (load changes)

Types of Power System Oscillations

• Power system oscillations are classified based on their frequency, damping, and mode shape
• Local plant mode oscillations involve a single generator or a group of generators at a generating station swinging against the rest of the system (0.7 to 2.0 Hz)
• Interarea mode oscillations involve two coherent groups of generators swinging against each other (0.1 to 0.7 Hz)
  • Characterized by the oscillation of many machines in one part of the system against machines in other parts
• Control mode oscillations are associated with generators and poorly tuned exciters, governors, HVDC converters, and SVC controls (0.7 to 2.0 Hz)
• Torsional mode oscillations involve the turbine-generator shaft system rotational components (10 to 46 Hz)
  • Interaction between series capacitors and turbine-generator torsional dynamics can lead to subsynchronous resonance
• Forced oscillations are caused by cyclic loads, control loops, or resonance effects (0.1 to 2.0 Hz)
• Identifying the dominant oscillation modes and their characteristics is crucial for designing effective damping controllers

Power System Stabilizer (PSS) Basics

• Power System Stabilizers (PSS) are supplementary control devices used to damp power system oscillations
• PSS provides an additional stabilizing signal to the excitation system of a synchronous generator
• The main objective of a PSS is to extend the stability limits by modulating the generator excitation to provide damping to the oscillations
• PSS uses local measurements (rotor speed, frequency, or power) as input signals
  • These input signals are processed through a series of lead-lag compensators and gain stages
• The output of the PSS is a stabilizing signal that is added to the reference voltage of the generator's excitation system
• Properly tuned PSS can significantly improve the damping of local plant mode and interarea mode oscillations
• PSS are most effective in damping local plant mode oscillations, but they can also contribute to damping interarea mode oscillations
• The effectiveness of a PSS depends on its location, input signal selection, and tuning of its parameters

PSS Design and Tuning Methods

• The design and tuning of PSS involve selecting the appropriate input signals, structure, and parameters
• Input signal selection is based on the observability and controllability of the dominant oscillation modes
  • Common input signals include rotor speed deviation, accelerating power, and frequency deviation
• The structure of a PSS typically consists of a washout filter, lead-lag compensators, and a gain stage
  • Washout filter removes steady-state bias and allows the PSS to respond only to oscillations
  • Lead-lag compensators provide the necessary phase compensation to ensure proper damping
  • Gain stage determines the amount of damping provided by the PSS
• Tuning methods for PSS include:
  • Pole-placement technique: Places the closed-loop poles at desired locations in the complex plane
  • Residue-based method: Maximizes the damping torque contribution of the PSS at the dominant oscillation frequencies
  • Optimization-based methods: Optimize PSS parameters based on a performance index (damping ratio, settling time)
• Robust tuning methods consider the uncertainties in the power system model and ensure satisfactory performance over a wide range of operating conditions
• Coordination with other control devices (AVR, FACTS) is essential to avoid adverse interactions and ensure optimal damping

Advanced PSS Technologies

• Adaptive PSS: Automatically adjust their parameters based on the current operating conditions of the power system
  • Use real-time measurements and online identification techniques to estimate the system parameters
  • Adapt the PSS gains and time constants to maintain optimal damping performance
• Wide-Area PSS (WAPSS): Utilize wide-area measurements from Phasor Measurement Units (PMUs) to improve the damping of interarea oscillations
  • PMUs provide synchronized measurements of voltage and current phasors across the power system
  • WAPSS use these measurements to estimate the system states and compute a stabilizing signal based on the overall system dynamics
• Multi-band PSS: Consist of multiple frequency bands, each designed to damp a specific range of oscillation frequencies
  • Effective in damping both local and interarea oscillations simultaneously
  • Each frequency band has its own gain, washout filter, and lead-lag compensators
• Fuzzy Logic PSS: Use fuzzy logic controllers to compute the stabilizing signal based on a set of linguistic rules
  • Fuzzy logic can handle the nonlinearities and uncertainties in the power system
  • Fuzzy PSS can provide robust performance over a wide range of operating conditions
• Neural Network PSS: Employ artificial neural networks to learn the optimal stabilizing signal based on training data
  • Neural networks can capture the complex relationships between the input signals and the required damping
  • Adaptive learning algorithms can update the neural network weights online to adapt to changing system conditions

Integration with Other Control Systems

• PSS are often integrated with other control systems in the power system to achieve optimal performance
• Coordination with Automatic Voltage Regulators (AVR) is essential to ensure stable operation and avoid negative interactions
  • AVR maintains the generator terminal voltage at a desired level
  • Proper coordination between PSS and AVR ensures that the damping provided by the PSS is not counteracted by the AVR
• Integration with Flexible AC Transmission Systems (FACTS) devices can enhance the overall system stability
  • FACTS devices (SVC, STATCOM, TCSC) provide fast and controllable reactive power support
  • Coordinated control of PSS and FACTS devices can improve the damping of interarea oscillations and enhance voltage stability
• Coordination with HVDC systems is necessary to avoid adverse interactions and ensure stable operation
  • HVDC converters can introduce additional oscillation modes and interact with the torsional dynamics of generators
  • Proper coordination between PSS and HVDC controls can mitigate these interactions and improve the overall system damping
• Integration with renewable energy sources (wind, solar) requires special consideration
  • Variable nature of renewable generation can introduce new oscillation modes and challenges for system stability
  • PSS tuning and coordination with the controls of renewable energy sources can help maintain stable operation and damping performance

Performance Analysis and Testing

• Performance analysis and testing are crucial to validate the effectiveness of PSS and ensure reliable operation
• Small-signal stability analysis is used to assess the damping of oscillation modes and identify potential stability issues
  • Eigenvalue analysis: Computes the eigenvalues and eigenvectors of the linearized system model to determine the oscillation modes and their damping
  • Participation factor analysis: Identifies the contribution of each state variable to a particular oscillation mode
• Time-domain simulations are used to evaluate the PSS performance under various disturbances and operating conditions
  • Simulate the power system response to faults, load changes, and other disturbances
  • Assess the damping of oscillations, settling time, and overall system stability
• Hardware-in-the-loop (HIL) testing allows the validation of PSS performance in a realistic environment
  • Real-time simulation of the power system is combined with physical hardware (PSS, excitation system)
  • HIL testing helps identify potential implementation issues and validate the PSS performance under realistic conditions
• Field testing and commissioning are essential to verify the PSS performance in the actual power system
  • Staged tests are conducted to gradually increase the PSS gain and evaluate its impact on the system damping
  • Continuous monitoring and fine-tuning of PSS parameters may be necessary to ensure optimal performance over time
• Performance benchmarking and comparison with other damping controllers can provide insights into the relative effectiveness of PSS
  • Compare the damping performance of PSS with other controllers (FACTS, HVDC) under similar operating conditions
  • Identify the strengths and limitations of PSS in different scenarios and explore potential improvements

Real-World Applications and Case Studies

• PSS have been widely implemented in power systems around the world to enhance stability and damping performance
• Case study: Western North American Power System (WNAPS)
  • WNAPS is a large interconnected power system spanning several states in the western United States and Canada
  • PSS have been installed on many generators in the WNAPS to damp local and interarea oscillations
  • Coordinated tuning of PSS has been performed to ensure optimal damping performance and avoid adverse interactions
• Case study: European Power System
  • The European power system is a highly interconnected network spanning multiple countries
  • PSS have been deployed on generators across Europe to improve the damping of interarea oscillations
  • Coordinated design and tuning of PSS have been carried out to ensure stable operation under various contingencies and operating conditions
• Case study: China Southern Power Grid (CSG)
  • CSG is one of the largest power grids in China, covering several provinces in the southern region
  • PSS have been installed on generators in the CSG to damp low-frequency oscillations and enhance system stability
  • Adaptive PSS and wide-area measurement-based PSS have been implemented to improve the damping performance under changing operating conditions
• Case study: Brazilian Interconnected Power System (BIPS)
  • BIPS is a large interconnected power system covering most of Brazil
  • PSS have been deployed on generators in the BIPS to damp local and interarea oscillations
  • Coordinated tuning of PSS and FACTS devices has been performed to enhance the overall system stability and damping performance
• Lessons learned from real-world applications:
  • Proper selection of input signals and PSS structure is crucial for effective damping performance
  • Coordination with other control devices (AVR, FACTS, HVDC) is essential to avoid adverse interactions and ensure optimal damping
  • Continuous monitoring and fine-tuning of PSS parameters may be necessary to maintain satisfactory performance over time
  • Advanced PSS technologies (adaptive, wide-area, multi-band) can provide improved damping performance under changing operating conditions