⚡Power System Stability and Control Unit 5 – Excitation Systems and AVRs

Fundamentals of Excitation Systems

• Excitation systems provide and control the field current of synchronous generators, which is essential for maintaining the generator's voltage and reactive power output
• The main components of an excitation system include the exciter, automatic voltage regulator (AVR), and various control and protection devices
• Excitation systems can be classified into three main categories: DC excitation systems, AC excitation systems, and static excitation systems
• The exciter generates the field current required by the generator's rotor windings to produce the magnetic field necessary for voltage generation
• The AVR monitors the generator's terminal voltage and adjusts the exciter's output to maintain the desired voltage level
  • This is achieved through a feedback control loop that compares the measured voltage with a reference value and generates an error signal
  • The error signal is processed by the AVR, which then adjusts the exciter's output accordingly
• Excitation systems play a crucial role in maintaining the stability of the power system by regulating the generator's voltage and reactive power output
• Proper design and operation of excitation systems are essential for ensuring the reliable and efficient operation of the power system

Types of Excitation Systems

• DC excitation systems utilize a DC generator (exciter) to supply the field current to the synchronous generator's rotor windings
  • The exciter is typically mounted on the same shaft as the main generator and is driven by the prime mover (turbine)
  • DC excitation systems are known for their simplicity and reliability but have limitations in terms of response time and control flexibility
• AC excitation systems employ an AC generator (exciter) and a rectifier to provide the field current to the main generator
  • The exciter is usually a brushless, three-phase, synchronous generator with a rotating rectifier assembly
  • AC excitation systems offer faster response times and better control capabilities compared to DC systems
• Static excitation systems use solid-state devices (thyristors or power electronics) to convert AC power from the generator's terminals or an auxiliary source into DC power for the field windings
  • Static exciters eliminate the need for rotating components, resulting in reduced maintenance requirements and improved reliability
  • They provide fast response times, high ceiling voltages, and excellent control flexibility
• Brushless excitation systems combine the advantages of AC and static systems by using a brushless exciter and a rotating rectifier assembly
  • This configuration eliminates the need for slip rings and brushes, reducing maintenance requirements and improving reliability
• Digital excitation systems incorporate microprocessor-based controllers and advanced control algorithms for enhanced performance and flexibility
  • These systems offer improved accuracy, adaptability, and diagnostic capabilities compared to analog systems

AVR Basics and Components

• The automatic voltage regulator (AVR) is a critical component of the excitation system, responsible for maintaining the generator's terminal voltage at a desired level
• The AVR continuously monitors the generator's output voltage and compares it with a reference value to generate an error signal
• The error signal is processed by the AVR's control algorithm, which determines the necessary adjustments to the exciter's output to maintain the desired voltage level
• The main components of an AVR include:
  • Voltage sensing and scaling circuits to measure the generator's output voltage and convert it to a suitable level for the AVR's input
  • A voltage reference source that provides a stable and adjustable reference value for the desired voltage level
  • A comparator or summing junction to generate the error signal by comparing the measured voltage with the reference value
  • A control amplifier or compensator to process the error signal and generate the appropriate control signal for the exciter
  • Output stages to amplify the control signal and drive the exciter's input
• AVRs can be classified into different types based on their control algorithms and implementation, such as:
  • Proportional-Integral-Derivative (PID) controllers
  • Proportional-Integral (PI) controllers
  • Lead-lag compensators
  • Fuzzy logic controllers
• Modern AVRs often incorporate additional features and functions, such as:
  • Limiters to prevent over-excitation or under-excitation of the generator
  • Stabilizers to improve the system's dynamic performance and damping
  • Reactive power and power factor control modes
  • Communication interfaces for remote monitoring and control

Control Loops and System Dynamics

• Excitation systems employ various control loops to regulate the generator's voltage and maintain system stability
• The primary control loop is the voltage regulation loop, which consists of the AVR, exciter, and generator
  • This loop aims to maintain the generator's terminal voltage at the desired level by adjusting the exciter's output based on the measured voltage error
  • The voltage regulation loop's performance is characterized by its response time, stability, and accuracy
• The stability of the voltage regulation loop is influenced by factors such as the generator's characteristics, the excitation system's parameters, and the power system's configuration
  • Poorly tuned or designed control loops can lead to oscillations, overshoot, or instability in the generator's voltage and reactive power output
• Additional control loops may be implemented to enhance the excitation system's performance and functionality, such as:
  • Reactive power control loop to maintain the generator's reactive power output at a desired level
  • Power factor control loop to regulate the generator's power factor
  • Field current limiting loop to protect the generator from over-excitation
  • Under-excitation limiting loop to prevent the generator from operating in an underexcited condition, which can lead to stability issues
• The dynamics of the excitation system and its control loops are influenced by the time constants and gains of the various components, such as the exciter, generator, and AVR
  • Proper tuning of these parameters is essential for achieving the desired performance and stability of the excitation system
• Advanced control techniques, such as adaptive control, robust control, and model predictive control, can be employed to improve the excitation system's performance and robustness in the presence of uncertainties and disturbances

Performance Criteria and Standards

• Excitation systems are designed and evaluated based on various performance criteria and standards to ensure their effectiveness and compatibility with the power system
• Key performance criteria for excitation systems include:
  • Voltage regulation accuracy: The ability to maintain the generator's terminal voltage within a specified tolerance range under varying load conditions
  • Response time: The speed at which the excitation system can respond to voltage deviations and restore the voltage to the desired level
  • Stability: The ability to maintain stable operation and avoid oscillations or instability in the generator's voltage and reactive power output
  • Ceiling voltage: The maximum voltage that the excitation system can provide to the generator's field windings, which determines the generator's overload capability
  • Reliability: The excitation system's ability to operate continuously and withstand various environmental and operational stresses without failure
• International standards, such as IEEE 421.1 and IEC 60034-16, provide guidelines and requirements for the design, testing, and performance of excitation systems
  • These standards specify the minimum requirements for voltage regulation accuracy, response time, ceiling voltage, and other key parameters
  • Compliance with these standards ensures the compatibility and interoperability of excitation systems with different generators and power systems
• Regional grid codes and utility-specific requirements may impose additional performance criteria and standards on excitation systems to ensure the reliable and stable operation of the power system
• Excitation system manufacturers and power system operators conduct extensive testing and commissioning procedures to verify the performance and compliance of excitation systems with the relevant standards and criteria
  • These tests include factory acceptance tests (FAT), site acceptance tests (SAT), and performance validation tests under various operating conditions
• Regular maintenance, calibration, and upgrades of excitation systems are necessary to ensure their continued compliance with performance criteria and standards throughout their operational lifetime

Modeling and Simulation Techniques

• Modeling and simulation play a crucial role in the design, analysis, and optimization of excitation systems and their impact on power system stability
• Excitation system models are developed to represent the dynamic behavior of the exciter, AVR, and associated control loops in power system stability studies
• The IEEE has standardized a set of excitation system models, known as the IEEE 421.5 models, which include:
  • DC excitation system models (DC1A, DC2A, etc.)
  • AC excitation system models (AC1A, AC2A, etc.)
  • Static excitation system models (ST1A, ST2A, etc.)
• These standardized models provide a common framework for representing excitation systems in power system simulation software, such as PSS/E, PSCAD, and DIgSILENT PowerFactory
• Excitation system models are typically represented using block diagrams, transfer functions, and state-space equations to capture the dynamic behavior of the system
  • The model parameters, such as time constants, gains, and limits, are derived from the actual excitation system's design and commissioning data
  • The models are validated against real-world measurements and performance data to ensure their accuracy and reliability
• Simulation studies are conducted to assess the impact of excitation systems on power system stability under various operating conditions and contingencies
  • These studies include small-signal stability analysis, transient stability analysis, and voltage stability analysis
  • The simulation results help identify potential stability issues, optimize excitation system settings, and evaluate the effectiveness of control strategies
• Advanced simulation techniques, such as hardware-in-the-loop (HIL) simulation and real-time digital simulation (RTDS), are used to test and validate excitation system controls in a realistic environment
  • HIL simulation involves connecting the actual excitation system controller to a real-time simulator that emulates the power system's behavior
  • RTDS allows for the detailed modeling and simulation of excitation systems and power systems in real-time, enabling the study of complex interactions and transient phenomena
• Modeling and simulation of excitation systems are essential for understanding their impact on power system stability, optimizing their performance, and ensuring the reliable and secure operation of the power grid

Impact on Power System Stability

• Excitation systems play a critical role in maintaining power system stability by regulating the voltage and reactive power output of synchronous generators
• Voltage stability is strongly influenced by the performance of excitation systems
  • Adequate voltage regulation and reactive power support from excitation systems help prevent voltage collapse and maintain voltage stability during disturbances
  • Excitation systems with fast response times and high ceiling voltages can provide the necessary reactive power support to stabilize the system during voltage dips or faults
• Excitation systems contribute to the damping of power system oscillations and enhance small-signal stability
  • Properly tuned AVR control loops and stabilizers can provide positive damping to local and inter-area oscillations, preventing them from growing and causing instability
  • Power system stabilizers (PSS) are often integrated with excitation systems to provide additional damping and improve the overall system stability
• Transient stability is affected by the excitation system's ability to maintain the generator's synchronism during large disturbances, such as faults or generator trips
  • Fast-acting excitation systems with high ceiling voltages can help generators maintain synchronism by providing the necessary field current to support the generator's voltage and power output
  • Excitation system limiters, such as over-excitation and under-excitation limiters, protect the generator from damage and ensure its stable operation during transient events
• Excitation systems can also impact the system's frequency stability by participating in frequency regulation and load-frequency control
  • Some excitation systems include frequency response functions that adjust the generator's active power output in response to frequency deviations, helping to maintain the system's frequency stability
• The coordination and compatibility of excitation systems with other power system controls, such as governors, FACTS devices, and HVDC systems, are essential for ensuring the overall stability and reliability of the power system
  • Poorly coordinated or incompatible control systems can lead to adverse interactions, oscillations, and instability
• The impact of excitation systems on power system stability is assessed through comprehensive stability studies, which consider various operating conditions, contingencies, and future expansion scenarios
  • These studies help identify potential stability issues, optimize excitation system settings, and guide the planning and operation of the power system to maintain a secure and stable operation

Practical Applications and Case Studies

• Excitation systems find practical applications in a wide range of power generation facilities, including thermal power plants, hydroelectric power plants, and renewable energy systems
• In thermal power plants, excitation systems are used to control the voltage and reactive power output of steam turbine-driven synchronous generators
  • These systems help maintain the generator's terminal voltage within the desired range, even under varying load conditions and grid disturbances
  • Case studies have demonstrated the effectiveness of modern excitation systems in improving the voltage regulation and stability of thermal power plants, particularly during grid faults and load rejections
• Hydroelectric power plants rely on excitation systems to regulate the voltage and reactive power output of hydro generators
  • The fast response times and high ceiling voltages of modern excitation systems are particularly valuable in hydroelectric applications, where rapid load changes and water hammer effects can cause significant voltage and power fluctuations
  • Case studies have shown how advanced excitation system controls, such as digital AVRs and power system stabilizers, have enhanced the stability and performance of hydroelectric power plants, even under challenging operating conditions
• Excitation systems are increasingly being applied in renewable energy systems, such as wind farms and solar photovoltaic (PV) power plants
  • In wind farms, excitation systems are used to control the voltage and reactive power output of doubly-fed induction generators (DFIGs) or full-converter wind turbines
  • In solar PV power plants, excitation systems are employed in the control of grid-connected inverters to provide voltage regulation and reactive power support
  • Case studies have demonstrated the benefits of excitation systems in improving the voltage stability, fault ride-through capability, and grid integration of renewable energy systems
• Excitation system retrofits and upgrades are common practical applications in existing power plants
  • Older analog excitation systems are often replaced with modern digital systems to improve performance, reliability, and flexibility
  • Case studies have shown significant improvements in voltage regulation accuracy, response times, and stability margins after excitation system retrofits, leading to enhanced power plant performance and grid stability
• Practical applications of excitation systems also extend to the realm of power system protection and control
  • Excitation systems are often integrated with generator protection systems, such as loss-of-excitation (LOE) protection and out-of-step (OOS) protection, to ensure the safe and reliable operation of the generator and the power system
  • Case studies have highlighted the importance of proper coordination between excitation systems and protection systems to prevent false trips, minimize fault impacts, and maintain power system stability during contingencies