12.3 Coordinated control of AVR and PSS
5 min read•august 1, 2024
Power system stability hinges on the coordinated control of Automatic Voltage Regulators (AVRs) and Power System Stabilizers (PSS). AVRs maintain generator voltage, while PSS dampen rotor oscillations. Without proper coordination, these systems can clash, causing instability and undamped oscillations.
Coordinated control ensures AVRs and PSS work together smoothly, improving transient and . It balances voltage regulation and oscillation damping, optimizing system performance. This coordination is crucial for maintaining power system stability under various operating conditions.
AVR and PSS Interaction in Excitation Control
Roles and Objectives of AVR and PSS
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- Automatic voltage regulators (AVRs) control the terminal voltage of synchronous generators by adjusting the excitation system
- Power system stabilizers (PSSs) provide additional damping to the generator rotor oscillations by modulating the excitation system
- AVRs maintain the generator terminal voltage, while PSSs enhance the damping of electromechanical oscillations
- AVRs have faster response times compared to PSSs, which can lead to potential conflicts in control actions
- Improperly tuned or coordinated AVR and PSS result in undamped oscillations or even instability in the power system
Necessity of Coordinated Control
- Coordinated control of AVR and PSS ensures that both controllers work in harmony to achieve their respective objectives without adversely affecting each other's performance
- Uncoordinated control of AVR and PSS exacerbates oscillations, leading to reduced stability margins and potential system instability
- Proper coordination of AVR and PSS improves the , small-signal stability, and of the power system
- Coordinated control optimizes the overall system performance by balancing the trade-offs between voltage regulation and oscillation damping
Coordinated Control for Stable Operation
Electromechanical Oscillations in Power Systems
- Power systems are prone to electromechanical oscillations, classified as local mode oscillations and inter-area mode oscillations
- Local mode oscillations involve a single generator or a group of generators oscillating against the rest of the system
- Inter-area mode oscillations involve the oscillation of a group of generators in one area against a group of generators in another area
- Coordinated control of AVR and PSS ensures adequate damping of oscillations and maintains system stability under various operating conditions
- Uncoordinated control of AVR and PSS exacerbates these oscillations, leading to reduced stability margins and potential system instability
Benefits of Coordinated Control
- Coordinated control improves the transient stability, small-signal stability, and voltage stability of the power system
- Proper coordination of AVR and PSS optimizes the overall system performance by balancing the trade-offs between voltage regulation and oscillation damping
- Efficient operation of power systems requires minimizing losses and maximizing the utilization of transmission capacity, achievable through coordinated control of AVR and PSS
- Coordinated control helps in maintaining the desired reactive power balance and minimizing reactive power losses in the system
Design Strategies for AVR and PSS
Modeling and Analysis Techniques
- Coordinated control strategies for AVR and PSS involve designing the control parameters and structures to achieve the desired performance objectives
- The design process involves modeling the power system, including the generators, excitation systems, and network topology
- Linear analysis techniques, such as eigenvalue analysis and frequency response analysis, assess the stability and damping characteristics of the system
- Time-domain simulations, such as transient stability analysis and dynamic voltage stability analysis, provide insights into the system's behavior under various scenarios
Control Parameter Tuning
- The control parameters of AVR and PSS, such as gains, time constants, and phase compensation, are tuned to optimize the system performance
- The objective is to achieve a balance between fast voltage regulation and adequate damping of oscillations
- The tuning process may involve iterative simulations and optimization algorithms to find the optimal parameter settings
- Advanced control techniques, such as , , and model predictive control, enhance the performance and robustness of the coordinated control system
Centralized and Decentralized Control Architectures
- Coordinated control strategies can be centralized or decentralized, depending on the architecture of the control system
- Centralized control involves a single controller that coordinates the actions of multiple AVRs and PSSs based on system-wide measurements
- Decentralized control relies on local measurements and control actions at each generator, with coordination achieved through proper tuning of the individual controllers
- The choice between centralized and decentralized control depends on factors such as system size, communication infrastructure, and reliability requirements
Impact of Coordinated Control on Power Systems
Transient Stability Improvement
- Coordinated control of AVR and PSS improves transient stability, which refers to the ability of the system to maintain synchronism after a large disturbance
- Properly tuned PSSs provide additional damping to the generator rotor oscillations, reducing the risk of loss of synchronism (generator tripping)
- Coordinated control minimizes the impact of disturbances (short circuits, line outages) and facilitates faster recovery of the system to a stable operating point
Enhanced Small-Signal Stability
- Small-signal stability, which deals with the system's ability to maintain synchronism under small perturbations, is enhanced by coordinated control
- Coordinated tuning of AVR and PSS parameters ensures adequate damping of low-frequency oscillations (0.1-2 Hz), improving the overall system stability
- Eigenvalue analysis assesses the damping of critical modes and evaluates the effectiveness of the coordinated control strategy
Voltage Stability Maintenance
- Voltage stability, which refers to the ability of the system to maintain acceptable voltage levels under varying load conditions, is positively influenced by coordinated control
- AVRs play a crucial role in regulating the generator terminal voltage and maintaining voltage stability
- Coordinated control ensures that the actions of AVRs and PSSs do not compromise voltage stability margins (maximum loadability)
- Proper coordination prevents voltage collapse scenarios and ensures stable operation under heavy loading conditions
Optimized Reactive Power Management
- Coordinated control of AVR and PSS optimizes reactive power management in the power system
- AVRs control the excitation system, which directly affects the reactive power output of the generators
- Coordinated control maintains the desired reactive power balance and minimizes reactive power losses in the system (transmission lines, transformers)
- Proper coordination ensures that the generators operate within their reactive power capability limits and avoid over-excitation (high field current) or under-excitation (low field current) conditions
Key Terms to Review (19)
Adaptive Control: Adaptive control refers to a type of control strategy that adjusts its parameters automatically in response to changes in system dynamics or operating conditions. This capability allows for improved performance and stability in various control systems, especially in environments where the system behavior is uncertain or variable.
Automatic Voltage Regulator (AVR): An Automatic Voltage Regulator (AVR) is an electronic device that automatically maintains the voltage levels of a generator or an electrical power system to ensure stable and reliable operation. By adjusting the excitation of the generator in response to changes in load and system conditions, AVRs help prevent voltage fluctuations and maintain system stability, which is crucial for effective generator modeling, compliance with industry standards, voltage collapse prevention, and coordinated control with Power System Stabilizers (PSS).
Damping Ratio: The damping ratio is a dimensionless measure describing how oscillations in a system decay after a disturbance. It indicates the level of damping in a system and is crucial for understanding the system's response to disturbances, influencing how quickly stability is achieved following changes in load or generation.
Gain Margin: Gain margin is a measure of the stability of a control system, indicating how much gain can be increased before the system becomes unstable. A higher gain margin suggests that the system can tolerate greater variations in gain without losing stability, making it essential for assessing system performance under varying conditions.
Integral Control: Integral control is a fundamental component of feedback control systems that continuously adjusts the control output based on the accumulated error over time, ensuring that any steady-state error is eliminated. This type of control is crucial for maintaining system stability and performance, especially in systems where precise regulation is needed. It connects directly to concepts like governor functions, speed-droop characteristics for load sharing, and coordinated controls among different system components.
Negative Feedback: Negative feedback is a control mechanism that reduces the output or activity of a system in response to changes in its environment, helping maintain stability and balance. This process is crucial in various systems, as it enables them to self-regulate and return to a desired state after experiencing disturbances or fluctuations.
Optimal Control: Optimal control refers to the process of determining a control policy that minimizes (or maximizes) a certain performance criterion over time, often applied in dynamic systems to achieve desired performance while considering constraints. This concept is essential in designing efficient and effective control strategies for power systems, ensuring stability and performance under varying conditions. By applying optimal control methods, engineers can fine-tune their systems for better response, stability, and efficiency, directly impacting the overall reliability of power generation and distribution.
Oscillatory disturbance: An oscillatory disturbance refers to a fluctuation or variation in system parameters, often resulting in oscillations around a stable operating point. In the context of power systems, these disturbances can arise from various sources such as changes in load, faults, or control actions, impacting system stability and performance. Understanding these disturbances is essential for effective control strategies that ensure a stable operation of power systems.
Pid tuning: PID tuning refers to the process of adjusting the proportional, integral, and derivative gains in a PID controller to achieve desired system performance. This adjustment ensures stability and optimal response in control systems, making it crucial for effective operation in various applications, including power system stabilizers and the coordinated control of automatic voltage regulators (AVR) and power system stabilizers (PSS). Effective PID tuning balances system responsiveness with stability, which is essential for maintaining the overall reliability of power systems.
Positive Feedback: Positive feedback is a process in control systems where an increase in a particular variable leads to further increases in that same variable, potentially resulting in exponential growth or system instability. This phenomenon can amplify small disturbances, leading to large deviations from the system's normal operating conditions. In power systems, it can be both beneficial in specific applications and detrimental if not properly managed.
Power System Stabilizer (PSS): A Power System Stabilizer (PSS) is a device installed in a power system that enhances the damping of generator oscillations by providing additional control signals to the generator's excitation system. It helps improve the overall stability of the power system by counteracting low-frequency oscillations that can lead to system instability. By integrating with the automatic voltage regulator (AVR), a PSS optimizes the performance of generators under varying load conditions, ensuring more reliable and efficient operation.
Proportional Control: Proportional control is a control strategy that provides an output response that is directly proportional to the error signal, which is the difference between a desired setpoint and the actual output. This method is foundational in feedback control systems, allowing for effective regulation of processes by adjusting system variables to maintain stability and performance.
Robust Control: Robust control refers to a control strategy designed to maintain performance and stability of a system in the presence of uncertainties and variations in system parameters. This approach is crucial for ensuring that power systems can effectively handle disturbances and changes without compromising reliability. It emphasizes the ability to cope with worst-case scenarios, making it particularly relevant in the context of linearized power system models and coordinated control mechanisms.
Small-signal stability: Small-signal stability refers to the ability of a power system to maintain its equilibrium under small disturbances or fluctuations, ensuring that the system returns to its original state without experiencing significant oscillations or instability. This concept is crucial for analyzing and designing control strategies in power systems, as it involves understanding how changes in load, generation, and system parameters affect the overall stability.
State-space representation: State-space representation is a mathematical modeling framework that describes a dynamic system by using a set of first-order differential equations. This approach captures the internal state of the system at any given time and relates it to its inputs and outputs, allowing for the analysis and control of complex systems in various fields, including power systems.
Transfer Function: A transfer function is a mathematical representation that relates the output of a system to its input in the frequency domain. It is typically expressed as a ratio of two polynomials, where the numerator represents the system's output and the denominator represents the input. This concept is vital for analyzing dynamic systems, especially in the context of stability and control.
Transient disturbance: A transient disturbance refers to a temporary change or fluctuation in a power system that can affect its stability and operational performance. These disturbances can be caused by events such as faults, sudden load changes, or switching operations, leading to rapid variations in voltage, frequency, or power flow. Understanding and managing these disturbances is essential for maintaining system stability, particularly when coordinating the Automatic Voltage Regulator (AVR) and Power System Stabilizer (PSS).
Transient Stability: Transient stability refers to the ability of a power system to maintain synchronism when subjected to a disturbance, such as a fault or sudden change in load. It focuses on the immediate response of the system after such disturbances and how well it can return to a stable operating condition. This concept is crucial in understanding system behavior during and after transient events, particularly in multi-machine environments.
Voltage Stability: Voltage stability refers to the ability of a power system to maintain steady voltage levels at all buses in the system after being subjected to a disturbance. This concept is crucial because voltage instability can lead to voltage collapse, where voltages drop significantly, causing widespread outages and affecting system reliability.