6.1 Governor types and control principles

Governors are crucial for maintaining power system stability. They regulate turbine speed and match generator output to electrical demand, keeping frequency within acceptable limits. Different types of governors, from mechanical-hydraulic to electronic, offer varying levels of precision and responsiveness.

Understanding governor control principles is essential for power system engineers. Negative feedback control and droop characteristics allow governors to respond to frequency deviations and share loads between generators. This knowledge is fundamental for designing and operating stable power systems.

Governor Types and Applications

Main Types of Governors

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• Mechanical-hydraulic governors rely on mechanical and hydraulic components to control the speed of the turbine and are commonly used in hydroelectric power plants and small to medium-sized thermal power plants
• Electro-hydraulic governors combine electronic control systems with hydraulic actuators, offer improved response times and accuracy compared to purely mechanical governors, and are used in larger thermal power plants
• Electronic governors, also known as digital governors, utilize microprocessors and digital control algorithms, offer high precision, flexibility, and advanced control features, and are used in modern power plants of various types and sizes (gas turbines, combined cycle plants)

Applications of Governor Types

• Mechanical-hydraulic governors are suited for hydroelectric power plants due to their ability to handle the large inertia of water turbines and the need for precise control of water flow through the wicket gates
• Electro-hydraulic governors are employed in larger thermal power plants (coal-fired, nuclear) where the increased accuracy and faster response times provided by the electronic control components are beneficial for maintaining stable operation
• Electronic governors are widely used in modern power plants, including gas turbines and combined cycle plants, as they offer advanced control capabilities, such as PID control, overspeed protection, and integration with plant control systems (distributed control systems, SCADA)

Governor Control Principles

Frequency Regulation and Load Matching

• Governors maintain the frequency of a power system within acceptable limits (typically ±0.5 Hz) by controlling the speed of the prime mover (turbine) in response to load changes
• The primary function of a governor is to adjust the mechanical power input to the generator to match the electrical power output, ensuring that the system frequency remains at the nominal value (50 Hz or 60 Hz)
• Governors continuously monitor the system frequency and compare it with the reference frequency, triggering corrective actions to adjust the prime mover's speed when deviations occur

Negative Feedback Control and Droop Characteristic

• Governors operate based on the principle of negative feedback control, where the system frequency is compared with the reference frequency, and any deviation results in a corrective action by the governor to adjust the prime mover's speed
• The governor's control action is proportional to the frequency deviation, with larger deviations resulting in more significant adjustments of the prime mover's speed, helping to stabilize the system frequency
• Governors have a droop characteristic that allows multiple generators to share the load proportionally, with the droop setting determining the percentage change in frequency that causes a 100% change in the generator's output (e.g., a 5% droop means a 5% change in frequency results in a 100% change in generator output)

Mechanical-Hydraulic Governors

Key Components

• Speed-sensing device (flyball mechanism) detects changes in the turbine's speed, with the flyballs moving outward due to centrifugal force as the speed increases, causing a displacement in the linkage system
• Hydraulic amplifier converts the small mechanical motion of the pilot valve into a larger hydraulic force, which is applied to the servomotor
• Pilot valve controls the flow of oil in the hydraulic amplifier based on the displacement of the linkage system caused by the speed-sensing device
• Servomotor actuates the control valves of the prime mover (steam admission valves in a steam turbine or wicket gates in a hydro turbine) in response to the amplified hydraulic force from the hydraulic amplifier
• Linkage system connects the servomotor to the prime mover's control valves and provides a mechanical feedback from the servomotor to the pilot valve, ensuring stable operation and preventing overshooting or hunting of the governor's response

Operation Principles

• The speed-sensing device detects changes in the turbine's speed, causing a displacement in the linkage system that affects the position of the pilot valve
• The pilot valve controls the flow of oil in the hydraulic amplifier, which converts the small mechanical motion into a larger hydraulic force applied to the servomotor
• The servomotor actuates the control valves of the prime mover, regulating the flow of the working fluid (steam or water) to control its speed
• The linkage system provides a mechanical feedback from the servomotor to the pilot valve, ensuring stable operation and preventing overshooting or hunting of the governor's response

Electronic vs Mechanical Governors

Advantages of Electronic Governors

• Higher accuracy in maintaining system frequency due to the use of digital control techniques and advanced control algorithms (PID control)
• Faster response times to load changes and frequency deviations compared to mechanical governors, improving system stability
• Greater flexibility in control algorithms and settings, allowing for customization and optimization of governor performance based on specific plant requirements
• Incorporation of additional control functions, such as overspeed protection, load limiters, and automatic generation control (AGC), enhancing overall system stability and reliability
• Reduced maintenance requirements and increased system lifespan due to the absence of mechanical wear and tear

Limitations of Electronic Governors

• Higher initial costs compared to mechanical governors due to the use of advanced electronic components and control software
• Increased complexity in design, implementation, and troubleshooting, requiring specialized knowledge and skills for maintenance and repair
• Potential vulnerability to electromagnetic interference (EMI) and cybersecurity threats, necessitating proper shielding, grounding, and security measures to ensure reliable operation
• Dependence on the quality and reliability of electronic components and control software, requiring thorough design, testing, and maintenance to prevent failures and ensure long-term performance