Excitation systems are crucial for controlling generator voltage and reactive power output. They consist of components like exciters, automatic voltage regulators, and power system stabilizers, working together to maintain stable voltage and improve system performance.

Different types of excitation systems, including DC, AC, and digital, offer varying advantages in terms of response time, maintenance, and control flexibility. Understanding these systems is key to optimizing power plant operation and ensuring grid stability.

Excitation System Components and Functions

Key Components and Their Roles

• The exciter supplies direct current (DC) to the field winding of the synchronous generator
  • Creates the magnetic field necessary for voltage generation in the generator
• The automatic voltage regulator (AVR) maintains the desired generator terminal voltage
  • Compares the generator terminal voltage to a reference value
  • Adjusts the exciter output accordingly to regulate voltage
• The power system stabilizer (PSS) enhances power system stability
  • Provides an additional input signal to the AVR
  • Dampens power system oscillations and improves overall stability
• Current and potential transformers provide feedback signals for voltage regulation
  • Current transformers measure the generator output current
  • Potential transformers measure the generator terminal voltage
  • Feedback signals are sent to the AVR for precise voltage control

Protection and Auxiliary Components

• The excitation transformer adapts voltage levels for the exciter input
  • Steps down the generator terminal voltage
  • Provides a suitable voltage level for the exciter to operate
• Limiters and protective devices ensure safe operation of the excitation system
  • Over-excitation limiters prevent excessive field current and overheating
  • Under-excitation limiters prevent loss of synchronism and stability issues
  • Protective devices safeguard the excitation system components from damage

Excitation System Types

DC and AC Excitation Systems

• DC excitation systems utilize a DC generator (exciter) for field current supply
  • Simple and reliable design, but slower response times
  • Require more maintenance due to the presence of brushes and commutators
• AC excitation systems employ an AC generator (exciter) and a rectifier
  • Provide faster response times compared to DC systems
  • Require less maintenance as they eliminate the need for brushes
  • Brushless AC systems mount the exciter and rectifier on the main generator shaft (eliminates slip rings and brushes)
  • Static AC systems use a separate excitation transformer and solid-state rectifiers (offers even faster response and improved reliability)

Digital Excitation Systems

• Digital excitation systems incorporate microprocessor-based controllers and power electronics
  • Provide precise control and monitoring of the excitation system
  • Offer advanced features such as adaptive control and self-tuning capabilities
  • Enable sophisticated protection, diagnostics, and communication functions
  • Require specialized knowledge for maintenance and troubleshooting

Excitation System Role in Voltage and Reactive Power Control

Voltage Regulation

• The excitation system regulates the generator terminal voltage
  • Controls the field current and magnetic field strength of the generator
  • AVR continuously monitors the generator voltage and adjusts excitation accordingly
  • Maintains the desired voltage setpoint despite changes in load or system conditions
• Excitation system responds to voltage fluctuations caused by various factors
  • Faults, load changes, or other disturbances in the power system
  • Helps maintain stability and prevent voltage collapse

Reactive Power Control

• By adjusting the excitation level, the generator controls its reactive power output
  • Reactive power is essential for maintaining voltage stability in the power system
• Overexcited operation (increased field current)
  • Generator supplies reactive power to the grid, helping to boost system voltage
• Underexcited operation (decreased field current)
  • Generator absorbs reactive power from the grid, helping to lower system voltage
• Excitation system enables the generator to support system voltage and stability

Excitation System Configurations: Advantages vs Disadvantages

DC Excitation Systems

• Advantages:
  • Simple design and reliable operation
  • Low initial cost compared to other excitation system types
• Disadvantages:
  • Slower response times, limiting the speed of voltage regulation
  • Higher maintenance requirements due to the presence of brushes and commutators
  • Limited control flexibility and adaptability to changing system conditions

AC Excitation Systems (Brushless)

• Advantages:
  • Faster response times compared to DC systems, enabling quicker voltage regulation
  • Lower maintenance requirements due to the absence of brushes
  • Improved reliability as a result of reduced mechanical components
• Disadvantages:
  • Higher initial cost and complexity compared to DC systems
  • Potential for shaft torsional oscillations, which may affect generator stability

AC Excitation Systems (Static)

• Advantages:
  • Even faster response times than brushless AC systems, enhancing voltage control
  • Improved reliability due to the absence of rotating components
  • Greater control flexibility and adaptability to system conditions
• Disadvantages:
  • Higher initial cost and complexity compared to brushless AC systems
  • Potential for harmonic distortion in the excitation current, affecting power quality

Digital Excitation Systems

• Advantages:
  • Precise control and regulation of the excitation system
  • Advanced monitoring, diagnostics, and protection capabilities
  • Adaptive control algorithms and self-tuning features for optimal performance
  • Integration with plant-wide control and communication systems
• Disadvantages:
  • Highest initial cost and complexity among all excitation system types
  • Requires specialized knowledge for maintenance, troubleshooting, and upgrades
  • Potential for cybersecurity vulnerabilities due to the reliance on digital components