Types of excitation system

Types of Excitation Systems for Generators: A Comprehensive Guide

In my previous post, I covered the basics of excitation systems for generators, including their operation and purpose. To build on that foundation, this post delves into the types of excitation systems commonly used in generators, providing a more detailed understanding of their design, functionality, and applications.

This guide is intended to offer basic knowledge about excitation systems. For detailed specifications and advanced guidance, it’s always best to consult your local generator supplier or manufacturer.

What Are the Types of Excitation Systems?

Generators rely on excitation systems to provide the field current required to produce electricity. The type of excitation system chosen depends on the generator’s size, application, and operational requirements. Below are the most common types of excitation systems:

1. AC Excitation System

• Overview:
  • Operates using an AC supply with frequencies ranging from 50 Hz to 250 Hz.
  • The AC output is rectified using diodes to produce the DC current needed for the rotor field winding.
  • This system eliminates the need for a commutator by using a solid-state rectifier.
• How It Works:
  • The AC exciter generates an output, which is fed through a rectifier bridge.
  • The rectified output is then delivered to the generator rotor via slip rings.
  • A pilot exciter (often a permanent magnet generator) supplies power to the main exciter.
• Key Features:
  • The rectifier uses diodes arranged in a bridge formation, with multiple diodes in parallel for increased capacity.
  • Each diode is individually fused to ensure protection in case of a fault, isolating the faulty diode without affecting the system.
• Applications:
  • Commonly used in medium to large generators where robust and reliable operation is required.
• Advantages:
  • Reliable and straightforward design.
  • Suitable for a wide range of applications.

2. Brushless Excitation System

• Overview:
  • An advanced version of the AC excitation system, designed to eliminate the need for slip rings and brushes.
  • Uses shaft-mounted rectifier diodes for a more compact and maintenance-free solution.
• How It Works:
  • The main exciter has its armature winding on the rotor and its field winding on the stator.
  • The output of the exciter rotor (rated between 100 Hz and 250 Hz) is rectified by shaft-mounted diodes.
  • The rectified current is directly fed to the generator rotor.
  • A permanent magnet pilot exciter is often used, operating at frequencies up to 400 Hz, to power the exciter field.
• Key Features:
  • Fully brushless design reduces wear and maintenance.
  • Compact and efficient with fewer moving parts.
• Applications:
  • Widely used in modern generators for industrial and utility applications, where reliability is paramount.
• Advantages:
  • Eliminates wear and tear associated with brushes and slip rings.
  • Reduces maintenance and downtime.

3. Thyristor Excitation System

• Overview:
  • Uses a thyristor rectifier to directly control the generator’s field current, providing precise and fast response to system demands.
  • Incorporates slip rings and brushes to connect the field current to the rotor.
• How It Works:
  • The thyristor rectifier controls the field current by adjusting the exciter field current.
  • Excitation power is supplied either from the generator terminals (via a step-down transformer) or directly through the main and pilot exciters.
  • Transformer ratios are designed to provide adequate output even during faults or voltage dips.
• Key Features:
  • Highly responsive system capable of handling critical and fault-sensitive applications.
  • Advanced control for real-time adjustments to maintain system stability.
• Applications:
  • Suitable for generators in critical infrastructure requiring fast response times and high stability.
  • Not commonly adopted globally due to complexity and higher costs.
• Advantages:
  • Exceptional response time, ideal for fault scenarios.
  • Provides precise control over field current and voltage stability.
• Disadvantages:
  • Complex design, requiring sophisticated control signals and equipment.
  • Less commercially available and challenging to maintain compared to other systems.

Comparison of Excitation Systems

Advancements in Excitation Technology

1. Digital Control Systems:
   • Modern excitation systems incorporate digital AVR controllers, offering advanced monitoring, diagnostics, and fault detection.
2. IoT Integration:
   • IoT-enabled exciters allow remote monitoring and predictive maintenance for reduced downtime.
3. Hybrid Excitation Systems:
   • Combine brushless and thyristor technologies for enhanced performance in demanding applications.

Conclusion

Excitation systems are the heart of generator operation, supplying the field current necessary for efficient and stable power generation. By understanding the types of excitation systems—AC, brushless, and thyristor—you can make informed decisions when designing or maintaining generator systems.

For standard applications, AC or brushless systems are reliable and cost-effective options. However, for critical applications requiring fast response and advanced control, a thyristor excitation system may be the best choice despite its complexity.

Always consult with generator manufacturers or suppliers for detailed specifications and recommendations tailored to your specific needs.