Types of Excitation system and how is it useful for power system

What is Excitation system

It is used to create and strengthen the magnetic field of the generator and it is done by passing a DC through the field winding. for excitation purpose, AC and DC exciters are used but the DC exciters can’t be used for large ratings because of the design problems of the commutator and the brush gear. Due to this problem, the DC exciters are replaced by AC exciters in case of generator

Why Excitation system required

By using excitation system it can provide stability of the power system large alternators are used in the power system. The excitation process can manage the voltage and the reactive power in the grid and thus it could ensure stability

Purpose of Excitation System

  • It basic function is to provide DC to the synchronous machine field winding
  • It can regulate the terminal voltage of the machine
  • Regulation of the load angle
  • Protection of the generator can be assured
  • Regulation of MVAR and power factor
  • It can utilize maximum machine capability
  • It can guard the machine against inadvertent tripping during transients
  • It can control the reactive power flow and it can do the sharing of the reactive load between the machines which operates parallel in the grid
  • It can improve dynamic and transient stability and by that, it can increase the availability

Required Elements for an Excitation system

  • Exciter
  • Regulator
  • Power system stabilizer
  • Limiters and protective circuit
  • Terminal voltage transducer and load compensator


Types of Excitation system

  • DC excitation system
  • High-frequency Excitation system
  • Static Excitation system
  • Brushless Excitation system

DC excitation system

In this system exciter is DC commutator generator, exciter can be driven by the direct coupling or through the reduction gear. This type of excitation can be used up to 100/110 MW. At first, a small rated output pilot exciter supplies excitation for the main exciter after that the main exciter supplies excitation to the main generator rotor, the output of the exciter is regulated by rheostat which is controlled by the AVR

High-frequency excitation system

It mainly consists of an AC exciter driven by generator shaft, static rectifier bridge assembly, permanent magnet generator, generator field circuit breaker and an automatic voltage regulator it is an AC exciter, it is an “inductive type alternator”

The field windings and the alternator are in the stator, it usually operates in high frequency and thus the machine size could be reduced, armature terminal is connected to the static rectifier assembly. Magnetizing and demagnetizing of the field winding is done by the AVR

Static Excitation system

It consists of a rectifier transformer, thyristor bridge cubicle, field breaker, field flashing circuit and an AVR

It has quick response time its supply is tapped from the generator terminals and the brushes are used to supply field currents. The supply of the DC voltage is done through thyristor-bridge. During the starting process, the starting field is supplied from an external source called field flashing


  • Fast response
  • Efficient and low maintenance
  • Periodic maintenance of the electric machine is eliminated

Brushless excitation system

It consists of an AC exciter, rotating rectifier ring, permanent magnet generator, and AVR

It has a 3-phase permanent magnet pilot exciter and its output voltage is rectified and controlled by the thyristor voltage regulator to provide DC current for the main exciter the rotating rectifier bridge is mounted on the rotor and there is no need for the slip rings and the output of the rectifier is directly connected to the field winding

How to choose an Excitation system

It mainly depends upon the complexity of the grid, loading pattern and the nature of the pollution level in the power plant and the equipment cost

Static Excitation can be used if

  • The power is transmitted through long power lines
  • If the loading pattern is peaking and cyclic

Brushless Excitation can be used if

  • If the system demands less maintenance
  • If the environment is polluted by the sulfur vapors and acidic fumes