What is an Excitation System – Basic Definition

The excitation system is responsible for supplying the necessary field current to the rotor winding of the synchronous machine. It can be defined as the system that generates the flux by passing current through the field winding. An excitation system should possess reliability under all service conditions, simple control, easy maintenance, stability, and fast transient response.

There are three main types of excitation systems:

  1. DC Excitation System: The DC excitation system comprises two exciters: the main exciter and a pilot exciter. An automatic voltage regulator (AVR) adjusts the exciter output to control the terminal voltage of the alternator. The current transformer connected to the AVR limits the alternator current during a fault. When the field breaker is open, a field discharge resistor is connected across the field winding to dissipate the stored energy in the highly inductive field winding.
  2. AC Excitation System: The AC excitation system consists of an alternator and a thyristor rectifier bridge directly connected to the main alternator shaft. The main exciter can be self-excited or separately excited. The AC excitation system can be broadly classified into the following two categories:
  3. Static Excitation System: In this system, the supply is obtained from the alternator itself through a 3-phase star/delta connected step-down transformer. The primary of the transformer is connected to the alternator bus, while the secondary supplies power to the rectifier. It also feeds power to the grid control circuit and other electrical equipment.

Each type of excitation system serves a specific purpose and is utilized based on the requirements of the synchronous machine. Understanding the characteristics and functionalities of these systems is crucial for ensuring optimal performance and operation.

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