Generator excitation system

Excitation:The process of generating a magnetic field by means of an electric current is called

excitation.

Purpose of generator excitation system:

1.The main purpose of excitation system for generator is to supply the power source to the rotor

field coil.2.The excitation system is used to control the excitation of the rotating field in the armature.

TYPES OF THE EXCITATION SYSTEM

1.Rotating2.Static

3.Brushless

Basic generator control loop

Rotating type

Within the family of rotating exciters, there are numerous types that

can be found operating on all types and sizes of generators. The basic kinds of rotating

exciters are motor or shaft driven and separately self-excited or bus-fed systems.

The shaft-driven rotating excitation system has been the most widely used excita-

tion source in the past. It is used for small and large turbine generators. The basic con-

figuration is a stationary armature and a rotating field. The ac output of the alternator is

rectified by stationary diodes physically located off the generator and fed to the main

rotor slip rings as dc current. Regulation of the current output is achieved by phase

control of thyristors in the alternator field power circuit. Figure 3.16

shows the typical basic circuit for a shaft-driven excitation system.

Static typeAn external source of power (often the generator

isolated phase bus) isused to supply ac power to an excitation

transformer. The transformer output is fed toa three-phase controlled rectifier bridge for

conversion to dc. The re-quired generator field voltage is obtained by properly

controlling the thyristor firing asin the rotating exciter. The standard control generally

consists of an ac (auto) controlmode for regulating generator terminal voltage, and

a dc (manual) control mode forregulating exciter field voltage (see Fig. 3.18).

1. Main equipments:

a. Rectifier transformer.b. Thyristor rectifier banks.

c. Excitation startup and field discharge equipment.d. Regulation and operation control circuit.

2.Advantages:a. Fast Response

b. More efficient and low maintenance

Rectify:to convert ac to dc.

fire : A transition from non-conduction to conduc- tion in an ionizing switching device.

Firing:The act or process of adding fuel and air to a furnace.

AVR

AVR means Automatic Voltage Regulator.

It is usually used inthe excitation of a generator.

AVR regulate the excitationvoltage of the generator according to the demand.

This device ismainly used for keeping the terminal volage of

generator atconstant level irrespective of load.

Brushless excitation system

The brushless excitation system is most widely used for gas turbine

units with air-cooled generators, but also can be found in steam generators of every

rating. It consists of a high-frequency ac generator with a rotating fused diode as-

sembly, a static voltage regulator for excitation control, and a transformer power supply. The exciter

is generally attached as an extension of the generator shaft.

Main parts

1.Pilot exciter2.main exciter

3.Rectifier wheel4.AVR

The three phase pilot exciter has a revolving field with permanent magnet poles.The armature winding

is housed on the stator. The three phase a.c. generated by the pilot exciter is rectified and

controlled by automatic voltage regulator to provide variable D.C. for exciting the main exciter. The three

phase main exciter has stationary field with revolving armature. Thus three phase a.c. power is

produced in main exciter which is rectified by rotating rectifier bridge and is fed to the field winding

of the rotor (turbo generator) through dc leads.

Brushless type