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Presentation outline Understanding basic principle
Types of excitation
Components of excitation system
Brief Description of most commonly used Excitation
systems in power generating plants:
Static Excitation system
Brushless Excitation System
Conclusion
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What is Excitation system?
Creating and strengthening the magnetic field of
the generator by passing DC through the field
winding.
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Why Excitation system?
With large alternators in the power system,
excitation plays a vital role in the management of
voltage profile and reactive power in the grid thus
ensuring Stability
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STATOR
ROTOR
EXCITATION PRINCIPLE
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STATOR
EXCITATION PRINCIPLE
ROTORN
S
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Stator induced Voltage
E = K. L. d/ dtK = constant
L = length exposed to flux
d
/ dt = rate of change of flux
Frequency of induced Voltage
F = NP / 120Magnitude of flux decides generated voltage and
speed of rotation decides frequency of
generated voltage
EXCITATION PRINCIPLE
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0 180
360
90
270
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The Equipment for supply, control and monitoring of thisDC supply is called the Excitation system
G
Flux in the generator rotoris produced by feedingDC supply in the fieldcoils, thus forming a 2
pole magnet of rotor
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TYPES OF EXCITATION
EXCITATIONSYSTEM
ROTATING
SYSTEMSTATIC
SYSTEM
ConventionalRotatingmachines
Highfrequencyexcitation
BrushlessExcitationSystem
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Functions of an ExcitationSystem To build up and maintain constant terminal
voltage i.e. regulate Vt with necessary accuracy
(90% to 110%).
Furnish d.c. with short time overload capacity Contribute to transient stability subsequent to a
fault
Communicate with power plant control system
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Functions of an ExcitationSystem (contd.) To supply reactive power to the system.
Other sources of reactive power are:
Synchronous condenser.
Series / shunt compensation.
To keep the generator parameters within the
Capability characteristics.
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EXCITATION SYSTEM
REQUIREMENT
Reliability
Sensitivity and fast response
Stability
Ability to meet abnormal conditions
Monitoring and annunciation of parameters
User friendliness
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COMPONENTS OF TYPICALEXCITATION SYSTEM
Input and output interface , Aux. power supply, FB
AVR: At least two independent channels
Follow up control and changeover
Excitation build up and Field Discharging system
Cooling / heat dissipation components
Limiters
Protective relays
Testing , Monitoring and alarm / trip initiation
Specific requirements :
Field Flashing, Stroboscope, PSS,
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AVR
AUTO
MAN
FDR
FF
415 v AC
STATIC EXCITATION SYSTEM ( 200 MW)
F B15.7
5kV
575 v
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Static excitation system
Excitation power from generator via excitation transformer. Protective relays forexcitation transformer
Field forcing provided through 415 v aux supply
Converter divided in to no of parallel (typically4 ) paths. Each one having separate
pulse output stage and air flow monitoring.
Two channels : Auto & manual, provision for change over from Auto to Manual
Limiters : Stator current limiter, Rotor current limiter, Load angle limiter etc.
Alternate supply for testing
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Field flashing For start up DC excitation is fed to the field from external source like
station battery or rectified AC from station Ac supply .
Filed flashing is used to build up voltage up to 30 %.
From 30 to 70 % both flashing and regulation remains in circuit.
70 % above flashing gets cut-off
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BRUSHGEAR
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Brushless excitation
PILOT EXCITER MAIN
EXCITER
GENERATOR
FIELD BREAKER
FIELD
(PM)
ARMATURE
ROTATINGDIODES
R
Y
B
For 500 MW
400 Hz, 16 Pole, 65 KW For 500 MW
150 Hz, 6 Pole, 3780 KW, 600 V
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Components of Brush lessExcitation SystemThree Phase Main Exciter.
Three Phase Pilot Exciter.
Regulation cubicleRectifier Wheels
Exciter Coolers
Metering and supervisory equipment.
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Brushless Excitation System
Eliminates Slip Rings, Brushgear and all problems associated with
transfer of current via sliding contacts
Simple, Reliable and increasingly popular system the world over,
Ideally suited for large sets
Minimum operating and maintenance cost
Self generating excitation unaffected by system fault/disturbances
because of shaft mounted pilot exciter
Multi contact electrical connections between exciter and
generator field
Stroboscope for fuse failure detection
Rotor Earth fault monitoring system
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Rotor E/F monitoring system
alarm 80 K, Trip 5 K
Stroboscope for thyristor fuse monitoring
(one fuse for each pair of diodes, )
Auto channel thyristor current monitor
For monitoring of thyristor bridge current , andinitiating change over to manual.
Auto to Manual changeover in case of Auto channelpower supply, thyristor set problem, or generator voltsactual value problem
Brushless Excitation system
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Excitation Power Requirement
Unit
capacity
MW
Excitation
Current at
Full Load
Excitatio
n Voltage
at full
load
Ceilin
g Volts
Ceiling
Curren
t
200/ 210
2600 (No
Load: 900
A)
310 (No
load: 100
V) 610 3900 A
500 6300 600 1000
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PMG
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DIFFERENCES BETWEEN BRUSHLESS ANDSTATIC EXCITATION SYSTEMS
More since slip rings and
brushes are required. Also
over hang vibrations are
very high resulting in faster
wear and tear.
Less since slip rings and brushes
are avoided.
Maintenance.5
No additional bearing and
increase in shaft length are
required.
One additional bearing and an
increase in the shaft length
are required.
Requirement of additional
bearing and increase of
turbo generator shaft
length.
4
Very fast response in the orderof 40 ms. due to the direct
control and solid state
devices employed.
Slower than static type sincecontrol is indirect (on the
field of main exciter) and
magnetic components
involved.
Response of the excitationsystem.
3
Field flashing supply required
for excitation build up.
No external source requirement
since pilot exciter has
permanent magnet field.
Dependency on external
supply.
2
Static excitation system uses
thyristors & taking supplyfrom output of the
generator
Brushless system gets activated
with pilot exciter, mainexciter and rotating diodes.
Type of system.1
Static ExcitationBrushless ExcitationDescriptionS.NO
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MAIN EXCITER
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EXCITER ROTOR
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Capability Curve Capability Curve relates to the limits in which a generator can
Operate safely. Boundaries of the Curve within with the machine will operate
safely
Lagging Power Factor/Overexcited region
Top Section Relates to Field Heating in Rotor Winding
Right Section Relates to Stator current Limit Straight line relates to Prime Mover Output
Leading Power Factor/ Underexicted region
Lower Side relates to Stator end ring Limit
Further down relates to Pole slipping
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THANK YOU
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