Excitation Ppf p

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8 February 2012 PMI Revision 00 2

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


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



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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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