Generator Excitaion & AVR

8/12/2019 Generator Excitaion & AVR

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3 November 2011 PMI Revision 00 2

Presentation outline

Types of excitation system

Static Excitation system

Brushless Excitation System

Components of excitation system

AVR


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What is Excitation system?

Creating and strengthening the magnetic field of

the generator by passing DC through the filed

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

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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The Equipment for supply, control and monitoring of thisDC supply is called the Exci tation 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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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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Providing variable DC current

with short time overload

capability

Controlling terminal voltage with

suitable accuracy

Ensure stable operation with

network and / or other machines

Contribution to transient stability

subsequent to a fault.

Communicate with the powerplant control system

Keep machine within permissible

operating range

AVR

Duties of an Excitation System
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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 systemCooling / 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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Brush Type Rotating Exciter Generator


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Static excitation system

Excitation power from generator via excitation transformer.

Protective relays for excitation transformer

Field forcing provided through 415 v aux supply

Converter divided in to no of parallel (typically4 ) paths. Eachone having separate pulse output stage and air flowmonitoring.

Two channels : Auto & manual, provision for change over

from Auto to Manual

Limiters : Stator current limiter, Rotor current limiter, Loadangle limiter etc.

Alternate supply for testing


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Static excitation system

voltage regulator

GT

EXC TRFR

18KV/700V

1500KVA

THYRISOR

BRIDGE

GENERATOR

FIELD

From TGMCC- C

415/40V,10KVA

Pre Excitation

Non linearresistor

Field Breaker

Field dischargeResistor

Crow Bar


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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

PILOTEXCITER

MAINEXCITER

GENERATOR

FIELD BREAKER

FIELD

(PM)

ARMATURE

ROTATINGDIODES

R

Y

B


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Components of Brush less

Excitation System

Three Phase Main Exciter.

Three Phase Pilot Exciter.

Regulation cubicleRectifier Wheels

Exciter Coolers

Metering and supervisory equipment.


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AVR

BRUSHLESS EXCITATION SYSTEM

(500 MW)

21 KV


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Brushless Excitation SystemEliminates 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 monitoringsystem

alarm 80 K, Trip 5 K

Stroboscopefor 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

Excitation

Voltage at

full load

Ceiling

Volts

200/ 210 2600 310 610

500 6300 600 1000


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PMG


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DIFFERENCES BETWEEN BRUSHLESS AND STATIC

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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EXCITER COOLINGVAPOUR EXHAUST

COOLER


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XG

EF VT

GENERATOR

Equivalent cir cuit of Generator

I

EF= I . XG+ VT


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GENERATOR

VT

IL

IL.Xd

Ef

Phasor diagram of the Generator


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GVbusVT

XTXd

Ef

GENERATOR

Generator + Generator Transformer Eq. Ckt.

G

GTGCB

GENERATOR


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Vbus

VT

EF

IL

Vector Diagram of Generator and GT

connected to an inf ini te bus

GENERATOR

IL .XT

IL.Xd

GENERATOR


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I n the equivalent Circuit and Phasor diagram, the notations used have

the following description:

Vbus : I nf ini te bus voltage

VT : Generator Terminal Voltage

EF : I nduced Voltage (behind synchronous

Impedance) of Generator, proportional

to excitation.

Xd : Direct axi s sync. Reactance assumed

same as quadrature axis sync.

Reactance

XT : Transformer reactance

IL : Load Curr ent

: Phase angle

: Torque Angle (rotor/load angle)

GENERATOR

GENERATOR


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Referr ing to the phasor diagram on slide no.14;Sin / IL.{Xd+XT} = Sin (90+ ) / EF

Putting Xd+XT=X,and multiplying both sides by VIL ,

V Sin /X = VILCos / EF

{Sin (90+ ) = Cos}

or,

(EF. V / X) Sin = VILCos = P

Pmax = EF. V / X

Note that the Electri cal Power Output varies as the Sin of Load angle

GENERATOR

POWER ANGLE EQUATION


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Torque angle diagram

0

0.2

0.4

0.6

0.8

1

1.2

0 30 60 90 120 150 180

Angle in degrees

Sind

elta

Torque angle diagram

0

0.2

0.40.6

0.8

11.2

030 60 90

120

150

180

Angle in degrees

Power

inpu


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ROTOR

STATOR

Rotor

mag.

axis

Stator

mag.

axis

N

S

S

N

red

yellow

blue

Physical

significance

of load angle


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O Vbus

EF1

EF2P1

P2

Locus of

Constant

ExcitationI2

I1

1

212

Excitation constant;

Steam flow increasedPower output P1to P2

ACTIVE POWER CHANGE


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O Vbus

EF1

EF2

Locus of P = const.

Locus ofConstant

ExcitationI2

I1

1

212

Steam Flow constant;

Excitation increasedPower output Constant

I Cos = Constant

EXCITATION CHANGE

E it ti C t l


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Excitation Control

Power Angle Diagrams for Different

Excitation Levels

0

0.2

0.4

0.60.8

1

1.2

1.4

0 30 60 90 120 150 180

Power Angle (delta), in degrees

Powerin

per

unit

P1

P2

P3


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AVR


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TYPES OF AVR SYSTEMS

Single channel AVR system

Dual channel AVR system

Twin channel AVR system


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Single channel AVR system

Here we have two controllers one is automatic and the other ismanual and both the controllers are fed from the same supply

The AVR senses the circuit parameters through current

transformers and voltage transformers and initiates the controlaction by initiating control pulses , which are amplified and sent

to the circuit components

The gate controller is used to vary the firing angle in order

to control the field current for excitation

In case of any fault in the automatic voltage regulator the control

can be switched on to the manual controller.


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Dual channel AVR system

Here also we have two controllers in the same manner as theprevious case i.e. one automatic voltage controller and one manual

controller

But here in contrary to the previous case we have different powersupply, gate control and pulse amplifier units for each of the

controllers

Reliability is more in this case than previous one since a fault in

either gate control unit or pulse amplifier or power supply in singlechannel AVR will cause failure of whole unit, but in dual channel

AVR this can be avoided by switching to another channel.


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This system almost resembles the dual channel AVR but the onlydifference is that here we have two automatic voltage regulators

instead of one automatic voltage regulator and one manual Voltage

regulator

This system has an edge over the previous one in the fact that in case

of failure in the AVR of the Dual voltage regulator the manual system

is switched on and it should be adjusted manually for the required

change in the system and if the fault in AVR is not rectified in

reasonable time it will be tedious to adjust the manual voltageregulator


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In Twin channel AVR both the AVRs sense the circuit parameters

separately and switching to other regulator incase of fault is much

easier and hence the system is more flexible than the other types.

Generally switching to manual regulator is only exceptional cases

like faulty operation of AVR or commissioning and maintenance

work and hence we can easily manage with one AVR and one

manual regulator than two AVRs. So Twin channel AVR is only

used in very few cases and generally Dual channel AVR is

preferred.



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AVR

The feedback of voltage and current output of the generator

is fed to avr where it is compared with the set point

generator volts se from the control room

There are two independent control systems

1. Auto control

2. Manual control

The control is effected on the 3 phase output of the pilot

exciter and provides a variable d.c. input to the main exciter


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AVR

The main components of the voltage Regulator are two closed

loop control systems each followed by separate gate control unit

and thyristor set and de excitation equipment

Control system 1 for automatic generator voltage control

(AUTO) comprises the following


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AVR

Excitation current regulator, controlling the field current of

the main exciter

Circuits for automatic excitation build-up during start up

and field suppression during shut-down

Generator voltage control

The output quantity of this control is the set point for a following.


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AVRThis equipment acts on to the output of the generator voltage,control, limiting the set point for the above excitation current

regulator. The stationary value of this limitation determines the

maximum possible excitation current set-point (field forcing

limitation);

Limiter for the under-excited range (under excitation limiter),

Delayed limiter for the overexcited range (over excitation limiter)


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AVRIn the under excitation range, the under

excitation ensures that the minimum excitation

required for stable parallel operation of thegenerator with the system is available and that

the under -excited reactive power is limited

accordingly


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AVRThe set-point adjuster of the excitation current

regulator for manual is tracked automatically (follow-

up control) so that, in the event of faults, change over

to the manual control system is possible without delay

Automatic change over is initiated by some special

fault condition. Correct operation of the follow-up

control circuit is monitored and can be observed on amatching instrument in the control room. This

instrument can also be used for manual matching.


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AVRFAULT INDICATIONS

The following alarms are issued from the voltage

regulator to the control room.

AVR fault

AVR automatic change over to MANUAL

AVR loss of voltage alarm


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AVR

There are 3 limiters

1.Under excitation limiter

2.Over excitation limiter

3. V/F limiter

The current feedback is utilized for active and

reactive power compensation and for limiters


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Ceiling Voltage It is the max. voltage that cab be impressed on the

field under specified conditions.

Ceiling voltageultimately determine how fast the

field current can be changed. For normal disturbances, ceiling condition prevails

for a 10 secs max. to either increase or decrease

the excitation untill the system returns to steady

state operating state.


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Ceiling Voltage Response: It is defined as the rate of increase (decrease) of the

excitation system out put voltage seen from the excitation voltage

time response curve.

The starting point for evaluating the rate of change shall be the initial

rated value.

Response ratiois the numerical value which is obtained whenthe excitation system response in volt/sec measured over first 0.5

sec. This is applied only for increasing excitation.


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Ceiling Voltage

o e

da

bed- ratedvoltage

eb-ceiling

voltageOe- 0.5 secs.

Capabilit C r e


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Capability CurveCapability 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 LimitStraight 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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LIMITERS

Over excitation limiter

Under excitation limiter

Rotor angle limiter

Stator current limiter V/F limiter


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Limiters OVER EXCITATION LIMITER (Rotor Current Limiter OR Field forcing

limiter)

It avoids thermal overloading of the rotor winding It limits the field current so that rotor temperature

does not cross the limit

Rotor current can go high due to low system voltage

and close in faults For low system voltage long time field forcing of

lesser degree is required

For close in faults very high degree of field forcing

for a short period is required to prevent collapse ofgrid voltage

The locus of the over-excitation limiter is a circle

having radius of maximum rotor current


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Limiters STATOR CURRENT LIMITER (SCL)

Avoids thermal overloading of the stator winding

It protects the generator against long duration of highstator currents

For excessive inductive current SCL acts over AVRafter a certain time lag and decreases the excitationcurrent to limit the inductive current to the limit value.

But for excessive capacitive current, SCL acts on theAVR without time delay to increase the excitation andthere by reduces the capacitive loading (There is arisk for the m/c falling out of step in the under excited

mode of operation) the locus of the stator current limiter is a circle having

radius of MVA or stator current


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LimitersUNDER EXCITATION LIMITER/Rotor Angle limiter

Operation of generator in under-excitation condition:

Flux density is low so coupling force stator androtor is low

Machine is operating in higher load angle, so

capability for absorbing disturbance is less

So with slight disturbance machine may go tounstable zone


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V/HZ limiter Under low frequency conditions, saturation oftransformers, PTs and unintended tripping due to

over voltages may occur if excitation is maintainedat rated frequency condition.

The circuit senses frequency and reduces thereference value when the frequency falls belowthe cut off value.

By this reduction in excitation the terminal voltage

is reduced.

Field failure protection


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Field failure protection

Loss of generator field excitation under normal running

conditions may arise due to any of the following condition.1. Failure of brush gear.

2.unintentional opening of the field circuit breaker.

3. Failure of AVR.

When generator on load loses its excitation , it starts tooperate as an induction generator, running above

synchronous speed.cylindrical rotor generators are not

suited to such operation , because they don't have damper

windings able to carry the induced currents, consequentlythis type of rotor will overheat rather quickly.


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THANK YOU

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Generator Excitaion & AVR