Excitation and Avrm

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20 July 2013 PMI Revision 00 1

GeneratorExcitation System&AVR


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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

AVR

Experience sharing

Conclusion


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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

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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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

5

kV

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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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 lessExcitation SystemThree 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 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

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 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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EXCITER COOLINGVAPOUR EXHAUST

COOLER


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XG

EF VT

GENERATOR

Equivalent circuit 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 descri ption:

Vbus : I nf in ite 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 axi s 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

PuttingXd+XT=X, and multiplying both sides byVIL ,

V Sin /X = VIL Cos / EF{Sin(90+ ) = Cos}

or,

(EF . V / X) Sin = VIL Cos = PPmax = EF . V / X

Note that the Electri cal Power Output var ies 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

EF2 P1

P2

Locus of

Constant

ExcitationI2

I1

1

212

Excitation constant;

Steam flow increasedPower output P1 to 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

Excitation Control


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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 systemHere 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 systemHere also we have two controllers in the same manner as the

previous 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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Twin channel AVR systemThis system almost resembles the dual channel AVR but the only

difference 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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Twin channel AVR systemIn 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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Excitation Interlocks

5s delay

Excitation ON command

N>90%

Protection Off

FCB Off feedback

External trip

GCB is OFF

ExcitationON

Preconditions for Excitation ON


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Excitation OFF Interlocks

Delay 1sec

Exc. OFF from Field flashing

Exc OFF command

GCB OFF

N>90%

External trip

Exc OFF

GCB OFF

C bilit C


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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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LIMITERS

Over excitation limiter

Under excitation limiter

Rotor angle limiter

Stator current limiter V/F limiter

O it ti li it


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Over excitation limiter

Line voltage drops due to more reactive power requirement ,

switching operations or faults AVR increases generator excitation to hold the voltage

constant

Line voltage drops , thermal over loading of generator canresult

OEL is automatic limitation of generator excitation by loweringthe generator voltage (otherwise the set point of generatorvoltage is reduced in time or the transformation ratio of the GTis to be adjusted )

OEL permits excitation values above the normal excitation and

extended to max excitation (for field forcing) for a limited time,so as to permit the generator to perform the grid stabilization inresponse to short drops in line voltage

When IF >110% of Ifn , the OEL and Field forcing limiter areactive

U d E it ti li it


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Under Excitation limiter

Function is to correct the reactive power when the excitation

current falls below minimum excitation current value requiredfor stable operation of generator

Activation of UEL takes over the control from the closed loopvoltage control, acting via a max selection

The limit characteristic is adjustable (shifted parallel)

I reactive ref is compared with the measured I reactive , theerror is fed to P- amplifier. When the value drops below thecharacteristic the amplified diff signal causes the field current toincrease

For commissioning purpose provision is made to mirror the

characteristic in the inductive range, this allowing both thedirection in which the control signal acts and the blocking of theset point generators is to be changed


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Rotor Angle Limiter

Stable operation rotor angle


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Stator current limiter During operation at high active power P and / low voltage the

stator current of the generator tends to rise beyond its ratedvalue and can cause the thermal overloading of stator, in spite

of the action of the UEL An additional stator current limiting controller acting on the

generator excitation is provided as a safe guard against suchstates of operation

SCL always monitors the stator current measured value for

crossing the rated stator current SCL permits small time over load but comes in action thereafter

and influences the effective generator voltage set point- toreduce the Q till the stator current is brought down below therated value

Change in generator voltage set point is not blocked when SCLactive

SCL does not operate near the unity PF because near thisvalue any limiter would cause oscillations


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V/F limiter Also known as over fluxing limiter

It is the protection function for the GT V/F ratio , eddy current , the local eddy current causes

thermal over loading of GT

In DVR mode V/F ratio is continuously monitors the limit

violation In case V/F ratio crosses the limit characteristic, the upper limit

as the effective AVR set point is reduced as a function of V/F

ratio

This limiter is used when it is required to keep the unit operatingeven in case of substantial frequency drops , for instance in

order to prevent complete breakdown of the system, a V/F

limiter is used to lower the voltage proportional with frequency

drop

PRIORITY STRUCTURE OF AVR


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Voltage regulatorUN-2010

3 rd priority

Stator current limiter

CapacitiveUN0027

Load angle limiter

UN1043

2 nd priority

Stator current li miter

inductive

UN0027

Rotor current limiter

UN1024

1st priority

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 thefollowing 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 abovesynchronous speed.cylindrical rotor generators arenot suited to such operation , because they don't have

damper windings able to carry the induced currents,consequently this type of rotor will overheat ratherquickly.


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Excitation and Avrm

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