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Generator

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 induced Voltage

E = K. L. dΦ/ dt

K = constant

L = length exposed to flux

dΦ/ dt = rate of change of flux

Frequency of induced VoltageF = NP / 120

Magnitude 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 this DC supply is called the Excitation system

G

Flux in the generator rotor is produced by feeding DC supply in the field coils, thus forming a 2 pole magnet of rotor

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TYPES OF EXCITATION

EXCITATION SYSTEM

ROTATING SYSTEM

STATIC SYSTEM

Conventional Rotating machines

High frequency excitation

Brushless Excitation System

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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 TYPICAL EXCITATION 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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AVRAUTO

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 for excitation 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 TRFR18KV/700V1500KVA

THYRISOR BRIDGE

GENERATOR

FIELD

From TGMCC- C

415/40V,10KVA

Pre Excitation

Non linear resistor

Field Breaker

Field discharge Resistor

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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BRUSH GEAR

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Brushless excitation

PILOT EXCITER

MAIN EXCITER

GENERATOR

FIELD BREAKER

FIELD

(PM)

ARMATURE

ROTATING DIODES

R

Y

B

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Components of Brush less Excitation System

•Three Phase Main Exciter.•Three Phase Pilot Exciter.•Regulation cubicle•Rectifier 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 KΏ, Trip 5 KΏ

• Stroboscope for thyristor fuse monitoring (one fuse for each pair of diodes, )

• Auto channel thyristor current monitor • For monitoring of thyristor bridge current , and initiating

change over to manual.

• ‘Auto’ to ‘Manual’ changeover in case of Auto channel power supply, thyristor set problem, or generator volts actual 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 order of 40 ms. due to the direct control and solid state devices employed.

Slower than static type since control is indirect (on the field of main exciter) and magnetic components involved.

Response of the excitation system.

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 supply from output of the generator

Brushless system gets activated with pilot exciter, main exciter 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

ф

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GVbusVT

XTXd

Ef

GENERATOR

Generator + Generator Transformer Eq. Ckt.

G

GTGCB

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Vbus

VT

EF

IL

ф

Vector Diagram of Generator and GT connected to an infinite bus

GENERATOR

IL.XT

IL.Xd

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In the equivalent Circuit and Phasor diagram, the notations used have the following description:

Vbus : Infinite bus voltage

VT : Generator Terminal Voltage

EF : Induced Voltage (behind synchronous Impedance) of Generator, proportional to excitation.

Xd : Direct axis sync. Reactance assumed same as quadrature axis sync. Reactance

XT : Transformer reactance

IL : Load Current

Ф : Phase angle

: Torque Angle (rotor/load angle)

GENERATOR

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Referring to the phasor diagram on slide no.14;

Sin / IL.{Xd+XT} = Sin (90+ Ф) / EF

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

V Sin /X = VIL Cos Ф / EF

{Sin (90+ Ф) = Cos Ф}

or,

(EF . V / X) Sin = VIL Cos Ф = P

Pmax = EF . V / X

Note that the Electrical Power Output varies as the Sin of Load angle

GENERATORPOWER 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

Sin

de

lta

Torque angle diagram

0

0.2

0.4

0.6

0.8

1

1.2

Angle in degrees

Po

we

r in

pu

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ROTOR

STATOR

δ

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 Excitation

I2

I1

ф1

ф212

•Excitation constant;

•Steam flow increased

•Power output P1 to P2

ACTIVE POWER CHANGE

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O Vbus

EF1

EF2

Locus of P = const.

Locus of Constant Excitation

I2

I1

ф1

ф212

•Steam Flow constant;

•Excitation increased

•Power output Constant

I Cos ф = Constant

EXCITATION CHANGE

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Excitation Control

Power Angle Diagrams for Different Excitation Levels

00.20.40.60.8

11.21.4

0 30 60 90 120 150 180Power Angle (delta), in degrees

Pow

er in

per

un

it

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 is manual 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 control action 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 the previous case i.e. one automatic voltage controller and one manual controller

But here in contrary to the previous case we have different power supply, 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 single channel 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 system

This 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 voltage regulator

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Twin channel AVR system

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 systems1. Auto control2. 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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AVR

This 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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AVR

In the under excitation range, the under excitation ensures that the minimum excitation required for stable parallel operation of the generator with the system is available and that the under -excited reactive power is limited accordingly

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AVR

The 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 a matching instrument in the control room. This instrument can also be used for manual matching.

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AVR

FAULT 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 Preconditions for Excitation ONON

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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

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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 safelyLagging 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

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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 can result

• OEL is automatic limitation of generator excitation by lowering the generator voltage (otherwise the set point of generator voltage is reduced in time or the transformation ratio of the GT is 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 in response to short drops in line voltage

• When IF >110% of Ifn , the OEL and Field forcing limiter are active

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Under Excitation limiter

• Function is to correct the reactive power when the excitation current falls below minimum excitation current value required for stable operation of generator

• Activation of UEL takes over the control from the closed loop voltage control, acting via a max selection

• The limit characteristic is adjustable (shifted parallel)• I reactive ref is compared with the measured I reactive , the

error is fed to P- amplifier. When the value drops below the characteristic the amplified diff signal causes the field current to increase

• For commissioning purpose provision is made to mirror the characteristic in the inductive range, this allowing both the direction in which the control signal acts and the blocking of the set point generators is to be changed

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Rotor Angle Limiter

• Stable operation rotor angle <900, for higher degree of stability a further margin of 10-12% is normally provided

• RAL gives the o/p as

permissible I reactive =F ( I active)

• Characteristic is shifted linearly as a function of generator voltage

• Permissible I reactive is compared with the measured value and is fed to the limit controller when the I reactive achieved value drops below the permissible value then the limiter comes in action and I reactive

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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 rated value 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 such states 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- to reduce the Q till the stator current is brought down below the rated value

• Change in generator voltage set point is not blocked when SCL active

• SCL does not operate near the unity PF because near this value 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 operating even 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

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PRIORITY STRUCTURE OF AVR

Voltage regulator

UN-2010

3 rd priority

Stator current limiter

Capacitive

UN0027

Load angle limiter

UN1043

2 nd priority

Stator current li miter inductive

UN0027

Rotor current limiter

UN1024

1st priority

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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 it’s excitation , it starts to operate 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, consequently this type of rotor will overheat rather quickly.

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