generator excitation

4 August 2011 PMI Revision 00 1

Generator

Excitation System

&

AVR


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


What is Excitation system?

• Creating and strengthening the magnetic field of

the generator by passing DC through the filed

winding.


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’


STATOR

ROTOR

EXCITATION PRINCIPLE


STATOR


ROTOR N

S


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 Voltage

F = NP / 120

Magnitude of flux decides generated voltage and

speed of rotation decides frequency of

generated voltage



0 180

360

90

270


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


EXCITATION SYSTEM

REQUIREMENT

• Regulate terminal voltage of the machine

•Meet excitation power requirements under all normal operating conditions

•Enable maximum utilisation of machine capability

•Guards the machine against inadvertent tripping during transients

•Improve dynamic & transient stability thereby increasing availability


EXCITATION SYSTEM

REQUIREMENT

• Reliability

• Sensitivity and fast response

• Stability

• Ability to meet abnormal conditions

• Monitoring and annunciation of parameters

• User friendliness


TYPES OF EXCITATION

EXCITATION SYSTEM

ROTATING SYSTEM

STATIC SYSTEM

Conventional Rotating machines

High frequency excitation

Brushless Excitation

System


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,


AVR

AUTO

MAN

FDR

FF

415 v AC

STATIC EXCITATION SYSTEM ( 200 MW)

F B 15.7

5 k

V

575 v


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


Static excitation system

voltage regulator

GT

EXC TRFR

18KV/700V

1500KVA

THYRISOR

BRIDGE

GENERATOR

FIELD

From TGMCC- C

415/40V,10KVA

Pre Excitation

Non linear resistor

Field Breaker

Field discharge Resistor

Crow Bar


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


BRUSH GEAR


Brushless excitation

PILOT EXCITER

MAIN EXCITER

GENERATOR

FIELD BREAKER

FIELD

(PM)

ARMATURE

ROTATING DIODES

R

Y

B


Components of Brush less

Excitation System

•Three Phase Main Exciter.

•Three Phase Pilot Exciter.

•Regulation cubicle

•Rectifier Wheels

•Exciter Coolers

•Metering and supervisory equipment.


AVR

BRUSHLESS EXCITATION SYSTEM (500 MW)

21 KV


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


• 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


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


PMG


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 Excitation Brushless Excitation Description S.NO


MAIN EXCITER


EXCITER ROTOR


EXCITER COOLING

VAPOUR EXHAUST

COOLER


XG

EF VT

GENERATOR

Equivalent circuit of Generator

I

EF = I . XG + VT


GENERATOR

VT

IL

IL.Xd

Ef

Phasor diagram of the Generator

ф


G Vbus VT

XT Xd

Ef

GENERATOR

Generator + Generator Transformer Eq. Ckt.

G

GT GCB


Vbus

VT

EF

IL

ф

Vector Diagram of Generator and GT

connected to an infinite bus

GENERATOR

IL.XT

IL.Xd


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


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

GENERATOR

POWER ANGLE EQUATION


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

del

ta

Torque angle diagram

0

0.2

0.4

0.6

0.8

1

1.2

0 30 60 90120

150180

Angle in degrees

Po

we

r in

pu


ROTOR

STATOR

δ

Rotor

mag.

axis

Stator

mag.

axis

N

S

S

N

red

yellow

blue

Physical

significance

of load angle


O Vbus

EF1

EF2 P1

P2

Locus of

Constant

Excitation I2

I1

ф1

ф2 1

2

•Excitation constant;

•Steam flow increased

•Power output P1 to P2

ACTIVE POWER CHANGE


O Vbus

EF1

EF2

Locus of P = const.

Locus of

Constant

Excitation I2

I1

ф1

ф2 1

2

•Steam Flow constant;

•Excitation increased

•Power output Constant

I Cos ф = Constant

EXCITATION CHANGE


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 180Power Angle (delta), in degrees

Po

wer

in p

er

un

it

P1

P2

P3


AVR


TYPES OF AVR SYSTEMS

• Single channel AVR system

• Dual channel AVR system

• Twin channel AVR system


Single channel AVR system

Here 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.


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.


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


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.


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


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


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.


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)


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


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.


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


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


Excitation Interlocks

5s delay

Excitation ON command

N>90%

Protection Off

FCB Off feedback

External trip

GCB is OFF

Excitation ON

Preconditions for Excitation ON


Excitation OFF Interlocks

Delay 1sec

Exc. OFF from Field flashing

Exc OFF command

GCB OFF

N>90%

External trip

Exc OFF

GCB OFF


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


LIMITERS

• Over excitation limiter

• Under excitation limiter

• Rotor angle limiter

• Stator current limiter

• V/F limiter


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


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


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


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


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


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


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.


THANK YOU

generator excitation

Documents

automatic voltage regulator

excitation current regulator

dual channel avr

manual voltage regulator

brushless excitation system

voltage regulator

field current

reactive power