8. Static Excitation System -Stage-2

8/19/2019 8. Static Excitation System -Stage-2

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STATIC EXCITATION SYSTEM

EXCITATION SYSTEMINTRODUCTION:

All synchronous machines excepting certain machines like permanentmagnet generators require a DC supply to excite their feld winding. As

synchronous machine is a constant speedy machine or a constant requency

supply, the output voltage o the machine depends on the excitation current.

The control o excitation current or maintaining constant voltage at

generator output terminals started with control through a feld rheostat, the

supply eing otained rom DC !xciter. The modern trend in interconnected

operation o power system or the purpose o reliaility and in increasing unitsi"e o generator or the purpose o economy has een mainly responsile

or the evolution o new excitation schemes.

#ormer practice, to have an excitation us ed y a numer o exciters

operating in parallel and supplying power to the felds o all the alternators in

the station, is now osolete. The present practice is unit exciter scheme, i.e.

each alternator to have its own exciter. $owever in some plants reserve us

exciter%standy exciter also provided in case o ailure o unit exciter ig.'(.

!xciter should e capale o supplying necessary excitation or

alternator in a reasonale period during normal and anormal conditions, so

that alternator will e in synchronism with the grid.

THE RATING OF THE EXCITER

)nder normal conditions, exciter rating will e in the order o *.+ to

*.- o generator rating &approx(. t/s rating also expressed in '*to '0 amps.

&approx.( per 12 at normal load. )nder feld orcing conditions exciter rating

will e ' to '.0-&approx( o the generator rating. Typical exciter rating or

various capacity o generators are as given elow3

EXCITER RATING DIFFERENT CAPACITYS OF GENERATORSINSTALLED IN INDIA (UPTO 210 MW)

T)456 7!8!4AT64

'1ax. Continuousrating &12(

9'* ''* '** * 0*

9 4ated :ower #actor *.;0 *.; *.;0 *.;=( '0.


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? 4ated Current &Amps( @*0*


3/38

T4A#6

?G!87T$ 6#1AC$8!

B$64T 1!D)1 1!D)1 G687!4

0C68T46G4!B:68B!

=!4F #ABT BG62!4 BG62!4 BG62!4

:46T!CT68B!G!CT=TF 766D 766D 766D 766D

<4!B:68B!4AT6B!G!CT68

#G!5G! G1T!D G1T!D G1T!D

;C61:68!8T4!I)4871A8T!8A8C!

BG:487BG:487 E

C611)TAT64BG: 487 868!

@#ABT D!H!CTAT68

F!B F!B F!B 86

INTRODUCTION TO STATIC EXCITATION EQUIPMENT, ITSSALIENT FEATURES AND COMPARISON WITH OTHERSYSTEMS:

At present various types o excitation systems, such as, conventional

DC, $igh requency AC, Btatic E 5rushless are eing adopted in ndia and

aroad.

The conventional DC !xciter was the unHchallenged source o 7enerator

!xcitation or nearly fty years till the rating o turoHgenerator reached

around '**12.n the last three to our decades, alternative arrangements

have een widely adopted ecause o limitations o the DC exciters. 2ith

increase in generator ratings, it is no longer enough to consider the exciter

used as earlier. nstead, the perormance o the whole excitation system

including the automatic voltage regulator and the response o the main

generator have to e considered. Techno economic considerations, grid

requirements, reliaility and mainte?nance have ecome prime

consideration.TYPES OF EXCITATION SYSTEMS (TYPICAL)1CONVENTIONAL DC EXCITER:

The earliest AC turine generators otain their excitation rom the

power station direct current distriution system. !ach machine had a

rheostat in series with its feld winding to adJustment o the terminal voltage

and reactive load. This method was suitale or machines, which needed

small feld power and low internal reactance. As generator si"es increased

excitation power requirements also increased and it ecame increasingly

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desirale or each unit to e sel suKcient or excitation and thus the shat

driven DC exciter was introduced.

2 AC (HIGH FREQUENCY) EXCITATION SYSTEM:

This system was developed to avoid Commutator and 5rush 7ear

assemly. n this system, a shat driven AC pilot exciter, which has a rotating

permanent magnetic feld and a stationary armature, eeds the DC feld

current o the main high requency AC exciter through controlled rectifers.

The high requency output o the stationary armature is rectifed y

stationary diodes and ed via slip rings to the feld o the main TuroH

7enerator. A response ratio o aout two can e achieved.

! BRUSHLESS SYSTEM:

Bupply o high current y means o slip rings involves considerale

operational prolem and requires suitale design o slip rings and rush gear.

n rushless excitation system diode rectifers are mounted on

generator shat and their output is directly connected to the feld o the

alternator, thus eliminating rushes and slip rings. This arrangement

necessitates the use o a rotating armature and stationary feld systems or

the main AC exciter. The voltage regulator fnal stage takes the orm o a

Thyristor ridge controlling the feld o main AC exciter, which is ed rom

:17 on the same shat. The response ratio o rushless excitation system is

normally aout two.

" STATIC EXCITATION SYSTEM:

n order to maintain system staility in interconnected system network

it is necessary to have ast acting excitation system or large synchronous

machines, which means the feld current must e adJusted extremely ast to

the changing operational conditions. 5esides maintaining the feld current

and steady state staility the excitation system is required to extend the

staility limits. t is ecause o these reasons the static excitation system is

preerred to conventional excitation system.

n this system, the AC power is trapped o rom the generator terminal

stepped down and rectifed y ully controlled Thyristor 5ridges and then ed

to the generator feld therey controlling the generator voltage output. A

high control speed is achieved y using an internal ree control and powerelectronic system. Any deviation in the generator terminal voltage is sensed

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y an error detector and causes the voltage regulator to advance or retard

the fring angle o the thyristors therey controlling the feld excitation o the

generator.

n #ig.9 B.86.&?( shows a lock diagram or a static excitation system.

Btatic !xcitation Bystem can e designed without any diKculty to achieve

high response ratio, which is required y this system.

This equipment controls the generator terminal voltage, and hence the

reactive load Low y adJusting the excitation current. The rotating exciter is

dispensed with and

Transormer E Bilicon controlled rectifers &BC4s( are used which directly eed

the feld o the Alternator.

DESCRIPTION OF STATIC EXCITATION SYSTEM#

Btatic !xcitation !quipment Consist o

'. 4ectifer Transormer.

9. BC4 output stage.

+. !xcitation start up E feld discharge equipment.

?. 4egulator and operational control circuits.

n the aove ',9,+ are power circuits o Btatic !xcitation Bystem E ? is

control circuit o Btatic !xcitation Bystem.

RECTIFIER TRANSFORMER:

The excitation power is taken rom generator output and ed through

the excitation &rectifer( transormer, which steps down to the required

voltage, or the BC4 5ridge and then ed through the feld reaker to the

generator feld. The rectifer transormer used in the B!! should have high

reliaility, as ailure o this will cause shutdown o unit%power station.

Dry type cast coil transormer is suitale or Btatic !xcitationapplications. The transormer is selected such that it supplies rated

excitation current at rated voltage continuously and is capale o supplying

ceiling current at the ceiling excitation or a short period o ten seconds.

SCR OUTPUT STAGE:

The BC4 output stage consists o a suitale numer o ridges

connected parallel. !ach Thyristor 5ridge comprises o six Thyristors,

working as a six pulses controlled ridge. Current carrying capacity o eachridge depends on the rating o individual Thyristor. Thyristors are designed

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such that their Junction temperature raise is well within its specifed rating.

5y changing the fring angle o the thyristor variale output is otained. !ach

ridge is controlled y one fnal pulse stage and controlled y a an.

These ridges are equipped with protection device and ailure o one

ridge causes alarm. there is a ailure o our Thyristor ridges &totally (

then the current will e limited to a predetermined value lesser than the

normal current. $owever, ailure o the fth ridge results in tripping and

rapid deHexcitation o the generator. The aove is applicale or ridges

thyristor with &nH'( principal operation.

Total numer o ridges M

&nH?( i.e., ? ridges ailN it will e reduced to predetermined value.

&nH0( i.e., 0 ridges ailN it will trip the system.

EXCITATION START UP AND FIELD DISCHARGE EQUIPMENT:

#or the initial uildHup o the generator voltage, feldHLashing

equipment is required. The rating o this equipment depends on the noHload

excitation requirement and feld time constant o the generator. #rom the

reliaility point o view, provisions or oth the AC feld Lashing are provided.

The feld reaker is selected such that it carries the ull load excitation

current continuously and also it reaks the max. #ield current when the three

phases short circuit occurs at the generator terminals.

The feld discharge resistor is normally o nonHlinear type or medium

and large capacity machines i.e. voltage dependent resistor.

To protect the feld winding o the generator against over voltage, an

over voltage protection along with a current limiting resistor is used to limit

the over voltage across the feld winding. The 6=: operates on the insulation

reak over principle. The voltage level at which 6=: should operate isselected ased on insulation level o feld winding o the generator.

REGULATOR $ OPERATIONAL CONTROL CIRCUITS (CONTROL

ELECTRONICS):

4egulator is the heart o the system. This regulates the generator

voltage y controlling the fring pulses to the Thyristor.

A)ERROR DETECTION $ AMPLIFIER

The generator terminal voltage is stepped down y a three phase :Tand ed to the A=4. The AC input is thus otained is rectifed, fltered and

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compared against a high staili"ed reerence value and the dierence is

amplifed d in dierent stage o amplifcation. The A=4 is designed with high

stale elements so that the variation in the amient temperature does not

cause any drit or change in the output level. Three CTs sensing the output

current o the generator eed proportional current across variale resistor in

the A=4. The voltage thus otained across the resistors, can e added

tectonically either or compensation can e adJusted as the resistors are o

variale type.

B)GRID % CONTROL UNIT

The output o the A=4 is ed to a grid control unit, it gets its

synchronous A.C. reerence through a flter circuit and generates six doule

pulses spaced *O electrical apart whose position depends on the output o

the A=4, i.e. the pulse position varies continuously as a unction o the

control voltage. Two relays are provided, y energi"ing, which, the pulses can

e either locked completely or shited to inverter mode o operation.

C)PULSE&AMPLIFIER

The pulse output o the P7rid Control )nitQ is amplifed urther at

intermediate stage amplifcation. This is also known as pulse intermediate

stage. The unit has a D.C. power supply, and a coarse staili"ed voltage =G.

A uilt in relay is provided which can e used or locking the six pulse

channels. n a two channel system &like Auto and 1anual(, the changeover is

aected y energi"ing%deHenergi"ing the relay.

D) PULSE FINAL STAGE

This unit receives input pulses rom the pulse amplifer and transmits

them through the pulse transormers to the gates o the thyristors. A uilt in

power supply provides the required D.C. supply to the fnal pulse and

amplifer. !ach thyristor ridge has its own fnal pulse stage. Thereore, even

i a thyristor ridge ails with its fnal pulse stage, the remaining thyristors

ridges can continue to cater to ull load requirement o the machine and

therey ensure &nH'( operation.

E)MANUAL CONTROL CHANNEL

A separate manual control channel is provided where the controlling

the D.C. signal is taken rom a staili"ed D.C. voltage through a manual

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reerence card. The D.C. signal is ed to a separate grid control unit whose

output pulses ater eing amplifed at an intermediate stage can e ed to

the fnal pulse stage. 2hen one channel is working, generating the required

pulses, the other remains locked. Thereore locking or releasing the pulses

o the corresponding intermediate stage aects a changeover rom PAutoQ to

P1anualQ control or vice versa. PA pulse supervision unit detects spurious

pulses or loss o pulses at the pulse us ar and transers control rom

Automatic Channel to manual channelQ.

F)FOLLOW & UP UNIT

To ensure a smooth changeover rom PAutoQ to P1anualQ control, it is

necessary that the position o the pulses on oth channels should e

identical. A pulse comparison unit detects any dierences in the position o

the pulses and with the help o a ollowHup unit actuates the motor operated

potentiometer on the P1anualQ Channel to turn in a direction so as to

eliminate the dierence.

$owever, while transerring control rom P1anualQ to PAutoQ mode any

dierence in the two control levels can e visually checked on a alance

meter and adJusted to otain null eore change over.

G)LIMIT CONTROLLERS

2hen a generator is running in parallel with the power network, it is

essential to maintain it in synchronism without exceeding the rating o the

machine and also without the protection system tripping. 6nly automatic

regulator cannot ensure this. t is necessary to inLuence the voltage

regulator y suitale means to limit the over excitation and under excitation.

This not only improves the security o the parallel operation ut also makes

operation o the system easier. $owever limiters do not replace theprotection system ut only prevent the protection system rom tripping and

unnecessarily under extreme transient conditions.

The A=4 also has a uiltHin requency dependent circuit so that when

the machine is running elow the rated requency rom the regulated voltage

should e proportional to requency. 2ith the help o a potentiometer

provided in the A=4, the circuit can e made to respond proportionally to

voltage aove a certain requency and proportional to a voltage elow the

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requency. The range o adJustment o this cut o requency lies etween ?*

and * $".

The static excitation system is equipped with three limiters, which act

in conJunction with the A=4. These limiters are as under3

• 4otor Current Gimiter

• 4otor Angle Gimiter

• Btator Current Gimiter

1 ROTOR CURRENT LIMITER

The unit asically comprises an actual value converter a limiter with

adJustale :D characteristics a reerence valueN dv%dt sensor and a

signali"ation unit. The feld current is measured on the A.C. input sign o the thyristor

converter and is converted into proportional D.C. voltages. The signal is

compared with an adJustale reerence value, amplifed, and with necessary

time lapse ed to the voltage regulator input.

4otor current limiter avoids thermal overloading o the rotor winding

and is provided to protect the generator rotor against excessively long

duration over loads. Thus ceiling excitation is limited to a predetermined limit

and is allowed to Low or a time, which is dependent upon the rate o raise o

feld o current o eore eing limited to the thermal limit value.

2 ROTOR ANGLE LIMITER

This unit is limits the angle etween the voltage o the network center

and the rotor voltage or it limits the angle etween the generator voltage

and the rotor voltage. t comprises an actual value converter, a limiting

amplifer which adJustale :D characteristic and a reerence value unit. The

limiting regulator operates as soon as the D.C. value exceeds the reerence

value. #or its operation the unit is given separate power supply rom a D.C.

power pack.

t generates a D.C. signal proportional to the load or rotor angle rom

the stator current and voltage y means o a simple analog circuit. The

device takes over as soon as the set limit angle is exceeded. ncreasing the

excitation and ignoring opposite control signals prevent the unit rom ailing

out o step.

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! STATOR CURRENT LIMITER

This unit unctions in conJunction with an integrator unit, which

provides the necessary dead time and the gradient that can e adJusted y

potentiometers. The regulator consists essentially o a measuring converter,

two comparators, two :D regulators and a D.C. power pack. A discriminator

in the circuit dierentiates etween inductive and capacitive current. The

positive and negative signals processed y two separate amplifers are

rought to the output stage and only that output which has to take care o

the limitations is made eective.

Btator current limiter avoids thermal overloading o the stator

windings. Btator current limiter is provided to protect the generator against

long duration o large stator currents. #or excessive inductive current it acts

over the A=4 ater a certain time lag and decreases the excitation current to

limit the inductive current to the limit value. 5ut or excessive capacitive

current it acts on the A=4 without time delay to increase the excitation and

there y reduce the capacitive loading. This is necessary as there is a risk or

the machine alling out o step during under excited mode o operation.

H) SLIP STABILI'ING UNIT

The slipHstaili"ing unit is used or the suppression o rotor oscillations

o the alternator to the additional inLuence o excitation. The slip as well as

acceleration signals needed or staili"ation are derived rom active power

delivered y the alternator. 5oth the signals, which are correspondingly

amplifed and summed up, inLuence the excitation o the synchronous

machine through A=4 in a manner as to suppress the rotor oscillations.

POWER SUPPLY

The voltage regulating equipments needs an A.C. supply +;*=, + :hase

power supply or its units that is delivered rom the secondary side o the

rectifer transormer through an auxiliary transormer. This voltage is reduced

to dierent levels required or the power packs y means o multi winding

transormers.

A separate transormer supplies the synchronous voltage + R +;* = or

the flter circuit o each channel and the voltage relay. During testing and

precommissioning activities when generator voltage is not availale, thestation auxiliary supply + phase, ?'0= can e temporarily connected through

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an auxiliary step down transormer or testing purpose with the help o a

regulator test%service switch.

The supply or the thyristor 5ridge an is taken rom an independent

transormer, which gets it, input supply rom the secondary o the excitation

transormer.

The control E protection relays need ?;= E 9?= DC which are delivered

rom the station attery y means o the Dc%Dc converters, which are

internally protected against overload.

PROTECTIONS

The ollowing protections are provided in the Btatic !xcitation

!quipment.

'. 4ectifer transormer over current instantaneous and delayed.

9. 4ectifer transormer over Temperature.

+. 4otor 6verHvoltage.

?. 4otor earth ault &in 74:(.

0. #use ailureHmonitoring circuit or thyristors.

. Goss o control voltage &?;= E 9?=(.


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There inormation particularly or limiting "ones o operations are

useul in setting the various limiters o Automatic =oltage 4egulator. 6ne

typical procedure or the construction o capaility diagram is given in

susequent paras%page. 6perational requirements o excitation system

essentially call or a ast response particularly $igh nitial 4esponse

!xcitation Bystem, $igh degree o 4eliaility and also suitale arrangement

or feld discharge.

RESPONSE

The astness o action o an !xcitation system is measured%expressed

y the term P4esponse 4atio o the !xcitation systemQ. The original defnition

o this y measuring the rise o exciter volts in frst *.0 second is well known

i.e. rate o rise o voltage%Bec. Btatic !xciter has very P$igh nitial 4esponseQ

as given in !!! BTDBH?9' and attains @0- o the ceiling voltage level within

*.' second or less. Thus it greatly helps or power system staility

consideration. Typical 4esponse time or static excitation !quipment is

Twenty 1illiHBeconds i.e., *.9 sec.

RELIABILITY

#or :ower Bystem application, 4eliaility is a very important criteria. To

ensure this, components are careully selected, lieral ratings wherever

required are used and redundancies uilt in. n Btatic !xcitation equipment

PnQ 8o. o Thyristor ridges are used, with &nH'( principle o operation i.e.

even with three o ridges out o operation, ull load requirement can met y

alance ridges in parallel. 2herever specifed%required y customers, 9R

'**- ridges are also given.

FIELD DISCHARGE

During load condition whenever the #ield reaker opens suddenly there

will e a surge voltage in the rotor, which will damage the rotor winding

insulation. To avoid this rotor winding is connected to the earth through feld

discharge 4esistor therey y passing the surge voltage to earth and limiting

the current to earth. #ield discharge greatly helps to limit the damages. 8onH

linear feld discharge resistance is used which helps in aster feld

suppression%discharge.

CAPABILITY DIAGRAM CONSTRUCTION

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Get us take an example o a '** 1w TuroH7enerator o *.;*p..

&8ominal( rating and having a BC4 o *.* Choosing suitale scales, 12

values are marked on FHaxis and 1=A4 values on Haxis. 4eer to #ig.? which

ahs een drawn on per unit asis and hence ases must e defned or

interpreting actual values. t is usual to defne the rated 1=A o the machine

as 5ase 1=A &i.e. 1=A( in which case rated 12 is *.; 1=A. n this case 1=A

M '90 and rated 12 M &*.; R '90( M '** 12. The other ase unit to defne

is the per unit excitation and this is usually taken as rotor A1:B to give rated

terminal voltage on openHcircuit on AirH7ap Gine. To otain actual values, the

p.u. #igures rom the capaility diagram must e multiplied y the ased

values Just given.

The various 12%1=A4 values and the excitation current &4otor Amps(

can e also e marked directly or the use o operators.

t should e noted that the diagram scaling is only correct or rated

machine terminal voltage and that all values must e appropriately adJusted

or dierent values o terminal voltage i.e., they must e multiplied y =9, so

that i the terminal voltage is say @*- o normal, then all scaling would have

to e multiplied y &*.@(9 M *.;', although excitation scaling would remain

the same.t is oviously undesirale to operate the machine up to theoretical

staility limits. 6perators have to e inormed through this diagram sae

limits or operation to allow or various unpredictale change such as sudden

power increase, a drit in 5usH5ar voltage due to lines or plant tripping etc. t

is usual to relate this saety actor to an increase in power demand with no

corresponding increase in excitation. The percentage o the power increase

used in this way defnes the shape and position o the P:ractical Btaility

Gimit GineQ.

4eerring ack to the example stated aove, let us assume that it is required

to have a '9.0 percent &or '.'90 p.u( power margin. This depends on the si"e

o the unit and operating practices. 6n Haxis mark point A such that 6A M

&1=A R BC4( i.e. in this case.

M &'90 R *.( M


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:ower intervals : equals to the required saety margin, in this case *.'90 p.u.

o rated power i.e., &*.; R *.'90( M *.'* p.u. o 1=A are marked on the

theoretical staility line SAB/ or the loads o *, *.9*, *.?*, *.* and *.;* p.u.

1=A i.e., at points e,d,c, and a. 2ith radii Aa, A. Ac, Ad and Ae arcs o

circles are drawn with A as centre to cut the *.;, *., *.?, *.9 and "ero power

lines. These intercepts are then Joined y a continuous curve # 5 7. This will

then e the P:ractical Btaility GineQ or a '9.0 - power margin.

The reasoning ehind this construction can e understood y taking

the case o SAa/ arc. This point ' &or 5( would e working point o the

machine at *.; p.u. 1=A power with an excitation o SAa/ Amps. Bince the

asis o the saety margin is that there should e provision or increase in

power without any cha ge in excitation, the working point ' would move

along arc o radius &fxed excitation( towards theoretical pullHout line, so that

it is Just suKcient to support *.@ 1=A i.e., '.'90 p.u. :ower &presuming

turine has the capaility( at a rotor angle o @*O. The same reasoning o

course applies to all other points such as 9,+,?,and 0 in the diagram.

8ext, with P6Q as centre draw a line 6! at an angle o Cos H' *.;* &+O(

&rated p.. angle( to the FHaxis to cut the rated 1w line &Turine limit line( at

!. 4ated 1=A is denoted y radius 6!. The line SA!/ represents the C14 excitation required. 2ith A as centre

and A! as radius, draw an arc o a circle !D representing excitation &or 4otor

heating( limit.

The diagram #5!D is the C*+-./ D3 o the machine.

USEFULNESS OF CAPABILITY DIAGRAM FOR EXCITATIONCONTROL SYSTEM:

As already mentioned, the inormation given y the capaility diagramregarding ull load rotor current &excitation( maximum rotor angle during

steady state leading p.. one operation etc., are essential or proper setting

o the various limiters in the excitation control system. n power system

operation, the importance and necessity o ast acting and reliale excitation

control system is well known. Capaility diagram gives the asic inormation

regarding the limiting ones o operation so that limiters can e

set%commissioned suitaly or sae operation o the units.

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PERFORMANCE AND CHARACTERISTICS OF STATICEXCITATION EQUIPMENT

The steady state and transient ehaviours o a synchronous machine

coupled to an infnite system must e matched to the desired operatingconditions y suitale selection o control unctions in the entire excitation

system.

The asic requirement o a closed loop excitation control system is to

hold the terminal voltage o a generator at a predetermined value

independent o the change has to contriute the ollowing unctions also.

a( 1aintenance o stale operation o a machine under steady state,

transient and dynamic conditions.

( Batisactory operation with other machines connected in parallel.

c( !ective utili"ation o machine capailities without exceeding machine

operating limits.

n order to understand the perormance o excitation system and to

achieve aoveHmentioned unctions, the ollowing parameters are necessary

to e studied.

CEILING VOLTAGE

t is the maximum voltage that can e impressed on the feld under

specifed conditions. Ceiling voltage ultimately determines how ast the feld

current can e changed. #or normal disturances, ceiling condition prevails

or a ew cycles &Ten seconds maximum( to either increase or decrease the

excitation until the system returns to steady operating state. #or Btatic

!xcitation, the ceiling voltage ranges rom '. to 9.* times the rated one,

which is considered to e adequate or a ast system response.

RESPONSE

4esponse is defned as the rate o increase &or decrease( o the

excitation system output voltage, which can e seen rom the excitation

voltage time response curve. The starting point or evaluating the rate o

change shall e the initial rated value. This is a rough measure o how ast

the exciter output circuit voltage will rise within a specifed time, when the

excitation control is adJusted in the maximum increasing direction. 4esponse

ratio is the numerical value, which is otained when the excitation system

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response in volts per sec. 1easured over frst *.0 sec. This applies only or

the increasing !xcitation. As the response is non linear the response ratio is

determined in terms o equivalent voltage time area or *.0 seconds as

shown in #ig.0. Area ad M Area acd, y approximation.

STEADY STATE ACCURACY

t is the degree o correspondence etween the controlled variale and

the ideal value under specifed steady state conditions. The accuracy o the

excitations system or changing the feld parameters to keep the generator

terminal voltage at a fxed level depends on its static gain and time

constants. 5y choosing a higher static gain or the system, the steady state

error can e minimi"ed apprecialy and therey improving the steady state

accuracy within U*.0*-. This can e reduced urther with proper integral

control.

OTHER SPECIFICATIONS

!xcitation system perormance could e Judged y the exciter voltage

=s time characteristic in response to a step change in the generated voltage

&Bee #ig.(.

The actors to e studied or optimum perormance are3

a. 6vershoot

. 4ise time

c. Bettling time

d. Damping ratio

#or ideal perormance, it should have one overshoot and one

undershoot with a quicker rise time to have a smaller steady state error.

Details o each o the parameters are not discussed here since the

requirement varies rom case to case.T RANSIENT AND DYNAMIC STABILITY LIMIT

The success o excitation control lies upon the extend o meeting the

requirement o capaility o the machine and there y giving the dynamic

perormance o the system. #ast excitation helps during disturances and

contriutes to the system staility y allowing the required transer o power

even during the disturances. Due to smaller time

constants in the excitation control loop, it is assumed that quick controleorts could e achieved through this.

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n transient staility the machine is suJected to a sever disturance

&during ault etc.( or a short time. This results in dip in the machine terminal

voltage and power transer. Taking one machine connected to infnite us,

the equation or power transer can e written as

: M =t R = Bin d

HHHHHHHHH

x

where =t M 1achine Terminal =oltage

= M nfnite 5us =oltage

x M nterconnected 4eactance

d M Goad Angle

#rom the aove equation i S=t/ is reduced S:/ is reduced y

corresponding amount. #or maintaining the power transer S:/ the excitation

should e ast acting enough to oost up the feld to ceiling and therey

holding the terminal voltage S =t/ at the desired value. Thus it is

advantageous to have higher speed and ceiling values in excitation control

circuitry. Bimilarly ater the ault is removed, the reactance Sx/ suddenly

changes therey causing unalanced condition due to power swings, which

in turn needs ast corrective action through excitation system to ring themachine to normal operating conditions.

1odern ast and high response excitation system helps in two ways y

reducing the severity o the machines frst swing during transient

disturances and also ensuring that the susequent swings are smaller than

the frst one. Thus it helps in increasing the transient staility limit. 2ith a

typical static excitation system, ceiling level can e achieved within 9*

milliseconds due to which it oers an improved transient staility limits.

#ollowing disturances, the group o machines operating in the same

control group experience smaller oscillations. 1oreover the oscillating control

group o machines reacts with each other reinorcing these oscillations. $ere

the change in excitation may not result in a stale operation &or slow acting

exciters( ecause y the time corrective action eing taken y the excitation

system &due to the inherent system delay( the oscillating system changes

causing separate excitation requirement to e met. Though aster excitation

system avoids this prolem to certain extent power system staili"ers as

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mentioned earlier are employed along with the automatic voltage regulators

to damp out the susequent smaller swings in the system. The staili"er gain

is adJusted to a value depending o the negative damping o the system and

other network parameters. :ower system damp out the susequent smaller

swings in the system. :ower Bystem staili"er helps to damp out inter area

oscillations explained aove and also machine system oscillations.

n addition to the aove, limiters are generally uilt into the excitation

system or large generators connected to the grid. This helps to extract

maximum operation output i.e., optimal utili"ation o the machine/s

capaility without Jeopardi"ing its staility. These limit controller act on oth

the lagging and leading side o the capaility diagram and set elow the

operating points o the protection relays. Thus they prevent unnecessary

tripling y keeping the system parameters well within the sae limits. The

limit controllers do not replace the unction o the protective relays. These

limiters enhance the staility o the machine, therey increasing its

availaility to the network. These cannot dispense with protection relays. The

staili"er gain is adJusted to a value depending on the negative.

EFFECT OF EXCITATION SYSTEM ON TRANSIENT STABILITY

Bince the transient staility prolems deal with the perormance o

power system when suJected to sudden disturances, sometimes leading to

loss o synchronism, it is worthwhile to study the ehavior during the frst

owing as the period is o very short duration. The maJor actors inLuencing

the outcome are the machine ehaviour and the power network dynamic

relations. #or this it is assumed that the mechanical power supplied y the

prime mover remains constant during the disturances. Thereore the eect

o the excitation control on this type o transient depends on its aility tohelp generator to maintain its output power in the aove period.

The main actors that aect the perormance during severe transients

are3

'( The disturance inLuence o impactN this includes the type o

disturances, its location and duration.

9( The aility o the transmission system to maintain synchroni"ing orce

during the transients.

+( Turine and generator parameters.

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These actors mainly aect the frst swing transient. The system

parameters inLuencing these actors are3

i( The synchronous machine parameters. 6 these, the most

important are3

a( The nertia constant

( The direct axis transient reactance

c( The direct axis open circuit time constant

d( The aility o the excitation system to hold the Lux level

o the synchronous machine and increase the output

during transients.

ii( The transmission system impedances under normal, aulted

and postHaulted conditions. $ere the Lexiility o the

switching aulted section is important such that the large

transer admittances etween synchronous machines are

maintained when ault is cleared.

iii( The protecting relaying scheme and equipment. The oJective

is to detect the ault and isolate the aulty section quickly with

minimum disruption.

During transients initiated y a ault, the armature reactionhas the tendency to reduce the Lux linkage. $ence the type o

excitation must e chosen as to have a ast speed o response

and high ceiling voltage &can e reerred to the static type( as

an aid to the transient staility. 2ith the help o aster ooster

up o the excitation, the internal Lux can e osetted and

consequently machine output power may e increased during

the frst swing. This results in reduction o accelerating power

and therey eects improvements o transient perormance o

the system.

THYRISTOR CHARACTERISTICS $ ITS APPLICATION INSTATIC EXCITATION SYSTEMINTRODUCTION

n the latest trend o excitation system neither the rheostatic mode o

the excitation control nor the magnetic amplifer type o control system is

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used as these are slugging in action and have an inherent dead and o the

operation ecause o their loe loop gains.

The use o BC4s at the power stage or the excitation system with

voltage regulator control the response o the system is much aster then the

conventional ones. The modern excitation system incorporating BC4s at their

power stages have a very low dead and.

SYSTEM DESCRIPTION

The excitation power eing ed rom the generator terminals or

auxiliary supplies through normally a stepdown transormer and then to the

input i the BC4s ridge. The voltage regulator having closed loop control

compares the actual terminal voltage o machine with that o the set

reerence value and orms an error signal, which controls the fring angle o

the Thyristor 5ridge. Busequently, the variale controlled DC voltage is

applied to the feld o the generator through a feld reaker. The BC4s 5ridge

orms an important integral part o the excitation system y providing an

accurate and ast feld DC voltage control.

THEORY OF DEVICE

The BC4 consists o our layers o : and 8 material and three Junctions

etween layers. This has got two locking states. 2hen the anode terminal is

iased positively with respect to the cathode, the Junctions V ' and V+ are

orward iased whereas V9 would e reverse iased. Bo that current Low is

locked and the BC4 is said to e in the orward locking state. Bimilarly,

with a negative voltage applied to the anode with respect to cathode,

Junction V' and V+ are reversed iased and Junction V9 is orward iased and the

device will not switch on. This state o the BC4 is called as reverse locking

state or high impedance state. The BC4 can e driven into conduction statewhen locking characteristic is erased and the BC4 continues to conduct

until the current level alls elow the certain lower value termed as holding

current o the BC4.

The BC4 e turned on y increasing the anode voltage suKciently to

exceed the reak over voltage, so that the reverse iased Vunction V9 reaks

down ecause o large voltage gradient across the depletion layers and the

orward current increases. 6nly the external resistance o the circuit limits it. The most convenient method o switching the BC4 is y applying a positive

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trigger pulse to the gate o the BC4 with lower positive anode voltage than

the reak down voltage. This is known as the gate control.

6nce the BC4 is 68, the orward current is to e maintained aove a

certain value known as latching current, so as to enale the BC4 to hold at

the conducting stage.

#or turning o a BC4, it is essential that the orward current though it

should e rought down elow the holding current value y reversing the

anode potential. #or using gate control methods to turn on the BC4 ollowing

conditions are to e ulflled or saer operation.

i( Appropriate anode to cathode voltage must e applied to ring the

device to the orward locking state.

ii( The gate signal must e removed once the device is turned 68. The

gate pulse duration is to e maintained in such a way that the gate

loss is less than that specifed or the device.

iii( 8o gate signal should e applied when the device is in the reverse

locking state.

iv( 2hen the device is in the o state, a negative voltage applied to the

gateHcathode Vunction will improve the reverse locking

characteristic o the device. Turn 68 time is dependent upon theload current and the rate i rise o gate pulse. Turn o time depends

on the recomination o charges near Junction V9. Bome typical

values o turn 68 and 6## times are ' to ? micro secs and '* to 90*

micro secs

v( 4espectively. #or power requency applications these turn 68 and

6## times does not pose any prolems.

SELECTION PROCEDURE OF SCR BRIDGES FOR STATICEXCITATION SYSTEM

The ollowing actors are taken into account,

'. :eak inverse voltage.

9. Vunction temperature.

+. dv%dt 4ating.

?. di%dt 4ating.

0. 7ate fring requirement.. Current rating.

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PARALLEL OPERATION

#or certain high current applications or or redundancy or the power

stage paralleling o the devices are required. #or such cases, ollowing points

must e careully oserved while designing the entire system.

'. #or paralleling, the connections, which are done y us ars and

cales etc., are to e kept symmetrical as ar as practicale.

9. Cooling or the devices are to e kept almost similar &i.e.( the

positions and type o mounting o the ridges and the cooling ans

are to e maintained identical.

+. 4C circuit should e so designed to keep the 4C discharge current

through the device within the specifed limit under all

circumstances. n addition to the aove, precautions are to e taken

to limit the rate o rise o 4C discharge current y providing

decoupling reactors in series with the device.

?. The aove series decoupling reactors with proper tolerances also

serve the purpose o reducing the miss haring actor or the parallelH

connected device. 2hile designing this, miss haring actor is to e

taken into account or the Junction temperature calculation.

SNUBBER CIRCUIT

The 4C 8etwork across the thyristor is known as snuer circuit. The

unction o snuer circuit is to limit the dv%dt with in maximum allowale

rating. The snuer could e polari"ed or unpolari"ed.

() P4-567

A orwardHpolari"ed snuer is suitale when a thyristor &or( transistor is

connected with an anti parallel diode. The resistor, 4 limits the orward dv%dt,

and 4' limits the discharge current o the capacitor when the device is turned

68.

() R68696&P4-567

A reverse polari"ed snuer, which limits the reverse dv%dt. 2here 4'

limits the discharge current o the capacitor. The capacitor does not

discharge through the device, resulting in reduced losses in the device.

() U*4-567

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2hen a pair o thyristors is connected in inverse parallel, the snuer

must e eective in either direction.

AVR % UN 2010

The Automatic voltage regulator type )8 9*'* is an electronic control

module specially designed or the voltage regulation o synchronous

machines. t primarily consists o an actual value converter, a control

amplifer with :D characteristics that compares the actual value with the set

reerence value and orms an output proportional to the dierence. The

output o this module controls the gate control circuit )8 '**'. The module

does not have an 85)GT power supply and devices its power rom )8 9**?,

the pulse intermediate stage and power supply unit. The A=4 works on W

'0= DC supply.

The main eatures o this module are listed elow3

a. The A=4 comprises o an input circuit which accepts + phases voltage

signals o ''*= AC and + phase current signals o 0A or 'A A.C. t is

thus necessary to use intermediate :T/s and CT/s to transorm the

generator voltage and current to the aove mentioned values. The

module itsel contains :T/s and CT/s with urther step down the signals

to make them compatile with electronic circuit. A C4C)T4F is

availale in the module or adding the current signals =!CT64AGF to

the voltage signals or providing compensation as a unction o active

or reactive power Lowing in the generator terminals.

. An actual value converting circuit or converting the AC input signal to

DC signal with minimum ripple with the aid o flter network.

c. A reerence value circuit using temperature compensated "ener

diodes. The output o which is taken to an external potentiometer thatprovide @*H''*- range o operation o the generator voltage.

d. A control amplifer, which comprises the reerence and actual value

and provides an output proportional to the deviation. Apart rom this, it

has the acility to accept other inputs or operation in conJunction with

various limiters and power system staili"er.

e. A voltage proportional to requency network, which reduces the

excitation current when requency alls elow the set level, thus

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keeping the air gap Lux constant. This prevents saturation o

connected transormers and possile over voltage.

LIMITERS IN STATIC EXCITATION SYSTEMLIMIT CONTROLLERS

2ith ever increasing si"e o generating units today, more stringent

requirements have to e met y excitation systems. Today, it is proven

eyond dout, that static excitation assures, stale operation under dynamic

and transient conditions, 7enerator running in parallel with the power

network even under extreme conditions must e remain in synchronism

without the maximum load limit on it eing exceeded and without the

protective relays operating. An automatic voltage regulator A=4 alone cannot

ensure this. 6ptimum utili"ation o the generator can e ensure only i the

asic A=4 is inLuenced y additional signals to limit the under excitation and

over excitation o the machine. Thus, limit controllers working in conJunction

with the A=4 ensure3

a( 6ptimum utili"ation o the machine.

( Becurity o parallel operation.

Gimit controllers simpliy the Jo o the operating sta and enales

stale operation close to the limiting values. 2ith limit controllers in service,operational errors and aults in the regulator lead only to the limit value

control and not to disconnection.

t has to e understood that limit controllers however are not mean to

replace the protection system ut they are only intended to prevent the

protection system rom operating under extreme transient conditions.

PARAMETERS FOR LIMITATIONS

Gimiters, whenever they intervene, inLuence the voltage regulatorsuitale or ring aout a corresponding change in the excitation. The

ollowing are the parameters which are to e limited3

'( Btator current under condition o over excitation and under excitation.

9( 4otor current.

+( 4otor angle or the Goad angle.

MECHANISM OF LIMITER INTERVENTION:

During overHexcitation, the rotor current and the stator current limiters

intervene to ring aout a reduction in excitation. 6n the other hand during

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under excitation the limitation o the rotor angle and stator current inLuence

to increase the excitation. 4otor and Btator current limiters must e designed

to intervene ater a certain deal so as to permit temporary over%ceiling

excitation, limiters do not impair the construction ehavior o the A=4 as

overHexcited condition can exist in the event o load surge or ecause o

short lived aults in the power supply network. The A=4 reacts to distance

ault &say + phase short circuit( and commands ceiling excitation to e

applied, therey increasing the synchroni"ing torque o the machine and

prevent rom the loss o synchronism. $owever, i the short circuit persists

and has not e cleared y system protection ater a set time, delayed rotor

current limiters comes into operation preventing the generator and the

excitation equipment rom eing suJected to thermal over load. An identical

situation prevails during such conditions o the system. The A=4 enale short

time ceiling excitation prevails so as to otain lower settling time.

The under excited mode, the rotor angle limiter and stator current

limiter must intervene instantaneously to increase the excitation to prevent

rom increment in the rotor angle.

n the under excited mode, stator current limiter is essentially used

multipleHpole synchronous condensers which run at suitale level o excitation increase the capacitive asorption capaility o the machine.

POWER DIAGRAM OF THE GENERATOR AND RANGE OFINFLUENCE OF LIMIT CONTROLLERS

The operational limits o the synchronous machine are shown in the

circle diagram. The application and range o inLuence o the limiters depends

on the conditions in the installation and the generator data. The possile

"one o intervention o the limiters is marked in the power chart%power circlediagram. #ig.<

ROTOR ANGLE LIMITER

Gine A5 represents the range o inLuence o the 4otor Angle, limiter the

maximum angle o which has een taken as ;0O. Although stale operation

can e ensured even eyond ;0O with the ast acting load angle limiter in

action and achieve greater possile reactive power asorption capaility, the

load angle is limited or practical purposes to ;0O ecause o the ollowingconsiderations3

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'. n the event o a short circuit in the systems, the generators may

accelerate owing to the arupt partial removal o the electrical load

and as the turine governor cannot act ast, the rotor angle increases

and the angle can ecome so large relative to the system vector that

the machine may all out o step.

9. The excitation system &A=4( switches over to manual mode in the

event o internal aults in the autoHmode. Changeover to manualHmode

signifes constant excitation and hence a stale operation up to a

maximum angle o @*O electrical only is possile.

The rotor angle limiter limits the load angle o the machine to an

acceptale present value. The load angle is the electrical angle etween the

voltage vector o the system and the vector o the machine voltage Se/ fg.;.

The system vector is derived rom the voltage vector o the generator )= y

adding to it the voltage drop in reactance external to the machine. This takes

into account the transormers and transmission lines etween the generator

and the system load centre. The rotor voltage is simulated adding the

inductive voltage drop in the machine xq. The system voltage at the load

centre is otained y sutracting e drop &4eactance drop in the

transmission line, transormers etc.( rom the generator terminal voltage. The phase angle etween Se/ and )8 is converted into a proportional

dc voltage. The actual value is compared with an adJustale reerence and

ed to the input o an operational amplifer. n case the angle exceeds the set

value the output signal immediately takes over the control o thyristor

network to uild up the generator airHgap Lux ast enough to avoid slipping.

t stands to reason that the output o the limiter acts directly over A=4 output

to avoid any loss o time due to flter time constants in the A=4. #ig.@

explains the operation o Automatic =oltage regulator in conJunction with

rotor angle limiter.

ROTOR CURRENT LIMITER

The A=4 drive the feld or the thyristor network into overload or one or

more o the ollowing reasons3

a. #aulty handling.

. Bystem voltage reduction.

c. Goss o sensing voltage to A=4 and

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d. #ailure within the controller.

The excitation limiter must prevent this overload rom persisting. 6n

the other hand, during dynamic disturances in the system the excitation

should not e reduced at once, ut ceiling excitation should e possile or

a limited time.

The limiter can e operated in three dierent modes as explained

elow to cater the aove requirements.

) S*-6 476

n this mode the excitation current is limited to a preset maximum value.

The limiter intervences with a time delay which is proportional to the

magnitude o the over load. 2hich the limiter in operation, the current is

limited steadily to the rated value.

) M;67 M476

during the aove period o limitation, the generator voltage dips

steeply or any reasons, the ceiling excitation limit is validated again. The

ceiling excitation current helps in increasing the short circuit current in the

ault "ones and hence aids selective tripping o the aulted section.

) S 476

n the switching mode the excitation is limited to the thermal or rated

current value. 6nly in case o sharp dip in the machine voltage, the ceiling

limit was unale momentarily. The limit switches ack to the rated value

ater the set time.

#igureH'* gives the lock diagram o a rotor current limiter acting in

conJunction with A=4 to limit the over excitation in the desired ashion.

STATOR CURRENT LIMITER

The stator current limiter has to inLuence the A=4 dierently

depending on whether the machine is overHexcited or underHexcited. The

excitation current is to e suitaly reduced to limit the inductive stator

current and is increased to limit the capacitive current. The rotor angle

limiter provides a more defnite protection in preventing the machine rom

alling out o step. Capacitive stator current limitation comes into play only

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with synchronous condensers which are to some extent negatively excited

with generators it prevents excessive leading 1=A4 loading corresponding to

any given 12 load.

The generator stator current is converted into polari"ed dc signal Wve

or Xve, depending upon whether the machine is overHexcited or underH

excited. This voltage orms the actual value or the controllers which process

each o the ipolar signal independently. 6ne o these controllers compares

the capacitive stator current against its reerence and acts directly on the

regulator via a deHcoupling diode to increase the excitation. The action o

second controller, which limits the inductive stator current, is delayed y

means o an integrator eore it inLuences the control nput o the A=4 so as

to reduce the excitation. The time lag oered is perectly acceptale as ar

as stator overheating is concerned ecause o the integrator time constant is

set one order less than the stator thermal time constant. #ig.'' shows an

A=4 operating in conJunction with a stator current limiter.

EXCITATION TRANSFORMERINTRODUCTION

4ectifer transormers directly connected to the generator terminals

and eeding power to the feld o the machine via thyristor converters, playsan important role in an excitation system and in turn power generation

4eliaility o this transormer has to e ensured in all respects.

mportance o rectifer transormer has een reali"ed ever since the

mercury arc converters came into existence or important applications like

large power drives and excitation systems. A gradual development has taken

place rom oil flled transormers to &resin( cast coil type transormers &dry

type( or !xcitation transormer.

6il and clophen%Bovtol flled transormers are still adopted or large

rating. $owever, in uran areas and thickly populated cities where pollution

control is also to e thought oN certain countries like 2est 7ermany have

rought oput regulation that oil immersed transormers can e used only

under special circumstances. #urther, use o clophen%Bovtol flled

transormers has already een anned almost in all advanced countries

ecause o poisonous gases emanating in case o damages. 1oreover, there

has een constant rise in price o oil in the international market, resulting in

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sustantial increase in the total price o transormer and its maintenance.

8ot only the aove reasons ut other ha"ards have led the scientists to think

o an alternate design which could gradually replace the oil and

clophen%Bovtol flled transormers. Accordingly vacuum impregnated dry type

transormers were taken up or large power and high voltage rating. The

results were however not satisactory ecause o many limitations like eect

o atmosphere, over voltages and the need or proper alternative and cast

resin moulding technique came into existence. The development o case

resin transormers has led to the production o dry insulated type

transormers up to + >=. These transormers have not only een ound

comparale to oil flled transormers ut also proved their superiority in all

respects. These transormers are o class S#/ insulation and indoor type.

VOLTAGE AND POWER RATING

The selection o the secondary voltage o excitation transormer

depends upon the flled orcing voltage. The primary voltage is the same as

that o generator terminal voltage. Current rating is dependent on the

maximum continuous current in the feld winding. 7enerally the power rating

o the !xcitation transormer used in Btatic !xcitation Bystem is around '-

o the rating o generator in 1=A.

ENCLOSURE AND COOLING

The enclosures are normally designed to ensure natural air

cooling%#orced airHcooling to the transormers. These enclosures are made to

:9*, :9+ depending upon the requirement. #orced cooling arrangement

provides increase in rating y ?*- than that with natural airHcooled

transormer. 8ormally this arrangement is switched on during peak load

period or in summer to deliver more current rom the same transormer. Thedescription that ollows compares resin cast coil, dry type transormers with

other transormers or various characteristics.

SALIENT FEATURESI SHORT CIRCUIT PROOF

The dynamic short circuit strength exceeds y ar that o oil immersed

transormers as well as that o conventional dry type transormers. n the

event o a short circuit the cast resin transormer is not endangered

mechanically, and only thermal damage can take place. The high mechanical

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strength is achieved y casting the coils in epoxy resin with a ferglass fller

to orm a compact tuular spool. An insulation thickness o 'H9 mm is quite

adequate to withstand the orce that occurs during operation.

II HIGH OVER LOAD CAPACITY

n certain applications where rectifer transormer is suJected to

intermittent loads like rolling mill, urnace, and traction and also in excitation

duly high current increase is ollowed y low current demand. t results in the

windings to e mechanically stressed to a greater extent.

n cast resin transormers all the windings are cast and thereore no

diKculties concerning mechanical strength due to repeated overloads.

8ormally $.=. E G.=. coils are cast separately, all the orces appearing on one

winding can e suitale asored y it. The resultant orces etween primary

and secondary windings can e made to asor y putting suitale support

locks etween the coils and rame. :osition o the support locks can e

conveniently designed to reduce the orces to a lower value in contrast to

conventional type transormer.

Conventional type, wound coil transormers consume a considerale

amount o insulation material like paper, which asors the expansion o

conductor and coils have to e recompressed ater certain periods. The cast

coils eing homogeneous, the coil structure expands and contracts as a

whole and the movement are taken care o y means o an elastic support.

4ecompression o the coils is thereore not required.

n synthetic liquid cooled transormer there is a rated temperature

Jump etween winding and cooling liquid o the order o 9* to 90OC with

current density + to ? A%Bq.mm. n contrast, in these transormers with class

# insulation the allowale temperature rise etween coil and air is o theorder o '**OC with the same current density. This clearly indicates the

heating time constant o cast resin, normally H'* times, higher than that o

oil flled transormers.

III RESISTANT AGAINST TEMPERATURE FLUCTUATION

The selected insulation material is ferglass reinorced epoxy resin,

which has got high tensile and ending strength. Thereore the transormer

can withstand the wide range o temperature Luctuations.IV MOISTURE PROOF

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The cast resin coils are impregnated and cast under vacuum which

ensures the void ness emedding o all windings into a system o uniorm

glass ferHepoxy laminate. This process helps the coil to oer an increased

protection against moisture.

Conventional dry type transormers are not moisture proo. The

winding do asor humidity and there is danger o Lashover once they are

put in service ater a long period.

V IMMEDIATE SWITCH ON

5ecause o the cast resin coil, the coils are homogeneously uilt in allrespects. There is no possiility o eect o moisture and amient

temperature Luctuations over coils. )nder such case the transormer can e

directly switched on without predrying the same ater long interruption rom

service.

VI IMPULSE STRENGTH

mpulse strength o these transormers is higher than that o

conventional dry type transormers and is comparale to that o oil cooled

transormers according to any international standard.

VII NON&INFLAMMABLE

Due to high quality o nonHhydroscopic material, it has een proved

that neither with welding cutting torches nor with welding electric arc the

cast coil resin could e induced to urn and as such is almost nonH

inLammale.

VIII PARTIAL DISCHARGE

During operation, there is no partial discharges inside the winding,

exceeding narrow and '* :.C. i.e. transormers are designed or long lie.

IX COMPACT INSTALLATION

Compared to oil and clophen%Bovtol flled transormers, the use o this

type transormer required less space, less weight and aove all, the cost or

the necessary erection o catchHpits no longer exist. 5ecause the cast resin

oils are nonHinLammale in nature a suHstation consisting o a numer o

such transormers can e installed in the same uilding near to the

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consumer end there y the power losses due to long distriution lines are

also avoided.

X NO LEA?ING

8othing can leak out rom these transormers in contrast top

clophen%sovtol and oil flled transormers where there is a possiility o the

liquid leaking. Thereore there is no need to make catchHpits at sites to avoid

contamination to the ground water.

XI MAINTENANCE FREE

Considering all aoveHmentioned eatures it can e concluded that

these transormers are virtually ree rom maintenance.

8o reHadJustment o the winding and no reHtensioning o the individual

coils are required to maintain the short circuit strength.

8o control o oil is required.

8o checking o electrical quality o used oil.

8o dry out is necessary even ater long interruption rom use.

XII OVER CURRENT PROTECTION

t is normally achieved with the help o current transormers mounted

on each phase on $.T. Bide o excitation transormer. #rom current

transormers current signals are given to two over current relays, one is

meant or instantaneous over current protectionN another is set or delayed

over current protection. The latter is set to suit the feld orcing

requirements.

XIII OVER TEMPERATURE PROTECTION

t is achieved with the help o temperature sensors kept near the hot

spot "one o the G.=. coils. The sensors have nonHlinear characteristic.

The resistance o the sensor is increased consideraly ater a certain

temperature limit. 8ormally two limits o over temperature are kept

depending on the class o insulating material used, one is the warning limit

and another one or tripping o the equipment. 5oth these limits are otained

y independent temperature sensor. The output o the sensors is rought to

the temperature monitoring equipment, which signali"es or calls or tripping.

XIV CONNECTION ARRANGEMENT

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8ormally the excitation transormer will have Dy0 vector group

connection to suppress harmonics. The angular displacement etween $T

and GT winding is '0*O !lectrical degrees.

XV OPERATING CONDITION

n spite o all advantages o the cast coil resin transormer mentioned

aove, it is recommended that this transormer should e mounted in an

enclosure installed away rom water, oil leaking sources, away rom sum rays

and heat dissipating equipments. Care has to e taken that suKcient ree

space all around is availale to maintain the amient temperature and

ventilation. The installation o the transormer has to e thought o in the

eginning itsel to avoid dust. $owever, dust%caron particles must e

removed during periodical shutdowns. 8ormally this transormer is located

Just elow the generator at exciter at S6/ meter level are at ?.0m level.

CONCLUSION

n excitation systems, practically cast resin dry type transormers are

used and there is no necessity presently o using o oil cooled transormers

with its inherent disadvantages o fre risk etc., as already mentioned.

#urther or indoor application it is preerale to use only dry type

transormers.

OPERATION OF STATIC EXCITATION EQUIPMENT

nitially the main Circuit 5reaker as well as #ield Circuit 5reaker is in

open condition. The signal lamp P!xcitation oQ shows that the machine is

not excited.

n order to start up the machine, machine should e frst rought to

nominal speed i.e. +*** 4:1. :reHselection to e done or selecting the

manual or auto control. The signal lamps on the control unit indicate whether

auto or manual control has een preHselected.

As soon as the nominal speed is reached, the ##5 ield #lashing

5reaker( E #5 ield 5reaker( to e closed. This is achieved through pressing

the luminous utton in the cuicle or y a parallel connected push utton

&remote(. Bince the remanance voltage o the machine is not suKcient to

operate the regulator, initially suitale station A.C. voltage via ull wave

ridge rectifer or suitale DC voltage rom station atteries to e suppliedthrough #ield #lashing 5reaker. The machine voltage rises to +*-, then the

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electronic regulation start unctioning y getting the released pulses, which

were locked till then. The locking o pulses is cancelled through voltage

relay and regulator takes over the unction o regulating the machine

voltage.

At


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)8


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?( 7uard the machine against inadvertent tripping during

transients

0( mprove dynamic and transient staility therey increasing

plant%machine availaility

( 4egulate 1=A4 loading within limits


37/38

0. During normal operation it is possile to work on any

thyristor%#use etc. y isolating it on A.C. E D.C. sides.

. #or initial !xcitation feldHLashing circuit is provided. A.C. supply

can e used. Circuit cuts o automatically when


38/38

'0. Blip staili"er unit helps to staili"e the power swings and thus

prevents the generator rom tripping.

'. 6ver current relays and temperature supervision protects the

rectifer transormer.

BIBILIOGRAPHY

'. G!CT)4! 86T!B 68 BTATC !CTAT68 BFBT!1 =6G. E H BHEL BANGALORE

9. 8BT4)CT68B #64 6:!4AT68 6# BTATC !CTAT68 BFBT!1H BHEL MANUAL

+. 8:T C6)4B! 1AT!4AG 68 T$!41AG :62!4 :GA8T #A1GA4BAT68

HVOLUME IV

131

8. Static Excitation System -Stage-2

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Transcript of 8. Static Excitation System -Stage-2