8/20/2019 Excitation Control(4)
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EXCITATION SYSTEMSEXCITATION SYSTEMS
Copyright © P. Kundur
This material should not be used without the author's consent
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Excitation SystemsExcitation Systems
1. Functions and Performancee!uirements
". E#ements of an Excitation System
$. Ty%es of Excitation Systems
&. Contro# and Protection Functions
'. Mode#in( of Excitation Systems
Out#ine
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Functions and PerformanceFunctions and Performance
e!uirements of Excitation Systemse!uirements of Excitation Systems
T)e functions of an excitation system are
to %ro*ide direct current to t)e sync)ronous
(enerator fie#d +indin(, and
to %erform contro# and %rotecti*e functions
essentia# to t)e satisfactory o%eration of t)e
%o+er system
T)e %erformance re!uirements of t)e excitation
system are determined -y
a /enerator considerations0
supply and ad!ust "ield current as the generatoroutput #aries within its continuous capability
respond to transient disturbances with "ield "orcing
consistent with the generator short term capabilities$
- rotor insulation "ailure due to high "ield #oltage
- rotor heating due to high "ield current
- stator heating due to high %& loading
- heating due to e(cess "lu( )#olts*+,
- Po+er system considerations0
contribute to e""ecti#e control o" system #oltage and
impro#ement o" system stability
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E#ements of an Excitation SystemE#ements of an Excitation System
Exciter 0 %ro*ides dc %o+er to t)e (enerator fie#d +indin(
Regulator 0 %rocesses and am%#ifies in%ut contro# si(na#sto a #e*e# and form a%%ro%riate for contro# of t)e exciter
Terminal voltage transducer and load compensator 0
senses (enerator termina# *o#ta(e, rectifies and fi#ters it
to dc !uantity and com%ares +it) a reference #oad com%
may -e %ro*ided if desired to )o#d *o#ta(e at a remote
%oint
Power system stabilizer: %ro*ides additiona# in%ut si(na#
to t)e re(u#ator to dam% %o+er system osci##ations
Limiters and protective circuits0 ensure t)at t)e
ca%a-i#ity #imits of exciter and (enerator are not
exceeded
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Ty%es of Excitation SystemsTy%es of Excitation Systems
C#assified into t)ree -road cate(ories -ased on t)e
excitation %o+er source0
0 2C excitation systems
0 AC excitation systems
0 Static excitation systems
1. 2C Excitation Systems0
0 uti#i3e dc (enerators as source of %o+er
dri*en -y a motor or t)e s)aft of main (enerator
se#f or se%arate#y excited
0 re%resent ear#y systems 415"6s to 1576s
#ost fa*or in t)e mid81576s -ecause of #ar(e si3e
su%erseded -y ac exciters
0 *o#ta(e re(u#ators ran(e from t)e ear#y non8
continuous r)eostatic ty%e to t)e #ater system
usin( ma(netic rotatin( am%#ifiers
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Fi(ure 98" s)o+s a sim%#ified sc)ematic of a ty%ica# dc
excitation system +it) an am%#idyne *o#ta(e re(u#ator 0 se#f8excited dc exciter su%%#ies current to t)e main
(enerator fie#d t)rou() s#i% rin(s
0 exciter fie#d contro##ed -y an am%#idyne +)ic)
%ro*ides incrementa# c)an(es to t)e fie#d in a -uc:8
-oost sc)eme
0 t)e exciter out%ut %ro*ides rest of its o+n fie#d -yse#f8excitation
". AC Excitation Systems0
0 use ac mac)ines 4a#ternators as source of %o+er
0 usua##y, t)e exciter is on t)e same s)aft as t)e
tur-ine8(enerator
0 t)e ac out%ut of exciter is rectified -y eit)er
contro##ed or non8contro##ed rectifiers
0 rectifiers may -e stationary or rotatin(
0 ear#y systems used a com-ination of ma(netic and
rotatin( am%#ifiers as re(u#ators most ne+ systems
use e#ectronic am%#ifier re(u#ators
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Fi(ure 9."0 3C e(citation system with amplidyne #oltage
regulators
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".1 Stationary rectifier systems0
0 dc out%ut to t)e main (enerator fie#d su%%#iedt)rou() s#i% rin(s
0 +)en non8contro##ed rectifiers are used, t)ere(u#ator contro#s t)e fie#d of t)e ac exciter Fi(.9.$ s)o+s suc) a system +)ic) is re%resentati*eof /E8A;TEEX system
0
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Fi(ure 9.$$ 6ield controlled alternator recti"ier e(citation
system
Fi(ure 9.&$ <ernator supplied controlled-recti"ier
e(citation system
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Fi(ure 9.'$ 9rushless e(citation system
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$. Static Excitation Systems0
0 a## com%onents are static or stationary
0 su%%#y dc direct#y to t)e fie#d of t)e main
(enerator t)rou() s#i% rin(s0 t)e %o+er su%%#y to t)e rectifiers is from t)e main
(enerator or t)e station auxi#iary -us
$.1 Potentia#8source contro##ed rectifier system0
0 excitation %o+er is su%%#ied t)rou() a
transformer from t)e main (enerator termina#s
0 re(u#ated -y a contro##ed rectifier
0 common#y :no+n as -us8fed or transformer8fed
static excitation system
0 *ery sma## in)erent time constant
0 maximum exciter out%ut *o#ta(e is de%endent on
in%ut ac *o#ta(e durin( system fau#ts t)e
a*ai#a-#e cei#in( *o#ta(e is reduced
Fi(ure 9.7$ Potential-source controlled-recti"ier e(citation system
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$." Com%ound8source rectifier system0
0 %o+er to t)e exciter is formed -y uti#i3in( currentas +e## as *o#ta(e of t)e main (enerator
0 ac)ie*ed t)rou() a %o+er %otentia# transformer
4PPT and a satura-#e current transformer 4SCT
0 t)e re(u#ator contro#s t)e exciter out%ut t)rou()
contro##ed saturation of excitation transformer
0 durin( a system fau#t, +it) de%ressed (enerator
*o#ta(e, t)e current in%ut ena-#es t)e exciter to
%ro*ide )i() fie#d forcin( ca%a-i#ity
An exam%#e is t)e /E SCT8PPT.
$.$ Com%ound8contro##ed rectifier system0
0 uti#i3es contro##ed rectifiers in t)e exciter out%ut
circuits and t)e com%oundin( of *o#ta(e and
current +it)in t)e (enerator stator
0 resu#t is a )i() initia# res%onse static system +it)
fu## >fau#t8on> forcin( ca%a-i#ity
An exam%#e is t)e /E /ENEEX system.
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Fi(. 9.?$ Compound-source recti"ier e(citation system
Fi(ure 9.9$ :E;EE< compound-controlled recti"ier
e(citation system ©=EEE751 >71?
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Contro# and Protecti*e FunctionsContro# and Protecti*e Functions
A modern excitation contro# system is muc) more
t)an a sim%#e *o#ta(e re(u#ator
It inc#udes a num-er of contro#, #imitin( and
%rotecti*e functions +)ic) assist in fu#fi##in( t)e
%erformance re!uirements identified ear#ier
Fi(ure 9.1& i##ustrates t)e nature of t)ese functions
and t)e manner in +)ic) t)ey interface +it) eac)
ot)er
any (i*en system may inc#ude on#y some or a## of
t)ese functions de%endin( on t)e s%ecific
a%%#ication and t)e ty%e of exciter
contro# functions re(u#ate s%ecific !uantities at
t)e desired #e*e#
#imitin( functions %re*ent certain !uantities from
exceedin( set #imits
if any of t)e #imiters fai#, t)en %rotecti*e functionsremo*e a%%ro%riate com%onents or t)e unit from
ser*ice
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Fi(ure 9.1&0 E(citation system control and protecti#e
circuits
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AC e(u#ator0
-asic function is to maintain (enerator stator *o#ta(e
in addition, ot)er auxi#iaries act t)rou() t)e ac
re(u#ator 2C e(u#ator0
)o#ds constant (enerator fie#d *o#ta(e 4manua#contro#
used for testin( and startu%, and +)en ac re(u#ator isfau#ty
Excitation System Sta-i#i3in( Circuits0 excitation systems +it) si(nificant time de#ays )a*e
%oor in)erent dynamic %erformance
un#ess *ery #o+ steady8state re(u#ator (ain is used,t)e contro# action is unsta-#e +)en (enerator is ono%en8circuit
series or feed-ac: com%ensation is used to im%ro*et)e dynamic res%onse
most common#y used form of com%ensation is aderi*ati*e feed-ac: 4Fi(ure 9.1'
Fi(ure 9.1'$ 3eri#ati#e "eedbac@ e(citation control system
stabili,ation
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Po+er System Sta-i#i3er 4PSS0
uses auxi#iary sta-i#i3in( si(na#s 4suc) as s)aft
s%eed, fre!uency, %o+er to modu#ate t)e
(enerator fie#d *o#ta(e so as to dam% system
osci##ations
;oad Com%ensator0
used to re(u#ate a *o#ta(e at a %oint eit)er +it)in
or externa# to t)e (enerator
ac)ie*ed -y -ui#din( additiona# circuitry into t)e
A@ #oo% 4see Fi(. 9.17
+it) C and XC %ositi*e, t)e com%ensator re(u#ates
a *o#ta(e at a %oint +it)in t)e (enerator
used to ensure proper sharing %&s between generatorsbussed together at their terminals
commonly used with hydro units and cross-compound
thermal units
+it) C and XC ne(ati*e, t)e com%ensator
re(u#ates *o#ta(e at a %oint -eyond t)e (enerator
termina#s
commonly used to compensate "or #oltage drop across
step-up trans"ormer when generators are connected
through indi#idual trans"ormers
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Fi(ure 9.17$ Schematic diagram o" a load compensator
T)e ma(nitude of t)e resu#tin( com%ensated *o#ta(e 4V c , +)ic) is
fed to t)e A@, is (i*en -y
( ) tcctc = !
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Bnderexcitation ;imiter 4BE;0
intended to %re*ent reduction of (eneratorexcitation to a #e*e# +)ere steady8state 4sma##8si(na# sta-i#ity #imit or stator core end8re(ion)eatin( #imit is exceeded
contro# si(na# deri*ed from a com-ination ofeit)er *o#ta(e and current or acti*e and reacti*e%o+er of t)e (enerator
a +ide *ariety of forms used for im%#ementation
s)ou#d -e coordinated +it) t)e #oss8of8excitation
%rotection 4see Fi(ure 9.1?
O*erexcitation ;imiter 4OX;
%ur%ose is to %rotect t)e (enerator fromo*er)eatin( due to %ro#on(ed fie#d o*ercurrent
Fi(. 9.19 s)o+s t)erma# o*er#oad ca%a-i#ity oft)e fie#d +indin(
OX; detects t)e )i() fie#d current condition and,after a time de#ay, acts t)rou() t)e ac re(u#atorto ram% do+n t)e excitation to a-out 116 ofrated fie#d current if unsuccessfu#, tri%s t)e acre(u#ator, transfers to dc re(u#ator, andre%ositions t)e set %oint corres%ondin( to rated*a#ue
t+o ty%es of time de#ays used0 4a fixed time, and4- in*erse time
+it) in*erse time, t)e de#ay matc)es t)e t)erma#ca%a-i#ity as s)o+n in Fi(ure 9.19
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Fi(ure 9.1?$ Coordination between AEB BDE relay and
stability limit
Fi(ure 9.19$ Coordination o" o#er-e(citation limiting with
"ield thermal capability
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@o#ts %er =ert3 ;imiter and Protection0
used to %rotect (enerator and ste%8u% transformer
from dama(e due to excessi*e ma(netic f#ux
resu#tin( from #o+ fre!uency andDor o*er*o#ta(e
excessi*e ma(netic f#ux, if sustained, can cause
o*er)eatin( and dama(e t)e unit transformer and
t)e (enerator core
Ty%ica# @D=3 #imitations0
@D=3 #imiter 4or re(u#ator contro#s t)e fie#d
*o#ta(e so as to #imit t)e (enerator *o#ta(e +)en
@D=3 exceeds a %reset *a#ue
@D=3 %rotection tri%s t)e (enerator +)en @D=3
exceeds t)e %reset *a#ue for a s%ecified time
Note0 T)e unit ste%8u% transformer #o+ *o#ta(e
ratin( is fre!uent#y ' -e#o+ t)e (enerator
*o#ta(e ratin(
%*+, )p.u. 7.2/ 7.2 7.7/ 7.78 7.8/
3amage Time ininutes
:E; 8.2 7.8 1.8 28.8 ∞
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Mode#in( of Excitation SystemsMode#in( of Excitation Systems
2etai# of t)e mode# re!uired de%ends on t)e
%ur%ose of study0
t)e contro# and %rotecti*e features t)at im%act
on transient and sma##8si(na# sta-i#ity studies are
t)e *o#ta(e re(u#ator, PSS and excitation contro#sta-i#i3ation
t)e #imiter and %rotecti*e circuits norma##y need
to -e considered on#y for #on(8term and *o#ta(e
sta-i#ity studies
Per Bnit System0
Se*era# c)oices a*ai#a-#e0
a %er unit system used for t)e main (enerator fie#d
circuit
chosen to simpli"y machine eFuations but not
considered suitable "or e(citer FuantitiesG under
normal operating conditions "ield #oltage in the order
o" 8.887 )too small
- %er unit system used for excitation system
s%ecifications
rated load "iled #oltage as one per unit
not con#enient "or system studies
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9.7." Mode#in( of Excitation System Com%onents
T)e -asic e#ements +)ic) form different ty%es of excitation
systems are t)e dc exciters 4se#f or se%arate#y excited acexciters rectifiers 4contro##ed or non8contro##ed
ma(netic, rotatin(, or e#ectronic am%#ifiers excitation
system sta-i#i3in( feed-ac: circuits si(na# sensin( and
%rocessin( circuits
Separately excited dc exciter
Fi(ure 9."7$ 9loc@ diagram o" a dc e(citer
Sel!excited dc exciter
T)e -#oc: dia(ram of Fi(. 9."7 a#so a%%#ies to t)e se#f8
excited dc exciter. T)e *a#ue of " E # )o+e*er, is no+ e!ua#
to R e $R g !% as com%ared to R e $R g for t)e se%arate#y excited
case.
T)e station o%erators usua##y trac: t)e *o#ta(e re(u#ator -y
%eriodica##y adustin( t)e r)eostat set%oint so as to ma:e
t)e *o#ta(e re(u#ator out%ut 3ero. T)is is accounted for -y
se#ectin( t)e *a#ue of " E so t)at t)e initia# *a#ue of V R is
e!ua# to 3ero. The parameter " E is thereore not ixed# but
varies with the operating condition&
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Fi(ure 9."9$ 9loc@ diagram o" an ac e(citer
Fi(ure 9.$6$ ecti"ier regulation model
'( Exciter and Rectiier
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Fi(ure 9.$&$ )a =ntegrator with windup limits
Fi(ure 9.$&$ )b =ntegrator with non-windup limits
e%resentation0
System e!uation0
;imitin( action0
e%resentation0
System e!uation0
;imitin( action0
)indup and *on!)indup Limits
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9.7.$ Mode#in( of Com%#ete Excitation Systems
Fi(ure 9.$5 de%icts t)e (enera# structure of a detai#ed
excitation system mode# )a*in( a one8to8one
corres%ondence +it) t)e %)ysica# e!ui%ment.
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Standard IEEE Mode#sStandard IEEE Mode#s
IEEE )as standardi3ed 1" mode# structures for
re%resentin( t)e +ide *ariety of excitation systems
current#y in use 4see IEEE Standard &"1.'8155"0
t)ese mode#s are intended for use in transient
and sma##8si(na# sta-i#ity studies
Fi(ures 9.&6 to 9.&$ s)o+ four exam%#es
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%& Type +(%' Exciter model
,& Type '(%' Exciter model
Fi(ure 9.&6$ =EEE type 3C7& e(citation system model. ©=EEE 7557>4?
Fi(ure 9.&1$ =EEE type &C7& e(citation system model. ©=EEE 7557>4?
T)e ty%e 2C1A exciter mode# re%resents fie#d contro##ed dc
communtator exciters, +it) continuous#y actin( *o#ta(e re(u#ators.T)e exciter may -e se%arate#y excited or se#f excited, t)e #atter ty%e
-ein( more common.
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/& Type '(0' exciter model
0& Type ST%' exciter model
T)e ty%e AC&A exciter mode# re%resents an a#ternator su%%#ied contro##ed
rectifier excitation system 8 a )i() initia# res%onse excitation systemuti#i3in( fu## +a*e t)yristor -rid(e circuit. Excitation system sta-i#i3ation is
usua##y %ro*ided in t)e form of a series #a(8#ead net+or: 4transient (ain
reduction. T)e time constant associated +it) t)e re(u#ator and firin( of
t)yristors is re%resented -y T '. T)e o*era## (ain is re%resented -y " '& T)e
rectifier o%eration is confined to mode 1 re(ion. ectifier re(u#ation effects
on exciter out%ut #imits are accounted for -y constant " ( .
T)e ty%e ST1A exciter mode# re%resents %otentia#8source contro##ed8rectifier
systems. T)e excitation %o+er is su%%#ied t)rou() a transformer from
(enerator termina#s t)erefore, t)e exciter cei#in( *o#ta(e is direct#y
%ro%ortiona# to (enerator termina# *o#ta(e. T)e effect of rectifier re(u#ation
on cei#in( *o#ta(e is re%resented -y " ( . T)e mode# %ro*ides f#exi-i#ity to
re%resent series #a(8#ead or rate feed-ac: sta-i#i3ation. ecause of *ery
)i() fie#d forcin( ca%a-i#ity of t)e system, a fie#d current #imiter is
sometimes em%#oyed t)e #imit is defined -y l LR and t)e (ain -y " LR .
Fi(ure 9.&"$ =EEE type &C& e(citation system model © =EEE 7557 >4?
Fi(ure 9.&$$ =EEE type ST7& e(citation system model © =EEE 7557 >4?
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Mode#in( of ;imitersMode#in( of ;imiters
Standard mode#s do not inc#ude #imitin( circuits
t)ese do not come into %#ay under norma#
conditions
T)ese are, )o+e*er, im%ortant for #on(8term and*o#ta(e sta-i#ity studies
Im%#ementation of t)ese circuits *aries +ide#y
mode#s )a*e to -e esta-#is)ed on a case -y case
-asis
Fi(ure 9.&? s)o+s as an exam%#e t)e mode# of a
fie#d current #imiter
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4a #oc: dia(ram re%resentation
4- ;imitin( c)aracteristics
Fi(ure 9.&?$ 6ield-current limiter model