1ref automatic frequency control

8/3/2019 1ref Automatic Frequency Control

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33EEE Transactions on Energy Conversion, Vol. 3, No. 1, March 1988

AUTOPIATIC G E N E R A T I O K CONTROL FOR H Y D R O SYSTEMS

G. L . Kusic, J . A. Su t te r f ie ld ,I E E E Senior I4ember I E E E MerrberADVANCED CONTROL SYSTEMS, I N C .Atl an ta , Georgia 30362

ABSTRACTModern Automatic Generation Control ( A G C ) i s imple-mented with digital computers t h a t periodically samplet i e li ne real power flows, l in e frequency, an d gener-at or power outputs. These analog sign als are usuallymeasured two second periodically an d combined withdesired interchange t o obtain the Area Control Error( A C E ) . The A CE digital quanti ty i s allocated t oregulating hydro turbines an d transmitted via tele-metry t o the remote terminal units ( R T U ) . The RTUsconvert the raise/lower megawatts ( M W ) into timed relaycontact closures t o the governor which result in wicketgate open/close movement t o change the generator o u t p u tpower.The o u t p u t power of each ge ne rat or i s monitored by th ed ig i ta l AG C which closes a feedback loop around thegovernor-turbine-generator t o assure the desired powerlevel is atta ined . This paper describe s the feedbackloop design, which is es sen ti al ly a sampled-datacon tro l. Additional feedback loops due to the AC E a ndload r egulati on are a lso analyzed. A method i spresented for all oca tin g water usage between rese r-voirs on a gen era tor command-time ba si s. The the or-eti cal designs are verified by on-line measurementspresented in the paper.I . INTRODUCTIONAn A G C an d hydro all oc at or were implemented as par t ofth e Energy Management System of th e Ci ty o f Tacoma,Washington Light Division ( T C L ) . The power network ofTC L consists of the greater Tacoma load area of sixhydro generating pla nts (18 gene rato rs) with a to ta lgenerating capacity of 760 M W. Four of the ge nera tingplants are i n the vicinity of Tacoma, while theMossyrock an d Mayfield si t es are remotely loc ate d. Thesystem is inte rconn ect ed with the Northwest Power Poolvia s ix in ter t ies . T CL also owns a share of the PriestRapids hydro fa c i li t y which is current ly scheduled asblocks of power a n d no t used t o regulate the Tacomaa rea.The energy contro l ce nt er i s loca ted in Tacoma a n demploys thre e ope rat or consoles with dual CRTs, li gh tpens, a n d keyboards as the man/m achine interfa ce. Oneof the consoles is used for generation/t ransmission a n danother for distr ibu ti on. The thi rd console i s usedfor train ing an d programmer system mainten ance. Sixad di ti on al CRTs used a s engineer's terminals are in thecontrol center. A 16-bit computer in a redundantbackup configuration provides the computational supportan d performs the Supervisory Control and Data Acqui-s i t io n ( S C A D A ) fun ct io ns. Ccnimunication between theenergy control center a n d the RTUs a t the generating

87 CY 2 3 4 - 8 A paper recommended and approvedhy the I E E E Po we r G e n e ra t i o n C o mmi t t ee of t h e IEEEP ow er E n x i n e e r i n g S o c i e t y Ear p r e s e n t a t i o n a t t h eI E E E / P E S 1 98 7 I J i n t p r Y e e t i n g , Ye w O rl e a n s , T ,o u i s i a n a ," e b r u a r y 1 - 6 , 1 98 7. Y a n u s c r i p t s u h m i t t e d9 u g u s t 2 7 , 1986; made a v a i l a h l e f o r p r i n t i n gDecember 10, 1986.

A . R . Caprez, J . L . Haneline, B. R . Bergman,I E E E I.leniber I E E E Non-Member ! E K E Non-MemberCITY OF TACOMA Light DivisionTacoma, Washington 98411

s i t e s is by means of dedicat ed microwave li nk s. Theremainder of the 32 RTU's are o r will be linked bymicrowave an d telephone lines. An additional d a t a linkt o Bonnevil le Power i s a ls o employed.The purpose of the digital AG C system i s t o assignpower generation t o the individual generating units ( ato ta l o f 18 u n i t s a t 6 s i t e s ) t o regulate the area,meet the desired interchange schedule, assist i nregulating the system frequency an d a l lo ca te waterconsumption among the six generating sit es .The basic control signal i s th e ACE:AC E = z P t i e ( i ) - P s + 10B (f-f,) + T . C .i

where the foliowing def ini tio ns are used:P t i e ( i ) = Power flow o n t i e l i ne i , b o t h areas , MWP, = Net scheduled interchange power, MWB = frequency bia s se tt in g, MW/.1 Hertzf , f S = Actual and scheduled frequencies, HertzT . C . = Time correction, M W , derived from the seconds oftime err or mul tiplied by a number, MW/secDiff ere nt operating moaes such as f l a t frequencycontro l , f l at t ie s , e tc . use por tions of the A C Eequation. Operating experience of TC L indicated t h a tperiodically allocating the AC E signal every 8 secondst o control the hydro gene rato rs would surpass the arearegulation criteria of the North American ElectricPeliability Council ( N E R C ) . The 8 second command timetakes i ntc consideration the load a n d interconnectionbehavior, governor action, a n d hydro generator re-sponse . The command time i s a basic design parameterfor the A G C , a n d in part determines the time-sharecomputational requirements of the di gi ta l computer.The AGC digital code computes AC E every 2 seconds, andexec utes genera tor contr ol every 8 seconds. However,between these int eger po ints , t he in terim time i s usedfo r normal Supervis ory Control a nd Data Acquisition( S C A D A ) functions o r the computer i s i dle .

C E N E R A T O ROUTPUTPOWER.

FIGURE 1 MAJOR DYNAMIC ELEMENTSOF A HYDRO GENERATOR

0885-8969/88/0300-0033$01W O 1988 IEEE


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3411. BASIC LOOP FOR AGCThe d i g i t a l AG C e s t a b l i s h e s a r e f e r en c e o r d e s i r e ds e t p o i n t p o w er o u t p u t f o r a g e n e r a t o r a n d c om pa re s t h em ea su re d o u t p u t w i t h t h i s v a lu e , s e nd i ng c o r r e c t i v er a i s e / l o w e r s i g n a l s t o t h e go v e r no r v i a t h e RTU. T heg o v e rn o r o f t h e t u r b i n e g e n e ra t o r d r i v e s a n e l e c t r o -h y d r a u l i c s e rv o me c ha ni s m t h a t o pe ns o r c l o s e s t h ew i c k e t g a te s . T v ) m a j o r d y na m ic e l em e n ts o f a h y d r ot u r b i n e - g e n e r a t o r a r e s how n i n F i g u r e 1, w h e r e t h e Kt e rm s a r e g a i n s , t h e T t er m s a r e t i m e c o n s t a n t s , a nd Si s t h e L a p l a c e t r a n s f o r m v a r i a b l e .T he s pe ed r e g u l a t i o n m e ch a ni s ms f o r TCL a r e p r i m a r i l yc e n t r i f u g a l f l y b a l l t y p e w i t h a d j u s t a b le l i n k a g e( d r oo p ) i n t o t h e e l e c t r o - h y d r a u l i c s e r v o s y s te m .S e v e r al r e c e n t d e s i g n s a r e m a g n et i c r e l u c t a n c e p i c k u pw i t h e l e c t r o n i c c i r c u i t s . The w a t e r c ol um n t i m ec o n s ta n t o f F i g u r e ( I ) depends upon the head o f w a t e r .The g ai n s an d t i m e c o n s t a n t s a l s o v a r y f o r d i f f e r e n th y d r o u n i t s .F o r e a ch h y d r o g e n e r a t o r , t h e g a i n s a n d t i m e c o n s t a n t so f t h e g o v e r n o r a n d h y d r a u l i c s e rv o m e ch a n is m s , a si n s t a l l e d , w ere s e t t o y i e l d " s u i t a b l e " r e sp on se s f o r ap o wer ch a ng e comma nd . The " s u i ta b l e " r e sp o n se , r e g a r d -l e s s o f t h e MW c a p a c i t y o f t h e g e n e r a t or , i s a s mo ot he x po n en t ia l t r a n s i t i o n ( f i r s t o r d e r t im e c on s t a nt ) f r o mt h e p r e v i o u s p o w e r t o t h e new po we r l e v e l w i t h o u to s c i l l a t i o n o r o v er s ho o t. T h e r ef o r e, f o r AGC d e s i g np u r po s e s, t h e c o m p l et e h y d ro t u r b i n e - g e n e r a t o r i sc h a r a c t e r i z e d b y a n i n p u t - o u t p u t r e l a t i o n s h i p . Thei n p u t i s a mo me nt ar y c o n t a c t c l o s u r e o f t h e r e l a y w h i c ha p p l i e s 5 12 5 VD C t o t h e g o v e r n or m o t o r , a n d t h e o u t p u ti s t h e g e n er a t o r o u t p u t p ow er . A t y p i c a l i n p u t - o u t p u tt e s t m e as ur em en t i s sho wn i n F i g u r e 2, b e f o r e t h er e s p o n s e w as a d j u s t e d t o s a t i s f y t h e AGC d e s i g nr e q u i r e m e n t s . In F i g u r e 2, t h i s h yd ro u n i t e x h i b i t s a n1 1. 3 s e co n d t i m e c o n s t a n t ( t i m e t o r e a c h . 63 2 o f t h et o t a l c h an g e) a n d ha s an e q u i v a l e n t g a i n o f 1 1. 7/ 3. 37 =3 . 27 M W /s ec on d o f c o n t a c t c l o s u r e . B o t h t h e g a i n an dr es p on s e t i m e o f t h e u n i t v a r y w i t h t h e s t e a d y - s t a t eo u t p u t p o we r l e v e l a n d f o r r a i s e c om pa re d t o l o w e rc om ma nd s. H o we v er , t h e r a n g e o f v a r i a t i o n i s c o n -s i d e r e d i n t h e AG C d e s i g n .

G E N E R A T O RO U T P U TP O W E R

4 0 5 01 RAISE P U L S E , 3 . 3 7 S E C O N D SF I G U R E 2 U N A D J U S T E D R E S P O N S E M E A S U RE DF O R C U S H M A N U N I T 821

T he b a s i c l o o p a r o u r d e a ch g e n e r a t i n g u n i t I S s t u d i e di n te rm s o f t h e v a r i a b l e r a i s e / l o w e r r e l a y c o n t a c t t i m ew h i c h i s a p u l s e - w id t h m o d ul d te d c o n t r o l , b u t t h ep r i n c i p l e s o f S er vo l o c p d e s i gn a r e c l e a r , a nd t h en e c e s s a r y c o m p e n s a t i o n E a s i l y d e r i v e d i f t h e r e l a yc l o s u r e i s t r e a t e d a s a s a ri p l e- a n d- h o ld d e v i c e a s s ho wn

i n F i g u r e 3. T he c l o s e d o p c o n t r o l i s t h e n a n a l y z e da s a s am p l ed - d at a s y st e m( % w i t h t h e d i g i t a l c o m p u te rn u m e r i c a l p r o c e s s r e p r e s e n t e d b y l i ( z ) a n d t h e h y d r ot u r b i n e - g e n e r a t o r a nd r e l a y r e p r e s e h e d b y H 2 ( z ) w h er eH ( z ) a n d h ( z ) a r e d i g i t a l t r a n s f e r f u n c t io n s i n te rm s0 ) z = exp( i3S) .The b l oc k d ia gr am t o t h e l e f t o f t h e d o t t e d l i n e i nF i g u r e 3 r e p r e s e n t s d i g i t a l c o m p ut e r c od e e x e c u t e de v e r y e i g h t s e c on d s . T he RT U a n d co mmu n i ca t i o n ch a n n e lc o n v er t s t h e r a i s e / l o w e r s i g n a l f r o m t h e d i g i t a lc o mp u te r i n t o r e l a y c o n t a c t c l o s u r e t im e . The r e l a y ,i n t u r n, a p p l i e s +125 v o l t s t o a DC s e r v om o t o r w hi c h i sp a r t o f t h e e l e f i r o h y d r a u l i c s er v o a nd o p en s /c l os e st h e h y d r o w i c k e t g a t e s.

D C M O T O R ,H ( t ) C - F A H 2 ( * )I N T E O R A T O R . S E R V 0 A ND

D E S I R E 0 _ -U N I TI O U T P U T

P O W E R

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G E N E R A T O RA G C IN ,-t-+ H Y D R O

D I G I T A L C O M P U T E R II

F I G U R E 3 D l Q l T A L C O N T R O L OF A H Y D R O G E N E R A T I N G U N I Tp_____--i t 1 o r d e r t o a n a ly z e t h e b a s i c c o n t r o l l o o p , a s am p le dd a t a f o r m u l a t i o n o r Z -T r a ns f o rm m u s t be o b t a i n e d f o rt h e l o o p . T he c o n t i n u o u s e l e m e nt s o f s e r vo m e c ha n i sma nd u n i t a r e c o n v e r t e d t o t h e Z - T r a n s fo r m H 2 ( z ) a s :

where T = 8 s e c on d s , t h e s a m p l i n g t i m e a n d T i s t h eu n i t ' s re s p on s e . U s i n g f r e q u e n c y d om a in b l o c k d i a g r a mc o n c e pt s , t h e c l o s e d l o o p r es p o ns e i s :

wh i ch i s a sequence of n u m e r i c a l v a l u e s i n ti m e .Because H ( 2 ) i s a p e r i o d i c a l l y e x ec ut ed d i g i t a l c o de ,a ny c o m b h a t i o n o f p r e s e n t a na p a s t t i m e h i s t o r ysamples, P ( t ) , o r p r e s e nt apd p a s t t i m e h i s t o r y o fi n p u t , P D ( ? ) , may be us ed t o c o n s t r u c t t h e d i g i t a lse q ue n ce. One e f fe c t i v e t o f H , ( z ) c o m p e n s a t io n i st o c a nc e l t h e d i g i t a l p o l eY f P o f e q u a t i o n 2 due t o t h ed om i na n t t im e c o n s t a n t w i t h a d i g i t a l z e ro :

w h er e T ' i s a n e s t i m a t e c f t h e u n i t d o mi na n t t i n ec o n s ta n t . T h i s i s o f t e n c a l l e d " mo de l d - f or w a r d" o rN o t e t h a tz-' r e p r e s e n t s t h e p r e v i o u s d i g i t a l sa mp le . T h e r e f o r e ,e q u a t i c n ( 4 ) a s a pp 1: ed t o t h e b l o c k d i ag r a m o f F i g u r e3 rieans: Multiply by K t h e p r es e nt d i f f e r c eb e t we e n d e s i r e d ar, d a c t u a P ' u n i t p o w er m i n us

" a t i c i p a t i o n " o r s e r i e s c o mp e ns a ti on ($7 .


3/7

35

t i m e s t h e p r e v i o u s ( 8 s e co n ds d g o ) d i f f e r e n c e a n dt r a n s m i t t h i s a s a r a i s e /l o w e r p u l s e. The B m u l t i p l i e ri s u se d t o i n v e s t i g a t e v a r i a t i on s i n T o r i n o t h e rw ords, t h e m is m a t c h be t w een c om pens a ti on and "p la n t " .The e f f e c t o f l o o p g a i n i s i n v e s t i g a t e d b y v a ry i n g KL e t t h e i n h e r e n t g a i n i n t h e g o v er n or a nd h y r o u n i t h ic o m b i n e d w i t h K so t h a t K = .1?5KLK2. The pr od uc t o fK K i s n o m i n d l i y 1.0 b ec au se t h e d i g i t a l c o de o f H i si!i %W w h i c h i s c o n v e r t e d t o a c o n t a c t c l o s u r e s i b n a ls e c t t o t h e RTU, a nd t h e H2 t r a n s f e r f u n c t i o n c o n v e r t st h e c o n t a c t c l o s u re t o MW .The denominator o f e q u a t i o n ( 3 ) i s :1 t ~ ~ ( 2 ) = 1+ K (z - ~e -T/T) [~(z-e-~l ' ) T(z-l)( l-e-T/T):

2 (z-1) (z-e-T/T) (5)so t h e c h a r a c t e r i s t i c p o ly no mi al o f t h e d i g i t a lt r a n s f e r f u n c t i o n i s :p(z) = z3 + ~ ' ( - l - e - ~ ' ~ K [ T- ~ (l -e - ~/ ') l )

+ [Te-T/T - ~ ( l - e - ~ ' ~ ) ]The r e sp cn se o f t h e d i g i t a l t r a n s f e r f u n c t i o n i s s t a b l eif h e z er oe s o f t h y 2 ) c h a r a c t e r i s t i c p o l yn o m i a l l i ei n s id e t he u n i t c i r c l e . The c l o s e r t h e p o l e s g e t t ou n i t c i r c l e , t h e l e s s t h e d y r m i c r es pc ns e i s damped.Any r o o t s o f t h e c h a r a c t e r i s t i c p o l y no m ia l o u t s i d e t h eu n i t c i r c l e r e p r e s e n t u n s t a b l e re sp on se .The r e s u l t s o f e v a l u a t in g t h e c h a r a c t e r i s t i c p o l yn o mi a lf o r a h yd ro u n i t w i t h T = 105 s econds ( r l os s y roc k U n i t51 ) a r e show n i n T a b l e 1. T he c om pens a t i on m is m a t c h i sv a r i e d a nd t h e l o o p g a i n i s v a r i e d t c s t u d y v a r i a t i o n si n h y d ro u n i t p a ra me te rs o r m i sniatch between cmpensa-t i o n an d t h e u n i t . The g a i n I( = . I 2 5 i s nom i r i a l .O b se rv e t h a t t h e r c o t s o f t h e c h a r a c t e r i s t , i c p o l y n o mi a la lw ay s r e ma i n i n s i d e t h e u t i i t c i r c l e f o r t h e v a l u es B I1.0. Hence, a s t i b l e r e sp o n se i s e x pe c te d . A m is m at c ho f B = 1.5 i s a l w a ys u n s t a b l e .

Underest imt ing T Exact Carpensation Ove res timti ng TG i n lmtllIroot~)mt31 ]root11ImtZ1 Iro0t31 lmtllIro0t21 Iro0t31

K.I2 5 .018 .955 .955 .039 .924 .927 .063 .718 1.103.500 .077 .934 .934 .235 .616 .927 .410 .410 1.195.875 .139 .921 .921 .503 .503 .927 .534 .534 1.2351.250 .1% .921 .921 .602 .602 .927 .632 .632 1.259Table 1. Variatio n o f the Roots o f the Cha racteristic Polyncniial withGain and Paraneter f o r T = 105 seconds, 5 = 11.5 W/SKA c o r r e s p o n d i n g a n a l y s i s wa s p e r fo r i m d f o r a u n i t wh ic hh a d a t i m e c o n s t a n t on t h e o r d er o f X seconds which i ss ho wn i n T a b l e 2. The r e s u l t s f o r t h i s u n i t a r em a r g i n a l i r i t h e se ns e t h a t t h e g a i n i s l i m i t e d t o t h r e et im es nom ina l , K i .375. Subsequenl ly , i t w i l l bes how n t ha t ii g a i n o f t h r c r i s e s sc rl l- ia i t o r a w e l lcfesiSntld AGC b e ca u se o f o t h e r f e e db a c k p a t h s b e s i d e s

L'ri&rest.irmt.ing T Exact Curpensdtion I)veres tir,eting T( d i n lmtl lrootil jroot31 lrootll ImoiYllkx~Imtil Irwt2/Imt31

K.125 . lo3 .m7 .m 5i4 .WI .w 462 .m.250 .140 .P3? .832 .38 .727 .i Z .611 .691 .h91.375 .I55 ,970 .S7U .36u .E90 .890 .%9 .a2 .862.W .162 1.094 1.094 .368 1.028 1.025 .E79 1.W 1 . W

B = . 5 3 = 1.0 B = 1.5

B = .5 B = 1.0 B = 1.5

Tahle i. Var ia t im c f t he Rmts ct ' the Chanic ter is t ic Pc lymid l b;ithGin and Paraw te r f o r T = 8 seconds.

t h e b a s i c c o n t r o l l o o p . A d o m in a nt t i m e c o n s t a n t o f~ 2 1 6 a s s t a b l e l o o p r es po ns e f o r g a i n s u p t o 1 0 ti m esn o m i n a l , K = 1.25 ana C < E 2 1 .0 . T h e r e f o r e , a l ls er vo me ch ap is ms we r e a d j u s t e d t o s a t i s f y t h e T 2 16s ec on d c r i t e r i o n .F i g u r e 4 shows t h e measu red c l os ed l o op res pons e o f t heCushman U n i t 2 1 a f t e r i t s s e rv o me c ha n is m wa s r e t a r d e dt o h a v e an o p en l o o p d o c i n a n t t i m e c o n s t a n t o f 1 7 .1s e co n d s. I n F i g u r e 4 , t h e d e s i r e d p o w e r s e t p o i n t w a sc hanged f ro m 13 t o 5 MW i n t h e d i g i t a l code t o i n t r o -d uc e t h e s t e p i n p u t . S u b se q ue n t t e s t s i n c o r p o r a t e dt h i s u n i t i n t o t h e c o tv p le t e AGC.

D I S P A T C H13-- - tI ( M W ) i

y.

---.- - +7 .' i C U S H M A N + 2 1 .&.71 ( M W )1 O U T P U T P O W E R5 1>ap---t- 8 0 1 6 3 * 4 8 6 4 8 0 ( ~ ~ ~ ~F I G U R E 4 M EA SU RE D C L O S E D L O O P R E S P O N S EO F C U S H M A N U N I T + 2 1 A F T E R

A D J U S T M E N T T O ' C = l l . l S E C O N D S ,U, ~ 3 . 2 7 W I S E C .

111.I n a d d i t i o n t o t h e b a s i c t ur b i ne - g e ne r a t or l o o p o f t h eAGC, t h e r e a r e t w o a d d i t i o n a l f e e d ba c k l o o p s d ue t o ACEand l oa d s ha r i ng . T he A C E c o n t r o l l o o p h a s b e en e x -t e n s i v e i 8 , S ju d i e d i n te r ms o f a n a r e a r es p on s e f o rt i n u o u s , a s w e l l a s t o s a m pl e d- d at a c o n t r o l sF i g u r e 5 i s a s c hem at i c b l oc k d iag r am s howing t he AGCw i t h ACE and BLD sha ri ng . K each sum t oun i t y . For des i gn purposes h'$&e," :t K y i approx im a t edt h a t g e n e r a t o r s o f t h e Tacoma AG C o p e r a te i n t o an" i n f i n i t e b u s" b e ca u se t h e l a r g e s t dy na m ic d i s tu r b a n c e sc o n s i d e r e d a r e on t h e o r de r o f 5 0 MW . T he d i s t u rbanc esa r e s m a l l c om p ar ed t o t h e 1 1 00 MW l o a d o f TCL, a n d t h eh i g h MW c a p a c i t y N o r t h w e s t P o we r Pool abs o rbs t he pow ers w in g s. A c c o r d i n g l y , t h e f r e q u e n c y r e m a in s c o n s t a n td e s p i t e t h e AG C d i st .u r ba n ce s a na t i e l i n e p ow er i s t h em e a su r ab l e o u t p u t v a r i a b l e f o r AGC d i s t u r b a n c e s .C m s - i d e r t h e s i c i p l e c a w w h e re o n l y o ne h y d r o g e nc i rd - to ri s p e r fo r m i ng t h e A C E r e g u l a t i o n f o r t h e a re a. W i tht h e i n f i n i t e b u s a s su m p ti o n, t h e ACE s i y n a l i s a c l o s e dl o o p ar ou nd t h e u n i t a s sho wn i n F i g u r e 6.If t h e v n i t u n d e r c o n t r o l h a s a d i g i t a l t r a n s f e rf u n c t i o n G ( z) g i v e n i n e q ca t i o n ( 3 ) , t h e n i t i s e a s i l ys ee n t h a t t h e t r a n s f e r f u n c t i o n o f t h e ACE c o n t r o l l o o pi s :

A C E A N D BASE LOAD DEVIATION (BLD) LOOPS

FY:

The A G C , vhe n c o n t r o l l in g t i e l i r e f l o w, e f f t c t i v e l yi n c r e a s e s t h e f e e d f o r w d r d g a i n o f t h e h y d r o g e n e r $ t o ru n i t t oG A J N = ( 1 + t:Ci)K

2 5 m n b e d e r iv e d f r om t h e c h a r a c t e r i s t i c p o l y n c n ia i o fe q u a t i o p ( 7 ) .


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36

E?

IIII

B L D S H A R EK B l K 1 3 Na - * c

% T O T A L G E N E R A T I O NS A M P L E R S

F O R rc D R O O P

I s R E Q U E N C YF I G U R E 5 M U L T I P L E G E N E R A T I N G U N I T S U N D E R A G C C O N T R O L W I T HA C E S H A R I N G A N D B L D S H A R I N G ( r - 1 . 0 F O R T C L )

As a re su lt , the basic hydro gene rato r loop must havestable response for a t least twice a s much ga in asdetermined from equati on (3 ). The gain K i s o f tenmade larger t h a n unity, af te r the point %here NERCperformarice i s measured. The la rge r gain , adjustedon-line, improves the area regulation performance.Additional units on AC E control hsve additionalfeeaback paths o n Figure 5 and AC E is divided amongthem.When Rase Load Leviation ( E L D ) i s considered, the AGCcomputes the load variation on every command cycle:

P Ri = l i =1 oiLD = z PDi - ( 9 )

for the N load regulating generators. P . an d P . are ,respectively the dispatch point and th e & t u a l p8der o funit i . A share of the B L D is al located t o each unitevery 8 second cormand time. Fo r thermal powersy st em , the share i s com t d rom the i nve rse slop eof the heat rat e curve^^^'^^^, b u t for the hydrounits, the share o f th e BL D between dispatch times i scomputed from water usage participation factors.Fo r T C L , there are tw o generdting sitps in tandem oneach of the three ri ve r systems. The discharge ra te ofthe upstremi plant can be greater t h a n the downstreamplant, increasing the loher pond level aboue desired.Nany other constraints sucb i s critical pohervibrationregions, power variations w i t h pond elevztion, sideflowa nd spill considerations, etc. are included i n th ehydro al loc dti cn program. tlcwever, the G L I j shdrefactors sre based u p o n a daily computed usage betweenthe th ree river systems, fo r example:

, R E G U L A T I N GU N I Tn;nI,q T I E T I E L I N E P O W E R )

- H Y D R OI U N I TA C E +-.-IA L L O C A T I O NF I G U R E 6 A C E C O N TR O L AR O UN D A U N IT

QDA Y L Y = [.2, .5, .3 ] (10)Among the three river systems, TC L employs a plantparticipation factor based u p o n th e KSFD water contentplus eleva tion of the s ix ponds, fo r example:P P P F = [ 6, . 4 , . 3 7 , .63 , .52, .48] (111

In equation 11, t h e f i r s t and second PPPFs, . 6 , an d.4, are upstream an d downstream sites corresponding t Gth e . 2 river entry in QDA Y L Y . Other entries ofequations 10 and 11 i r e ordered correspondinyly. TheF L C share o f uni t i i s conlputed as:

JJ = 1

E L D i = (QDAYIY i ) (PPPFi ) l b lPX i /X PMAXj ( 1 2 )


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37where PMAX. i s t h e m ax im um p ow e r a v a i l a b l e ( d e p en d e n ton head) fAr u n i t i, n d t h e P M A X . a r e a l l g e n e r at o rso n - l i n e a t t h e same s i t e .T he p o we r d i s p a t c h p o i n t f o r e a ch g e n e r a t o r , P i sbased upon QDAYLY and th e PPPF ta bl es , and comp&b bya n a l l o c a t o r e xe c ut ed f o r t h e f o l l o w i n g f o u r c o nd i -t i o n s :

J

H y d r o A l l o c a t o r O p e r a t i o n1)2 )3 )4 ) Operator demand

10 m i nu t es e l a ps e d s in c e l a s t a l l o c a t i o nBLD ex c eeds s om e t h re s h o ld l e v e lG e n e r a ti n g u n i t c ha ng es s t a t u s (o n o r o f f - l i n e )E x ce p t f o r t h e e l a ps e d ti m e i n t e r v a l , t h e h y d roa l l o c a t o r i s o p e r a t e d e x a c t l y t h e same a s a n e c on om icd i s p a t c h p r o g r a m f o r t h e r m al u n i t s . T he B LD . f a c t o r sa r e r e co m pu t ed ea ch t i m e t h e h y d r o a l l o c a t o r i k r un .BLD c o r r e c t i o n s f o r a g e n e r a t o r a d d a n o t h e r f e e d b ac kl o o p a r o u n d t h e g e n e r a t o r t h r o u g h t h e AGC. When onlyo ne g e n e r a t o r i s c o n t r o l l i n g t h e p o w er sy st em , t h e AGCi s s hown i n F i g u r e 7 .

F I GU R E 7 ON E G E N E R A T O R C O N T R O L L I N G A C E A N D B L D

Th e BLD l o o p 8 - s ec o n d p e r i o d i c a l l y i n t r o d u c e sc o r r e c t i v e s i g n a l s t o t h e h y dr o g en e r a t or s . When o n l yo ne u n i t i s BLD r e g u l d t i n g , t h e e f f e c t i v e g a i n i s :G A I N = (1 + KA + KB)K

where KB i s t h e BLD c c n t r i b u t i o n .It i s c l e a r t h a t K an d K c a n ea c h be a s l a r g e a st i n i t y , so t h e b a s i t h y d r o \ e n e r a t o r l c o p mu st h a ve ag a i n m a r y i n o f a t l e a s t 3.0. T h i s i s a fu n d a me n t a ld e s i g n c h a r a c t e r i s t i c .For d g e n e r a t o r w i t h s l o w r e s p c n s e , i t i s p o s s ib l e t oi n c r ea s e t h e l o o p g a i n u s i n g K, e q u a t i o n 4 and improvet h e r a t e o f r e sp o ns e . F i g u r e 8 s h o w s c a l c u l a t e d r e -s po ns es f o r a T = 105 s e c o n d t i m e c o n s t a n t g e n e r a t o rw i t h n o m i n a l a r id i n c r e a s e d l c o p g a i n w he n t h i s g en -e r a t or a lo ne i s c o n t r o l l i n g AC E dnd BLD.I V . MULTIPLE UNITS UNDER AG C CONTROLA number o f app rox ima t ion s have been made t o a n a l y z ee ac h t u r b i n e g e n e r a t o r i n AC E ar!d BLD co nt ro l. Theres pons e o f i n d i v i d u a l u n i t s mu st be s a t i s f a c t o r y -i . e . , d am pe d, m o n o t o n i c t r a n s i t i o n s f r o m o ne p c w erl e v e l t c a n o t h e r - hecause sinal 1 i r ia y n it u de d y r m i ct e r m s a s w e l l a s t r a n s m i s s i o n n e t w o r k c o u p l i n g h a v eS ee n n e g l e c t e d . When m u l t i p l e i l n i t s a r e u n d e r c o n t r o l ,(?dct i u n i t ad ds a d d i t i o n d l l o o p s ( F i g u r e 5 ) , s o w ofw h ic h c ou ld caus e und es i r ab l e beha v io r . As an ex am p le ,a T = E s ec ond res ponse t im e u r i i t when a lone i n AC E d n dP L D c o n t r o; h a s d am pe d o s c i l l a t o r y b e h a v i o r d ti e t o a

d i s tu r b a n c e, a nd v e r y l i t t l e g a i n m a rg i n. F i g u r e 9s ho ws t h e c a l c u l a t e d r e s p o n s e o f an 8 second and 105s e co n d u n i t w he n p r o p o r t i o n a t e l y s h a r i n g b o t h BLD a ndACE. The s u st a in e d o s c i l l a t i o n s i n t h e 8 s e c o n d u n i ta r e o b j e c t i o n a b l e .

M O S S Y R O C K U N I T # 5 1O U T P U T P O W E R1

4 0 I I I I I I I I I I I I0 1 0 0 2 0 0 3

T I M E: S E C )I O+

FIGURE 8 CALCULATED RESPONSEFOR A SYSTEM LOAD DECREASE WITHMOSSYROCK UNIT U51 IN ACE PLUS BLDREGULATION

U N I T'(=lo5a

O U T P U TP O W E R

50

300

U N I TZ= 8

Il i l i ~ l l ~ l ~ l l ~ i0 200 300 400

T I M E ( S E C T -

- 2 5 . 0

- 2 2 . 5

-20.0

.17.5

-15 . 0

F I G U R E 92 = 8 A N D T=105 U N I T S S H A R I N G A L O A DC A L C U L A T ED R E S P O N S E O FLO SS. R A T I O O F B L D A N D T IE F L O WS H A R I N G IS 1 8 / 8 2 R E S P E C T I V E L Y .


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38

- 1 6 . ~- 2 0 . -

M E A S U R E UA C E ( M W )

X X

J1 1 5 . -

1 I O - -

1 0 5

-x- - --MOSSYROCK + 5 1P O W E R ( M W ) *- x --x- - *- --X-x---y.-- -+!--.x/ - , , - t

0 4 0 8 0

- 43 1

F I G U R E 1 0 M E A S U R E D A C E R E S P O N S E W I T H C U S H M A N U N I T R 2 1( Y = 1 7 . 1 ) A N D M O S S Y R O C K # 5 1 ( ' C= 1 05 ) R E G U L A T I N GA - 2 0 M W S T E P C H A N G E . R A T I O O F B L D A N D A C ES H A R I N G I S 1 8 / 8 2 R E S P E C T I V E L Y .

Each generating unit o f the TC L system was checked fo rdynamic response on an individual basis. A minimumtime constant of T = 16 seconds over al l operatingpower conditions was obtained by adjusting the dampingo f th e hydraul ic-mechanical servomechanism which dri ve sthe wicket gates. As a result, any combination ofgenerators on control yi eld s excellent AG C performance.One on-line measurement performed using together the" fas tes t" a n d "slowest" ge nerat ors i s shown i n Figure10 .The "disturbance" used fo r the t e s t of Figure 10 was acomputer-entered numerical o ff se t of -2 0 MW. Thedistu rbanc e was entered several seconds before thef i r s t AG C coininand ti me . Since Figure 10 i s an on- l inerieasurement, th ere a re continuous load distu rbanc eswhich appear in the two second measured and smoothedAC E sign al. The disturbances res ul t in continuedraise/lower pulses being se nt to the units. Observe ir;Figure 10 th a t th e f as te r r espo nse un i t i n i t ia l ly&sorbs more o f the 20 MW di st ur ba nc e. The Mossyrockgen era tor i s in a remote area while the Cushinan ljni t i sin the same vicinity as the T C L load, such that the AC Esignal represents the sum of t i e li ne flows from thetwo di sj oi nt dreas.

V. CONCLUSIONSThis paper has presented the major concepts which areused t o design an AG C system for a hydro type ofut il it y. The basic loop to control each hydro unit i sa sampled-data control system. The e ff e c t of the AC Ereg ula tio n and load shar-ing on AG C design was shown.The s im i la ri t i es between hydro ar;d thermal systems werepointed out.Experimental re su lt s were presented to verif y thetheoretical calculations and methods developed in thepape r .ACKPICWLEDGEIIENTSThe aut hors wish t o thank th e personnel of Tacoma CityLight who partic ipa ted in the in st al la ti on and checkoutof the Energy Management System, a n d who cooper ated t oobtain extra te st material used in th is paper. Specialcon tr ib ut ors ar e Willard Freeman who formulated t hewater balance method, M ax Enirick who supervised theunit pulse tes ts a n d servomechanism adjustments, andBill Barcus, in charge of dispatching a n d control roonioperations.


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REFERENCESE P R I ( E l e c t r i c P o w e r R e s e a r c h I n s t i t u t e ) ," T r a n s i e n t - M i d t e r m S t a b i 1i y Program", ResearchPr o j ec t 745 , J une 1979.G.F. F r a n k l i n a n d J.D. P o we ll , " D i g i t a l C o n tr o l o fDynamic Systems". Reading, Mass ach use tts:Addison-Wes ley Pub l is h i ng Company, Inc . , 1980.G.L. Kus i c , "S t ru c t u re and Sam pled-D at a As pec t s o fAGC f o r Power Sys tems", I E E E Summer Power Meeting,Vanco uver, B.C.: Paper A79 510-9, J u ly 1979.Ew ar t , D.N., " A u t o m a t i c G e n e r a t i o n C o n t r o l -Performance Under Normal Condi t ions" , U.S. ERDAConference 750867, Henr i iker N.H., 1975.Wood, A.J. and Wo lle nb er g, B.F., "Powe rGenera t i on , Ope ra t i o n and C o n t r o l " , New York ,N.Y.: John Wi ley and Sons, 1984Deble l lo , F.P. a n d B ' R e l l s , " A u t o m a t i c G e n e r a ti o nC o n t r o l - P a r t 11, D i g i t a l C o n t r o l T e c hn i qu e s" ,Paper T72487-7, I E E E PAS-92, Mar /Ap r 1973

1ref automatic frequency control

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