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3 7E E E Transactions on Power Apparatus an d S y s t e m s , V o l . PAS-99, N o . 1 Jan./Feb. 1980OPERATION OF THE L A R GE I N TE R CO N NE C TE D POWER SYSTEM
BY DECISION AND CONTROL
John Zaborszky Krishna Prasad Keh-Wen Whang
Department o f S y s t e m s Science and MathematicsW a s hin gt o n Un i ver si t yS t . L o u i s , Missouri 63130
A BST R A CTA c om pr eh en si ve a pp ro ac h t o th e control of thelarge interconnected power s y s t e m i n c on di ti on s r a n g -in g from normal to disintegrated i s outlined. Thebasic decision i s b et ween normal and abnormal condi-tions o r , more precisely, on t h e degree o f abnormalitywith n or m al i ty r at in g as zero degree abnormality. Theapplicable control algorithms would then b e aimed a tmaintaining normalcy when it i s p r e s e n t or leading t h es y s t e m back to norm alcy wit h t h e least disruption whent h e condition i s abnormal. In a sense one could sayt h a t in De c i s i o n and C o n t r o l o p e r a t i o n a system is a l -
ways in a r es to r at or y s t a te seeking t h e best way backto normalcy - w h e n it finds n o t h i n g t o restore thes y s t e m i s in a norm al con dition . To lead t he systemback to normalcy f i v e Regimes o f C o n t r o l are d e f i n e dand t h e measures and means used in each are definedand develop ed. N ew ancillary algorithms to use ast o o l s in th ese o p e r a t i o n s h a v e b e en d ev el op ed . A n il-lustration o f a s e q u e n c e of e m e r g e n c y e v e n t s u n d e r D e-cision and * Control Operation i s also included. Ameasure of th e effectiveness of the new techniques is aconsistent tenfold increase of critical clearing t i m e s .This indicates a l a r g e potential sav ing in new trans-mission equipment.
1 . INTRODUCTIONIn 1 9 7 5 E R D A gave o u t four c o n t r a c t s f o r th e studyo f c o m p u t e r control o f t h e large interconnected powers y s t e m during emergencies with t h e ai m of preservingthe system's integrity as m u c h as possible and a t anyr a t e preventing a total breakdown and blackout. One o fthese c o n t r a c t s came to Washington U n i v e r s i t y and this
paper p r e s e n t s t h e principal accomplishments o f t h elast three years of r es ea rc h e ff or t in this area in-cluding work outside th e scope of the E R D A contract .I n th e course o f this research, i t soon becamea p p a r e n t that really meaningful results could be ob-tained only in th e c o n t e x t of the entire operation oft h e large interconnected p o w e r system. E xt e ns i ve t hi nk -i n g and research finally boiled d o w n t o an overall in-s i g h t and organization of th e p o w e r s y s t e m operationwhich is referred to as operating w i t h Decision an dControl. T h e f u n d a m e n t a l s o f this type of operation areintroduced in the next Section.Let i t only be n m e n t i o n e dhere th at wi t h the increasing size an d complexity ofth e s y s te m s , i n cr eas i ngl y a ut oma ti c c o m p u t e r controlleda ct io n b ec om es necessary, especially a t th e f a s t end ofth e e v e n t s where t he time scale is a few cycles or sec-onds a nd even w h e n i t i s a f e w m in utes. I n f a c t , t hefastest Control Regime in emergencies, on the few cy-cles level, was always a u t o m a t i c s i n c e it was e n t r u s t e dThis r e s e a r c h was supported in part by the EnergyResearch a n d Development A d m i n i s t r a t i o n u n d e r C o n t r a c t#EX76-C-01-2073, a n d in part by th e Department o f E n e r -gy under Contract #ET-78-D-01-3090.F 79 68 9-1 A paper reconmended and approved by the IEEEPower System Engineering Committee of t h e IEEE PowerEn gin eer i n g S o ci ety fo r presentation a t t h e IEEE P E SS u m m e r Meeting, Vancouver, British Columbia, r C a n a d a ,J ul y, 1 5 - 2 0 , 1979. Manuscript subDmitted February 1 ,1 9 7 9 ; m a d e a v a i l a b l e fo r printing May 1 7 , 1 9 7 9 .
t o t h e selective protection r e l a y s . These i n t o d a y ' sterminology are s p e c i a l purpose microcomputers - origi-nally e l e c t r o m e c h a n i c a l , l a t e r p a r t i a l l y electronic a n di n c r e a ' s i n g l y s o l i d s t a t e . The d e s i r a b l e development f o rt h e future seems t o b e t o w a r d s e x p a n d i n g t h i s automaticrange t o longer time p e r i o d s a n d, t o more c o m p l e x t a s k so f p r es er v in g s y st em w id e V ia b il i ty an d S t a b i l i t y . Theeventual system r e s t o r a t i o n , h o w e v e r , will always havet o remain t h e responsibility o f t h e operator a i d e d b yt h e computer in s u p p l y i n g d a t a a n d o p t i m i z e d solutionst h r o u g h a m u l t i p l i c i t y o f a n c i l l a r y a l g o r i t h m s likestate e s t i m a t i o n , l o a d f l o w s , security e v a l u a t i o n s ,A G C , e t c .For e x p a n d i n g automatic operations into a w i d erange o f emergency c o n t r o l , i t becomes necessary t od e v e l o p a number of n ew a n c il l a r y t e ch ni q ue s. One s u c hf u n d a m e n t a l new t o o l i s the " O b s e r v a t i o n Decoupled( L o c a l E q u i l i b r i u m ) State S p a c e " . T h i s new s ta te s pa cewa s shown t o be e q u i v a l e n t t o t h e conventional statespace but u n l i k e t h e l a t t e r it s c o m p o n e n t s ca n b el o c a l l y estimated in a small fraction o f t h e time i tt a k e s to estimate t h e conventional state c o m p o n e n t s [ 4 ] .Fast l o c a l c o n t r o l action utilizing t h e new state spacewa s s h o w n t o be c a p a b l e o f e f f e c t i v e l y stabilizing t h esystem in e m e r g e n c i e s , at l e a s t t e m p o r a r i l y . A n addi-t i o n a l new a n c i l l a r y t e c h n i q u e which is r e p o r t e d in acompanion paper [ 5 ] makes i t possible t o e v a l u a t e ' t h esystem V i a b i l i t y i n m i l l i s e c o n d s . Another n ew tech-nique allows th e e s t i m a t i o n , based on t h e ObservationD e c o u p l e d ( L o c a l E q u i l i b r i u m ) State S p a c e , o f lastingc h a n g e s in bus i n j e c t i o n s while t h e system i s in atransient s t a t e . T h i s a n d another new t e c h n i q u e f o rs e l e c t i n g c o n t r o l measures t o Viabilize t h e system t e l a -p o r a r i l y within a few s e c o n d s ar e p r e s e n t e d in d e t a i li n [ 4 ] .
2 . OPERATION OF TH E LARGE INTERCONNECTEDPOWER SYSTEM BY D EC IS IO N A ND CONTROLFunctions, p a r t i c u l a r l y computer control andsystemoperating f u n c t i o n s , on t h e l a r g e interconnected powersystem f a l l into two broad c a t e g o r i e s .
2 . 1 . H o u s e k e e p i n g o r A n c i l l a r y ProcessesThese i n c l u d e c o m p u t a t i o n a l , a l g o r i t h m i c a n d o t h e rt o o l s a n d solutions which b a s ic a l l y p ro vi de a ux il ia ryi n f o r m a t i o n and o p e r a t i n g d a t a . T h u s , t h e y would serve
sometimes as t h e basis o f decisions or t h e y may estab-l i s h future o p e r a t i n g c o n d i t i o n s , b u t t h e y are no t real-l y part o f t h e on l i n e operation. Some c us to ma ry p ro -cesses o f t h i s t y p e are1 . Load f l o w2 . State estimation3 . System security evaluation4 . Economic d i s p a t c h5 . Unit conmitment6 . Optimal l o a d f l o w a n d Optimal dynamic l o a d f l o w7 . Load f o r e c a s t i n g8 . Load managementA d d i t i o n a l processes were i n t r o d u c e d as a r e s u l t o Lt h e r esea rc h a c ti vit i es p r e s e n t e d here. These a r e9 . Computation o f Area L o a d Excess ( A L E ) [ 6 ] ,1 0 . Computation o f Economic Target Curves f o r Unit0 0 1 8 - 9 5 1 0 / 8 0 / 0 1 0 0 - 0 0 3 7 $ 0 0 . 7 5 1 9 8 0 IEEE
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3 8Controls [ 6 ]1 1 . Computation o f Coordinated Unit Control t of o l l o w t h e t ar ge t c ur ve s [ 6 ]1 2 . Computation of t h e Observation Decoupled ( L o -c a l E q u i l i b r i u m ) State Vector [ 4 ]1 3 . Computation o f local load imbalance from th eObservation Decoupled ( L o c a l E q u i l i b r i u m )State [ 4 ]
1 4 . Computation o f Sectional or Area power i m b a l -ance [ 4 ]1 5 . Fast contingency evaluation by Concentric Re-laxation [ 4 ] , [ 5 ]2 . 2 . Operating Processes. Degrees o f A b n o r m a l i t y
Given a l a r g e comprehensive power system operat-i n g with t h e ai d o f such advanced devices as multiter-minal DC n e t w o r k s , microwave c o m m u n i c a t i o n s , m i c r o ,m i n i , and macro computer e s t a b l i s h m e n t s , al l u nd er t h ea u t h o r i t y o f a Control Center with satellite LocalControl Centers, a Decision a n d Control a p p r o a c h e m e r -ges as t h e natural way to a p p r o a c h system o p e r a t i o n .T he D e ci s io n Phase o f t h e process consists o f acontinuous s u r v e i l a n c e , m o n i t o r i n g and d e c i s i o n o n t h ec o n d i t i o n o f t h e system and a l s o t h e decisions o n s e-l e c t i n g th e best R e g i m e o f a c t i o n s , s p e c i f i c a l l y , c o n -t r o l actions in order to m o v e t h e system towards i t snormal c o n d i t i o n .T he C on tr ol Phase t h e n p r o c e e d s with t h e a l g o -rithms comprising t h e Control Regime s e l e c t e d d u r i n gt h e Decision Phase a n d carries ou t t h e control actionwhich are commanded b y t h e a l g o r i t h m s .S o m e t h i n g o f t h i s nature, i n f a c t , i s i m p l i c i t int h e f a m o u s D y L i a c c o d i a g r a m [ 1 ] ( F i g u r e 1 ) and itsmodified v e r s i o n s , t h e m o s t re cen t a n d p e r t i n e n t o fw h i c h i s t h e Fink-Carlsen d i a g r a m [ 2 ] shown in F i g u r e2 a n d ev en in c u r r e n t o p e r a ti n g p r ac t ic e s. It needst o be f o l l o w e d m o r e c o n s c i o u s l y in t h e f u t u r e in t h econtext o f much m o r e advanced devices an d a l g o r i t h m s .
Figure 1
A s expressed here Decision a n d Control then b e-com es th e b as ic f or ma t o f t h e system operation. A newdecision on s ys te m c on di ti on s a n d applicable algo-rithms i s made at every time s t e p and t h e next commands t e p of t h e currently applicable c on tr ol a l go r it h m i scarried ou t at every time s t e p .The basic Decision i s between normal and abnormalconditions o r , more p re ci se ly , o n t h e Degree o f Abnor-mality with Normality rating a s Zero Degree Abnormal-i t y . T h e applicable control a lg or it hm s w ou l d then beaimed at maintaining normalcy wh en i t i s present orleading t h e system back t o normalcy through a leastobjectionable path when t h e condition i s abnormal. Ina sense one could sa y that i n Decision a nd C on tr olOperation a system i s always in a Restoratory condi-tion s e e k i n g t h e best way back t o normalcy - when i tfinds nothing t o restore t h e system i s in a normalc o n d i t i o n . The word abnormal i s used t o avoid seman-tic arguments over th e meaning of emergency. F or in-s t a n c e , one c o u l d argue whether losing a line or agenerator when adequate reserves a re p r es en t is , or i sn o t , an emergency. I t clearly i s a n a bn or m al c on di -t i o n , h o w e v e r . Abnormal c o n d i t i o n s simply mean thate v e r y t h i n g i s not a s expected. There are m an y degreesand many time scales o f abnormality which can beclassed as f o l l o w s :2 . 2 . 1 . Degree # 0 . Normal Operating Conditions.Conditions are normal when t h e y are a s expected - allequipment working which was supposed t o w o r k , l o a d s ,f u e l supplies, water and weather c on di ti on s w it hi n t h ee x p e c t e d ranges. T he p ri nc ip al applicable algorithmsare
1 . Monitoring and Estimation o f the load andgeneration2 . S ta ti c S ta te Estimation3 . Monitoring o f th e s y s t e m l oa d in g c on d it io nsa n d Security4 . E c o n o m i c Dispatch or O pti mal L oad F lo w5 . Unit Commitment6 . Automatic Generation Control7 . Load M a n a g e m e n t2 . 2 . 2 . Degree # 1 . Normal O p e r a t i n g Conditions
with Structural D e f e c t . Conditions a r e n o r m a l . Thesystem i s Secure a nd V ia bl e except t h a t its s t r u c t u r ehas been altered b y a n earlier e v e n t from what wa s ex-p e c t e d - a l i n e i s m i s s i n g , f o r e x a m p l e .The p r i n c i p a l o p e r a t i n g a l g o r i t h m s are as in D e-gree # 0 b u t , in a d d i t i o n , future consequences o f t h es t r u c t u r a l c h a n g e m u s t be evaluated a n d , i f necessary,r e m e d i e d . F or i n s t a n c e , t h e structural c h a n g e mayforeshadow S e c u r i t y or V i a b i l i t y problems d u r i n g ana p p r o a c h i n g p e a k .2 . 2 . 3 . Degree # 2 . S e c u r i t y Defect or AlertState. Conditions are still normal and t h e system i sViable but t h e s e c u r i t y margin is smaller than d e s i r e d .A p p l i c a b l e a l g o r i t h m s would include t h o s e f o rDegree # 0 b ut overruled in some instances in order t orestore s e c u r i t y , s p e c i f i c a l l y in t h e area o f EconomicD i s p a t c h a n d Load Management which may then be re-p l a c e d b y s p ec i al a l g or i th m s s u c h as1 . R e a d j u s t m e n t s o f t h e network l o a d f l o w s b yt h e u se o f t h e DC network
2 . Modified t i e line s c h e d u l e s , modified loadd i s p a t c h , e t c .3 . Bringing on new generation or other equip-mentO f course, i f e f f e c t i v e control in crisis condi-tions i s available many situations w hi ch w ou ld b e i n -secure without such control would become secure. Thisi s part o f t h e f i n a n c i a l benefit r e s u l t i n g f r o m in-s t a l l i n g s u c h c o n t r o l .2 . 2 . 4 . Degree # 3 . S t a b i l i t y Crisis. The systemi s in a momentary d y n a m i c s t a t e , n o r m a l l y caused b y af a u l t , which i s s u f f i c i e n t l y violent t o e n d a n g e r t h ei n t e g r i t y o f t h e s y s t e m .. F i g u r e 2
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DE CISI O N P H A S ES E L E C T I O N O F ACTION O ND E G R E E O F AB N O R M AL I TY CONTROL REGIM E
I START AT E A C HS A M P L E TIMET a b l e 1 . S t r u c t u r e of the O p e r a t i o n with D e c i s i o n a n d C o n t r o l .
N O R M A L f I \lNORMAL WITH ' TSTRUCTURAL , | rD E F E C T , , I
S E C U R I T Y DE F E CT 'iI t e 1 o L id l
S T A B I L I T Y C R I S I S
F i g u r e 3 . S o m e Fictitious Case H i s t o r i e s .
C O N T R O L P H A S ES E L E C T I O N OFCONTROL M EA NS CONTROL AC TI O N
3 9
( S E E TABLE 2 )
DEGREE # 0NORM A L
NO
DEGREE # 1 R EG # 0STRUCTURAL Y E SD E F E C T ?
N O 7DEGREE # 2 R EG # 0SECURITY Y E SDEF ECT ?
N O R E G # 1NO
DEGREE # 3 RE G # 2S T A R I L T Y Y E S NCRISIS? N O t YE S RG#~ ~ ~ ~ ~ ~ ~ R E GO /GREG 3 _
N ODEGREE # 4 YE SVIABILITY Y E S RE G # 4CRISIS? N O t i YE S RE G # 5N O R E G #3
N O Y E S, D E G R E E #5 RE G # 4I I N T E G R I T Y Y E S I)CRISIS? NO I YE S-_ _ R E D 05
REGIME # 01 .NORMAL OPERATION2.NORMAL WITH M INORR E S TO R ATI O N
REGIM E #1SELECTIVE P R O T E C T I O N ,S TAB I L I ZI N GSINGLE M A C H I N EF IRST SWING( 2 5 C Y C L E S ) - '
REGIM E # 2 _PRELIMINARYSTABILIZING _M ULT I M A C H IN E o (M ULT I S W I N G _(1 2 CYCLES-3 S E C O N D S )C)REGIM E # 3 zPRELIMINARYVIABILIZING 0 ' , I(0.5- 60 S E C O N D S ) V_---_ _ _ _ _ uiA N D _STABILIZING a Z( O 5 - 3 S E C O N D S ) v * 0
R E G I M E #4 C ) xV I A B I L I Z I N G- 60 MINUTES
REGIME # 5RESTORATION
K
RE G # O
I L i
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REGIME
#l
REGIME
AZ
REGIME
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REGIME
M4
REGIME
MO
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TOELIMINATE
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SUCCESS:
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-
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-
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S
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ORMORE
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5CHOICE
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2.
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0
.
CHOOSE
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TO
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A
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-
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MINUTE
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4.
CHOOSE
QUANTITATIYE
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PERMITTED
IN
REGIME
A3
TO
MOOROVER-
ANDNORMAL
AUTOMATIC
GEISERATION
CONTROL
LOADS
PRINCIPAL
CONTROL
SOME
TOOLS
SOME
TOOLS
SOME
TOOLS
IN
ORDER
OFUNDESIRABILITY.
SAMEUS
IN
REGIME
AS
BUT
MOORDOVER-
ALLAVAILABLE
RESOURCES
SF
TUE
PONER
TOOLS
I)
SELECTIVE
NETWORK
PROTECTION
EQUIP-
I)
BRAKING
RESISTOR,
0)
LOAD
SKIPPING,
I)
FREQUENCY
REDUCTION
(AGC).
2)
USE
LOADS
ONLY
SYSTEM,
TELEPMONE
MENT,
01
BRAKING
RESISTOR,
3)
SNUNTAND
3)
SERIES
OR
SNUNT
CAPACITOR
SWITCHNGA,
OF
GENERATOR
RESERVES
(NUT
AND
COLD)
OR
SERIES
CAPACITOR
SWITCHING,
4)
FAST
4)
NUDE
LINE
CONTROL,
U)
FAST
VALVING
FAST
GENERATOR
RUNBACK,
0)
EARLY
RETURN
VALVING
OF
EQUIPMENT
FROM
MAINTENANCE,
A)
TIE
LINE
RESERVE
HELP,
A)
USING
SNORtT
TIME
(U
-
10
MINUTES)
OVERLOAD
CAPACITY
OF
EQUIPMENT,
A)
VOLTAGE
REDUCTION,
7)
LOADDROPPING,
0)
ISLANDING.
AVAILABILITY,
SPEED,
AND
DISRUPTIONS
ARE
FACTORS
IN
CHOICE
MEASUREMENTS
VOLTAGEANDCURRENT
MEASUREMENT,
AND
CONVENTIONAL
MEASUREMENTSOF
VOLTAGE,
CONVENTIONAL.MEASUREMENTS
OF
VOLTAGE.
CONVENTIONAL
MEASUREMENTS
OF
VOL7AGE,
CONVENTIONAL
SYSTEM
INSTRUMENTATION
ACCELEROMETER
OR
FREQUENCY
MEASUREMENT
CURRENT
AND
POWER
CURRENT,
AND
POWER
CURRENT,
AND
POWER
C
N
N
E
P
IS
N
N
B
E
R
C
T
BE
C
ANDTELEMETERINGPROTECTION
~CENTER
OF,
THE
CONTROL
CENTER
SF
THE
STATUS
OP
MAIN
THE
CONTROL
CENTERSF
THE
STATUS
OFMAIN
I.
LOCAL
EQUILIBRIUM
STATE
OR
LOAD
TRANSMISSION
BREAKERS
AND
GENERATORS.
TRANSMISSION
AREAKERS
AND
GENERATORS.
I
CONVENTIONAL
TELEMETERING
TO
THE
CENTER
C
ELEMETERING
TOTHE
CENTER
2.
STATUSOF
MAINTRANSMISSION
OF
UOLTAGE
AND
PONER
VALVES
FOR
STATE
OF
VOLTAGE
AND
POWER
VALVES
FOR
STATE
BREAKERS
AND
GENERATORS
(BUT
NOT
ESTIMATION
ESTIMATION.
OF
SUBTRANSMISSION
OR
LOADS)
SCONOMIC
DISPATCH
INACTIVE
INACTIVE
INACTIVE
INACTIVE
RESTORED
4AUTOMATIC
GENERATION
INACTIVE
INACTIVE
ACTIVE.
WILL
TEND
TO
ALLEVIATE
LOAD
ACTIVE
RETURNS
TONORMALOPERATION
CONTROL
EXCESS
BYFREQUENCY
ADJUSTMENT
OF
THE
SYSTEM
ON
LINE
STATE
INACTIVE
INACTIVE.
SUPERSEZ..U
BO
LOCAL
EVUILIB-
INACTIVE:SUPERSEDED
BY
LOCAL
EQUILI-
REACTIVATED
ACTIVE
ESTIMATION
AIUM
STATE
ESTIMATION
AT
THE
INOIVIDUAL
BRIUM
STATE
ESTIMATION
AT
THE
INDIVI-
VUSSES
DUAL
BUSSES
Table2.
TheControlRegimes.
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4 1Principal a l g o r i t h m i c features can b e1 . L oc al s tr uc tu ra l control or protective r e-laying t o eliminate f a u l t y c o n p o n e n t s andl o c a l control action ( R e gi me # 1 ) * to pre-serve s t a b i l i t y o f a n individual g e n e r a t o r .2 . Stability augmentation u t i l i z i n g t h e DC net-w o r k , b r ak i ng r e s is t o r s , load s k i p p i n g , etc.( R e g i m e # 2 ) .3 . Load D r o p p i n g or m a j o r Structural S u r g e r y to
separate t h e system in t h e least o b j e c t i o n -a b l e , manner when i n t e g r i t y c a n n o t b e main-t a i n e d ( R e g i m e # 3 ) .Note t h a t 3 . p u s h e s t h e system into a n I n t e g r i t yCrisis ( e xp l a in ed b el o w) .2 . 2 . 5 . D e a r e e # 4 . V i a b i l i t y C r i s i s . The systemin i t s p r e s e n t c o n d i t i o n i s i n c a p a b l e o o p e r a t i n g c o n -t i n u o u s l y o r o f s e t t l i n g back to normal o p e r a t i o n be-cause t h e l o a d s , t h e available g e n e r a t i o n , a n d t h ea v a i l a b l e transmission c a p a b i l i t y a r e n o t i n balance.This condition c a n result f r o m a fault a n d i s thenfrequently preceded b y a Stability Crisis. N o q n v i ablec o n d i t i o n s can a l s o , h o w e v e r , come on q u i e t l y ; f o r in-s t n a c e , m a j o r g e n e r a t i o n or other e q u i p m e n t d o e s n o tb e co m e a v ai l ab l e when i t i s s c h e d u l e d t o come o n linef o r a p e a k .P r i n c i p a l a l g o r i t h m i c f ea tu re s i nc lu d e i n a r o u g ho r d e r o f i n c r e a s i n g u n d e s i r a b i l i t y ( R e g i m e # 3 ) :1 . Frequency reduction o f t h e AGC t y p e2 . U se o f s p i n n i n g reserves or cold reserves3 . S p e c i a l m e a s u r e s such as f a s t turbine r u n -b a c k4 . H e l p from n e i g h b o r i n g a r e a s5 . Drawing o n time-limited overload c a p a b i l i t yo f t h e e q u i r m e n t6 . S p e c i a l m e a s u r e s such as voltage reduction7 . First stage structural control such a s ad -d i n g n ew g e n e r a t i o n or r et ur ni ng e q u ip me ntf ro m m a i nt e na n ce or load d r o p p i n g ( f o r min-utes or h o u r s )
8 . S e c o n d stage structural c o n t r o l - I s l a n d i n gNote t h a t 7 . or 8 . m o v e s t h e system into a n I n t e g -rity C r i s i s .2 . 2 . 6 . Degree # 5 . - I n t e g r i t y Crisis or " I n Ex-t r e m i s " C o n d i t i o n . Th e i n t e g r i t y o f t h e system i sv i o l a t e d ; f o r i n s t a n c e , load wa s d r o p p e d or t h e systemi s i s l a n d e d .P r in c i p a l a l go r it h mi c measures i n c l u d e :
1 . Restoration o f t h e V i a b i l i t y o f t h e indivi-dual i s l a n d s as i n D e g r e e # 42 . Reconnection o f t h e islands3 . Restoration o f l o a d s , etc.
3 . SOME FICTITIOUS C ASE HISTORIESTo elucidate t h e o p e r a t i n g process visualized agroup o f q u a l i t a t i v e case histories are shown in Figure3 . Broken lines indicate i m p o s e d events, a n d s o l i dlines represent control a l g o r i t h m i c o p e r a t i o n s .Case D A short circuit o c c u r s o n a transmissionline which m o m e n t a r i l y e n d a n g e r s th e s t a b i l i t y ( D e g r e e# 3 ) but t h e selective p r o te c t i o n r e la ys and circuitbreakers c l e a r t h e f a u l t ( R e g i m e # 1 ) a n d s u c c e s s f u l l yreclose t h e l i n e . Back t o n or ma l a lm os t i n s t a n t l y .Case 9 S a m e as in C a s e Db u t with a f a i l e d r e-c l o s i n g a n d ultimate loss of t h e l i n e . R e g i m e 1 # 2 i s s u c -c e s s f u l in r e t a i n i n g system s t a b i l i t y but t h e l o s s o ft h e line l e a v e s t h e s y s t e m with a S e c u r i t y Defect( D e g r e e # 2 ) until t h e DC system i s used to s h i i f ts u f f i c i e n t load to r e s t o r e s e c u r i t y a l t h o u g h t h e str uc-t ure i s still altered from normal ( D e g r e e # 1 ) .C a s e ( Q Same initial events b u t t h e r e c l o s i n gf a i l s a n d s o d o e f f o r t s f o r s t ab i li t y a u g m e n t a t i o n i nR e g i m e # 2 . The s y s t e m b r e a k s u p , f a l l s into an Inte-
(Regime # 5 ) b u t h a s i n s u f f i c i e n t security (Degree # 2 )u ntil C ontrol i s used to shift loads t o e l i m i n a t e d e-f ic ie nt s ec ur it y. F in al ly , t h e faulty line is restoredto operation and everything goes b a c k to normal ( D e -gree # 0 ) .Case Same initial e v e n t a t peak load wit h afailed breaker an d s ub sequ en t b ac ku p clearing. T h i sc r e a t e s a Stability Crisis (Degree # 3 ) whi c h is s u c c e s s -fully handled by th e E mer gen cy C ont ro l (Regime #2).The s y s t e m integrity i s m o me n ta r il y p r es er v ed ( n o loadsare l o s t , no islanding) and t h e s y s t e m remains viablealthough insecure u n t i l later a h ea v il y l oa de d tie froma neighboring s y s t e m i s lost in an independent i n c i d e n t .T hi s l ea ve s the s y s t e m t e mp o ra r il y n o nv i ab l e (Degree#4)with n o t e no ug h g en er at io n to cover the loads. Inte-g ri ty C ri si s ( l o a d dropping or breakup) i s prevented b yEmergency Control - Regime # 2 subsequently refined b yRegime # 3 using a judicious combination o f spinning re-serves, short time overloads, shifting o f loads b y theDC network, and voltage red u ction until t h e waning o fthe peak load ( i t was decided to wait this o u t ratherthan try to bring in new g eneration) and t h e gradualr e t u r n to normal i f n o t quite fully secure o p e r a t i o n(Degree # 2 ) . At this point, another fault on a t r a n s -mission line c r e a t e s a new Stability C ri si s ( De gr ee # 3 )with a subsequent breakup o f t h e s y s t e m wi th loss o fload (Degree # 5 ) .C a s e D G e n e r a t i o n s c h e d u l e d to come on linej u s t before th e peak rise becomes unavailable u n e x p e c t -e d l y . Th is c r e a t e s the development of a Viability Cri-sis (Degree # 4 ) as the loa d rises withou t, however,causing any s ta bi li ty p ro bl em s. Load dropping i sa v o i d e d by Emergency C o n t r o l in Regime # 3 , th at i s , byusing reserves a n d voltage r e d u c t i o n as well as in-creased tie line help until a fault on a neighboringarea c u t s down on the availability of the tie line helpin the face of still i nc re as in g p ea k load. Th is d a n -geroulsly o v e r l o a d s the generators w h i c h f o r c e s loaddropping an d thus a c c e p t a n c e of an Integrity Crisis( D e g r e e # 5 ) in o r d e r to a v o i d shutting o ff the g e n e r a -t i o n . Eventually the l o a d s are gradually r e c o n n e c t e das more g en er at io n b ec om es a va il ab le an d the loa d b e -gins to drop taking th e s y s t e m b a c k t ow ar ds n or ma l op-e r a t i o n (Degree # 0 ) .In these examples as i n the literature StabilityCrises play a dom inant role. I n p ractice p r ob ab lyCaseG is more typical of the e v e n t s th an the othercases because m o s t s ystem s are n o t susceptible toStability Crises. In this co untr y , the W e s t e r n Systemsare p r o n e to Stability Crises mostly.
4 . STRUCTURE O F D E C I S I O N A N D CONTROL; REGIMESOF C ONTROL A L G O R I T H M SA f t e r t h i s b r i e f i n t r o d u c t i o n a n d i l l u s t r a t i o n o fth e D e c i s i o n a n d C o n t r o l Operation i d e a s , some d e t a i l sof t h e s t r u c t u r e of t h i s operation are now i n order.T h i s s t r u c t u r e is s k e t c h e d i n T a b l e 1 . Th e o p e r a -t i o n i s d i v i d e d i n t o a Decision Phase a n d a C o n t r o lPh ase. E a c h in t u r n s u b d i v i d e s into a S el ec t io n T as kan d an A c t i o n Task.I n t he D e c i s i o n Phase, f i r s t s e l e c t i o n is p e r -formed a m o n g s ix Degrees of A b n o r m a l i t y w h i c h were a l-ready described. Two of these Degrees are described
as D e fe c t s (Structural a n d Security respectively).T h e s e require c o rr e ct i ve a c ti o n b u t n o t a " s c r a m b l e tocorrect" since they d o n o t lead to f ur th er d et er io ra -t i o n of th e s it uat io n wi th ou t a dd it ion al adverseoccurences. Three Degrees are described as Crises(Stability, Viability and Integrity). These requirei m m e d i a t e action because, in the absence o f i t t h esi tu ati on wi ll degenerate, possibly into an ultimatetotal blackout. Th e urgency and th e n a t u r e of thiscontr ol a ct io n depends on th e Degree o f Abnormalityan d furth er specifics wi thin each Degree. Dependingon this judgement the Decision Phase in its A c t i o nTas k assigns the s i t u a t i o n to one of the six Regimes
g r i t y Crisis ( D e g r e e # 5 ) a n d i s later r e s y n c h r o n i z e d* The s i x r e g i m e s o f control w i l l b e described i ndetail i n Section 4 .
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4 2o f Control fo r handling.Each Regime o f Control i s c o m p r i s e d o f a set o fa p p l i c a b l e control t o o l s and a l g o r i t h m s which providea choice o f measures a n d means d e a l i n g with a s e t o fc ir cu ms ta nc es r equ i ri ng r e m e d i e s within a given timef r a m e ( T a b l e 2 ) .The Selection T a s k o f t h e Control Phase ( T a b l e 2 )will pick t h e b est route t o remedy t h e problem basedo n t h e available choices o f t o o l s , t he ir l oc at io n, andt h e i r d i s r u p t i v e consequences o n s er vi ce . Optimalityi n Regime # 1 - 4 i s mainly i n terms o f minimal disrup-tion a n d particularly in t h e f a s t Regimes l i k e Regime# 1 or 2 i t will have t o be f o u n d from l imi ted i n fo r ma -t i o n a n d t h e co nt rol wi ll h av e t o b e p er fo rm ed locally.The A c t i o n Task i s finally initiated t o carry ou t t h ec o n t r o l .4 . 1 . Regime # 0 : This i s not shown in Table 2and consists o f t h e normal op erating p rocedu res sucha s A G C , E c on o mi c D i sp a tc h , S ta te E s ti m at io n, e t c . a swere l i s t e d i n S e c t i o n 2 . 2 . 1 . A d d i t i o n a l l y , i n t h ecase o f Structural o f S e c u r i t y D e f e c t special d e c i s i o na l g o r i t h m s are r e q u i r e d t o establish t h e need f o r rem-e d y i n g t h e s e d e f e c t s and t h e way o f remedying t h e m .Time i s usually not p ressing here and s o relativelye l a b o r a t e algorithms may b e p e r m i s s i b l e [ 6 ] .
4 . 2 . Regime # 1 : I t controls t h e strictly localaspects o f a s i tu a ti o n w h er e a f a u l t ma y occu r on someequipment. T h i s e qu i p me nt m u st then be selectivelyr e m o v e d by t h e s el ec ti ve p ro tec ti on relays which ar es p e c i a l purpose computers ( c l a s s i c a l l y electromechani-c a l , i n c r e a s i n g l y m i c r o p r o c e s s o r s ) s o t h a t only t h ef a u l t y equipment i s r e m o v e d and - in case o f transmis-sion l i n e s - r e t e s t e d i n about 2 0 c y c l e s b y r e c l o s i n g .While a s h o r t curcuit essentially shuts o f f power f l o wby r e d u c i n g t h e voltage t o nearly z e r o , a l o c a l gen-erator may s p e e d u p enough t o pull out o f synchronismu n l e s s c h e c k e d by t a k i n g out a quick pulse o f energyf r o m i t s rotor by some means such as a d a m p i n g resis-t o r . T h i s operation i s controlled s t r i c t l y l o c a l l y b ya microprocessor much i n t h e nature o f s e l e c t i v e pro-tection r e l a y i n g a n d shares R e g i m e # 1 with t he l at te r.4 . 3 . R e g im e # 2: This c o n t r o l s a system w i d e .problem on a very short time scale o f a f e w s e c o n d s( l e s s t h a n 3 s e c o n d s ) . T h i s arises when a d i s t u r b a n c ep uts t h e sy stem into a violent d y n a m i c state whicht h r e a t e n s t o break i t i n t o segments ( M u l t i m a c h i n e orMultiswing I n s t a b i l i t y ) . The time s c a l e d o e s notpermit c e n t r a l l y c o o r d i n a t e d a c t i o n . Each unit mustac t alone u n d e r t h e control o f i t s own minicomputerbu t t h e c o l l e c t i v e action m u s t e f f e c t i v e l y s t a b i l i z et h e s y s t e m . These c o n t r a d i c t o r y r e q u i r e m e n t s can b er e c o n c i l e d , a s will be s h o w n , b y u s i n g a n ew states p a c e , Observation D e c o u p l e d S t a t e S p a c e , introducedb y t h e a u t h o r s . I t i s important t o n o t e , however,t h a t a l t h o u g h m e a s u r e s in R e g i m e # 2 preserve the sys-tem transient s t a b i l i t y i n t h e sense t h a t t h e y preventou r o f s t e p c o n d i t i o n s on t h e system, t h i s i s accom -p l i s h e d with s h o r t range d a m p i n g d e v i c e s like b r a k i n gr e s i s t o r s . S o t h i s R e g i m e cannot b e maintained i n -d e f i n i t e l y ; i t must e n d in a very f e w s e c o n d s . I fat t h a t t i m e , t h e system i s not viable f o r eithers t a b i l i t y or overload reasons then i t would s t i l ld i s i n t e g r a t e i f remedial measures are not t a k e n . Thef e w s e c o n d s g a i n e d in R e g i m e # 2 m u s t then be used tog e t r e a d y f o r a c t i v a t i n g t h e remedial m e a s u r e s o fRegime # 3 .4 . 4 . R e g i m e # 3 : This i s d e s i g n e d to d e a l with asystem which i s s e v e r e l y nonviable ei ther b ec au se o fi n s t a b i l i t y or because o f o v e r l o a d . Since t h e c o n d i -tion i s s e v e r e ( a s a l w a y s in i n s t a b i l i t y based V i a b i l i t yC r i s e s ) f a s t action i s n e e d e d within a f e w s ec on d s ( f o ri n s t a b i l i t y ) or a t most h a l f a minute ( f o r severe o v e r -l o a d ) . T h e action m u s t b e c e n t r a l but i t will have t obe b a s e d on f a s t c o n t i n g e n c y evaluation u s i n g l i m i t e di n f o rm a t i o n r e g a r di n g t h e imbalances. N ew a n c i l l a r yt e c h n i q u e s f o r these w e r e d e v e l o p e d i n t h e co urse o f
t h i s project a n d are recorded in a c om p an io n p ap er [ 5 ] .T he se t ec hn iqu es which ar e based on t h e ObservationDecoupled State Concept make i t possible t o t a k e ac-tion in Regime # 3 which will make t h e system viableprovisionally although there may still p ersist over-loads which cannot be allowed f o r more than 5 t o 10min.4 . 5 . Regime # 4 : During th e 5 -1 0 mi nu tes g ai nedin Regime # 3 , an operating condition s hou ld b e devel-o p e d which assures Viability f o r an hour or m or e u nt ilf u l l r e st o ra t io n b e co m es p o ss i bl e. I n a l e s s severeViability Crisis, Regime # 4 can b e d ir ec tl y addressedw it h ou t p r ec ed i ng i t with Regime # 3 . In either c a s e ,in Regime # 4 , on th e time scale o f 5 - 1 0 m in ut es , S ta ti cState Estimation can b e r e s t o r e d , Load F lows an d othera lg or it hm s r equ ir in g a few minutes can be performeda n d s o an o v er al l s ol u ti on ca n b e re ac he d w it h c onf i-dence w hi ch w il l make t h e system viable fo r t h e desiredduration o f an hour or more. I t i s a l s o possible t oselect those measures which are l e a s t undesirable,t h a t i s , least d i s r u p t i n g ; f o r i n s t a n c e , bringing inpower from t h e neighbors or starting gas turbines i sl e s s d i s r u p t i n g t h a n load d r o p p i n g o r i s l a n d i n g . Inf a c t , i t should be possible f o r instance t o put backi n Regime # 4 some o f t h e customer loads which wered r o p p e d in a p r e c e d i n g Regime # 3 because f a s t actionwas r e q u i r e d i n t h e l a t t e r . On t h e other h a n d , someequipment l o a d s may need t o be reduced f u r t h e r becausel o a d s which can b e tolerated f o r a f ew m in u te s may notbe a c c e p t a b l e f o r an h o u r .4 . 6 . R e g i m e # 5 : F i n a l l y comes an eventual r e c o n -struction l e a d i n g back t o normal c o n d i t i o n . This wouldb est b e carried o ut by t h e operator aided b y th e Con-t r o l Center computer, both d r a w i n g on all th e reserveso f t h e system.The structure sketched in Table 1 i s mea n t to bes c a n n e d a t every t i m e s a m p l e p o i n t d u r i n g t h e o p e r a t i o n .For i n s t a n c e , a t t h e t i m e s a m p l e indicated a s t 1 inFigure 3 f o r Case Q th e D ec is io n a nd Control scan mayrun a s shown in Table 1 b y t h e dashed l i n e leading t oR e g i m e # 3 a n d t h e output c on tr ol o rd er m ay c on si st o fo r d e r s t o ru n back certain generators f a s t . Some timea f t e r such preliminary restoration o f Viability, a tanother t i m e s a m p l e , sa y t 2 , t h e Decision Phase may optf o r Regime # 4 resulting i n an " o p t i m a l " s y st em v i ab l ef o r a n hour or s o until f u l l restoration t o normalcybecomes p o s s i b l e i n Regime # 5 .
5 . DEMONSTRATION OF PERFORMANCE BYA SIMULATED CASE HISTORY; AN EXAMPLESeveral years o f research work i s represented byt h e D ec is io n a nd Control type system o p e r a t i o n s u m m a -r i z e d in t h i s paper. Muc h detailed work i n s o l u t i o n s ,n ew i n s i g h t s , new concepts a n d n ew c o m p u t a t i o n a l or al-g o r i t h m i c t o o l s were d e v e l o p e d in t h e course o f t h i sr e s e a r c h . Some o f t he se r es u l ts were previously pub-l is h ed , p a rt ic u l a r l y those r e l a t i n g t o t h e ObservationD e c o u p l e d State S p a c e . Other d e t a i l s will be giveni n c o m i n g p u b l i c a t i o n s [ 4 ] , [ 6 ] , a n d i n a companion
paper [ 5 ] . With r e f e r e n c e t o T a b l e 1 , detailed re-s u l t s are n o w a v a i l a b l e f o r Control R e g i m e s # 0 , # 1 , # 2# 3 , a n d - # 4 . Because o f t h e sheer bulk o f th e researchr e s u l t s i t i s i m p o s s i b l e t o cover them even s k e t c h i l yin one paper. All that i s p o s s i b l e here i s t o illus-trate b y s i m u l a t i o n , o n a r e l a t i v e l y s i m p l e emergency,t h e sequence o f e v e n t s which t a k e s p l a c e in Decisiona nd C o n t r ol Type O p e r a t i o n .The system k n o w n a s t h e IEEE 1 1 8 bu s test system[ 4 ] a n d shown in F i g u r e 4 will be used f o r t h i sd e m o n s t r a t i o n . S i n c e , h o w e v e r , t h e IEEE 1 1 8 b u s sys-t e m , which d e r i v e s from the network o f a l a r g e Mid-w e s t e r n U t i l i t y as i t was some years a g o , i s not su s-c e p t i b l e to either S t a b i l i t y o r V i a b i l i t y C r i s i s ,( o n c e a t o r n a d o t o o k ou t several lines onthis systemwith n o serious d i s r u p t i o n o f s e r v i c e ) , i t i s n e c e s -s a r y to m o d i f y t h e system t o illustrate a S t a b i l i t y
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I M P E D A N C E S ( p . u . )0 . 1 9 + j O . 6 20 . 3 + j o . 9 8 80 . 1 4 + j O . 5L O A D ( M V A )3 6 + j 2 76 + j 92 4 + j 1 01 5 + j 1 237+j224 + j 22 0 + j 1 0O + j O3 0 0 + j 9 3O + j OO + j O
B U S GENERA T ION ( M W )10 4501 2 8 52 5 5 3 4
Figure 4 . T h e M o d i f i e d IEEE Systemand Viability C r i s i s . Specifically, slort 1.neo andtransformers were consolidated f o r computational pur- 1 4 0 -poses and impedances of lines 15-33,19-34 a d d Z 3 - 2 4 weremodified as shown in Figure 4 . Also shown in Figure 4 1 0 5i s t h e modified l o a d s , and generation on t h e l e f t sideof the dotted line. O t h e r base case line, g e n e r a t o r , 70 10and load data areavailable in [4 ] among other sources.With th e modified line i m p e d a n c e s , t h e seg ment of t h es y s t e m to th e left of the dotted line becomes an A r e aconnected to th e larger s y s t e m on t h e right by a se tof f ou r r at he r weak ti e lines. In f a c t , th e transient Q os t a b i l i t y o f t h i s combination i s s o m a r g i n a l t h a t onewould b e u n l i k e l y t o o p e r a t e such a s y s t e m i n t h est ate of art. -One point of t h e following illustration 0 . oi s that operating such s y s t e m s becomes possible byusing Decision and Control t e c h n i q u e s . This woulda m o u n t to great savings in installations of new equip- Figure 5 . W v i t h cm e n t . t he system brea k EN o w the f o ll o wi n g c o nt i ngenc y is assumed:1 . A total 3 phase short circuit occurs a tb us 1 7 .
2 . Regime #1 controls initially consist of se-lective network protection which springs into actionautomatically a nd locally to isolate the faulted equip-m e n t . However, a c u r c u i t breaker fails to function andt h e b ack up clearing takes a total of 12 c y c l e s . Alsoline 1 7 -38 and the t ot al load o f A P= 3 0 0 MW a t b u s 17 arelost during this operation. Th e stability augmentationpart of Regime # 1 does n o t come into operation sincebus 17 contains no generator.U p to this point everything i s c o n v e n t i o n a l inthis partitular contingency. If no further actionswere t a k e n t h e n as the time history o f b u s phase an-gles plotted in Figure 5 s h o w s , the system would b eviolently unstable and would break into two islands -aclassic case of m u l t i ma c hi n e m u l t i w i n g instability.In contrast Figure 6 shows what happens i f Deci-sio n and Control operation runs its course as follows:3 . Through all the Regimes ( # 0 - 5 ) computation ofthe Observation Decoupled State 6 proceeds at a fixedsampling r a t e on s p e c i a l , dedicated microprocessors
7 0
3 5
-v< 5
- 7 0 -
TIME ( S E C O N D S )
only Conventional Selective P r o t e c t i o ns u p i n t o two i s l a n d s .
0. 6 i. 2 1. 8TIME ( S E C O N D S )
Ftgsoe 6 . W i t h f u l l D e c i s i o n an d C o n t r o l Operation se-quence t h e s y s t e m Stability a nd V i ab i li t y is r e s t o r e d .
L I N E1 5 - 3 31 9 - 3 42 3 - 2 4B U Sl234671 51 61 71 81 9
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( a b o u t 5-10 milliseconds per computation on IB M 3 6 0 / 6 5 ) .The output is near zero everywhere f o r 6 d u r i n g normalo p er a ti on b u t a t the instant o f t h e fault nonzero valuesor no solutions are obtained a t some busses because ofvoltage collapse. Th e same m i c r o p r o c e s s o r s also com-p u t e ( i n a b o u t 1 0 m i l l i s e c o n d s on the I B M 3 6 0 / 6 5 ) f r o mthe O b s er v at i on D e co u p le d State data any sudden c h a n g e s ,A P , in the load balance a t t h e local b us ( S e e [ 4 ] ) .
4 . Information on 6 and/or A P and on t h e actiono f t h e selective protection in Regime # 1 i s flashed tot h e Decision Phase in th e Control Center w h i c h at thenext sample t i m e n o t e s that an Abnormality is in p r o -gress in the vicinity o f bu s 1 7 and g i v e s alarm to theo p e r a t o r . No action i s , however, taken by the o p e r a -tor. The De c i s i o n Phase orders Regime # 2 into a c t i o n( T a b l e 1 ) by doing nothing R f e g i m e # 2 i s local andautomatic.
TIME ( S E C O N D S )t i g u r c - 9 . W i t l h only local s t a a i l i t y augmentation ini ' e g i m e # 2 th e system s t a b i l i t y i s momentarily preserv-e d even w h e n t h e f a u l t is cleared after 30 cycles.
35
0
- 3 5T I M E ' ( S E C O N D S )
-7 0
F i g u r e 7 . V i i ; oil;7 Local S t a b i l i t - T - A u i - : nt. on i i iRegime # 2 t h e s y s t e m Stability i s momentarily p r e s e r v -e d b u i t t h e s y s t e m is n o t Viable.
.0 0.6 1.2 1.8 2 . 4TIME ( S E C O N D S )
Figure 8 . ' I i t h only c o n v e n t i o n a l selective ar-otectiona ct in g w it hi n 3 cycles the system breaks u p into twoi s l a n d s .5 . Regime # 2 waits till 0.2 s ec on ds a ft er theo n s e t of the fault that i s l on g en ou gh t o let Regime# 1 take care of the post fault switching and clearingof the fa ult (Figure 6 ) . A t th at time it comes intoaction ( T a b l e 1 ) using braking resistors a n d sh ort t i m eload skipping a t the v a r i o u s busses. T h e commandingof these control tools is strictly local using a normreducing c o n t r o l l a w [ 4 ] . The Regime #2 c o n t r o l byitself is effective in stabilizing the system tempo-rarily as illustrated b y Fig ure 7 . In fact the effec-tiveness of this c on tro l in stabilizing t h e system i struly r e m a r k a b l e . Figure 8 s ho ws the course o f t h i se m e r g e n c y when no Regime # 2 c o n t r o l is u s e d b u t t h efault is cleared in 3 cycles. Th e system is still v i o -lently unsta ble. O n the o th er h an d Figure 9 s hows t ha tRegime # 2 c o n t r o l s r ea di ly k e ep t he s ys te m t og eth ereven when the fault persists for 30 c y c l e s . T h i s r e p r e -sents a m o r e t h a n 1 0 times increase o f t h e c r i t i c a lclearing time. T h i s is a d r a m a t i c i l l u s t r a t i o n o f t h eeffectiveness o f Regime #2 c o n t r o l s w h i c h are t h e re-sult of th is project.
o 0. 6 1. 2 1 . 8 2. 4TIME ( S E C O N D S ).igure 1 0 . h i l h e n L oc al S ta bi li ty A u gm en t at i on in R eg i me1 2 i s turned off and n o t followed by Regime # 3 , insta-bility recurs.It can b e further o bs er ve d, h ow ev er , i n Figure 1 0that t h e s y s t e m will r e t u r n to instability i f theRegime # 2 control i s turned off -in this case it i sturned off a t 1 . 5 seconds. The point i s that Regime # 2m u s t e ve nt ua ll y b e turned o f f because t h e resistorsheat u p . S o , it i s important that appropriate measuresbe t a k e n in this case b e f o r e turning off the resistors.T hi s c al ls for Regime # 3 c o n t r o l . H o w i s then t h e de-cision reached to t u r n to Regime # 3 , t h i s latter beingcentrally rather than locally c o n t r o l l e d ?A reliable value for A P at b u s 17 of a b o u t 3 0 0 1 4 Wbecomes a va il ab l e a bo ut 0 . 5 to 0 . 7 5 seconds after t h eo n s e t of the f a u l t . ( O f course, t h e experimenter knowsit i s 300MW lost b ut t h e D ec i si o n P ha se o nl y f in ds outa t t h i s point). A t t h i s point th e Decision Phase per-forms a t t h e next sample ( s e e dashed line in T a b le 1 )th e C o nc e nt r ic R e la xa t io n Algorithm [ 5 ] around b u s 1 7 .This i s estimated to take about 0 . 1 - 02 s ec ond s inthe C on tr ol C en te r com p uter . The conclusion i s reachedfrom this computation that the p o s t fault steady s t a t etorque angles on the lines of the dashed line c u t s e t inFigure 4 range 60-70 degrees i f no change i s made on theaffected a r e a . (This pa r ti c ula r c o mput at i o n i s pre-sented in detail in a companion p a p e r [ 5 ] ) . This istaken b y the Decision Phase as e v i d e n c e of a ViabilityCrisis on a cc ou nt of instability s i n c e it is clear that
even a sm all transient c a n n o t s e t t l e o u t at t h e s e largetorque angles. I m m e d i a t e V i a b i l i z a t i o n i s called fo ron the affected area and the Degree # 4 D e ci si on b lo ckin Table 1 p u t s the Control Regime #3 in charge asshown by the dashed l i n e . T hi s c ho ic e only takes afew microseconds. N o t e th at th e existence of a weakc u t s e t wou ld b e k n o wn and so this c u t s e t would betested as a m a t t e r of routine.5 . Regime #3 scans t h s available control meanson th e area to the left of th e dashed c u t s e t in F i g u r e4 since vi a b i l i z a t i o n clearly requires action on t h i ssegment given the j us t c om pu te d large s t e a d y statetorque angles without such action. It finds t ha t f a s t
4 4
0"vs
4
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2. 1T I M E ( SEC O N D S )
Figure 1 1 . Regime 1 3 alone would make t h e r y s t e m Via-ble but i t cannot restore S t a b i l i t y .generator runback o f 1 5 0 M W each i s available at units# 1 0 and # 2 5 . T h i s measure i s c h o s e n b y t h e controlalgorithm [ 4 ] since t h e o n l y l e s s objectionable mea-s u r e , f r e q u e n c y reduction ( T a b l e 2 ) , i s no t e f f e c t i v ein a Viability Crisis c a u s e d b y i n s t a b i l i t y . Thet o r q u e a n g l e s with runback o n t h e d a s h e d cutset arec h e c k e d b y Concentric Relaxation [ 5 ] again a n d aref o u n d t o b e in t h e range 8 0 t o 1 4 0 ( a n o t h e r 0 . 1 - 0 . 2s e c o n d s ) . This i s q u i t e s a t i s f a c t o r i l y s t a b l e . Con-s e q u e n t l y runback o f 1 5 0 M W each on u n i t s # 1 0 a n d # 2 5 i sordered and carried out a t 2 s e c o n d s a s shown i n Figure6 with clearly very s a t i s f a c t o r y r e s u l t s . I n contrastFigures 1 0 and 1 1 show that n e i t h e r o f Regime # 2 o r# 3 alone i s effective in s t a b i l i z i n g t h e s y s t e m .6 . U s i n g a combination o f Regimes # 2 a n d # 3 asin Figure 6 t h e s y st em r em ai ns s ta bl e a nd viable butf u r t h e r a d j u s t m e n t s are needed i n a matter o f minutessince f a s t runback l e a v e s th e units in a conditionwhich i s o n l y t e m p o r a r i l y a d m i s s i b l e . T h u s R e g i m e # 4will b e initiated and i t will o r d e r some action liket h e n o r m a l i z i n g o f t h e unit # 1 0 a n d # 2 5 generation att h e i r reduced l e v e l or somewhat h i g h e r - d r a w i n g somea d d i t i o n a l power t h r o u g h t h e t i e s . A GC will need t o b er e a d j u s t e d t o d o t h e l a t t e r .
7 . Restoration - Regime # 5 will se e t o t h e recon-nection o f t h e l o s t load and a r i s e back t o normal o ft h e g e n e r a t i o n after t h e f a u l t e d e q u i p m e n t i s checkedout or p o s s i b l y b y p a s s e d .Regime # 4 a n d Regime # 5 a l g o r i t h m s have not yetbeen d e v e l o p e d . This remains f o r f u r t h e r r e s e a r c h .6 . CONCLUSIONS
The p r e c e d i n g case history i l l u s t r a t e d just on er e l a t i v e l y uncomplicated emergency situation where asystem which would not b e p ra ct ic al w it h state o f a r toperating t e c h n i q u e s i s maintained s t a b l e an d operatingby t h e use o f t h e Decision and Control O p e r a t i o n . Ad -option o f a f u l l y developed operating practice alongt h e s e l i n e s would e x t e n d decisively t h e operating rangeand u n d i s r u p t e d t i m e s o f t h e large i n t e r c o n n e c t e d powers y s t e m . T h i s , o f c o u r s e , would r e s u l t in major s a v i n g sby r e d u c i n g t h e cost o f equipment especially expansioncost o f t r a n s m i s s i o n . There s o u l d be o f f s e t t i n g c o s tin t e l e c o m m u n i c a t i o n and c o n t r o l equipment but i t i sc o n j e c t u r e d t h a t t h e added e xp e ns e w o ul d be dwarfed byt h e s a v i n g s .
7 . APPENDIXDEFINITIONS O F A F EW T E R H S
S t a b i l i t y Crisis i s an ongoing d y n a m i c conditiono f t h e system which will lead t o system breakup unlessemergency measures are t a k e n .System Security i s t h e ability o f t h e system t oremain Viable i f stricken by any o f a s e t o f preselectedpotential d i s t u r b a n c e s ( e . g . : First c o n t i n g e n c i e s ) .
S e cu r it y D e fe ct i s t h e condition o f t h e systemwhen t h e system i s not Viable f o r some contingencieswithin t h e set o f p r e s e l e c t e d potential d i s t u r b a n c e sused to define S e c u r i t y .System Viability i s t h e ability of t h e systemt o operate i n a given condition without t h e l o s s o fl o a d or stability and with t h e f r e q u e n c y , t h e v o l t a g e s ,t h e currents, etc. r em ai ni ng wi th in tolerances whichare p e r m i s s i b l e f o r a given t i m e period. The system i st h e n viable i n t h i s g i v e n condition f o r t h i s given t i m ep e r i o d .Viability Crisis i s an ongoing condition wheret h e t i m e p e r i o d o f Viability i s s o short ( i n c l u d i n gz e r o ) t h at i t requires emergency action t o lengthen i ts u f f i c i e n t l y f o r continued system operation. Typicalranges:0 - 0 . 1 seconds range-usually connected with sta-b i l i t y or torque a n g l e p r o b l e m s ( R e g i m e # 3 )1 minute range-usually connected with thermal overl o a d or undervoltage ( R e g i m e # 3 )1 0 minute r a n g e - u s u a l l y connected withoverload o r undervoltage ( R e g i m e # 3 ) t h e r m a lSystem Integrity i s a condition of the system
where all load s are supplied according t o their demandand a l l parts o f th e system are e n e r g i z e d and inter-connected as s c h e d u l e d .Integrity Crisis i s a condition where system In-t e g r i t y i s l o s t in some respect f o r i n s t a n c e , loadshave been d r o p p e d , some parts of the system are deen-ergized or t h e system i s islanded into disconnectedsegments.Note: Loss o f individual pieces o f equipment sucha s a generator, a transmission l i n e , etc. i s considereda Structural Defect ( p o s s i b l y combined with a higherlevel of Abnormality - a Viability or I n t e g r i t y C r i s i s ,fo r i n s t a n c e ) b ut not per se as I n t e g r i t y C r i s i s .Structural Control consists of structural changes( s w i t c h i n g o f l i n e s , g e n e r a t o r s , i s l a n d i n g ) ordered byt h e control computer and aimed at a l t e r i n g th ebehaviour o r t h e Viability o f t h e s y s t e m . Two levels1 . Local or First S t a g e Structural Control:Structural Changes carried out l o c a l l y based onl o ca l i nf o rm a ti o n. E . g . th e f u n c t i o n i n g o f selec-tive protection or load s k i p p i n g .2 . Second S t a g e Structural C o n t r o l : StructuralChanges ordered b y the Control Center C o m p u t e r .E . g . : Intentional i s l a n d i n g
8 . REFERENCES1 . T . E . Dy L i a c c o , " T h e Adaptive R e l i a b i l i t y ControlS y s t e m " , IEEE Transactions on P A S , May 1 9 6 7 , p p .5 1 7 - 5 3 1 .2 . - L . H . Fink a n d K . C a r l s e n , " O p e r a t i n g under Stressand S t r a i n " , IEEE S p e c t r u m , March 1 9 7 8 , p p . 4 8 - 5 3 .3 . J . Z a b o r s z k y , A . K . S u b r a m a n i a n , T . J . Tarn and K . M .L u , "A New State S p a c e f o r E m e r g e n c y C o n t r o l i n
th e Interconnected Power S y s t e m " , IEEE Transac-tions on Automatic C o n t r o l , A u g u s t 1 9 7 7 , p p . 5 0 5 -5 1 7 .4 . J . Z a b o r s z k y , K . W . Whang and K . V . P r a s a d , "Moni-t o r i n g , Evaluation a n d Control o f Power SystemE m e r g e n c i e s " , Report N o . SSM 7 9 0 7 , Department ofS y s t e m s Science and M a t h e m a t i c s , W a s h i n g t o n Uni-v e r s i t y , S t . L o u i s , M i s so u r i, 6 3 1 3 0 .5 . J . Z a b o r s z k y , K . W . W h a n g a n d K . V . P r a s a d , " F a s tC o n t i n g e n c y Evaluation U s i n g Concentric Relaxa-t i o n " , Companion p a p e r .6 . J . Z a b o r s z k y , H . M u k a i , a n d J . S i n g h , " C o n t r o l o fPower System in t h e Normal S t a t e " , Report N o . SSM7 9 0 2 , D e p a r t m e n t o f S y s t e m s Science a n d Mathemat-i c s , W a s h i n g t o n U n i v e r s i t y , S t . L o u i s , MO 6 3 1 3 0
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