Operation of Large Interconnected PSys by Decision and Ctrl_1980

7/28/2019 Operation of Large Interconnected PSys by Decision and Ctrl_1980

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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

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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.


5/9

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 .


6/9

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


7/9

4 3

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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4 5

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

Operation of Large Interconnected PSys by Decision and Ctrl_1980

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