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How the Power Grid Behaves
Tom Overbye
Department of Electrical and Computer EngineeringUniversity of Illinois at Urbana-Champaign
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Presentation Overview
Goal is to demonstrate operation of large scalepower grid.
Emphasis on the impact of the transmission syste.
Introduce basic power flow concepts throughsmall system examples.
Finish with simulation of Eastern U.S. System.
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PowerWorld Simulator
PowerWorld Simulator is an interactive,Windows based simulation program, originally
designed at University of Illinois for teaching
basics of power system operations to non-powerengineers.
PowerWorld Simulator can now study systems of
just about any size.
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Zoomed View of Midwest
CEI
CINCIPS
CONS
DPL
IMPA
IP
IPL
NI
NIPS
TE
BRECSIGE
CILCO
CWLP
HE
EMO
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Power System Basics
All power systems have three major components:Generation, Load and Transmission.
Generation: Creates electric power. Load: Consumes electric power. Transmission: Transmits electric power from
generation to load.
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One-line Diagram
Most power systems are balanced three phasesystems.
A balanced three phase system can be modeled asa single (or one) line.
One-lines show the major power systemcomponents, such as generators, loads,
transmission lines. Components join together at a bus.
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Eastern North American High Voltage
Transmission Grid
828MW293MVR273MVR829MW
250MVR1093MW
1094MW250MVR
9MVR300MW9MVR300MW9MVR300MW
300MW9MVR300MW9MVR320MW9MVR
-114MVR893MW
897MW-110MVR
-127MVR801MW
0MVR
0MVR1129MW183MVR
0MVR
0MVR
340MVR
143MVR
294MVR
348MW262MVR
0MW 0MVR
286MVR
145MVR
250MW45MVR
0MW 0MVR
45MVR250MW 0MVR
0MW
294MVR
-202MVR
-210MVR
146MVR
676MW50MVR676MW50MVR
RiverheadWildwood
Shoreham
Brookhaven
PortJefferson
HolbrookHoltsville
Northport
PilgrimSyossetBethpage
RulandRd.NewbridgeLcst.Grv.
07MEROM5
KEYSTONE
01YUKON
CONEM-GH
JUNIATA
SUNBURY
SUSQHANA
WESCOVLE
ALBURTISHOSENSAK
BRANCHBG
ELROY
WHITPAINLIMERICK
DEANSSMITHBRG
3MILEI
RAMAPO5
HUNTERTN
CNASTONE
PEACHBTMKEENEY
BRIGHTON
WCHAPEL
CLVTCLFCHALK500
BURCHES
8POSSUM
8OX8CLIFTON
8LOUDON08MDWBRK
8MORRSVL
8MTSTM
8VALLEY
8DOOMS
8BATHCO
8LEXNGTN
8NOANNA8LDYSMTH
8ELMONT
8MDLTHAN
8CHCKAHM
8CARSON8SEPTA
8YADKIN8FENTRES
8SURRY
8PERSON8MAYO1
8PARKWOD
8WAKE
8PLGRDN
8CUMBERL
8RICHMON
8MCGUIRE
8JOCASSE
8BADCRK
8OCONEE
8NORCROS8BULLSLU
8BIGSHA
8BOWEN
8KLONDIK
8UNIONCT
8VILLAR
8WANSLEY
8SNP
8WBNP1
8ROANE8BULLRU
8VOLUNTE
8SULLIVA
8PHIPPB
05NAGEL
8WILSON
8MONTGOM
8DAVIDSO
8MARSHAL
8SHAWNEE
8JVILLE
8WEAKLEY
8JACKSON
8SHELBY
8CORDOVA
8FREEPOR
WM-EHV8
8UNION
8TRINITY
8BFNP8LIMESTO
8BNP28MADISON8BNP1
8WIDCRK
8RACCOON
8FRANKLI
8MAURY
8MILLER
8LOWNDES
8WPOINT
MCADAM8
8S.BESS
8FARLEY
8SCHERER
8HATCH8
8ANTIOCH
8CLOVER
ROCKTAV
COOPC345ROSETON
FISHKILL
PLTVLLEY
HURLEY3
LEEDS3
GILB345
FRASR345
N.SCOT99ALPS345
REYNLD3
EDIC
MARCYT1
MASS765
OAKDL345
WATERC345
STOLE345
LAFAYTTE
DEWITT3ELBRIDGE
CLAY
VOLNEYSCRIBA
JAPITZP9MIPT1INDEPNDC
OSWEGO
PANNELL3ROCH345
KINTI345
NIAG345BECKA
BECKB
NANTICOK
MIDD8086
MILTON
TRAFALH1TRAFALH2
CLAIRVIL
HAWTHORN
ESSABRUJB561
BRUJB569
BRUJB562
LONGWOOD
Barrett
E.G.C.
ValleyStream
LakeSuccessRainey
Jamaica
Greenwood
FoxHillsFreshKillsGoethals
CogenTechGowanusFarragut
E15thSt.W49thSt.
TremontShoreRd.
DunwoodieSprainBrook
EastviewPleasantville
Millwood
BuchananIndianPoint
Dvnpt.NKHmp.Harbor
VernonCorona
GreelawnElwood
Figure shows
transmission
lines at 345
kV or abovein Eastern
U.S.
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Zoomed View of Midwest
1115 MW-185 MVR
600 MW-41 MVR
200 MW 6 MVR
500 MW25 MVR
05COOK
05GRNTWN
05JEFRSO
05ALLEN
03LEMOYN
05BEATTY
05BENTON
07BLOMNG
05BREED
17BUROAK
05CORR
03DAV-BE
06DEARBN
05DEQUIN
05DESOTO
05DUMONT
05E LIMA
05EELKHA
19MADRD
05EUGENE
05FALL C
05FOSTOR
16GUION
16HANNA
05HAYDEN
17HIPLE
05HYATT
05JACKSR
05MARQUI
05MARYSV
05OLIVE
06PIERCE
05REYNOL
05ROB PK
05ROBERT
16ROCKVL
05SORENS
17STLWEL
16STOUT
16SUNNYS
05SW LIM
05TANNER
16THOMPS
05TWIN B
07WORTHN
60%
69%
07MEROM5
05KENZIE
SLINE;BSLINE;R
17SHEFLD
17SCHAHF
17DUNACR
17MCHCTY
17BABCOK
17TWRRD
17CHIAVE
BURNH;B
BURNH;0R
17LKGORG
17MUNSTR
GACR;T
17GRNACRSJOH;T
17STJOHN
DAVIS; B
DAVIS; R
BRAID; B
BRAID; R
LASCO; B
LASCO; R
PLANO; B
PLANO; R
ELECT; B
ELECT; R
ZION ; B
ZION ; R
SILVE; R
LIBER; R
DRESD; B
DRESD; R
LOCKP; B
LOCKP; R
GOODI;3B
GOODI;2RGOODI;4B
GOODI;1R
B ISL; R
NELSO; B
H471 ;
TAZEWELL
POWER; B
POWER; R
DUCK CRK
PONTI;
BROKA; T
LATHA; T
KINCA;
08CAYUGA
08CAY CT
BUNSONVLSIDNEY
CASEY
KANSAS
08DRESSR
62%
08WHITST
08NUCOR
?????
?????
08BEDFRD
08ALENJT
08COLMBU
08GWYNN
08OKLND
08GRNBOR
08NOBLSV?????
08WESTWD
17LESBRG
08WALTON
08DEEDSV
05COLNGW
05S.BTLR
56%
05SULLVA
12GHENT
06CLIFTY
08BUFTN1
08EBEND
08M.FTHS
0 8 M. F OR T 0 8 RE D BK 1
08REDBK2
08TERMNL
08SGROVE
08P.UNON
08WODSDL
08TDHNTR
08FOSTER
?????
09CLINTO09NETAP
09KILLEN
09BATH
?????
09GIVENS
08ZIMER
????? 09CARGIL
09URBANA?????
62%
02TANGY
19MAJTC
03BAY SH
02GALION
COFFEEN
PAWNEE
COFFEN N
PANA
RAMSEY
NEOGA
NEWTON
CLINTON
MAROA W MAROA E
OREANA E
RISING
PLANO;
COLLI;
WILTO;
PAD 345
WEMPL; R
WEMPL; B
BYRON; BBYRON; R
CHERR; B
CHERR; R
53%WAYNE;R
?????
W407M;9T
W407K;9T
W407K;R
LOMBA;B
LOMBA;R
ELMHU;B
ELMHU;R
ITASC;1M
DP46;B
DP46;R
PH117;R
NB159;1M
NB159;B
SK88;R
SK88;B
GOLF;R
GOLF;B
LISLE;B
LISLE;R
JO29;B
JO29;R
MCCOO;B
MCCOO;R
COLLI; R
WILTO;
E FRA; B
E FRA; R
BLOOM; R
TAYLO;B
TAYLO;RCRAWF;B
CRAWF;R
BEDFO;R
BEDFO;RT
GARFI;B
CALUM;B
BURNH;4M
BURNH;1R
Arrows
indicate MW
flow on the
lines;
piecharts
show
percentage
loading of
lines
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Example Three Bus System
Bus 2 Bus 1
Bus 3
200 MW
100 MVR
150 MWMW
150 MWMW
35 MVRMVR
114 MVRMVR
100 MW50 MVR
1.00 pu
-17 MW 3 MVR
17 MW-3 MVR
-33 MW10 MVR
33 MW
-10 MVR17 MW-5 MVR
-17 MW
5 MVR
1.00 pu
1.00 pu
100 MW
2 MVR
100 MWAGC ON
AVR ON
AGC ON
AVR ON
Generator
LoadBus
Circuit Breaker
Pie charts
show
percentage
loading of
lines
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Generation
Large plants predominate, with sizes up to about1500 MW.
Coal is most common source, followed by hydro,nuclear and gas.
Gas is now most economical. Generated at about 20 kV.
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Loads
Can range in size from less than a single watt to10s of MW.
Loads are usually aggregated. The aggregate load changes with time, with
strong daily, weekly and seasonal cycles.
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Transmission
Goal is to move electric power from generation toload with as low of losses and cost as possible.
P = V I or P/V = I Losses are I2R Less losses at higher voltages, but more costly to
construct and insulate.
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Transmission and Distribution
Typical high voltage transmission voltages are500, 345, 230, 161, 138 and 69 kV.
Transmission tends to be a grid system, so eachbus is supplied from two or more directions.
Lower voltage lines are used for distribution, witha typical voltage of 12.4 kV.
Distribution systems tend to be radial. Transformers are used to change the voltage.
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Other One-line Objects
Circuit Breakers - Used to open/close devices; redis closed, green is open.
Pie Charts - Show percentage loading oftransmission lines.
Up/down arrows - Used to control devices. Values - Show current values for different
quantities.
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Power Balance Constraints
Power flow refers to how the power is movingthrough the system.
At all times the total power flowing into any busMUST be zero!
This is know as Kirchhoffs law. And it can notbe repealed or modified.
Power is lost in the transmission system.
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Basic Power Control
Opening a circuit breaker causes the power flowto instantaneously(nearly) change.
No other way to directly control power flow in atransmission line.
By changing generation we can indirectly changethis flow.
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Flow Redistribution Following Opening
Line Circuit Breaker
Bus 2 Bus 1
Bus 3
200 MW
100 MVR
150 MWMW
150 MWMW
36 MVRMVR
111 MVRMVR
100 MW50 MVR
1.00 pu
-50 MW11 MVR
50 MW-9 MVR
0 MW 0 MVR
0 MW
0 MVR50 MW-14 MVR
-50 MW
16 MVR
1.00 pu
1.00 pu
101 MW
6 MVR
100 MWAGC ON
AVR ON
AGC ON
AVR ONPower Balance must
be satisfied at each bus
No flow on
open line
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Indirect Control of Line Flow
Bus 2 Bus 1
Bus 3
200 MW
100 MVR
150 MWMW
250 MWMW
8 MVRMVR
118 MVRMVR
100 MW50 MVR
1.00 pu
16 MW-3 MVR
-16 MW 3 MVR
-66 MW21 MVR
67 MW
-19 MVR83 MW-23 MVR
-82 MW
27 MVR
1.00 pu
1.00 pu
2 MW
30 MVR
100 MWAGC ON
AVR ON
OFF AGC
AVR ONGenerator MWoutput changed
Generator change
indirectly changes
line flow
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Transmission Line Limits
Power flow in transmission line is limited by anumber of considerations.
Losses (I2R) can heat up the line, causing it tosag. This gives line an upper thermal limit.
Thermal limits depend upon ambient conditions.Many utilities use winter/summer limits.
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Overloaded Transmission Line
Bus 2 Bus 1
Bus 3
359 MW
179 MVR
150 MWMW
150 MWMW
102 MVRMVR
234 MVRMVR
179 MW90 MVR
1.00 pu
-152 MW37 MVR
154 MW-24 MVR
-57 MW18 MVR
58 MW
-16 MVR-87 MW29 MVR
89 MW
-24 MVR
1.00 pu
1.00 pu
343 MW
-49 MVR
104% 104%
100 MWAGC ON
AVR ON
AGC ON
AVR ON
Thermal limit
of 150 MVA
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Interconnected Operation
Power systems are interconnected across largedistances. For example most of North American
east of the Rockies is one system, with most of
Texas and Quebec being major exceptions Individual utilities only own and operate a small
portion of the system, which is referred to an
operating area (or an area).
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Operating Areas
Areas constitute a structure imposed on grid. Transmission lines that join two areas are known
as tie-lines.
The net power out of an area is the sum of theflow on its tie-lines.
The flow out of an area is equal to
total gen - total load - total losses = tie-flow
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Three Bus System Split into Two Areas
Bus 2 Bus 1
Bus 3Home Area
Area 2
Scheduled Transactions
214 MW
107 MVR
150 MWMW
150 MWMW
41 MVRMVR
124 MVRMVR
107 MW53 MVR
1.00 pu
-29 MW 6 MVR
29 MW-6 MVR
-35 MW11 MVR
35 MW
-10 MVR 8 MW-2 MVR
-8 MW
2 MVR
1.00 pu
1.00 pu
121 MW
-3 MVR
100 MWAGC ON
AVR ON
AGC ON
AVR ON
0.0 MWMW
Off AGC
Net tie flow
is NOT zero
Initially
area flow
is not
controlled
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Area Control Error (ACE)
The area control error mostly the differencebetween the actual flow out of area, and
scheduled flow.
ACE also includes a frequency component. Ideally the ACE should always be zero. Because the load is constantly changing, each
utility must constantly change its generation tochase the ACE.
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Home Area ACE
Bus 2 Bus 1
Bus 3Home Area
Area 2
Scheduled Transactions
255 MW
128 MVR
227 MWMW
150 MWMW
57 MVRMVR
135 MVRMVR
128 MW64 MVR
1.00 pu
-12 MW 2 MVR
12 MW-2 MVR
-17 MW 5 MVR
17 MW
-5 MVR 6 MW-2 MVR
-6 MW
2 MVR
1.00 pu
1.00 pu
106 MW
-1 MVR
100 MWOFF AGC
AVR ON
AGC ON
AVR ON
0.0 MWMW
Off AGC
06:30 AM 06:15 AMTime
-20.0
-10.0
0.0
10.0
20.0
AreaControlError(M
W)
ACE changes with time
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Inadvertent Interchange
ACE can never be held exactly at zero. Integrating the ACE gives the inadvertent
interchange, expressed in MWh.
Utilities keep track of this value. If it getssufficiently negative they will pay back the
accumulated energy.
In extreme cases inadvertent energy is purchasedat a negotiated price.
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Automatic Generation Control
Most utilities use automatic generation control(AGC) to automatically change their generation to
keep their ACE close to zero.
Usually the utility control center calculates ACEbased upon tie-line flows; then the AGC module
sends control signals out to the generators every
couple seconds.
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Three Bus Case on AGC
Bus 2 Bus 1
Bus 3Home Area
Area 2
Scheduled Transactions
214 MW
107 MVR
150 MWMW
171 MWMW
35 MVRMVR
124 MVRMVR
107 MW53 MVR
1.00 pu
-22 MW 4 MVR
22 MW-4 MVR
-42 MW13 MVR
42 MW
-12 MVR22 MW-6 MVR
-22 MW
7 MVR
1.00 pu
1.00 pu
100 MW
2 MVR
100 MWAGC ON
AVR ON
AGC ON
AVR ON
0.0 MWMW
ED
With AGC on, net
tie flow is zero, butindividual line flows
are not zero
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Generator Costs
There are many fixed and variable costsassociated with power system operation.
Generation is major variable cost. For some types of units (such as hydro and
nuclear) it is difficult to quantify.
For thermal units it is much easier. There are four
major curves, each expressing a quantity as afunction of the MW output of the unit.
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Generator Cost Curves
Input-output (IO) curve: Shows relationshipbetween MW output and energy input in Mbtu/hr.
Fuel-cost curve: Input-output curve scaled by afuel cost expressed in $ / Mbtu.
Heat-rate curve: shows relationship between MWoutput and energy input (Mbtu / MWhr).
Incremental (marginal) cost curve shows the costto produce the next MWhr.
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Example Generator Fuel-Cost Curve
0 150 300 450 600Generator Power (MW)
0
2500
5000
7500
10000
Fuel-cost($/hr)
Current generator
operating point
Y-axis
tells
cost to
produce
specifiedpower
(MW) in
$/hr
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Example Generator Marginal Cost
Curve
0 150 300 450 600Generator Power (MW)
0.0
5.0
10.0
15.0
20.0
Incrementalcost($/MWH)
Current generator
operating point
Y-axis
tells
marginal
cost toproduce
one more
MWhr in
$/MWhr
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Economic Dispatch
Economic dispatch (ED) determines the least costdispatch of generation for an area.
For a lossless system, the ED occurs when all thegenerators have equal marginal costs.
IC1(PG,1) = IC2(PG,2) = = ICm(PG,m)
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Power Transactions
Power transactions are contracts between areas todo power transactions.
Contracts can be for any amount of time at anyprice for any amount of power.
Scheduled power transactions are implemented bymodifying the area ACE:
ACE = Pactual,tie-flow- Psched
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Implementation of 100 MW Transaction
Bus 2 Bus 1
Bus 3Home Area
Area 2
Scheduled Transactions
340 MW
170 MVR
150 MWMW
466 MWMW 9 MVRMVR
232 MVRMVR
170 MW85 MVR
1.00 pu
-31 MW 6 MVR
31 MW-6 MVR
-159 MW55 MVR
163 MW
-41 MVR133 MW-35 MVR
-130 MW
44 MVR
1.00 pu
1.00 pu
1 MW
38 MVR112%
112%
100 MWAGC ON
AVR ON
AGC ONAVR ON
100.0 MWMW ED
Net tie flow is
now 100 MW fromleft to right
Scheduled Transaction
Overloaded
line
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Security Constrained ED
Transmission constraints often limit systemeconomics.
Such limits required a constrained dispatch inorder to maintain system security.
In three bus case the generation at bus 3 must beconstrained to avoid overloading the line from bus
2 to bus 3.
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Security Constrained Dispatch
Bus 2 Bus 1
Bus 3Home Area
Area 2
Scheduled Transactions
340 MW
170 MVR
177 MWMW
439 MWMW15 MVRMVR
223 MVRMVR
170 MW85 MVR
1.00 pu
-22 MW 4 MVR
22 MW-4 MVR
-142 MW49 MVR
145 MW
-37 MVR124 MW-33 MVR
-122 MW
41 MVR
1.00 pu
1.00 pu
-0 MW
37 MVR100%
100%
100 MWOFF AGC
AVR ON
AGC ONAVR ON
100.0 MWMW ED
Net tie flow is
still 100 MW from
left to right
Gens 2 &3
changed to
remove
overload
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Multi-Area Operation
The electrons are not concerned with areaboundaries. Actual power flows through the
entire network according to impedance of the
transmission lines. If Areas have direct interconnections, then they
can directly transact up their tie-line capacity.
Flow through other areas is known as parallelpath or loop flows.
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Seven Bus, Thee Area Case One-line
Top Area Cost
Left Area Cost Right Area Cost
1
2
3 4
5
6 7
106 MWMW
168 MWMW
200 MWMW 201 MWMW
110 MW
40 MVR
80 MW30 MVR
130 MW
40 MVR
40 MW
20 MVR
1.00 pu
1.01 pu
1.04 pu1.04 pu
1.04 pu
0.99 pu1.05 pu
62 MW
-61 MW
44 MW -42 MW -31 MW 31 MW
38 MW
-37 MW
79 MW -77 MW
-32 MW
32 MW
-14 MW
-39 MW
40 MW-20 MW20 MW
40 MW
-40 MW
94 MWMW
200 MW 0 MVR
200 MW 0 MVR
20 MW -20 MW
AGC ON
AGC ON
AGC ON
AGC ON
AGC ON
8029 $/MWH
4715 $/MWH4189 $/MWH
Case Hourly Cost
16933 $/MWH
Area
Top
has 5
buses
Area Left has one bus Area Right has one bus
ACE for
each area
is zero
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Seven Bus Case: Area View
Area Losses
Area Losses Area Losses
Top
Left Right
-40.1 MW 0.0 MWMW
0.0 MWMW
0.0 MWMW
40.1 MW
40.1 MW
7.09 MW
0.33 MW 0.65 MW
Actual
flow
between
areas
Scheduled
flow
between
areas
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Seven Bus Case with 100 MW Transfer
Area Losses
Area Losses Area Losses
Top
Left Right
-4.8 MW 0.0 MWMW
100.0 MWMW
0.0 MWMW
104.8 MW
4.8 MW
9.45 MW
0.00 MW 4.34 MW
Losses
went up
from
7.09MW
100 MW Scheduled Transfer from Left to Right
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Seven Bus Case One-line
Top Area Cost
Left Area Cost Right Area Cost
1
2
3 4
5
6 7
106 MWMW
167 MWMW
300 MWMW 104 MWMW
110 MW
40 MVR
80 MW30 MVR
130 MW
40 MVR
40 MW
20 MVR
1.00 pu
1.01 pu
1.04 pu1.04 pu
1.04 pu
0.99 pu1.05 pu
106%
60 MW
-60 MW
45 MW -44 MW -27 MW 27 MW
40 MW
-39 MW
106 MW -102 MW
-35 MW
36 MW
-24 MW
-4 MW
5 MW-50 MW52 MW
5 MW
-5 MW
97 MWMW
200 MW 0 MVR 200 MW 0 MVR
52 MW -50 MW
AGC ON
AGC ON
AGC ON
AGC ON
AGC ON
8069 $/MWH
2642 $/MWH5943 $/MWH
Case Hourly Cost
16654 $/MWH
Transfer
also
overloads
line in Top
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Transmission Service
FERC Order No. 888 requires utilities providenon-discriminatory open transmission access
through tariffs of general applicability.
FERC Order No. 889 requires transmissionproviders set up OASIS (Open Access Same-
Time Information System) to show available
transmission.
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Transmission Service
If areas (or pools) are not directly interconnected,they must first obtain a contiguous contract
path.
This is NOT a physical requirement. Utilities on the contract path are compensated for
wheeling the power.
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Power Transfer Distribution Factors
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Power Transfer Distribution Factors
(PTDFs)
PTDFs are used to show how a particulartransaction will affect the system.
Power transfers through the system according tothe impedances of the lines, without respect toownership.
All transmission players in network could be
impacted, to a greater or lesser extent.
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PTDF for Transfer from WE to TVA
CINCIPS
CONS
DECO
DPL
IP
IPL
NI
NIPS
TE
CILCO
CWLP
8%
7%
16% 39%
13%
WPL
WEP
WPS
MGE
7%
7%
55%22%
10%
55%54%
NSP
19%
IPW
DPC
8%
10%
MEC
IESC
MPW
9%
8%
7%
8%
7%
SMP
100% of
transfer
leavesWisconsin
Electric
(WE)
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PTDFs for Transfer from WE to TVA
TVA
SOUTHERN
20%
CPLW
DUKE
EKPC
KU
LGE
SIPC
I
SCE&G
DOE
25%
10%
6% 7% 8%19%
11%
YADKI
HARTWELL
SEPA-JST
-
About
100% of
transfer
arrives at
TVA
But flow
does NOT
follow
contract
path
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Contingencies
Contingencies are the unexpected loss of asignificant device, such as a transmission line or a
generator.
No power system can survive a large number ofcontingencies.
First contingency refers to loss of any one device.
Contingencies can have major impact on PowerTransfer Distribution Factors (PTDFs).
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Available Transfer Capability
Determines the amount of transmission capabilityavailable to transfer power from point A to point
B without causing any overloads in basecase and
first contingencies. Depends upon assumed system loading,
transmission configuration and existing
transactions.
R i P
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Reactive Power
Reactive power is supplied bygeneratorscapacitors
transmission linesloads
Reactive power is consumed by
loadstransmission lines and transformers (very high losses
R ti P
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Reactive Power
Reactive power doesnt travel well - must besupplied locally.
Reactive must also satisfy Kirchhoffs law - total
reactive power into a bus MUST be zero.
R ti P E l
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Reactive Power Example
Bus 2 Bus 1
Bus 3
359 MW
179 MVR
150 MWMW
150 MWMW
102 MVRMVR
234 MVRMVR
179 MW90 MVR
1.00 pu
-152 MW37 MVR
154 MW-24 MVR
-57 MW18 MVR
58 MW
-16 MVR-87 MW29 MVR
89 MW
-24 MVR
1.00 pu
1.00 pu
343 MW
-49 MVR
104% 104%
100 MWAGC ON
AVR ON
AGC ON
AVR ON
Reactive
power
must also
sum tozero at
each bus
Note
reactive
line losses
are about13 Mvar
V lt M it d
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Voltage Magnitude
Power systems must supply electric power withina narrow voltage range, typically with 5% of a
nominal value.
For example, wall outlet should supply120 volts, with an acceptable range from 114 to
126 volts.
Voltage regulation is a vital part of systemoperations.
R ti P d V lt
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Reactive Power and Voltage
Reactive power and voltage magnitude are tightlycoupled.
Greater reactive demand decreases the bus
voltage, while reactive generation increases thebus voltage.
V lt R l ti
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Voltage Regulation
A number of different types of devices participatein system voltage regulation
generators: reactive power output is automatically
changed to keep terminal voltage within range.capacitors: switched either manually or automatically
to keep the voltage within a range.
Load-tap-changing (LTC) transformers: vary their off-
nominal tap ratio to keep a voltage within a specifiedrange.
Five Bus Reactive Power Example
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Five Bus Reactive Power Example
Bus 3Bus 4
Bus 5
200 MW
100 MVR
MWMW
MVRMVR
100 MWMW
50 MVRMVR
1.000 pu
143 MW
5 MVR
-60 MW
5 MVR
61 MW
-2 MVR
1.00 pu
0.994 pu
100 MW
12 MVR
100 MWAGC ON
AVR ON
79 MVRMVR
0.982 pu
0.995 pu
100 MWMW
0 MVRMVR
3 L
-40 MW
24 MVR
100 MW
10 MVR
Voltagemagnitude
is
controlled
by
capacitor
LTC
Transformer
is
controlling
load voltage
Voltage Control
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Voltage Control
Voltage control is necessary to keep systemvoltages within an acceptable range.
Because reactive power does not travel well, it
would be difficult for it to be supplied by a thirdparty.
It is very difficult to assign reactive power andvoltage control to particular transactions.
Conclusion
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Conclusion
Talk has provided brief overview of how powergrid operates.
Educational Version of PowerWorld Simulator,
capable of solving systems with up to 12 buses,can be downloaded for free at
www.powerworld.com
60,000 bus commercial version is also available.