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1
How the Power Grid Behaves
Tom Overbye
Department of Electrical and Computer Engineering
University of Illinois at Urbana-Champaign
2
Presentation Overview
• Goal is to demonstrate operation of large scale power grid.
• Emphasis on the impact of the transmission syste.
• Introduce basic power flow concepts through small system examples.
• Finish with simulation of Eastern U.S. System.
3
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-power engineers.
• PowerWorld Simulator can now study systems of just about any size.
4
Eastern Interconnect Operating Areas
T V A
SOUTHERN
AEP
CPLW
AP
JCP&L
PECO
AE
PSE&G
AEC
SMEPA
CEI
CINCIPS
CONS
DECO
CPLE
DLCO
DPL
DUKE
EKPC
IMPA
IP
IPL
KU
NI
NIPS
OE
OVEC
TE
VP
METED
PENELEC
PEPCO
PJM500
BG&E
PP&L
BREC
LGE
SIGE
SIPC
CILCO
CWLP
HE
EEI
EMO
CORNWALL
NYPP
SCE&G
SCPSA
ONT HYDR
DOE
DPL
ENTR
NEPOOL
WPLWEP
WPS
MGE
YADKIN
HARTWELL
SEPA-JST
SEPA-RBR
TAL
JEA
SECCELE
LAFA
SWEP
SWPA
PSOK
GRRD
OKGE
KAMO
WEFA
OMPA
EQ-ERCOT
WERE
NSP
IPW
DPC
MEC
IESC
MPW
NPPD OPPD
SMP
LES
MIPUSTJO
KACY
KACP
ASEC
SPRM
INDN
EMDE
MIDW
Ovals represent operating
areas
Arrows indicate
power flow in MW between
areas
5
Zoomed View of Midwest
CEI
CINCIPS
CONS
DPL
IMPA
IP
IPL
NI
NIPS
OVEC
TE
BRECSIGE
SIPC
CILCO
CWLP
HE
EMO
WPLWEP
6
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.
7
One-line Diagram
• Most power systems are balanced three phase systems.
• A balanced three phase system can be modeled as a single (or one) line.
• One-lines show the major power system components, such as generators, loads, transmission lines.
• Components join together at a bus.
8
Eastern North American High Voltage Transmission Grid
8 2 8 M W2 9 3 M V R2 7 3 M V R8 2 9 M W
2 5 0 M V R1 0 9 3 M W
1 0 9 4 M W2 5 0 M V R
9 M V R3 0 0 M W 9 M V R3 0 0 M W 9 M V R3 0 0 M W
3 0 0 M W 9 M V R3 0 0 M W 9 M V R3 2 0 M W 9 M V R
- 1 1 4 M V R8 9 3 M W
8 9 7 M W- 1 1 0 M V R
- 1 2 7 M V R8 0 1 M W
0 M V R
0 M V R1 1 2 9 M W 1 8 3 M V R
0 M V R
0 M V R
3 4 0 M V R
1 4 3 M V R
2 9 4 M V R
3 4 8 M W 2 6 2 M V R
0 M W 0 M V R
2 8 6 M V R
1 4 5 M V R
2 5 0 M W 4 5 M V R
0 M W 0 M V R
4 5 M V R 2 5 0 M W
0 M V R 0 M W
2 9 4 M V R
- 2 0 2 M V R
- 2 1 0 M V R
1 4 6 M V R
6 7 6 M W 5 0 M V R 6 7 6 M W 5 0 M V R
R i v e r h e a dW i l d w o o d
S h o r e h a m
B r o o k h a v e n
P o r t J e f f e r s o n
H o l b r o o k
H o l t s v i l l e
N o r t h p o r t
P i l g r i mS y o s s e t
B e t h p a g eR u l a n d R d .N e w b r i d g e
L c s t . G r v .
0 7 M E R O M 5
K E Y S T O N E
0 1 Y U K O N
C O N E M - G H
J U N I A T A
S U N B U R Y
S U S Q H A N A
W E S C O V L E
A L B U R T I SH O S E N S A K
B R A N C H B G
E L R O Y
W H I T P A I NL I M E R I C K
D E A N SS M I T H B R G
3 M I L E I
R A M A P O 5
H U N T E R T N
C N A S T O N E
P E A C H B T M
K E E N E Y
B R I G H T O N
W C H A P E L
C L V T C L F
C H A L K 5 0 0
B U R C H E S
8 P O S S U M
8 O X
8 C L I F T O N
8 L O U D O N0 8 M D W B R K
8 M O R R S V L
8 M T S T M
8 V A L L E Y
8 D O O M S
8 B A T H C O
8 L E X N G T N
8 N O A N N A8 L D Y S M T H
8 E L M O N T
8 M D L T H A N
8 C H C K A H M
8 C A R S O N8 S E P T A
8 Y A D K I N8 F E N T R E S
8 S U R R Y
8 P E R S O N 8 M A Y O 1
8 P A R K W O D
8 W A K E
8 P L G R D N
8 C U M B E R L
8 R I C H M O N
8 M C G U I R E
8 J O C A S S E
8 B A D C R K
8 O C O N E E
8 N O R C R O S
8 B U L L S L U8 B I G S H A
8 B O W E N
8 K L O N D I K
8 U N I O N C T
8 V I L L A R
8 W A N S L E Y
8 S N P
8 W B N P 1
8 R O A N E
8 B U L L R U
8 V O L U N T E
8 S U L L I V A
8 P H I P P B
0 5 N A G E L
8 W I L S O N
8 M O N T G O M
8 D A V I D S O
8 M A R S H A L
8 S H A W N E E
8 J V I L L E
8 W E A K L E Y
8 J A C K S O N
8 S H E L B Y
8 C O R D O V A
8 F R E E P O R
W M - E H V 8
8 U N I O N
8 T R I N I T Y
8 B F N P8 L I M E S T O
8 B N P 2
8 M A D I S O N 8 B N P 1
8 W I D C R K
8 R A C C O O N
8 F R A N K L I
8 M A U R Y
8 M I L L E R
8 L O W N D E S
8 W P O I N T
M C A D A M 8
8 S . B E S S
8 F A R L E Y
8 S C H E R E R
8 H A T C H 8
8 A N T I O C H
8 C L O V E R
R O C K T A V
C O O P C 3 4 5R O S E T O N
F I S H K I L L
P L T V L L E Y
H U R L E Y 3
L E E D S 3
G I L B 3 4 5
F R A S R 3 4 5
N . S C O T 9 9
A L P S 3 4 5
R E Y N L D 3
E D I C
M A R C Y T 1
M A S S 7 6 5
O A K D L 3 4 5
W A T E R C 3 4 5
S T O L E 3 4 5
L A F A Y T T E
D E W I T T 3
E L B R I D G E
C L A Y
V O L N E Y
S C R I B A
J A P I T Z P9 M I P T 1I N D E P N D C
O S W E G O
P A N N E L L 3R O C H 3 4 5
K I N T I 3 4 5
N I A G 3 4 5
B E C K A
B E C K B
N A N T I C O K
M I D D 8 0 8 6
M I L T O N
T R A F A L H 1T R A F A L H 2
C L A I R V I L
H A W T H O R N
E S S A
B R U J B 5 6 1
B R U J B 5 6 9
B R U J B 5 6 2
L O N G W O O D
B a r r e t t
E . G . C .
V a l l e y S t r e a m
L a k e S u c c e s sR a i n e y
J a m a i c a
G r e e n w o o d
F o x H i l l sF r e s h K i l l sG o e t h a l s
C o g e n T e c hG o w a n u s
F a r r a g u tE 1 5 t h S t .
W 4 9 t h S t .
T r e m o n t
S h o r e R d .
D u n w o o d i eS p r a i n B r o o k
E a s t v i e wP l e a s a n t v i l l e
M i l l w o o d
B u c h a n a n
I n d i a n P o i n t
D v n p t . N KH m p . H a r b o r
V e r n o n
C o r o n a
G r e e l a w nE l w o o d
Figure shows transmission lines at 345 kV or above in Eastern
U.S.
9
Zoomed View of Midwest
1115 MW-185 MVR
600 MW-41 MVR
200 MW 6 MVR
500 MW 25 MVR
05COOK
05GRNTWN
05JEFRSO
05ALLEN
03LEMOYN
05BEATTY
05BENTON
07BLOMNG
05BREED
17BUROAK
05CORRID
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
70%
S L I N E ; BS L I N E ; R
1 7 S H E F L D
1 7 S C H A H F
1 7 D U N A C R
1 7 M C H C T Y
1 7 B A B C O K
1 7 T W R R D
1 7 C H I A V E
B U R N H ; B
B U R N H ; 0 R
1 7 L K G O R G
1 7 M U N S T R
G A C R ; T
1 7 G R N A C RS J O H ; T
1 7 S T J O H N
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
08M.FORT 08REDBK1
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%W A Y N E ; R
? ? ? ? ?
W 4 0 7 M ; 9 T
W 4 0 7 K ; 9 T
W 4 0 7 K ; R
L O M B A ; B
L O M B A ; R
E L M H U ; B
E L M H U ; R
I T A S C ; 1 M
D P 4 6 ; B
D P 4 6 ; R
P H 1 1 7 ; R
N B 1 5 9 ; 1 M
N B 1 5 9 ; B
S K 8 8 ; R
S K 8 8 ; B
G O L F ; R
G O L F ; B
L I S L E ; B
L I S L E ; R
J O 2 9 ; B
J O 2 9 ; R
M C C O O ; B
M C C O O ; R
COLLI; R
WILTO;
E FRA; BE FRA; R
BLOOM; R
T A Y L O ; B
T A Y L O ; RC R A W F ; B
C R A W F ; R
B E D F O ; R
B E D F O ; R T
G A R F I ; B
C A L U M ; B
B U R N H ; 4 M
B U R N H ; 1 R
Arrows indicate MW flow on the
lines; piecharts
show percentage loading of
lines
10
Example Three Bus System
Bus 2 Bus 1
Bus 3
200 MW
100 MVR
150 MWMW
150 MWMW 35 MVRMVR
114 MVRMVR
100 MW 50 MVR
1.00 pu
-17 MW 3 MVR
17 MW -3 MVR
-33 MW 10 MVR
33 MW-10 MVR
17 MW -5 MVR
-17 MW
5 MVR
1.00 pu
1.00 pu
100 MW 2 MVR
100 MWAGC ONAVR ON
AGC ONAVR ON
Generator
LoadBus
Circuit Breaker
Pie charts show
percentage loading of
lines
11
Generation
• Large plants predominate, with sizes up to about 1500 MW.
• Coal is most common source, followed by hydro, nuclear and gas.
• Gas is now most economical.
• Generated at about 20 kV.
12
Loads
• Can range in size from less than a single watt to 10’s of MW.
• Loads are usually aggregated.
• The aggregate load changes with time, with strong daily, weekly and seasonal cycles.
13
Transmission
• Goal is to move electric power from generation to load with as low of losses and cost as possible.
• P = V I or P/V = I
• Losses are I2 R
• Less losses at higher voltages, but more costly to construct and insulate.
14
Transmission and Distribution
• Typical high voltage transmission voltages are 500, 345, 230, 161, 138 and 69 kV.
• Transmission tends to be a grid system, so each bus is supplied from two or more directions.
• Lower voltage lines are used for distribution, with a typical voltage of 12.4 kV.
• Distribution systems tend to be radial.
• Transformers are used to change the voltage.
15
Other One-line Objects
• Circuit Breakers - Used to open/close devices; red is closed, green is open.
• Pie Charts - Show percentage loading of transmission lines.
• Up/down arrows - Used to control devices.
• Values - Show current values for different quantities.
16
Power Balance Constraints
• Power flow refers to how the power is moving through the system.
• At all times the total power flowing into any bus MUST be zero!
• This is know as Kirchhoff’s law. And it can not be repealed or modified.
• Power is lost in the transmission system.
17
Basic Power Control
• Opening a circuit breaker causes the power flow to instantaneously(nearly) change.
• No other way to directly control power flow in a transmission line.
• By changing generation we can indirectly change this flow.
18
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 MW 50 MVR
1.00 pu
-50 MW 11 MVR
50 MW -9 MVR
0 MW 0 MVR
0 MW 0 MVR
50 MW-14 MVR
-50 MW
16 MVR
1.00 pu
1.00 pu
101 MW 6 MVR
100 MWAGC ONAVR ON
AGC ONAVR ON
Power Balance mustbe satisfied at each bus
No flow onopen line
19
Indirect Control of Line Flow
Bus 2 Bus 1
Bus 3
200 MW
100 MVR
150 MWMW
250 MWMW 8 MVRMVR
118 MVRMVR
100 MW 50 MVR
1.00 pu
16 MW -3 MVR
-16 MW 3 MVR
-66 MW 21 MVR
67 MW-19 MVR
83 MW-23 MVR
-82 MW
27 MVR
1.00 pu
1.00 pu
2 MW 30 MVR
100 MWAGC ONAVR ON
OFF AGCAVR ON
Generator MW output changed
Generator change indirectly changes
line flow
20
Transmission Line Limits
• Power flow in transmission line is limited by a number of considerations.
• Losses (I2 R) can heat up the line, causing it to sag. This gives line an upper thermal limit.
• Thermal limits depend upon ambient conditions. Many utilities use winter/summer limits.
21
Overloaded Transmission Line
Bus 2 Bus 1
Bus 3
359 MW
179 MVR
150 MWMW
150 MWMW102 MVRMVR
234 MVRMVR
179 MW 90 MVR
1.00 pu
-152 MW 37 MVR
154 MW-24 MVR
-57 MW 18 MVR
58 MW-16 MVR
-87 MW 29 MVR
89 MW
-24 MVR
1.00 pu
1.00 pu
343 MW-49 MVR
104% 104%
100 MWAGC ONAVR ON
AGC ONAVR ON
Thermal limit of 150 MVA
22
Interconnected Operation
• Power systems are interconnected across large distances. 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).
23
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 the flow on its tie-lines.
• The flow out of an area is equal to
total gen - total load - total losses = tie-flow
24
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 MW 53 MVR
1.00 pu
-29 MW 6 MVR
29 MW -6 MVR
-35 MW 11 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 ONAVR ON
AGC ONAVR ON
0.0 MWMW
Off AGC
Net tie flowis NOT zero
Initially area flow
is not controlled
25
Area Control Error (ACE)
• The area control error mostly the difference between 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 to “chase” the ACE.
26
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 MW 64 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 AGCAVR ON
AGC ONAVR ON
0.0 MWMW
Off AGC
06:30 AM 06:15 AMTime
-20.0
-10.0
0.0
10.0
20.0
Are
a C
on
trol Err
or
(MW
)
ACE changes with time
27
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 gets sufficiently negative they will “pay back” the accumulated energy.
• In extreme cases inadvertent energy is purchased at a negotiated price.
28
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 ACE based upon tie-line flows; then the AGC module sends control signals out to the generators every couple seconds.
29
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 MW 53 MVR
1.00 pu
-22 MW 4 MVR
22 MW -4 MVR
-42 MW 13 MVR
42 MW-12 MVR
22 MW -6 MVR
-22 MW
7 MVR
1.00 pu
1.00 pu
100 MW 2 MVR
100 MWAGC ONAVR ON
AGC ONAVR ON
0.0 MWMW
ED
With AGC on, net tie flow is zero, but
individual line flowsare not zero
30
Generator Costs
• There are many fixed and variable costs associated 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 a function of the MW output of the unit.
31
Generator Cost Curves
• Input-output (IO) curve: Shows relationship between MW output and energy input in Mbtu/hr.
• Fuel-cost curve: Input-output curve scaled by a fuel cost expressed in $ / Mbtu.
• Heat-rate curve: shows relationship between MW output and energy input (Mbtu / MWhr).
• Incremental (marginal) cost curve shows the cost to produce the next MWhr.
32
Example Generator Fuel-Cost Curve
0 150 300 450 600Generator Power (MW)
0
2500
5000
7500
10000Fu
el-
cost
($
/hr)
Current generatoroperating point
Y-axis tells
cost toproduce specified
power (MW) in
$/hr
33
Example Generator Marginal Cost Curve
0 150 300 450 600Generator Power (MW)
0.0
5.0
10.0
15.0
20.0In
crem
enta
l co
st (
$/M
WH
)
Current generatoroperating point
Y-axis tells
marginal cost to
produce one more MWhr in $/MWhr
34
Economic Dispatch
• Economic dispatch (ED) determines the least cost dispatch of generation for an area.
• For a lossless system, the ED occurs when all the generators have equal marginal costs.
IC1(PG,1) = IC2(PG,2) = … = ICm(PG,m)
35
Power Transactions
• Power transactions are contracts between areas to do power transactions.
• Contracts can be for any amount of time at any price for any amount of power.
• Scheduled power transactions are implemented by modifying the area ACE:
ACE = Pactual,tie-flow - Psched
36
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 MW 85 MVR
1.00 pu
-31 MW 6 MVR
31 MW -6 MVR
-159 MW 55 MVR
163 MW-41 MVR
133 MW-35 MVR
-130 MW
44 MVR
1.00 pu
1.00 pu
1 MW 38 MVR112%
112%100 MW
AGC ONAVR ON
AGC ONAVR ON
100.0 MWMW
ED
Net tie flow isnow 100 MW from
left to right Scheduled Transaction
Overloaded line
37
Security Constrained ED
• Transmission constraints often limit system economics.
• Such limits required a constrained dispatch in order to maintain system security.
• In three bus case the generation at bus 3 must be constrained to avoid overloading the line from bus 2 to bus 3.
38
Security Constrained Dispatch
Bus 2 Bus 1
Bus 3Home Area
Area 2
Scheduled Transactions
340 MW
170 MVR
177 MWMW
439 MWMW 15 MVRMVR
223 MVRMVR
170 MW 85 MVR
1.00 pu
-22 MW 4 MVR
22 MW -4 MVR
-142 MW 49 MVR
145 MW-37 MVR
124 MW-33 MVR
-122 MW
41 MVR
1.00 pu
1.00 pu
-0 MW 37 MVR100%
100%
100 MWOFF AGCAVR ON
AGC ONAVR ON
100.0 MWMW
ED
Net tie flow isstill 100 MW from
left to right
Gens 2 &3changed to
removeoverload
39
Multi-Area Operation
• The electrons are not concerned with area boundaries. 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 “parallel path” or “loop flows.”
40
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 MW 30 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 MW 20 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 $/MWH 4189 $/MWH
Case Hourly Cost 16933 $/MWH
Area “Top”has 5 buses
Area “Left” has one bus Area “Right” has one bus
ACE foreach area
is zero
41
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
42
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
Losseswent up
from7.09 MW
100 MW Scheduled Transfer from Left to Right
43
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 MW 30 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 MW 52 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 $/MWH 5943 $/MWH
Case Hourly Cost 16654 $/MWH
Transfer also
overloadsline in Top
44
Transmission Service
• FERC Order No. 888 requires utilities provide non-discriminatory open transmission access through tariffs of general applicability.
• FERC Order No. 889 requires transmission providers set up OASIS (Open Access Same-Time Information System) to show available transmission.
45
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.
46
Eastern Interconnect Example
T V A
SOUTHERN
AEP
CPLW
AP
JCP&L
PECO
AE
PSE&G
AEC
CEI
CINCIPS
CONS
DECO
CPLE
DLCO
DPL
DUKE
EKPC
IMPA
IP
IPL
KU
NI
NIPS
OE
OVEC
TE
VP
METED
PENELEC
PEPCO
PJM500
BG&E
PP&L
BREC
LGE
SIGE
SIPC
CILCO
CWLP
HE
EEI
EMO
CORNWALL
NYPP
SCE&G
SCPSA
ONT HYDR
DOE
DPL
ENTR
NEPOOL
WPL
WEP
WPS
MGE
YADKIN
HARTWELL
SEPA-JST
SEPA-RBR
SWEP
SWPA
PSOK
GRRD
KAMO
NSP
IPW
DPC
MEC
IESC
MPW
OPPD
SMP
MIPUSTJO
KACY
KACP
ASEC
SPRM
INDN
EMDE
Arrows indicate
the basecase
flow between
areas
47
Power Transfer Distribution Factors (PTDFs)
• PTDFs are used to show how a particular transaction will affect the system.
• Power transfers through the system according to the impedances of the lines, without respect to ownership.
• All transmission players in network could be impacted, to a greater or lesser extent.
48
PTDFs for Transfer from Wisconsin Electric to TVA
T V A
SOUTHERN
20%
AEP
CPLW
AP
PECO
CEI
CINCIPS
CONS
DECO
CPLE
DLCO
DPL
DUKE
EKPC
IMPA
IP
IPL
KU
NI
NIPS
OE
OVEC
TE
VP
METED
PENELEC
PEPCO
PJM500
BG&E
PP&L
BREC
LGE
SIGE
SIPC
CILCO
CWLP
HE
EEI
EMO
CORNWALL
NYPP
SCE&G
SCPSA
ONT HYDR
DOE
ENTR
25%
7%
10%
6% 7% 8%
9%
9%
8%
7%
16% 39%
6%
19%
5% 6%
13%
WPLWEP
WPS
MGE
7%
7%
11%
55% 22%
10%
55% 54%
YADKIN
HARTWELL
SEPA-JST
SEPA-RBR
SWPA
PSOK
GRRD
OKGE
KAMO
6%
WEFA
OMPA
WERE
NSP
19%
IPW
DPC
8%
10%
MEC
IESC
MPW
9%
8%
7% 8%
NPPDOPPD
7%
SMP
LES
MIPUSTJO
6%
KACY
KACP
11%
8%
ASEC 13%
11%
SPRM
INDN
EMDE
MIDW
Piecharts indicate
percentage of transfer that will
flow between specified
areas
49
PTDF for Transfer from WE to TVA
CINCIPS
CONS
DECO
DPL
IP
IPL
NI
NIPS
TE
CILCO
CWLP
8%
7%
16% 39%
6%
13%
WPLWEP
WPS
MGE
7%
7%
55% 22%
10%
55% 54%
NSP
19%
IPW
DPC
8%
10%
MEC
IESC
MPW
9%
8%
7%
8%
OPPD
7%
SMP
MIPUSTJO
100% of transfer leaves
Wisconsin Electric (WE)
50
PTDFs for Transfer from WE to TVA
TVA
SOUTHERN
20%
CPLW
DUKE
EKPC
KU
BREC
LGE
SIGE
SIPC
EEI
SCE&G
SCPSA
DOE
25%
10%
6% 7% 8% 19%
11%
YADKIN
HARTWELL
SEPA-JST
SEPA-RBR
About 100% of transfer
arrives at TVA
But flow does NOT
follow contract
path
51
Contingencies
• Contingencies are the unexpected loss of a significant device, such as a transmission line or a generator.
• No power system can survive a large number of contingencies.
• First contingency refers to loss of any one device.
• Contingencies can have major impact on Power Transfer Distribution Factors (PTDFs).
52
Available Transfer Capability
• Determines the amount of transmission capability available 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.
53
Reactive Power
• Reactive power is supplied by–generators
–capacitors
– transmission lines
– loads
• Reactive power is consumed by– loads
– transmission lines and transformers (very high losses
54
Reactive Power
• Reactive power doesn’t travel well - must be supplied locally.
• Reactive must also satisfy Kirchhoff’s law - total reactive power into a bus MUST be zero.
55
Reactive Power Example
Bus 2 Bus 1
Bus 3
359 MW
179 MVR
150 MWMW
150 MWMW102 MVRMVR
234 MVRMVR
179 MW 90 MVR
1.00 pu
-152 MW 37 MVR
154 MW-24 MVR
-57 MW 18 MVR
58 MW-16 MVR
-87 MW 29 MVR
89 MW
-24 MVR
1.00 pu
1.00 pu
343 MW-49 MVR
104% 104%
100 MWAGC ONAVR ON
AGC ONAVR ON
Reactive power
must also sum to zero at
each bus
Note reactive
line losses are about 13 Mvar
56
Voltage Magnitude
• Power systems must supply electric power within a narrow voltage range, typically with 5% of a nominal value.
• For example, wall outlet should supply 120 volts, with an acceptable range from 114 to 126 volts.
• Voltage regulation is a vital part of system operations.
57
Reactive Power and Voltage
• Reactive power and voltage magnitude are tightly coupled.
• Greater reactive demand decreases the bus voltage, while reactive generation increases the bus voltage.
58
Voltage Regulation
• A number of different types of devices participate in 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 specified range.
59
Five Bus Reactive Power Example
Bus 3Bus 4
Bus 5
200 MW
100 MVR
405 MWMW
96 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
Voltage magnitude
is controlled
bycapacitor
LTC Transformer
is controlling
load voltage
60
Voltage Control
• Voltage control is necessary to keep system voltages within an acceptable range.
• Because reactive power does not travel well, it would be difficult for it to be supplied by a third party.
• It is very difficult to assign reactive power and voltage control to particular transactions.
61
Conclusion
• Talk has provided brief overview of how power grid operates.
• Educational Version of PowerWorld Simulator, capable of solving systems with up to 12 buses, can be downloaded for free atwww.powerworld.com
• 60,000 bus commercial version is also available.
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