Power Systems Analysis

1. EE 369
POWER SYSTEM ANALYSIS
Lecture 16
Economic Dispatch
Tom Overbye and Ross Baldick
1

2. Announcements
Read Chapter 12, concentrating on sections
12.4 and 12.5.
Read Chapter 7.
Homework 12 is 6.59, 6.61, 12.19, 12.22,
12.24, 12.26, 12.28, 12.29, 7.1, 7.3, 7.4, 7.5,
7.6, 7.9, 7.12, 7.16; due Thursday, 12/3.
2

3. Economic Dispatch: Formulation
The goal of economic dispatch is to
determine the generation dispatch that
minimizes the instantaneous operating cost,
subject to the constraint that total
generation = total load + losses
T
1
1
Minimize C ( )
Such that
m
i Gi
i
m
Gi D Losses
i
C P
P P P
=
=
= +
∑
∑
@
Initially we'll
ignore generator
limits and the
losses
3

4. Unconstrained Minimization
This is a minimization problem with a single
equality constraint
For an unconstrained minimization a
necessary (but not sufficient) condition for a
minimum is the gradient of the function
must be zero,
The gradient generalizes the first derivative
for multi-variable problems:
1 2
( ) ( ) ( )
( ) , , ,
nx x x
 ∂ ∂ ∂
∇  ∂ ∂ ∂ 
f x f x f x
f x K@
( )∇ =f x 0
4

5. Minimization with Equality Constraint
When the minimization is constrained with an
equality constraint we can solve the problem
using the method of Lagrange Multipliers
Key idea is to represent a constrained
minimization problem as an unconstrained
problem.
That is, for the general problem
minimize ( ) s.t. ( )
We define the Lagrangian L( , ) ( ) ( )
Then a necessary condition for a minimum is the
L ( , ) 0 and L ( , ) 0
T
=
= +
∇ = ∇ =xλ
f x g x 0
xλ f x λ g x
xλ x λ 5

6. Economic Dispatch Lagrangian
G
1 1
G
For the economic dispatch we have a minimization
constrained with a single equality constraint
L( , ) ( ) ( ) (no losses)
The necessary conditions for a minimum are
L
( , )
m m
i Gi D Gi
i i
Gi
C P P P
dC
P
λ λ
λ
= =
= + −
∂
=
∂
∑ ∑P
P
1
( ) 0 (for 1 to )
0
i
Gi
Gi
m
D Gi
i
P i m
dP
P P
λ
=
− = =
− =∑
6

7. Economic Dispatch Example
1 2
2
1 1 1 1
2
2 2 2 2
1
1
1
What is economic dispatch for a two generator
system 500 MW and
( ) 1000 20 0.01 $/h
( ) 400 15 0.03 $/h
Using the Lagrange multiplier method we know:
( ) 20 0.0
D G G
G G G
G G G
G
G
P P P
C P P P
C P P P
dC
P
dP
λ
= + =
= + +
= + +
− = + 1
2
2 2
2
1 2
2 0
( ) 15 0.06 0
500 0
G
G G
G
G G
P
dC
P P
dP
P P
λ
λ λ
− =
− = + − =
− − =
7

8. Economic Dispatch Example,
cont’d
1
2
1 2
1
2
1
2
We therefore need to solve three linear equations
20 0.02 0
15 0.06 0
500 0
0.02 0 1 20
0 0.06 1 15
1 1 0 500
312.5 MW
187.5 MW
26.2 $/MW
G
G
G G
G
G
G
G
P
P
P P
P
P
P
P
λ
λ
λ
λ
+ − =
+ − =
− − =
− −    
    − = −
    
− − −        
 
  =
 
   h
 
 
 
   8

9. Economic dispatch example, cont’d
• At the solution, both generators have the
same marginal (or incremental) cost, and this
common marginal cost is equal to λ.
• Intuition behind solution:
– If marginal costs of generators were different,
then by decreasing production at higher marginal
cost generator, and increasing production at
lower marginal cost generator we could lower
overall costs.
– Generalizes to any number of generators.
• If demand changes, then change in total costs
can be estimated from λ. 9

10. Economic dispatch example, cont’d
• Another way to solve the equations is to:
– Rearrange the first two equations to solve for PG1
and PG2 in terms of λ,
– Plug into third equation and solve for λ,
– Use the solved value of λ to evaluate PG1 and PG2.
• This works even when relationship between
generation levels and λ is more complicated:
– Equations are more complicated than linear when
there are maximum and minimum generation
limits or we consider losses. 10

11. Lambda-Iteration Solution Method
• Discussion on previous page leads to “lambda-
iteration” method:
– this method requires a unique mapping from a value
of lambda (marginal cost) to each generator’s MW
output:
– for any choice of lambda (common marginal cost),
the generators collectively produce a total MW
output,
– the method then starts with values of lambda below
and above the optimal value (corresponding to too
little and too much total output), and then
iteratively brackets the optimal value. •11
( ).Gi
P λ

12. Lambda-Iteration Algorithm
L H
1 1
H L
M H L
H M
1
L M
Pick and such that
( ) 0 ( ) 0
While Do
( )/2
If ( ) 0 Then
Else
End While
m m
L H
Gi D Gi D
i i
m
M
Gi D
i
P P P P
P P
λ λ
λ λ
λ λ ε
λ λ λ
λ λ λ
λ λ
= =
=
− < − >
− >
= +
− > =
=
∑ ∑
∑
12

13. Lambda-Iteration: Graphical View
In the graph shown below for each value of lambda
there is a unique PGi for each generator. This
relationship is the PGi(λ) function.
13

14. Lambda-Iteration Example
1 1 1
2 2 2
3 3 3
1 2 3
Consider a three generator system with
( ) 15 0.02 $/MWh
( ) 20 0.01 $/MWh
( ) 18 0.025 $/MWh
and with constraint 1000MW
Rewriting generation as a function of , (
G G
G G
G G
G G G
Gi
IC P P
IC P P
IC P P
P P P
P
λ
λ
λ
λ
= + =
= + =
= + =
+ + =
1 2
3
),
we have
15 20
( ) ( )
0.02 0.01
18
( )
0.025
G G
G
P P
P
λ
λ λ
λ λ
λ
λ
− −
= =
−
=
14

15. Lambda-Iteration Example, cont’d
m
i=1
m
i=1
1
H
1
Pick so ( ) 1000 0 and
( ) 1000 0
Try 20 then (20) 1000
15 20 18
1000 670 MW
0.02 0.01 0.025
Try 30 then (30) 1000 1230 MW
L L
Gi
H
Gi
m
L
Gi
i
m
Gi
i
P
P
P
P
λ λ
λ
λ
λ λ λ
λ
=
=
− <
− >
= − =
− − −
+ + − = −
= − =
∑
∑
∑
∑
15

16. Lambda-Iteration Example, cont’d
1
1
Pick convergence tolerance 0.05 $/MWh
Then iterate since 0.05
( )/ 2 25
Then since (25) 1000 280 we set 25
Since 25 20 0.05
(25 20)/ 2 22.5
(22.5) 1000 195 we set 2
H L
M H L
m
H
Gi
i
M
m
L
Gi
i
P
P
ε
λ λ
λ λ λ
λ
λ
λ
=
=
=
− >
= + =
− = =
− >
= + =
− = − =
∑
∑ 2.5
16

17. Lambda-Iteration Example, cont’d
H
*
*
1
2
3
Continue iterating until 0.05
The solution value of , , is 23.53 $/MWh
Once is known we can calculate the
23.53 15
(23.5) 426 MW
0.02
23.53 20
(23.5) 353 MW
0.01
23.53 18
(23.5)
0.025
L
Gi
G
G
G
P
P
P
P
λ λ
λ λ
λ
− <
−
= =
−
= =
−
= 221 MW=
17

18. Thirty Bus ED Example
Case is economically dispatched (without considering
the incremental impact of the system losses).
18

19. Generator MW Limits
Generators have limits on the minimum and
maximum amount of power they can
produce
Typically the minimum limit is not zero.
Because of varying system economics
usually many generators in a system are
operated at their maximum MW limits:
Baseload generators are at their maximum
limits except during the off-peak.
19

20. Lambda-Iteration with Gen Limits
,max
,max
In the lambda-iteration method the limits are taken
into account when calculating ( ) :
if calculated production for
then set ( )
if calculated production for
Gi
Gi Gi
Gi Gi
P
P P
P P
λ
λ
>
=
,min
,minthen set ( )
Gi Gi
Gi Gi
P P
P Pλ
<
=
20

21. Lambda-Iteration Gen Limit Example
1 2
3
1 2 3
1
In the previous three generator example assume
the same cost characteristics but also with limits
0 300 MW 100 500 MW
200 600 MW
With limits we get:
(20) 1000 (20) (20) (20) 10
G G
G
m
Gi G G G
i
P P
P
P P P P
=
≤ ≤ ≤ ≤
≤ ≤
− = + + −∑
1
00
250 100 200 1000
450 MW (compared to 670MW)
(30) 1000 300 500 480 1000 280 MW
m
Gi
i
P
=
= + + −
= − −
− = + + − =∑ 21

22. Lambda-Iteration Limit Example,cont’d
Again we continue iterating until the convergence
condition is satisfied.
With limits the final solution of , is 24.43 $/MWh
(compared to 23.53 $/MWh without limits).
Maximum limits will always caus
λ
1
2
3
e to either increase
or remain the same.
Final solution is:
(24.43) 300 MW (at maximum limit)
(24.43) 443 MW
(24.43) 257 MW
G
G
G
P
P
P
λ
=
=
=
22

23. Back of Envelope Values
$/MWhr = fuelcost * heatrate + variable O&M
Typical incremental costs can be roughly
approximated:
– Typical heatrate for a coal plant is 10, modern
combustion turbine is 10, combined cycle plant is 6
to 8, older combustion turbine 15.
– Fuel costs ($/MBtu) are quite variable, with current
values around 2 for coal, 3 to 5 for natural gas, 0.5
for nuclear, probably 10 for fuel oil.
– Hydro costs tend to be quite low, but are fuel (water)
constrained
– Wind and solar costs are zero. 23

24. Inclusion of Transmission Losses
The losses on the transmission system are a
function of the generation dispatch.
In general, using generators closer to the
load results in lower losses
This impact on losses should be included
when doing the economic dispatch
Losses can be included by slightly rewriting
the Lagrangian to include losses PL:
G
1 1
L( , ) ( ) ( )
m m
i Gi D L G Gi
i i
C P P P P Pλ λ
= =
 
= + + − ÷
 
∑ ∑P
24

25. Impact of Transmission Losses
G
1 1
G
The inclusion of losses then impacts the necessary
conditions for an optimal economic dispatch:
L( , ) ( ) ( ) .
The necessary conditions for a minimum are now:
L
(
m m
i Gi D L G Gi
i i
Gi
C P P P P P
P
λ λ
= =
 
= + + − ÷
 
∂
∂
∑ ∑P
P
1
, ) ( ) 1 ( ) 0
( ) 0
i L
Gi G
Gi Gi
m
D L G Gi
i
dC P
P P
dP P
P P P P
λ λ
=
 ∂
= − − = ÷∂ 
+ − =∑
25

26. Impact of Transmission Losses
th
Solving for , we get: ( ) 1 ( ) 0
1
( )
1 ( )
Define the penalty factor for the generator
(don't confuse with Lagrangian L!!!)
1
1 ( )
i L
Gi G
Gi Gi
i
Gi
GiL
G
Gi
i
i
L
G
Gi
dC P
P P
dP P
dC
P
dPP
P
P
L i
L
P
P
P
λ λ
λ
 ∂
− − = ÷∂ 
=
 ∂
− ÷∂ 
=
 ∂
− ∂

÷

The penalty factor
at the slack bus is
always unity!
26

27. Impact of Transmission Losses
1 1 1 2 2 2
The condition for optimal dispatch with losses is then
( ) ( ) ( )
1
. So, if increasing increases
1 ( )
the losses then ( ) 0 1.0
This makes generator
G G m m Gm
i Gi
L
G
Gi
L
G i
Gi
L IC P L IC P L IC P
L P
P
P
P
P
P L
P
λ= = =
=
 ∂
− ÷∂ 
∂
> ⇒ >
∂
appear to be more expensive
(i.e., it is penalized). Likewise 1.0 makes a generator
appear less expensive.
i
i
L <
27

28. Calculation of Penalty Factors
Unfortunately, the analytic calculation of is
somewhat involved. The problem is a small change
in the generation at impacts the flows and hence
the losses throughout the entire system. However,
i
Gi
L
P
using a power flow you can approximate this function
by making a small change to and then seeing how
the losses change:
1
( )
1
Gi
L L
G i
LGi Gi
Gi
P
P P
P L
PP P
P
∂ ∆
≈ ≈
∆∂ ∆ −
∆ 28

29. Two Bus Penalty Factor Example
2 2
2 2
0.37
( ) 0.0387 0.037
10
0.9627 0.9643
L L
G
G G
P P MW
P
P P MW
L L
∂ ∆ −
= − = = −
∂ ∆
= ≈ 29

30. Thirty Bus ED Example
Now consider losses.
Because of the penalty factors the generator incremental
costs are no longer identical.
30

31. Area Supply Curve
0 100 200 300 400
Total AreaGeneration(MW)
0.00
2.50
5.00
7.50
10.00
The area supply curve shows the cost to produce the
next MW of electricity, assuming area is economically
dispatched
Supply
curve for
thirty bus
system
31

32. Economic Dispatch - Summary
Economic dispatch determines the best way to
minimize the current generator operating costs.
The lambda-iteration method is a good approach for
solving the economic dispatch problem:
– generator limits are easily handled,
– penalty factors are used to consider the impact of losses.
Economic dispatch is not concerned with determining
which units to turn on/off (this is the unit commitment
problem).
Basic form of economic dispatch ignores the
transmission system limitations.
32

33. Security Constrained ED
or Optimal Power Flow
Transmission constraints often limit ability to
use lower cost power.
Such limits require deviations from what would
otherwise be minimum cost dispatch in order to
maintain system “security.”
Need to solve or approximate power flow in
order to consider transmission constraints.
33

34. Security Constrained ED
or Optimal Power Flow
The goal of a security constrained ED or
optimal power flow (OPF) is to determine the
“best” way to instantaneously operate a
power system, considering transmission
limits.
Usually “best” = minimizing operating cost,
while keeping flows on transmission below
limits.
In three bus case the generation at bus 3
must be limited to avoid overloading the line 34

35. Security Constrained Dispatch
Bus 2 Bus 1
Bus 3Home Area
Scheduled Transactions
357 MW
179 MVR
194 MW
448 MW
19 MVR
232 MVR
179 MW
89 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 MW
OFF AGC
AVR ON
AGC ON
AVR ON
100.0 MW
Need to dispatch to keep line
from bus 3 to bus 2 from overloading
35

36. Multi-Area Operation
In multi-area system, “rules” have been established
regarding transactions on tie-lines:
– In Eastern interconnection, in principle, up to “nominal”
thermal interconnection capacity,
– In Western interconnection there are more complicated
rules
The actual power that flows through the entire network
depends on the impedance of the transmission lines,
and ultimately determine what are acceptable patterns
of dispatch:
Can result in need to “curtail” transactions that otherwise
satisfy rules.
Economically uncompensated flow through other areas
is known as “parallel path” or “loop flows.”
Since ERCOT is one area, all of the flows on AC lines are
inside ERCOT and there is no uncompensated flow on
AC lines. 36

37. Seven Bus Case: One-line
Top Area Cost
Left Area Cost Right Area Cost
1
2
3 4
5
6 7
106 MW
168 MW
200 MW 201 MW
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 MW20 MW
40 MW
-40 MW
94 MW
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
System has
three areas
Left area
has one
bus
Right area has one
bus
Top area
has five
buses
37
No net
interchange
between
Any areas.

38. Seven Bus Case: Area View
System has
40 MW of
“Loop Flow”
Actual
flow
between
areas
Loop flow can result in higher losses
Area Losses
Area Losses Area Losses
Top
Left Right
-40.1 MW
0.0 MW
0.0 MW
0.0 MW
40.1 MW
40.1 MW
7.09 MW
0.33 MW 0.65 MW
Scheduled
flow
38

39. Seven Bus - Loop Flow?
Area Losses
Area Losses Area Losses
Top
Left Right
-4.8 MW
0.0 MW
100.0 MW
0.0 MW
104.8 MW
4.8 MW
9.44 MW
-0.00 MW 4.34 MW
100 MW Transaction
between Left and Right
Transaction has
actually decreased
the loop flow
Note that
Top’s
Losses have
increased
from
7.09MW to
9.44 MW
39