ECE 333
Renewable Energy Systems
Lecture 7: Power System Operations,
Wind as a Resource
Prof. Tom Overbye
Dept. of Electrical and Computer Engineering
University of Illinois at Urbana-Champaign
overbye@illinois.edu
Announcements
•
•
Start reading Chapter 7; also read Prof. Sauer's article on
course website explaining reactive power
HW 3 is posted; it will be covered by an in-class quiz on
Thursday Feb 13
–
Material from Power Systems history and operations will be
covered on exams (such as true/false)
1
Power Flow
•
A common power system analysis tool is the power flow
–
•
Solves sets of non-linear equations enforcing
"conservation of power" at each bus in the system (a
consequence of KCL)
–
–
•
It shows how real and reactive power flows through a network,
from generators to loads
Loads are usually assumed to be constant power
Used to determine if any transmission lines or transformers are
overloaded and system voltages
Educational version PowerWorld tool available at
–
http://www.powerworld.com/gloversarmaoverbye
2
PowerWorld Simulator Three Bus
System
Load with
green
arrows
indicating
amount
of MW
flow
Bus 2
20 MW
-4 MVR
Bus 1
1.00 PU
204 MW
102 MVR
1.00 PU
106 MW
0 MVR
150 MW AGC ON
116 MVR AVR ON
-14 MW
-34 MW
10 MVR
4 MVR
34 MW
-10 MVR
Home Area
Used
to control
output of
generator
-20 MW
4 MVR
14 MW
-4 MVR
Bus 3
1.00 PU
102 MW
51 MVR
100 MW
Note the
power
balance at
each bus
150 MW AGC ON
37 MVR AVR ON
Direction of arrow is used to indicate
direction of real power (MW) flow
3
Area Control Error (ACE)
•
The area control error is the difference between the
actual flow out of an area, and the scheduled flow.
• Ideally the ACE should always be zero.
• Because the load is constantly changing, each utility
must constantly change its generation to “chase” the ACE.
MISO ACE|
(in MW) from
9/19/12. At
the time the
MISO load
was about
65GW
https://www.misoenergy.org/MarketsOperations/RealTimeMarketData/Pages/ACEChart.aspx
4
Automatic Generation Control
•
•
BAs use automatic generation control (AGC) to
automatically change their generation to keep their
ACE close to zero.
Usually the BA control center calculates ACE based
upon tie-line flows; then the AGC module sends
control signals out to the generators every couple
seconds.
5
Three Bus Case on AGC
Bus 2
-40 MW
8 MVR
40 MW
-8 MVR
Bus 1
1.00 PU
266 MW
133 MVR
1.00 PU
101 MW
5 MVR
150 MW AGC ON
166 MVR AVR ON
-39 MW
-77 MW
25 MVR
12 MVR
78 MW
-21 MVR
Home Area
100 MW
39 MW
-11 MVR
Bus 3
1.00 PU
133 MW
67 MVR
250 MW AGC ON
34 MVR AVR ON
6
Generator Costs
•
•
•
•
•
There are many fixed and variable costs associated
with power system operation.
The major variable cost is associated with generation.
Cost to generate a MWh can vary widely.
For some types of units (such as hydro and nuclear) it
is difficult to quantify.
Many markets have moved from cost-based to pricebased generator costs
7
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)
8
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
9
100 MW Transaction
Bus 2
8 MW
-2 MVR
-8 MW
2 MVR
Bus 1
1.00 PU
225 MW
113 MVR
1.00 PU
0 MW
32 MVR
150 MW AGC ON
138 MVR AVR ON
-92 MW
-84 MW
27 MVR
30 MVR
85 MW
-23 MVR
Home Area
93 MW
-25 MVR
Bus 3
Scheduled Transactions
100.0 MW
100 MW
1.00 PU
113 MW
56 MVR
291 MW AGC ON
8 MVR AVR ON
Scheduled 100 MW
Transaction from Left to Right
Net tie-line
flow is now
100 MW
10
Security Constrained Economic
Dispatch
• 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.
11
Security Constrained Dispatch
Bus 2
-22 MW
4 MVR
22 MW
-4 MVR
Bus 1
1.00 PU
357 MW
179 MVR
1.00 PU
0 MW
37 MVR
100%
194 MW OFF AGC -142 MW
49 MVR
232 MVR AVR ON
145 MW 100%
-37 MVR
Home Area
Bus 3
Scheduled Transactions
100.0 MW
-122 MW
41 MVR
100 MW
124 MW
-33 MVR
1.00 PU
179 MW
89 MVR
448 MW AGC ON
19 MVR AVR ON
Dispatch is no longer optimal due to need to keep
12
Line from bus 2 to bus 3 from overloading
Multiple Area Operation
•
•
•
If Areas have direct interconnections, then they may
directly transact up to the capacity of their tie-lines.
Actual power flows through the entire network
according to the impedance of the transmission
lines.
Flow through other areas is known as “parallel
path” or “loop flows.”
13
Seven Bus Case One-line Diagram
44 MW
-42 MW
-31 MW
0.99 PU
3
1.05 PU
1
System has
three areas
106 MW -37 MW
AGC ON
62 MW
79 MW
2
40 MW
20 MVR
Top Area Cost
8029 $/MWH
1.00 PU
-32 MW
Case Hourly Cost
16933 $/MWH
32 MW
80 MW
30 MVR
4
110 MW
40 MVR
38 MW
-61 MW
1.04 PU
31 MW
-77 MW
5
40 MW
-39 MW
94 MW
AGC ON
-14 MW
Area top
has five
buses
1.01 PU
130 MW
40 MVR
168 MW AGC ON
-40 MW
1.04 PU
6
Area left
has one
bus
20 MW
-20 MW
40 MW
1.04 PU
20 MW
200 MW
0 MVR Left Area Cost
4189 $/MWH
200 MW AGC ON
-20 MW
7
200 MW
Right Area Cost
0 MVR
4715 $/MWH
201 MW AGC ON
Area right has one
bus
14
Seven Bus Case: Area View
Top
40.1 MW
0.0 MW
System
has
40 MW of
“Loop
Flow”
-40.1 MW
0.0 MW
Left
Area Losses
0.33 MW
Area Losses
7.09 MW
Actual
flow
between
areas
Scheduled
Area Losses flow
Right
40.1 MW
0.0 MW
0.65 MW
Loop flow can result in higher losses
15
Seven Bus System – Loop Flow?
Transaction
has actually
decreased
the loop
flow
Top
4.8 MW
0.0 MW
-4.8 MW
0.0 MW
Left
Area Losses
-0.00 MW
Area Losses
9.44 MW
Right
104.8 MW
100.0 MW
100 MW Transaction
between Left and Right
Area Losses
4.34 MW
Note that
Top’s
Losses have
increased
from
7.09MW to
9.44 MW
16
Pricing Electricity
•
•
•
•
•
Cost to supply electricity to bus is called the locational
marginal price (LMP)
Presently PJM and MISO post LMPs on the web
In an ideal electricity market with no transmission
limitations the LMPs are equal
Transmission constraints can segment a market,
resulting in differing LMP
Determination of LMPs requires the solution on an
Optimal Power Flow (OPF)
17
Three Bus Case LMPs: Line Limit NOT
Enforced
Gen 2’s
cost
is $12
per
MWh
Bus 2
60 MW
60 MW
Bus 1
10.00 $/MWh
0 MW
10.00 $/MWh
120 MW
120%
180 MW
0 MW
Gen 1’s
cost
is $10
per
MWh
60 MW
Total Cost
1800 $/hr
120%
120 MW
60 MW
10.00 $/MWh
Bus 3
180 MW
0 MW
Line from Bus 1 to Bus 3 is over-loaded; all
buses have same marginal cost
18
Three Bus Case LMPS: Line Limits
Enforced
Bus 2
20 MW
20 MW
Bus 1
10.00 $/MWh
60 MW
12.00 $/MWh
100 MW
80%
100%
120 MW
0 MW
80 MW
Total Cost
1921 $/hr
80%
100%
100 MW
80 MW
14.01 $/MWh
Bus 3
180 MW
0 MW
Line from 1 to 3 is no longer overloaded, but now
the marginal cost of electricity at 3 is $14 / MWh
19
Generation Supply Curve
As the load goes up so does the price
Price ($ / MWh)
80
Natural
Gas Generation
60
Base Load
Coal and Nuclear
Generation
40
20
0
0
10000
20000
30000
40000
Generation (MW)
Renewable Sources Such as Wind Have Low
Marginal Cost, but they are Intermittent
20
MISO LMPs on Sept 19, 2012
(11:50am EST which is CDT)
Available on-line at https://www.misoenergy.org/LMPContourMap/MISO_All.html
21
MISO LMPs on Feb 6, 2015, 1pm Central
Available on-line at https://www.misoenergy.org/LMPContourMap/MISO_All.html
22
MISO Annual Load Duration Curves
https://www.misoenergy.org/Library/Repository/Report/Annual%20Market%20Report/20
13%20Annual%20Market%20Assessment%20Report.pdf
23
MISO Average Prices and Wind Output
https://www.misoenergy.org/Library/Repository/Report/Annual%20Market%20Report/20
13%20Annual%20Market%20Assessment%20Report.pdf
24
Wind Power Systems
Photos taken Kate Davis near Moraine View State Park, IL 25
Historical Development of Wind
Power
• The first known wind turbine for producing
electricity was by Charles F. Brush turbine, in
Cleveland, Ohio in 1888
•
•
12 kW
Used electricity
to charge
batteries in the
cellar of the
owner’s mansion
Note the
person
http://www.windpower.org/en/pictures/brush.htm
26
Historical Development of Wind
Power
• First wind turbine outside of the US to generate
electricity was built by Poul la Cour in 1891 in Denmark
•
Used electricity
from his wind
turbines to
electrolyze water
to make hydrogen
for the gas lights
at the
schoolhouse
http://www.windpower.org/en/pictures/lacour.htm
27
Historical Development of Wind
Power
• In the US - first wind-electric systems built in
•
•
•
•
the late 1890’s
By 1930s and 1940s, large numbers in rural
areas not served by the grid for pumping
water and sometimes electricity generation
Interest in wind power declined as the utility
grid expanded and as reliable, inexpensive
electricity could be purchased
Oil crisis in 1970s created a renewed interest in
wind until US government stopped giving tax
Renewed interest again since the 1990s
Photo: www.daviddarling.info/encyclopedia/W/AE_wind_energy.html
28
Global Installed Wind Capacity
Total worldwide electric capacity is 4500GW, so
wind, at almost 250GW, is 5.6% of total
Source: Annual Market Update 2013, Global Wind Energy Council,
29
Wind Capacity Additions by Region
Source: Annual Market Update 2013, Global Wind Energy Council,
30
Top 10 Countries - Installed Wind
Capacity (as of the end of 2013)
Source: Annual Market Update 2013, Global Wind Energy Council,
31
US Wind Resources
http://www.windpoweringamerica.gov/pdfs/wind_maps/us_windmap.pdf
http://www.windpower.org/en/pictures/lacour.htm
32
US Wind Capacity by State, 12/31/14
33
Wind Map for Illinois at 80m
34
Worldwide Wind Resource Map
Source: www.ceoe.udel.edu/WindPower/ResourceMap/index-world.html
35
Types of Wind Turbines
•
•
•
•
“Windmill”- used to grind grain into flour or pump
water
Many different names - “wind-driven generator”,
“wind generator”, “wind turbine”, “wind-turbine
generator (WTG)”, “wind energy conversion system
(WECS)”
Can have be horizontal axis wind turbines (HAWT)
or vertical axis wind turbines (VAWT)
Groups of wind turbines are located in what is
called either a “wind farm” or a “wind park”
36
Vertical Axis Wind Turbines
•
•
•
•
•
Darrieus rotor - the only vertical axis machine with
any commercial success
Wind hitting the vertical blades, called aerofoils,
generates lift to create rotation
No yaw (rotation about vertical axis)
control needed to keep them facing
into the wind
Heavy machinery in the nacelle is
located on the ground
Blades are closer to ground where
windspeeds are lower
http://www.absoluteastronomy.com/topics/Darrieus_wind_turbine
37
Horizontal Axis Wind Turbines
•
•
•
“Downwind” HAWT – a turbine with the blades
behind (downwind from) the tower
No yaw control needed- they naturally orient
themselves in line with the wind
Shadowing effect – when a blade swings behind the
tower, the wind it encounters is briefly reduced and
the blade flexes
38
Horizontal Axis Wind Turbines
•
•
•
•
“Upwind” HAWT – blades are in front of (upwind
of) the tower
Most modern wind turbines are this type
Blades are “upwind” of the tower
Require somewhat complex yaw control to keep
them facing into the wind
–
•
•
Need to search for the wind to start turning
Operate more smoothly and deliver more power
Largest turbines are on the order of 6 MW with 1.5
MW a quite common design
39
Number of Rotating Blades
•
Windmills have multiple blades
–
–
–
–
•
•
need to provide high starting torque to overcome weight of the
pumping rod
must be able to operate at low wind speeds to provide nearly
continuous water pumping
a larger area of the rotor faces the wind
Note, most seem to write “wind speed” as two words
Turbines with many blades operate at much lower
rotational speeds - as the speed increases, the turbulence
caused by one blade impacts the other blades
Most modern wind turbines have two or three blades
40
Worldwide Wind Energy Company
Market Share, 2013 Installations
Source:http://www.statista.com/statistics/272813/market-share-of-the-leading-wind-turbine-manufacturers-worldwide/
41
Vestas Stock Price
https://uk.finance.yahoo.com/echarts?s=VWS.CO#symbol=VWS.CO;range=my
42