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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
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)
2
Power Flow
• A common power system analysis tool is the power flow – It shows how real and reactive power flows through a network,
from generators to loads
• Solves sets of non-linear equations enforcing "conservation of power" at each bus in the system (a consequence of KCL)– 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
3
PowerWorld Simulator Three Bus System
Bus 2 Bus 1
Bus 3Home Area
204 MW
102 MVR
150 MW
150 MW 37 MVR
116 MVR
102 MW 51 MVR
1.00 PU
-20 MW 4 MVR
20 MW -4 MVR
-34 MW 10 MVR
34 MW-10 MVR
14 MW -4 MVR
-14 MW
4 MVR
1.00 PU
1.00 PU
106 MW 0 MVR
100 MWAGC ONAVR ON
AGC ONAVR ON
Load with
green
arrows
indicating
amount
of MW
flow
Used
to control
output of
generator Direction of arrow is used to indicate
direction of real power (MW) flow
Note the
power
balance at
each bus
4
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.
https://www.misoenergy.org/MarketsOperations/RealTimeMarketData/Pages/ACEChart.aspx
MISO ACE|(in MW) from 9/19/12. Atthe time the MISO loadwas about 65GW
5
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.
6
Three Bus Case on AGC
Bus 2 Bus 1
Bus 3Home Area
266 MW
133 MVR
150 MW
250 MW 34 MVR
166 MVR
133 MW 67 MVR
1.00 PU
-40 MW 8 MVR
40 MW -8 MVR
-77 MW 25 MVR
78 MW-21 MVR
39 MW-11 MVR
-39 MW
12 MVR
1.00 PU
1.00 PU
101 MW 5 MVR
100 MWAGC ONAVR ON
AGC ONAVR ON
7
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 price-
based generator costs
8
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)
9
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
10
100 MW Transaction
Bus 2 Bus 1
Bus 3Home Area
Scheduled Transactions
225 MW
113 MVR
150 MW
291 MW 8 MVR
138 MVR
113 MW 56 MVR
1.00 PU
8 MW -2 MVR
-8 MW 2 MVR
-84 MW 27 MVR
85 MW-23 MVR
93 MW-25 MVR
-92 MW
30 MVR
1.00 PU
1.00 PU
0 MW 32 MVR
100 MWAGC ONAVR ON
AGC ONAVR ON
100.0 MW
Scheduled 100 MW
Transaction from Left to Right
Net tie-line flow is now
100 MW 11
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.
12
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 MWOFF AGCAVR ON
AGC ONAVR ON
100.0 MW
Dispatch is no longer optimal due to need to keep
Line from bus 2 to bus 3 from overloading 13
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.”
14
Seven Bus Case One-line Diagram
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 MW 20 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 $/MWH 4189 $/MWH
Case Hourly Cost 16933 $/MWH
System has
three areas
Area left
has one
bus Area right has one bus
Area top
has five
buses
15
Seven Bus Case: Area View
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
System has
40 MW of
“Loop Flow”
Scheduled
flow
16
Seven Bus System – 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
Note that
Top’s
Losses have
increased
from
7.09MW to
9.44 MW
Transaction has actually decreased
the loop flow
17
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)
18
Three Bus Case LMPs: Line Limit NOT Enforced
Bus 2 Bus 1
Bus 3
Total Cost
0 MW
0 MW
180 MWMW
10.00 $/MWh
60 MW 60 MW
60 MW
60 MW120 MW
120 MW
10.00 $/MWh
10.00 $/MWh
180 MW120%
120%
0 MWMW
1800 $/hr
Line from Bus 1 to Bus 3 is over-loaded; all buses have same marginal cost
Gen 1’s
cost
is $10
per
MWh
Gen 2’s
cost
is $12
per
MWh
19
Three Bus Case LMPS: Line Limits Enforced
Bus 2 Bus 1
Bus 3
Total Cost
60 MW
0 MW
180 MWMW
12.00 $/MWh
20 MW 20 MW
80 MW
80 MW100 MW
100 MW
10.00 $/MWh
14.01 $/MWh
120 MW 80% 100%
80% 100%
0 MWMW
1921 $/hr
Line from 1 to 3 is no longer overloaded, but now
the marginal cost of electricity at 3 is $14 / MWh 20
Generation Supply Curve
0
20
40
60
80
0 10000 20000 30000 40000
Generation (MW)
Pri
ce (
$ /
MW
h)
Base Load
Coal and Nuclear
Generation
Natural
Gas Generation
As the load goes up so does the price
Renewable Sources Such as Wind Have Low Marginal Cost, but they are Intermittent 21
MISO LMPs on Sept 19, 2012 (11:50am EST which is CDT)
Available on-line at https://www.misoenergy.org/LMPContourMap/MISO_All.html22
MISO LMPs on Feb 6, 2015, 1pm Central
23Available on-line at https://www.misoenergy.org/LMPContourMap/MISO_All.html
MISO Annual Load Duration Curves
24https://www.misoenergy.org/Library/Repository/Report/Annual%20Market%20Report/2013%20Annual%20Market%20Assessment%20Report.pdf
MISO Average Prices and Wind Output
https://www.misoenergy.org/Library/Repository/Report/Annual%20Market%20Report/2013%20Annual%20Market%20Assessment%20Report.pdf 25
Wind Power Systems
Photos taken Kate Davis near Moraine View State Park, IL 26
Historical Development of Wind Power
• The first known wind turbine for producing electricity was by Charles F. Brush turbine, in Cleveland, Ohio in 1888
http://www.windpower.org/en/pictures/brush.htm
• 12 kW• Used electricity
to charge batteries in the cellar of the owner’s mansion
Note the person
27
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 28
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 utilitygrid 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 1990sPhoto: www.daviddarling.info/encyclopedia/W/AE_wind_energy.html 29
Global Installed Wind Capacity
Source: Annual Market Update 2013, Global Wind Energy Council,
Total worldwide electric capacity is 4500GW, sowind, at almost 250GW, is 5.6% of total
30
Wind Capacity Additions by Region
Source: Annual Market Update 2013, Global Wind Energy Council, 31
Top 10 Countries - Installed Wind Capacity (as of the end of 2013)
Source: Annual Market Update 2013, Global Wind Energy Council, 32
US Wind Resources
http://www.windpower.org/en/pictures/lacour.htmhttp://www.windpoweringamerica.gov/pdfs/wind_maps/us_windmap.pdf 33
US Wind Capacity by State, 12/31/14
34
Wind Map for Illinois at 80m
35
Worldwide Wind Resource Map
Source: www.ceoe.udel.edu/WindPower/ResourceMap/index-world.html36
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”
37
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 lowerhttp://www.absoluteastronomy.com/topics/Darrieus_wind_turbine 38
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
39
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
40
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
41
Worldwide Wind Energy Company Market Share, 2013 Installations
Source:http://www.statista.com/statistics/272813/market-share-of-the-leading-wind-turbine-manufacturers-worldwide/42
Vestas Stock Price
https://uk.finance.yahoo.com/echarts?s=VWS.CO#symbol=VWS.CO;range=my43