Wind Energy Honors CourseSpring 2011
Iowa State University
Wind Power and Power Balance in the Grid
James D. McCalleyHarpole Professor of Electrical &
Computer Engineering
Outline1. Basic problems, potential solutions2. Wind power equation3. Variability4. System Control5. Comments on potential solutions
Basic problems with wind & power balance1. Wind is a variable resource when maximizing
energy productiona. Definition: NETLOAD.MW=LOAD.MW-WIND.MWb. Fact: Wind increases NETLOAD.MW variability in gridc. Fact: Grid requires GEN.MW=NETLOAD.MW alwaysd. Fact: “Expensive” gens move (ramp) quickly, “cheap” gens
don’t, some gens do not ramp at all.e. Problem: Increasing wind increases need for more and
“faster” resources to meet variability, increasing cost of wind.
2. Wind is an uncertain resourcea. Fact: Market makes day-ahead decisions for “unit
commitment” (UC) based on NETLOAD.MW forecast.b. Fact: Large forecast error requires available units compensate.c. Problem: Too many (under-forecast) or too few (over-
forecast) units may be available, increasing the cost of wind.
Solutions to variability & uncertainty
1. We have always dealt with variability and uncertainty in the load, so no changes are needed.
2. Increase MW control capability during periods of expected high variability via control of the wind power.
3. Increase MW control capability during periods of expected high variability via more conventional generation.
4. Increase MW control capability during periods of expected high variability using demand control.
5. Increase MW control capability during periods of expected high variability using storage.
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• Groups of 2-3, 10 minutes• Identify your preferred approach to the variability problem• Consider the below solutions, one, or combination, or other• Identify reasons (e.g., economics, effectiveness, sustainability)
and have one person report to class at end of 10 minutes
Power productionWind power equation
v1 vt v2
v
x
Swept area At of turbine blades:
The disks have larger cross sectional area from left to right because• v1 > vt > v2 and• the mass flow rate must be the same everywhere within the streamtube:
ρ=air density (kg/m3)
Therefore, A 1 < At < A 2
2211
21 vAvAvA
QQQ
tt
t
Mass flow rate is the mass of substance which passes through a given surface per unit time.
Power productionWind power equation
ttt
t vAtxA
tmQ
3. Mass flow rate at swept area:
22
212
1 vvmKE
1. Wind velocity:txv
xAm 2. Air mass flowing:
4a. Kinetic energy change:
5a. Power extracted: 2
221
22
21 2
121 vvQvvtm
tKEP t
6a. Substitute (3) into (5a):)()2/1( 2
221 vvvAP tt
4b. Force on turbine blades: 21 vvQv
tm
tvmmaF t
5b. Power extracted: 21 vvvQFvP ttt
6b. Substitute (3) into (5b):)( 21
2 vvvAP tt
ttttt vvvvvvvvvvvvvvvvv ))(2/1()())(()2/1()()()2/1( 12212
21212122
221
7. Equate
8. Substitute (7) into (6b): ))((4
)()))(2/1(( 2122
2121
221 vvvvAvvvvAP t
t
9. Factor out v13: )1)()(1(
4 1
22
1
231
vv
vvvAP t
Power productionWind power equation
10. Define wind stream speed ratio, a: 1
2vva
)1)(1(4
231 aavAP t
11. Substitute a into power expression of (9):
12. Differentiate and find a which maximizes function:
1,3/10)1)(13(0123122
0)1()1(24
222
231
aaaaaaaaa
aaavAaP t
This ratio is fixed for a given turbine & control condition.
13. Find the maximum power by substituting a=1/3 into (11):
278
34
98
4)
34)(
911(
4
31
31
31 vAvAvAP ttt
Power productionWind power equation
14. Define Cp, the power (or performance) coefficient, which gives the ratio of the power extracted by the converter, P, to the power of the air stream, Pin.
)1)(1(4
231 aavAP t
31
211
211
21 2
121
210
21 vAvvAvQvtm
tKEP ttin
power extracted by the converter
power of the air stream
)1)(1(21
21
)1)(1(4 2
31
231
aavA
aavA
PPC
t
t
inp
15. The maximum value of Cp occurs when its numerator is maximum, i.e., when a=1/3:
5926.02716)
34)(
98(
21
in
p PPC
The Betz Limit!
312
1 vACPCP tPinp
Power productionCp vs. λ and θ
Tip-speed ratio:11 vR
vu
u: tangential velocity of blade tipω: rotational velocity of blade R: rotor radiusv1: wind speed
Pitch: θ
GE SLE 1.5 MW
Power productionWind Power Equation
31),(
21 vACPCP tPinp
So power extracted depends on 1.Design factors:
• Swept area, At 2.Environmental factors:
• Air density, ρ (~1.225kg/m3 at sea level)• Wind speed v3
2. Control factors: • Tip speed ratio through the rotor speed ω• Pitch θ
Power productionCp vs. λ and θ
Tip-speed ratio:11 vR
vu
u: tangential velocity of blade tipω: rotational velocity of blade R: rotor radiusv1: wind speed
GE SLE 1.5 MW
Important concept #1:The control strategy of all US turbines today is to operate turbine at point of maximum energy extraction, as indicated by the locus of points on the black solid line in the figure.
Important concepts #2:• This strategy maximizes the energy produced by a given wind turbine.• Any other strategy “spills” wind !!!
Important concepts #3:• Cut-in speed>0 because blades need minimum torque to rotate.• Generator should not exceed rated power• Cut-out speed protects turbine in high winds
Power productionUsable speed range
Cut-in speed (6.7 mph) Cut-out speed (55 mph)
STOPPED HERE
Wind Power Temporal & Spatial Variability
13
JULY2006JANUARY2006
Notice the temporal variability:• lots of cycling between blue and red;• January has a lot more high-wind power (red) than July;
Notice the spatial variability• “waves” of wind power move through the entire Eastern Interconnection;• red occurs more in the Midwest than in the East
Blue~VERY LOW POWER; Red~VERY HIGH POWER
MW-Hz Time Frames
-100
-80
-60
-40
-20
0
20
40
60
80
100
07:00 07:20 07:40 08:00 08:20 08:40 09:00 09:20 09:40 10:00
REG
ULAT
ION
IN M
EGAW
ATTS
Regulation
=
+
Load Following Regulation
Source: Steve Enyeart, “Large Wind Integration Challenges for Operations / System Reliability,” presentation by Bonneville Power Administration, Feb 12, 2008, available athttp://cialab.ee.washington.edu/nwess/2008/presentations/stephen.ppt.
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How Does Power System Handle Variability
15
Turbine-Gen 1Turbine-Gen 2Turbine-Gen …Turbine-Gen N
∆f∆Ptie
ACE=∆Ptie +B∆f
Primary control provides regulation
Secondary control provides load following
Characterizing Netload Variability∆T HISTOGRAMMeasure each ∆T variation for 1 yr (∆T=1min, 5min, 1 hr)Identify “variability bins” in MWCount # of intervals in each variability binPlot # against variability binCompute standard deviation σ.
Regulation
Load following
Ref: Growing Wind; Final Report of the NYISO 2010 Wind Generation Study, Sep 2010.www.nyiso.com/public/webdocs/newsroom/press_releases/2010/GROWING_WIND_-
_Final_Report_of_the_NYISO_2010_Wind_Generation_Study.pdf
Solutions to variability & uncertainty1. Do nothing: fossil-plants provide reg & LF (and die ).2. Increase control of the wind generation
a. Provide wind with primary control• Reg down (4%/sec), but spills wind following the control • Reg up, but spills wind continuously
b. Limit wind generation ramp rates• Limit of increasing ramp is easy to do• Limit of decreasing ramp is harder, but good forecasting
can warn of impending decrease and plant can begin decreasing in advance
3. Increase non-wind MW ramping capability during periods of expected high variability using one or more of the below:a. Conventional generation b. Load controlc. Storaged. Expand control areas
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%/min $/mbtu $/kw LCOE,$/mwhr
Coal 1-5 2.27 2450 64
Nuclear 1-5 0.70 3820 73
NGCC 5-10 5.05 984 80
CT 20 5.05 685 95
Diesel 40 13.81
Why Does Variability Matter? NERC penalties for poor-performance Consequences of increased frequency variblty:
Some loads may lose performance (induction motors) Relays can operate to trip loads (UFLS), and gen (V/Hz) Lifetime reduction of turbine blades Frequency dip may increase for given loss of generation Areas without wind may regulate for windy areas
Consequences of increased ACE variability (more frequent MW corrections):
Increased inadvertent flows Increase control action of generators
Regulation moves “down the stack,” cycling!
Increasing wind penetration causes cycling
Hydro peaking: http://hydropowerstation.com/?tag=hydropower-peaking-operations
How to decide?First, primary frequency control for over-frequency conditions, which requires generation reduction, can be effectively handled by pitching the blades and thus reducing the power output of the machine. Although this action “spills” wind, it is effective in providing the necessary frequency control. Second, primary frequency control for under-frequency conditions requires some “headroom” so that the wind turbine can increase its power output. This means that it must be operating below its maximum power production capability on a continuous basis. This also implies a “spilling” of wind.Question: Should we “spill” wind in order to provide frequency control, in contrast to using all wind energy and relying on some other means to provide the frequency control? Answer: Need to compare system economics between increased production costs from spilled wind, and increased investment, maint, & production costs from using storage & conventional gen.
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