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 vAt

xA

t

mQ

3. Mass flow rate at swept area:

22

212

1vvmKE

1. Wind velocity:t

xv

xAm 2. Air mass flowing:

4a. Kinetic energy change:

5a. Power extracted: 2

221

22

21 2

1

2

1vvQvv

t

m

t

KEP t

6a. Substitute (3) into (5a):)()2/1( 2

221 vvvAP tt

4b. Force on turbine blades:

21 vvQvt

m

t

vmmaF 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 vvvv

AvvvvAP t

t

9. Factor out v13: )1)()(1(

4 1

22

1

231

v

v

v

vvAP t

Power productionWind power equation

10. Define wind stream speed ratio, a: 1

2

v

va

)1)(1(4

231 aavA

P 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

aaaa

aaaaa

aaavA

a

P t

This ratio is fixed for a given turbine & control condition.

13. Find the maximum power by substituting a=1/3 into (11):

27

8

3

4

9

8

4)

3

4)(

9

11(

4

31

31

31 vAvAvA

P 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 aavA

P t

31

211

211

21 2

1

2

1

2

10

2

1vAvvAvQv

t

m

t

KEP ttin

power extracted by the converter

power of the air stream

)1)(1(2

1

21

)1)(1(4 2

31

231

aavA

aavA

P

PC

t

t

inp

15. The maximum value of Cp occurs when its numerator is maximum, i.e., when a=1/3:

5926.027

16)

3

4)(

9

8(

2

1

inp P

PC

The Betz Limit!

312

1vACPCP tPinp

Power productionCp vs. λ and θ

Tip-speed ratio:11 v

R

v

u u: tangential velocity of blade tip

ω: rotational velocity of blade

R: rotor radiusv1: wind speed

Pitch: θ

GE SLE 1.5 MW

Power productionWind Power Equation

31),(

2

1vACPCP 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 v

R

v

u u: tangential velocity of blade tip

ω: 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

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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

RE

GU

LA

TIO

N I

N M

EG

AW

AT

TS

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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