Smart Rotor Control of Wind Turbines

Using Trailing Edge Flaps

Matthew A. Lackner and Gijs van KuikJanuary 6, 2009

Technical University of DelftUniversity of Massachusetts Amherst

2

Agenda

• Background and Objectives• Simulation Environment• Control Design• Fatigue Load Reduction Results• Conclusions

3

Objective of Smart Rotor Control

• Objective: Significant reduction of blade loads by applying spanwise-distributed load control devices.

• Faster, local load control is possible.

• Active feedback based on local measurements.

4

Motivation

• Turbines are becoming very large, and so are loads.– 5 MW turbine has a 126 m diameter.

• Strong motivation to reduce blade fatigue loads for longer lifetime and lower cost.

5

Sources of Loads

• Result is large loads on the blades.

• Loads especially pronounced at integer multiples of the rotation frequency: 1P, 2P, etc…

Turbulence Wind Shear Tower Shadow

6

Aerodynamic Load Control Devices

• Shifting of Cl curve or change in α

flaps

adaptive geometry

active twist

microtabs*

*Van Dam 2001

7

Research Objectives

• Evaluate the use of trailing edge flaps for fatigue load reductions.– Compare to baseline controller.– Compare to individual pitch control.

• Investigate combined blade pitch and trailing edge flap approach.

8

Agenda

• Background and Objectives• Simulation Environment• Control Design• Fatigue Load Reduction Results• Conclusions

9

Simulation and Analysis of Smart Rotor

• Bladed:– Aero-elastic design

code.– Couple aerodynamics,

structural dynamics, and control into a simulation.

– Calculate performance and loads.

• Simulation of NREL 5 MW turbine in Garrad-Hassan ‘Bladed’ program.

10

Modeling a Smart Rotor In Bladed

Planform showing blade chord and pitch axis

m

Radius (m)

-0.5-1.0-1.5-2.0

0.00.51.01.52.02.53.0

20 40 60 80

-8 -6 -4 -2 0 2 4 6 8 10 12-1

-0.5

0

0.5

1

1.5

2

Angle of Attack (degrees)

CL

+10+6

+2

0

-2

-6-10

• Can include trailing edge flaps in a variable speed, pitch controller turbine in Bladed.

• 70% to 90% span length, 10% chord length.• Aerodynamic data for flaps generated using XFOIL.

11

Simulations Performed in Bladed

• 600 second simulations.

• Normal and extreme turbulence levels used.

• Mean wind speeds of 8, 12, 16, and 20 m/s.

• Baseline, individual pitch, flap, and combined blade pitch-flap control used.

12

Agenda

• Background and Objectives• Simulation Environment• Control Design• Fatigue Load Reduction Results• Conclusions

13

Basic Control Objective

• Reduce blade root flapwise bending moment loads:

• Utilize flap deflection or pitch angle:

• Problem: Rotating reference frame.

1 2 3, ,y y yM M M

1 2 3, ,

14

Multi-Blade Transformation

• Map variables in rotating coordinate system into fixed coordinate system.

• Multi-Blade (Coleman) Transformation.

• Variables now mapped into “yaw-wise” and “tilt-wise” axes (independent).

• Can approximate as time invariant system (LTI).

15

Feedback Control

1. Measure blade loads.2. Transform to fixed coordinate system.3. Two SISO systems for load reduction.4. Transform back into rotating coordinates.

16

Agenda

• Background and Objectives• Simulation Environment• Control Design• Fatigue Load Reduction Results• Conclusions

17

Time Domain Visualization

• Segment of the results of the 20 m/s simulation: Flap Control Individual Pitch Control

18

Fatigue Load Reduction Results

• Quantify using damage equivalent load of the root flapwise bending moment.

• Normal turbulence results:

8 12 16 200

0.2

0.4

0.6

0.8

1

Mean Wind Speed

Nor

mal

ized

DE

QL

SC

IFC

IPC

19

Load Reductions in the Frequency Domain

• Large 1P peak.• Effectiveness

depends on frequency of the loads.

• Most energy in the low frequency range

• Flaps have much higher bandwidth.

20

Hybrid Controller Results

• Also, two hybrid controllers that utilize individual pitch and flap control tested:– HYB1: Reduced individual pitch action, flaps focus on high frequency

loads.– HYB2: Identical individual pitch action, flaps focus on high frequency

loads.

• Fatigue Loads reduced even more than IFC or IPC.

• Sizeable load reduction across all frequencies.

21

Hybrid Controller Time Series Results

110 112 114 116 118 120 122 124 126 128 130-2

0

2

4

6

Mz1

(Nm

*106 )

SC

IFC

HYB 1

HYB 2

110 112 114 116 118 120 122 124 126 128 130

-10

-5

0

5

10

Time

Bla

de 1

TE

F D

efle

ctio

n (d

egre

es)

IFC

HYB 1

HYB 2

110 112 114 116 118 120 122 124 126 128 130-2

0

2

4

6

Mz1

(Nm

*106 )

SC

IPC

HYB 1

HYB 2

110 112 114 116 118 120 122 124 126 128 13016

17

18

19

20

21

22

23

Time

Bla

de 1

Pitc

h A

ngle

(de

gree

s)

IPC

HYB 1

HYB 2

• Same segment of the 20 m/s simulation.• Significantly different flap behavior.• Potentially reduced pitch usage.

Compare to IFC Compare to IPC

22

Limitations of Analysis: Unsteady Aerodynamics?

• Bladed assumes quasi-steady aerodynamic behavior.• No lag between changes in angle of attack or flap angle and

lift/drag/moment coefficient.• Reduced frequency quantifies degree of unsteadiness:

• k = 0: Steady.• 0 < k < 0.05: Quasi-steady.• k > 0.05: Unsteady.

2

ck

U

23

Evaluation of Unsteadiness

• Calculate flap deflection spectrum as a function of the reduced frequency.

• Integrate spectrum in steady and unsteady region.

0 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09 0.10

1000

2000

3000

PS

D(T

EF

Def

lect

ion)

[de

gree

s2 ]

NTM

8 m/s

12 m/s16 m/s

20 m/s

0 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09 0.10

500

1000

1500

2000

2500

3000

Reduced Frequency []

PS

D(T

EF

Def

lect

ion)

[de

gree

s2 ]

ETM

8 m/s

12 m/s16 m/s

20 m/s

24

Agenda

• Background and Objectives• Simulation Environment• Control Design• Fatigue Load Reduction Results• Conclusions

25

Conclusions

• Flaps are effective at reducing fatigue loads.

• IPC and IFC differ in how they reduce loads, specifically in which frequency ranges.

• A hybrid system offers an interesting combination of IPC and IFC.

• Question: What is the optimal balance between load reduction and blade pitch usage in a combined control approach?

26

Questions?

27

Basic Current Control Approaches

• Below Rated: Generator torque used to control rotor speed.• Above rated: Generator torque is constant, and blade pitch

used to control speed for constant power.

Below Rated: Generator Torque Control for Variable Speed Operation

Above Rated: Torque and Pitch Control for Constant Power Output

28

Advanced Current Control Approaches

• Individual pitch control (IPC) can also be used for load reduction.

• Problem: Increased demand on the pitch system.

*van Engelen 2005

29

Control Implementation

• Control code utilizing multi-blade transformation written in Fortran.

• PID controllers used.

• Compiled as “.dll” file to externally control the Bladed model.

• Gain scheduling used for different operating points.

• Collective flap angle also used to help with rotor speed control.

30

Collective Flap Angle

• Extra degree of freedom when using flaps.

• Collective pitch angle is already used for rotor speed control.

• But, can use collective flap angle as well to help with rotor speed control (above rated).

31

Dynamic Stall Limitations

• No dynamic stall model used for flap simulations.

-10 -8 -6 -4 -2 0 2 4 6 8 100

2

4

6

8

10

12O

ccur

renc

e (%

)

Angle of Attack (degrees)

NTM

-10 -8 -6 -4 -2 0 2 4 6 8 100

2

4

6

8

10

12

Occ

urre

nce

(%)

Angle of Attack (degrees)

ETM

32

Agenda

• Background and Objectives• Simulation Environment• Control Design• Fatigue Load Reduction Results• Effects of Integrating Trailing Edge Flaps• Conclusions

33

Effects on Pitch System

• Different approaches affect the pitch system differently.• Use 16 m/s simulation to investigate.

140 142 144 146 148 150 152 154 156 158 1609

10

11

12

13

14

15

Pitc

h A

ngle

(de

gree

s)

IPC 1

IPC 2

IPC 3SC Col.

IFC Col.

34

Summary Pitch System Effects

• Pitch Angle:

• Pitch Rate:

35

Agenda

• Background and Objectives• Simulation Environment• Control Design• Fatigue Load Reduction Results• Effects of Integrating Trailing Edge Flaps• Extreme Loads• Conclusions

36

Extreme Loads: Global Step Change• Step increase in wind speed from 15 m/s to 20 m/s.

• Uniform across the rotor face.

• No wind shear, tower shadow, turbulence, or gravity loads.

• Isolate effects of the gusts. 0 5 10 15 20 25 3010

12

14

16

18

20

22

Time (s)

Win

d S

peed

(m

/s)

0.5 s

1.5 s

3 s5 s

7.5 s

10 s

37

Global Step Change Results• Range used to quantify load

reductions: R = Max – Min.• IFC reduces range, IPC does not.• Reduction due to collective flap

angle.• Large scale gusts don’t produce

tilt/yaw moments.

10 15 200

2

4

6

8

10

Mz1

(Nm

*106 )

1.5 Second Step

SC

IPCIFC

10 15 200

2

4

6

8

105 Second Step

SC

IPCIFC

10 15 20-10

-5

0

5

10

15

20

Pitc

h or

TE

F A

ngle

(de

g)

Time (s)10 15 20

-10

-5

0

5

10

15

20

Time (s)

38

Extreme Loads: Local Step Change

• Wind speed is 15 m/s at almost all points.

• At 2, 4, or 6 points, increase it to 20 m/s.

• Simulates a local gust on a scale close to the blade length.

-65 -54.2 -43.3 -32.5 -21.7 -10.8 0 10.8 21.7 32.5 43.3 54.2 650

25

35.8

46.7

57.5

68.3

79.2

90

100.8

111.7

122.5

133.3

144.2

155

Rotor Center

Outer 6 PointsOuter 4 Points

Outer 2 Points

39

Extreme Loads: Local Step Change• Now IFC and IPC are both effective.

• Local gusts do generate tilt/yaw moment.

• For faster, more local gusts, IFC is much better due to higher bandwidth.

54.5 55 55.5 56 56.5 57 57.54.5

5

5.5

6

6.5

7

7.5

8

Mz1

(Nm

*106 )

2 Point Local Step

SC

IPCIFC

54.5 55 55.5 56 56.5 57 57.54.5

5

5.5

6

6.5

7

7.5

86 Point Local Step

SC

IPCIFC

54.5 55 55.5 56 56.5 57 57.5-5

0

5

10

Pitc

h or

TE

F A

ngle

(de

g)

Time (s)54.5 55 55.5 56 56.5 57 57.5-5

0

5

10

Time (s)

40

Fatigue Load Reduction Results• High turbulence levels:

41

Dimensionalize Load Reduction Capabilities

• At a very basic level, goal of flaps or IPC is to generate a moment on the blade root.

• Depends on position, length, and lift change of control surface.

• Helps explain effectiveness of IPC, especially at low frequencies.

L

r lM C

R R

42

Effect on Power Production – Region III

• Trying to produce constant power output.• Average power is unaffected in all cases.• Use of collective flap angle results in less variability.• IPC results in more variability.

43

Effect on Power Production – Region II• Reduced power below rated: ~1%.• Due to reduced torque on the flap section.

80 82 84 86 88 90 92 94 96 98 1000

100

200

300

400

500

600

700

800

900

1000

Time (s)

In P

lane

Aer

o Lo

adin

g (N

/m)

SC

IFC

44

Explanation for Reduced Power Production – Region II

• Average operating angle of attack is not optimal on the flap section.

Actual Optimal

-2 -1 0 1 2 3 4 5 6 7 8

60

80

100

120

140

160

180

200

220

Angle of Attack (deg)

Lift

-Dra

g R

atio

Delta = 0 deg

Delta = -10 deg

Delta = 10 deg

-2 -1 0 1 2 3 4 5 6 7 8

60

80

100

120

140

160

180

200

220

Angle of Attack (deg)

Lift

-Dra

g R

atio

Delta = 0 deg

Delta = -10 deg

Delta = 10 deg

45

Airfoil Data Extrapolation

-1.50

-1.00

-0.50

0.00

0.50

1.00

1.50

2.00

-180 -135 -90 -45 0 45 90 135 180

CL

CD