alan d. wright lee j. fingersh national renewable energy laboratory karl a. stol university of...

Alan D. Wright Alan D. Wright Lee J. FingershLee J. Fingersh

National Renewable Energy LaboratoryNational Renewable Energy Laboratory

Karl A. StolKarl A. StolUniversity of AucklandUniversity of Auckland

2828thth ASME Wind Energy Symposium ASME Wind Energy SymposiumOrlando, Fl.Orlando, Fl.

January 5, 2009January 5, 2009

Field Testing Controls to Mitigate Fatigue Loads in the Controls Advanced Research Turbine

A. Wright 1/5/09 2

Presentation Scope

Show design of region 3 generator torque and blade pitch controller.

Goals: – Region 3 speed regulation– active damping of tower side-side and fore-aft motion

Describe field implementation and tests in the Controls Advanced Research Turbine.

Compare state-space control results to baseline PID control results.

A. Wright 1/5/09 3

Commercial Turbine Control

Generator Speed

Generator Torque

Rotor Collective Pitch

Region 2

Region 3

Wind Disturbances

PID Pitch Controller

Drive-train Damper

T = kw^2

Nonlinear Turbine

Generator Torque

Nacelle Yaw

Blade Pitch

Control Actions

2T k

A. Wright 1/5/09 4

Control of Flexible Modes

Blade2 edge

Tower Side-Side

Rotor Rotation

(b) Frontview

Blade1 edge

Blade-1 Flap

Tower Fore-Aft

Rotor Teeter

(a) Sideview

Blade-2 Flap

Generator Rotation

Drive-train Torsion

A. Wright 1/5/09 5

Region 3 Control Design

Control Actuators: Collective blade pitch, generator torque

Goals:– Collective Blade Pitch Control:

• Speed regulation

• Tower fore-aft damping

– Generator Torque Control:• Tower side-side damping

• Drive-train torsion damping

Two separate control loops:– Collective blade pitch

– Generator torque

A. Wright 1/5/09 6

Questions

Can separate control loops be used to add active damping to two closely spaced modes?

– Tower 1st fore-aft mode (0.87 Hz.)

– Tower 1st side-side mode (0.88 Hz.)

Will these separate loops destabilize each other?

– Two separate control loops:

• Collective blade pitch

• Generator torque

Will adding damping reduce tower fatigue loads?

A. Wright 1/5/09 7

Controller Structure

CART

C onventional Variable - Sp eed

Controller

S tate - Space Torque Controller

S tate - Space P itch Controller

Nominal Generator Torque

Total Gen. Torque

Rotor Coll. Pitch Rate

Added Gen. Torque

y y

y

y

A. Wright 1/5/09 8

Regulate rotor-speed in the presence of wind-speed disturbances and stabilize turbine modes.– Stabilize flexible modes through full state

feedback.– Use state estimation to provide the controller with

needed states (including wind-speed).– Account for uniform wind disturbances

2-Multiple input/single output control loops

Full State Feedback Control

A. Wright 1/5/09 9

Rotor Collective Pitch Control Model

perturbed generator speed

rotor collective pitch angles

perturbed tower-top fore-aft acceleration

y

d d

d d

x Ax Bu B u

y Cx Du D u

rotor collective pitch rateu q

qx

Perturbed structural dofs & rates:

tower f-a

rotor collective flapgenerator-speed

Pitch actuator states

A. Wright 1/5/09 10

Disturbance model

( ) ( )

( ) ( )d d

d d

z t Fz t

u t z t

du uniform wind disturbance

A. Wright 1/5/09 11

Open-loop and Closed-loop Pole Locations

Open-loop poles Closed-loop poles

First flap symmetric mode

−3.63 ± 13.81i −3.66 ± 13.85i

Tower first f-a mode −0.07 ± 5.52i −1.27 ± 5.32i

Generator speed −0.1943 −2.50

( ) ( ) ( ) d du t x t G zG t

A. Wright 1/5/09 12

Generator Torque Control Design Model

perturbed generator speed

perturbed tower-top side-side accelerationy

x Ax Bu

y Cx Du

generator torqueu q

x q

w

Perturbed structural dofs & rates:

tower s-sdrive-train torsiongenerator-speed

filter states

A. Wright 1/5/09 13

Open-loop and Closed-loop Pole Locations

Open-loop poles Closed-loop poles

Tower first s-s mode

−0.002 ± 5.54i −0.140 ± 5.54i

Generator speed −0.102 −0.102

Drive train first torsion

−0.01 ± 22.47i −1.07 ± 22.45i

( ) ( )Gu t x t

A. Wright 1/5/09 14

Testing Strategy

Using two separate uncoupled control loops to add active damping to two closely spaced tower modes

Will these two control loops interact and destabilize the turbine?

Test the tower f-a damping with collective pitch first.

Add tower s-s damping with generator torque.

A. Wright 1/5/09 15

Controllers:

PID State-space1 State-space2

Speed regulation

yes yes yes

Tower f-a damping

no yes yes

Tower s-s damping

no no yes

Drive-train torsion damping

no yes yes

A. Wright 1/5/09 16

Controller Structure

CART

C onventional Variable - Sp eed

Controller

S tate - Space Torque Controller

S tate - Space P itch Controller

Nominal Generator Torque

Total Gen. Torque

Rotor Coll. Pitch Rate

Added Gen. Torque

y y

y

y

A. Wright 1/5/09 17

Results

Statistics and Performance

Measure

Baseline PID Control

(Undamped)(5530 sec.)

State-Space 1 (tower f-a

damping only) (2500 sec.)

State-Space 2 (tower f-a and s-s damping) (3700 sec.)

Tower fore-aft bending(kNm)

mean 1527.5fatigue DEL

1342.2

mean 1525.6fatigue DEL

944.4

mean 1498.9fatigue DEL

928.6

Tower side-side bending(kNm)

mean 122.1fatigue DEL

920.5

mean 293.3fatigue DEL

926.4

mean 360.1fatigue DEL

680.7

Blade pitch rate (deg/s)

avg mag 2.02max 15.1min -15.5

avg mag 2.32max 14.7min -13.9

avg mag 2.33max 14.7min -12.9

Generator torque (Nm)

Std 0.00max 3524min 3524

std 23.1max 3631min 3423

std 115.8max 4177min 2942

A. Wright 1/5/09 18

Results

10 12 14 16 18 20 22 24 26 28 300

0.05

0.1

0.15

0.2

0.25

Windspeed

PDF o

f W

indsp

eed

41.8 42 42.2 42.4 42.6 42.8 43 43.2 43.4 43.6 43.80

0.5

1

1.5

2

2.5

Rotorspeed

PDf of

Rot

orsp

eed

State-space1State-space 2PID

Probability Density Functions:

A. Wright 1/5/09 19

Results

0

0.2

0.4

0.6

0.8

1

1.2

1.4

Low-speed ShaftTorque DEL

(Nm)

Tower s-sbending DEL

(kNm)

Tower f-abending DEL

(kNm)

Blade flap-bending DEL

(kNm)

Load Category

No

rma

lize

d V

alu

e

PID State-space 1

A. Wright 1/5/09 20

Results

0

0.2

0.4

0.6

0.8

1

1.2

1.4

Low-speed ShaftTorque DEL

(Nm)

Tower s-sbending DEL

(kNm)

Tower f-abending DEL

(kNm)

Blade flap-bending DEL

(kNm)

Load Category

No

rma

lize

d V

alu

e

PID State-space 1 State-space 2

A. Wright 1/5/09 21

Results

Tower Bending Moments:

1490 1492 1494 1496 1498 1500 1502 1504 1506 1508

0

500

1000

1500

2000

2500

Time

Tow

er for

e-af

t (k

Nm

)

1490 1492 1494 1496 1498 1500 1502 1504 1506 1508 1510

-500

0

500

1000

Time

Tow

er s

ide-

side

(kNm

)

State-space1

State-space2PID

A. Wright 1/5/09 22

Results

Power Spectral Densities:

0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2

105

102

108

Frequency, Hz

PSD o

f Tow

er F

ore-

aft

Bend

ing

(kNm

)2/H

z

0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2

105

102

108

Frequency, Hz

PSD o

f Tow

er S

ide-

side

Be

ndin

g (k

Nm

)2/H

z

State-space1State-space 2PID

1stside-side

1stfore-aft

2P

2P

1P

1P

A. Wright 1/5/09 23

Results

500 510 520 530 540 550 560 570 580 590 600-20

-10

0

10

20

Time

pit

ch1ra

te(d

eg/

s)

500 510 520 530 540 550 560 570 580 590 6002800

3000

3200

3400

3600

3800

4000

4200

Time

GenTor

que

(Nm

)

State-space 1

State-space 2PID

Pitch Rate and Generator Torque:

A. Wright 1/5/09 24

Conclusions

Designed and performed field tests of two separate control loops:

– Rotor collective pitch control• active tower f-a damping • Region 3 speed regulation

– generator torque control• active tower s-s damping• active drive-train torsion damping

Field tests demonstrate 30% reduction in tower f-a and 26% reduction in tower s-s fatigue loads compared to simple PID controls.

Reasons for lack of drive-train torque load mitigation not resolved.

Results showed no undesirable interactions between these separate control loops.

A. Wright 1/5/09 25

Future Work

Resolve issue with drive-train torsion damping and re-test controller.

Implement and field-test independent blade pitch for shear mitigation and generator torque control.

Investigate alternative sensors for independent pitch control – look ahead Lidar

– additional blade sensors

– hub or shaft sensors

A. Wright 1/5/09 26

Acknowledgements

Dr. Michael Robinson – NREL management support

Garth Johnson, Scott Wilde – CART maintenance and support

Marshall Buhl – MCrunch data analysis scripts

A. Wright 1/5/09 27

Questions?

Dr. Alan D. Wright

303-384-6928

alan_wright@nrel.gov