Implications on loads by up-scaling towards 20 MW size

EWEA annual event 2012Copenhagen, Denmark

Implications on loads by up-scaling towards 20 MW size

V.A.Riziotis & S.G.Voutsinas

National Technical University of Athens

E.S. Politis

Centre for Renewable Energy Sources

H.A. Madsen & F. Rasmussen

EWEA annual event, Copenhagen 16-19 April 2012

Scope of the work

- Objective of the work

Assess the loads of up-scaled turbines taking into account

• parameters of the control system

• turbulent inflow characteristics

EWEA annual event, Copenhagen 16-19 April 2012 2/17

- Analysis performed in the context of geometric similarity

simple pathway to obtain a first approximation of the critical operational and structural properties

- As baseline turbine of the analysis the 5MW offshore RWT of UPWIND project is used

Background of geometric up-scaling

For any geometric scale factor s

• the rotor speed decreases with 1/s - tip speed remains constant.

• the power increases with s2 rule.

• the aerodynamic loads and the rotor thrust scale up with s2 while moments scale with s3.

• gravitational forces and mass follow s3 scaling while gravitational moments scale with s4

• the sectional bending stiffness follows s4 rule and section moment of resistance s3.

• stresses due to aerodynamic bending loads are scale invariant - weight induced stresses scale linearly.

• wind turbine natural frequencies normalized with the rotational frequency values remain unchanged - absolute values decrease with 1/s scale.


proportional gain Kp [s]

dam

pin

gin

log

.dec

rem

ent[

%]

0 0.01 0.02 0.03 0.04

0

20

40

60

80

100

120

140

13m/s-1st tower fore aft15m/s19m/s23m/s13m/s-controller mode15m/s19m/s23m/s

Controller response in up-scaled turbines


tuning of controller parameters using a linear eigenvalue servoaeroelastic stability tool

A limit on tower damping is set for defining consistent Kp values

0.000

0.005

0.010

0.015

0.020

0.025

0.00 5.00 10.00 15.00 20.00 25.00

prop

ortio

nal g

ain

Kp

[s]

pitch angle [deg]

Kp Ziegler-Nichols schedule

fit to Ziegler-Nichols

Kp final schedule

fit to final schedule

tuning of proportional gain


interaction of controller modes with tower 1st fore-aft bending modes leads to unstable behaviour


t/T

pit

ch

an

gle

[d

eg

]

With up-scaling, natural frequencies decrease and come closer to controller frequencies

5 MW

10 MW

20 MW



proportional gain of upscaled turbines

Proportional gain is preserved with upscaling

proportional gain Kp [s]

dam

pin

gin

log

.dec

rem

ent[

%]

0 0.01 0.02 0.03 0.04

0

20

40

60

80

100

120

1405MW - 1st fore-aft mode5MW - controller mode

20MW - 1st fore-aft mode20MW - controller mode

.dp

constd

p i set

dpK K

dt

pK

P-I equation

K

constant

0.00

0.20

0.40

0.60

0.80

1.00

1.20

120 160 200 240 280 320

Ki/

Ki_

ref

D [m]

predicted1/s

0.000

0.002

0.004

0.006

0.008

0.010

0.012

0.014

0.016

0.018

0.00 5.00 10.00 15.00 20.00 25.00

inte

gra

l ga

in K

i

pitch angle [deg]

Ki Ziegler-Nichols schedule

fit to Ziegler-Nichols

Ki final - 5MW

Ki final- 10MW

Ki final - 20MW

integral gain Ki

dam

pin

gin

log

.dec

rem

ent[

%]

0 0.005 0.01 0.015-10

0

10

20

30

40

50

60

70

8013 m/s - 1st tower mode fore-aft15m/s19m/s23m/s

integral gain Ki

dam

pin

gin

log

.dec

rem

ent[

%]

0 0.005 0.01 0.015-10

0

10

20

30

40

50

60

70

80 13 m/s - 1st tower mode fore-aft15m/s19m/s23m/s



integral gain of up-scaled turbines

Integral gain follows an 1/s scaling

5 MW

20 MW

Effect of turbulent inflow


In up-scaled turbines less coherent wind is experienced by

points at the same dimensionless distance

1

2

U

DfAexp)f(Coh



Effect of up-scaling on purely aerodynamic loads and power sdv

Lower loads are obtained as a result of the less coherent wind over the rotor disk of bigger diameter



Energy from the wind is concentrated to p-multiples in

the rotating frame.

With up-scaling, rotational frequency decreases and gets closer to the frequency range where wind spectrum contains

the maximum energy



Energy from the wind is concentrated to 3p-multiples for

tower loads

A more pronounced 3p peak is obtained

0.000

5.000

10.000

15.000

20.000

25.000

30.000

4 6 8 10 12 14 16 18 20 22 24 26

roto

r thr

ust f

orce

at h

ub/D

^2

[N/m

^2]

wind speed [m/s]

5MW

10MW

20MW

Aeroelastic loads of up-scaled turbines


normalized thrust force

Decrease in loads as a result of spatial averaging of the wind over the rotor disk due to

lower coherency

Increase in loads as a result of slower response

of pitch control. If the response of the controller is made faster interaction with tower modes will take

place

1Hz equivalent load



blade flapwise and edgewise moments

tower bottom fore-aft bending moment

1Hz equivalent load

-20.0

-15.0

-10.0

-5.0

0.0

5.0

10.0

15.0

20.0

4 6 8 10 12 14 16 18 20 22 24 26

(%) v

aria

tion

of d

imen

sion

less

bla

de

flapw

ise

mom

ent

wind speed [m/s]

10MW

20MW

10MW stiff tower

20MW stiff tower

-20.0

-15.0

-10.0

-5.0

0.0

5.0

10.0

15.0

20.0

4 6 8 10 12 14 16 18 20 22 24 26

(%) v

aria

tion

of d

imen

sion

less

tow

er

bott

om fo

re-a

ft b

endi

ng m

omen

t

wind speed [m/s]

10MW

20MW

10MW stiff tower

20MW stiff tower

loads variation with respect to 5MW turbine

0.0

10.0

20.0

30.0

40.0

50.0

60.0

70.0

80.0

90.0

100.0

4 6 8 10 12 14 16 18 20 22 24 26

(%) v

aria

tion

of d

imen

sion

less

bla

de

edge

wis

e m

omen

t

wind speed [m/s]

10MW

20MW

10MW stiff tower

20MW stiff tower



Lower variations of power and rotor speed in the partial load

region

0

20

40

60

80

100

120

4 9 14 19 24

norm

aliz

ed p

ower

wind speed [m/s]

5MW meanminmax

10MW meanminmax

20MW meanminmax

0

20

40

60

80

100

120

4 9 14 19 24

norm

aliz

ed r

otor

spe

ed

wind speed [m/s]

5MW meanminmax

10MW meanminmax

20MW meanminmax

0

2

4

6

8

10

12

4 9 14 19 24

norm

aliz

ed s

dv o

f rot

or s

peed

wind speed [m/s]

5MW sdv

10MW sdv

20MW sdv

Conclusions

• Interaction of the control system modes with the structural modes is likely to occur as the rotor size increases which implies that a time scaling must be introduced in the control system parameters.

• as the rotor size increases spatial coherency of the incoming wind, from the blade root to the blade tip decreases and as a result rotor and tower loads become lower in the partial load region

• better power quality and lower rotor speed fluctuations are obtained for the up-scaled turbines

• the energy of the wind concentrates mainly on multiples of the rotational frequency which indicates that the wake induced effects will have a strong variation with the azimuth


Acknowledgements

The work has been partially financed by the EC within the FP6 UpWind project


Thank you for your Attention


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Implications on loads by up-scaling towards 20 MW size