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
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- 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.
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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
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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
Controller response in up-scaled turbines
interaction of controller modes with tower 1st fore-aft bending modes leads to unstable behaviour
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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
Controller response in up-scaled turbines
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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
Controller response in up-scaled turbines
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integral gain of up-scaled turbines
Integral gain follows an 1/s scaling
5 MW
20 MW
Effect of turbulent inflow
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In up-scaled turbines less coherent wind is experienced by
points at the same dimensionless distance
1
2
U
DfAexp)f(Coh
Effect of turbulent inflow
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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
Effect of turbulent inflow
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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
Effect of turbulent inflow
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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
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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
Aeroelastic loads of up-scaled turbines
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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
Aeroelastic loads of up-scaled turbines
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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
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Acknowledgements
The work has been partially financed by the EC within the FP6 UpWind project
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Thank you for your Attention
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