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EWEC2007 Milano, 8 May 2007
Extrapolation of extreme loads acc. to IEC 61400-1 Ed.3 in comparison with the
physics of real turbine response
Dirk Steudel, Martin von Mutius REpower Systems
Introduction
1. Identification of real turbine response and nonlinear effects
2. Nonlinear effects and load extrapolation acc. to IEC 61400-1 ed.3
3. Proposal regarding the combination of extrapolation and real turbine behaviour
Example
0
0.800 0.850 0.900 0.950 1.000 1.050 1.100
Pitch regulatedVariable speed13m/s rated wind speed25m/s cut out wind speedOver speed trigger directly above highest rotor speed in NTM-regime*
14m/s mean wind speed TI=16.1%
Normalized generator speed
Over speed trigger
22m/s mean wind speedTI=14.1%
Rated generator speed
Artificial onshore turbine:
Simplified control system:
*NTM-Regime = Normal Turbulence Model acc. to IEC 61400-1
Turbine response in time domain
Out of Plane Blade Bending Moment
Turbine response is linear in turbulence intensity around NTM-Regime
0.8
0.9
1
1.1
1.2
1.3
1.4
1.5
10% 15% 20% 25% 30% 35% 40% 45% 50% 55% 60%
NTM
Normalized Load
14m/s mean wind speed
22m/s mean wind speed
Normalized Load 0.95
1
1.05
1.1
1.15
1.2
10% 15% 20% 25% 30% 35% 40% 45% 50% 55% 60%
NTM
Turbulence Intensity
Turbulence Intensity
9.E+03
1.E+04
1.E+04
1.E+04
1.E+04
1.E+04
1.E+04
10% 20% 30% 40% 50% 60%
1200
1220
1240
1260
1280
1300
1320
1340
1360
1380
1400
10% 20% 30% 40% 50% 60%
Nonlinearity of the control system
22m/s mean wind speed
Turbulence Intensity
Generator Speed
NTM
Out Of Plane Bending Moment
The control system may cause significant nonlinearities in higher TI-Regime
Turbulence Intensity
NTM
Over speed trigger
Nonlinearity of aerodynamics
The aerodynamics may cause significant nonlinearities in higher TI-Regime
-1
-0.5
0
0.5
1
1.5
-30 -20 -10 0 10 20 30
CL/
CLm
ax
AOA[deg]
ETM
-1
-0.5
0
0.5
1
1.5
-30 -20 -10 0 10 20 30
CL/
CLm
ax
AOA[deg]Angle of attack
Normalizedlift coefficient curve
NTM
Static polar
Load Extrapolation
• What is load extrapolation acc. to 61400-1 ed.3?• How does it work?
4000
5000
6000
7000
8000
9000
10000
11000
12000
0 100 200 300 400 500 600
Input : Extreme values in normal operation (NTM)
1e-005
0.0001
0.001
0.01
0.1
1
11000 11100 11200 11300 11400 11500 11600 11700 11800
Output: Long period extremevalues corresponding to extreme turbulent events
Load extrapolation concludes from NTM-regime to extreme events assuming constant system properties
Plausibility check of extrapolation results
1. Time domain simulation with appropriate turbulence intensity
2. Compare simulated load with extrapolated load
Extrapolated load should correspond to turbine response in time domain
=>
But how to determine the appropriate turbulence intensity?
Extrapolation of turbulence intensity
Mean wind speed
TINTM TIETM TIXTM
14m/s 16.29% 23.50% 23.54%
22m/s 14.73% 17.90% 18.71%
• Extreme value extrapolation for
wind speed at hub height
• Scaling of turbulence intensity:
meanNTM
meanXTMNTMXTM vv
vvTITI
XTMv
Result:
Extrapolated TIXTM is slightly above TIETM in these bins
Procedure:
Turbulence Intensity NTM XTM
Linear i
ncrease
of turb
ine resp
onse
Extrapolation of turbulence intensity
Extrapolated Load
Timedomain
simulation
LoadExtra-
polation
Plausibility check of extrapolation results
Time domain simulation and load extrapolation should be comparable
1.0
1.1
1.2
1.3
1.4
InPlan
eBen
ding
OOPlan
eBen
ding
Rotor T
orqu
e
Tower
Base B
ending
Load Extrapolation
Simulation with extrapolated TI
More or less acceptable differences
14m/s mean wind speed
Differences between load extrapolation and time domain up to ~10%
1.0
1.1
1.2
1.3
1.4
InPlan
eBen
ding
OOPlan
eBen
ding
Rotor T
orqu
e
Tower
Base B
ending
Load Extrapolation
Simulation with extrapolated TI
Unacceptable differences
22m/s mean wind speed
Differences between load extrapolation and time domain up to ~30%!
NTM XTM
0.8
0.9
1
1.1
1.2
1.3
1.4
1.5
10% 15% 20% 25% 30% 35% 40%
Explanation for big deviations
Out of Plane Blade Bending Moment
Bad correspondence between extrapolation and real physical state at 22m/s
22m/s mean wind speed
14m/s mean wind speed
Extrapolated Load
NTM XTM
0.8
0.9
1
1.1
1.2
1.3
1.4
1.5
10% 15% 20% 25% 30% 35% 40%
Extrapolated Load
Time Domain Load(XTM)
Time Domain Load(XTM)
Summary
• Load extrapolation acc. to IEC61400-1 can lead to overly conservative loads which differ significantly from real turbine response
• Load extrapolation is not valid if the system properties change between normal and extreme ambient conditions
• Time domain simulations are able to follow all kinds of changes of system properties
• The proposed approach considers the real turbine physics and leads to realistic loads in extreme ambient conditions
The famous last sentence
The completion of load
extrapolation by additional
extrapolation of turbulence
intensity
has recently been introduced by
REpower into the actual
amendment discussions of MT01