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