Extreme Response for Wind Turbines

1

• Extreme Response for Wind Turbines
• Authors
• Henrik Stensgaard Toft, Aalborg University,
  Denmark
• John Dalsgaard Sørensen, Aalborg University /
  Risø-DTU, Denmark

2
Contents

• Introduction
• Load Extrapolation based on Extreme Response in
  Storms
• Numerical Example
• Conclusion
• Future Work

3
Introduction

• The extreme load can occur in two situations
• Standstill position The wind turbine is parked
  and behave like a normal civil engineering
  structure (wind speed gt 25m/s).
• Operational condition The wind turbine is
  producing power and behave like a machine (wind
  speed 3-25m/s).

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4
Introduction

• The extreme load during operation is dependent
  on
• The mean wind speed.
• The turbulence intensity.
• The type and settings of the control system.

Tower Mudline Moment Pitch controlled wind turbine
Flap Bending Moment Pitch controlled wind turbine
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Introduction

• IEC 61400-1 Extreme load determined from 10min.
  simulations of the
• response during operation over the range of
  significant wind speeds.
• The method is based on the following assumptions
• Local extremes in the response are independent.
• Individual 10min. time series are independent.
• In the present paper are these assumptions
  investigated.

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6
Introduction

• In the present paper is a new method for
  estimation of the extreme load presented.
• By comparing the extreme load determine by this
  method and the method in IEC 61400-1 can the
  assumptions about independence be studied.
• The new method uses a long measured time series
  which rarely is available Therefore is the new
  method only used for validation of the existing
  method.

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Load Extrapolation based on Extreme Response in
Storms

• The new method for estimating the extreme load is
  based on
• Existing methods for estimating e.g. the extreme
  wave height.
• Assumption about independent storms.

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8
Load Extrapolation based on Extreme Response in
Storms

• A storm wind speed Ustorm should be defined so
  extreme loads occur for mean wind speeds above
  Ustorm.

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Load Extrapolation based on Extreme Response in
Storms

• Ustorm is defined from the nominal wind speed
  Unom (typically 10-15 m/s) and the standard
  deviation for the turbulence ?1.
• Independence of storms
• Storms are combined if the mean wind speed
  between them not are below a percentile of
  Ustorm.
• Storms are separated a minimum number of hours.

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Load Extrapolation based on Extreme Response in
Storms

1. The measured time series is divided into
   independent storms.
2. For each storm is the extreme response extracted.
3. To the 25-30 largest extreme responses is a
   distribution function fitted.
4. The characteristic load is calculated for the
   probability

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Numerical Example

• Stall controlled onshore wind turbine.
• Dataset for response measured over 62 days in
  winter and spring.
• (52 days of complete measurements)
• Dataset for mean wind speed measured over 4
  years.
• Site Less severe than class III in IEC 61400-1.
  Turbulence intensity Iref 0.12.

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Numerical Example

• Investigation of storm definition.
• Storms are combined if the mean wind speed
  between them not are below 80 of Ustorm.
• Storms are separated at least 2 hours.
• Higher nominal wind speed leads to less and
  shorter storms.

Nominal wind speed Unom Storm wind speed Ustorm Storms per year ? Length of storm Ts
14m/s 9.5m/s 202 8.0h
15m/s 10.2m/s 165 7.4h
16m/s 11.0m/s 135 6.7h
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Numerical Example

• Load extrapolation according to IEC 61400-1.
• Extremes extracted by the Peak Over Threshold
  method.
• Characteristic loads calculated with/without
  statistical uncertainty (Hessian matrix).
• Normalized characteristic loads.

Threshold Characteristic load Characteristic load
without stat. unc. with stat. unc.
1.4 1.000 1.037
2.0 1.041 1.091
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Numerical Example

• Load extrapolation based on extreme response in
  storms.
• 25 largest extremes from independent storms are
  used.
• Measured time series are short 52 days of
  complete measurements
• Choice of distribution function has a significant
  influence on the results.

Distribution Characteristic load Characteristic load
without stat. unc. with stat. unc.
Weibull 1.106 1.348
Gumbel 1.303 1.401
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Conclusion

• New method for calculation of the extreme
  response is proposed.
• Definition of a storm is proposed.
• Higher characteristic load using the new method
  which could indicate that the 10min. time series
  or local extremes are dependent.
• Based on a stall controlled wind turbine and a
  short measured time series.

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16
Future Work

• Refinement of the storm definition based on
• Longer measured time series.
• More severe sites.
• Estimation of the correct long-term
  distribution for the extreme response
• Weibull
• Gumbel
• Etc.
• Comparison of the new method and IEC 61400-1 for
  a long measured time series.

17

• Extreme Response for Wind Turbines
• Authors
• Henrik Stensgaard Toft, Aalborg University,
  Denmark
• John Dalsgaard Sørensen, Aalborg University /
  Risø-DTU, Denmark