EWEC2007 Milano, 8 May 2007 Extrapolation of extreme loads acc. to IEC 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 ed.3 3.Proposal regarding the combination of extrapolation and real turbine behaviour

Example  Pitch regulated  Variable speed  13m/s rated wind speed  25m/s cut out wind speed  Over 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 speed TI=14.1% Rated generator speed Artificial onshore turbine: Simplified control system: *NTM-Regime = Normal Turbulence Model acc. to IEC

Turbine response in time domain Out of Plane Blade Bending Moment Turbine response is linear in turbulence intensity around NTM-Regime NTM Normalized Load 14m/s mean wind speed 22m/s mean wind speed Normalized Load NTM Turbulence Intensity

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 ETM Angle of attack Normalized lift coefficient curve NTM Static polar

Load Extrapolation What is load extrapolation acc. to ed.3? How does it work? Input : Extreme values in normal operation (NTM) Output: Long period extreme values corresponding to extreme turbulent events Load extrapolation concludes from NTM-regime to extreme events assuming constant system properties

Plausibility check of extrapolation results  Time domain simulation with appropriate turbulence intensity  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 speedTI NTM TI ETM TI XTM 14m/s16.29%23.50%23.54% 22m/s14.73%17.90%18.71% Extreme value extrapolation for wind speed at hub height Scaling of turbulence intensity: Result: Extrapolated TI XTM is slightly above TI ETM in these bins Procedure:

Turbulence Intensity NTM XTM Linear increase of turbine response Extrapolation of turbulence intensity Extrapolated Load Time domain simulation Load Extra- polation Plausibility check of extrapolation results Time domain simulation and load extrapolation should be comparable

More or less acceptable differences 14m/s mean wind speed Differences between load extrapolation and time domain up to ~10%

Unacceptable differences 22m/s mean wind speed Differences between load extrapolation and time domain up to ~30%!

NTM XTM 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 Extrapolated Load Time Domain Load(XTM)

Summary Load extrapolation acc. to IEC 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