A Review of Wind Turbine Noise

DEWI Magazin Nr. 28, Februar 2006 IEC-NEWS: Power Performance and Acoustic Noise H. Klug, DEWI Wilhelmshaven The author, Dr. Helmut Klug, is a member of the IEC MT12 Power Performance and convener of the IEC Acoustic Noise MT11. 1. Power Performance 61400 –12 The revision of the IEC 61400-12 Power Performance Measurement standard is finished. The Final Draft International Standard FDIS 61400-12-1 Power Performance Measurements of Electricity Producing Wind Turbines is accepted by the national
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  30 DEWI Magazin Nr. 28, Februar 2006  The author, Dr. Helmut Klug, is a member of the IECMT12 Power Performance and convener of the IECAcoustic Noise MT11. 1.Power Performance 61400 –12 The revision of the IEC 61400-12 Power PerformanceMeasurement standard is finished. The Final DraftInternational Standard FDIS 61400-12-1 Power Perform-ance Measurements of Electricity Producing Wind Tur-bines is accepted by the national committees and in thenear future the standard will be available.The IEC Power Performance MT12 is now focusing onthe documents ã 61400-12-2 Power Performance Verification ã 61400-12-3 Wind Farm Performance TestingThe new document –12-2 will describe alternativemethods for power performance testing of individual windturbines. The standard will encompass two separatemethodologies: power performance measurement, nacel-le anemometry and numerical site calibration. The nacel-le anemometry method will be published as a standard and numerical site calibration will be published eit-her as part of the standard or as an informative annex to the standard depending on the national com-ments. Since the wind speed at the nacelle is affected by the rotor (i.e. slowed down), the measured windspeed needs to be corrected for this. Procedures for determining that correction will be included in themethodology. The numerical site calibration method will allow site calibration after the erection of the windfarm, based on numerical modelling. This numerical site calibration will replace the normal site calibrationrequirements of IEC 61400-12-1. After this replacement, the rest of IEC 61400-12-1 may be applied. Themain effort will focus on validating the models and defining criteria for acceptable models based on thisvalidation.The table 1 shows example results based on DEWI measurements of nacelle anemometry. In the firstexample a wind farm of nine turbines has been tested for power performance. At the first turbine a IEC61440-12 compliant power curve measurement has been performed together with the determination of the nacelle anemometer correction function. This function has been transferred to all other turbines of similar type in the farm to test the power performance. In terms of annual energy production (AEP) maxi-mum deviations of -2.7% to +3.3%, with a standard deviation of 2% have been observed. Furthermorethe test of the procedure at the turbine that has been assessed following IEC and nacelle anemometrydelivered results with a deviation of 0.3% in AEP.With respect to numerical site calibration, DEWI performed some exemplary case studies regarding theuncertainty of site calibration calculations and comparisons with results from site calibration measurementand nacelle anemometry. These results show that a high accuracy can be achieved (Tab. 2 and Fig. 1).The Round Robin Test „Numerical Flow Modelling in Wind Energy“, which is currently being performed byDEWI, is assumed to deliver further valuable information to the expected uncertainties of these methods.In the document –12-3 the wind farm will be treated as a single power plant and a method is proposedfor testing the performance of a complete wind farm in relation to a reference point (location of a met IEC-NEWS: Power Performance and Acoustic Noise H. Klug , DEWI Wilhelmshaven Tab. 1:Power Curve measurements on 9 wind turbi-nes. WT1 is measured according to IEC61400-12 determining as well the relation bet-ween free wind speed and nacelle anemome-ter wind speed. Using this relation for all 9wind turbines the nacelle anemometer basedpower curve was measured and the AEPwasdetermined. The results differed less than 2% in terms of standard deviation of the AEP. WTGSEvaluatedwind speedsignal AEPextrapolatedRelative toav. AEP[MWh][%]1met mast top5377.099.9%1nacelle5388.3100.2%2nacelle5390.2100.2%3nacelle5356.299.6%4nacelle5338.999.2%5nacelle5255.697.7%6nacelle5544.6103.1%7nacelle5555.8103.3%8nacelle5338.199.2%9nacelle5252.497.6% Av nac. based5380.0100.0%Sigma108.42.0%  31 DEWI Magazin Nr. 28, Februar 2006  mast). DEWI has calculated the windfarm performance matrix for a windfarm in Spain which was used as anexample for the draft standard whichwill be circulated in the near future asa CD (committee draft). 2.Acoustic Noise 61400-11 and TS61400-14 The revised standard 61400-11 ed. 2was issued end of 2002. The revisionallows for the determination of thesound power level and an objectivereproducible tonal assessment in thewind speed range 6 to 10m/s at 10m height. Right now an amendment tothis document is circulated for votingas an FDIS (Final draft internationalstandard). This amendment to 61400-11 ed. 2 addresses special caseswhere 95 % of rated power is reachedbelow 10 m/s at 10 m height and for sites where wind speeds of 10 m/s at10 m height are very rare. Furthermore a clarification on regression analysis and frequency weighting isincluded.Aproposal for a second revision is circulated for national comments at the moment. The primary focusareas in the revision 61400-11 ed. 3 are expected to be: ã Reference height for wind speed. ã Averaging periods during the measurement should be reviewed ã Amore detailed description of the regressionanalysis is needed. ã Customers and authorities demand stan-dardized data in the wind speed range from3 to 14 m/s. The methods in the standardshould be usable at a broader range of windspeeds (In principle all wind speeds) ã Demands on reduced wind speed ranges for verification purposes should be introduced. ã Improvements in the procedure for 1/3octave data are desirable. This could be aconsequence of altered averaging periods. ã Improvements in the procedure for tonalityanalysis are desirable. ã Improvements in the uncertainty analysisshould be introduced. ã Improvements in the demands for the docu-mentation of measurement results should bemade. (mode of operation of the turbine e.g.low noise operation). ã Clarification of the use of power curves in theanalysis of measurement results should beincluded.(air density corrections etc.) ã Small wind turbines. ã Off-shore wind turbines. ã Considerations on the use of the nacelleanemometer for background noise measure-ments. Wind directionSector [°]simulatedmeas meanmeas sigma330 to 3400.97280.97690.0344340 to 3500.98450.97950.0239350 to 3600.98850.98560.0179Site calibration factor  Tab. 2 and Fig. 1: Comparison of measured and numerical site calibration sho-wing good agreement even in complex terrain. 0.800.820.840.860.880.900.920.940.960.981. 330 340 350 360   v_   W   T   G   S   /  v_  r  e   f   [  -   ] dir [deg]Mean and Standard Deviation of Measured DataSimulatedMeasured Data For more information: Individual anemometers canbe calibrated according to MEASNET and IEC 61400-121 88/163/CDA100 Series Porton™ Anemometers Visit our Stand 149a at:  32 DEWI Magazin Nr. 28, Februar 2006  1.Introduction In many countries the noise radiation is still the major limitation in the tremendous development of windenergy over the last years. New designs resulted in considerable noise reductions of both aerodynamicnoise from the blades and machinery noise. The sound power levels of variable speed machines can beadjusted even after they have been put into operation and after the sound pressure levels at the nearestdwellings have been verified.Some national codes work with absolute noise limits, while in some other countries the limits are basedon the ambient noise. The nature of the wind turbine noise and the wind induced background noise arevery important for defining masking criteria. The national codes for noise regulations have to be consis-tent with the international standards of measuring the wind turbine noise including the assessment of tonality and the standards for noise propagation.The IEC standard 61400-11 Wind Turbines – Part 11‚Acoustic Noise Measurement Techniques‘ was revisedrecently in order to present a procedure expected toprovide accurate results that can be replicated by oth-ers. Immission measurements are not within the scopeof this IEC standard. The different measurement proce-dures of noise immission from wind turbines at noise re-ceptor locations are described in an IEARecommendation.In this general review the history and the state of the artof wind turbine noise is given with special emphasis on: ã Noise sources ã Propagation effects ã Standards and Recommendations ã Noise reduction ã Measurement procedures at high wind speeds ã Noise characteristics (e.g. tonality) ã Declaration and verification of sound levels 2.Noise Sources In order to assess the noise at the receptor locations (nearestdwellings) we have to distinguish between noise generation(noise sources) noise propagation (propagation conditions,prediction standards) and sound pressure levels at the recep-tor location. The noise sources can be split up into the aero- ã Other aspects of noise are being investigated these years (low frequency noise, infra sound etc.) ã Wind farm noise verification. (number of turbines to be documented)The Technical Specification TS 61400-14 was issued in March 2005. The intention of this TS is to deter-mine declared noise emission values from a sample of turbines of the same type. The declaration willincrease the reliability of wind farm planning and shall facilitate the comparison of sound power levelsand tonality values of different types of wind turbines. AReview of Wind Turbine Noise Dr. Helmut Klug , DEWI Wilhelmshaven Fig. 1:Wind turbine noise assessment factors Source: Sheperd, K. P.; Grosveld, F. W.;Stephens, D. G.: Evaluation of Human Exposureto the Noise from; Large Wind TurbinesGenerators. Noise Control Engineering Journal,Vol. 21, No. 1 pp. 30-37, July-August 1983 Noise Generation  Aerodynamic sourcesMechanical sources Propagation DistanceWind Gradient AbsorptionTerrain Reception  Ambient NoiseIndoor / Outdoor ExposureBuilding Vibrations   Fig 2:Schematic of the flow around the outer part of the rotor blade Quelle: Blake, W. K.: Mechanics of Flow-Induced Sound and Vibration, Vol II: Complex Flow Structure Interactions. ACADEMIC Press INC., Harcourt Brace Jovanovich, Publishers pp. 426 - 973, 1986  33 DEWI Magazin Nr. 28, Februar 2006  dynamic noise sources [1] and the machinerynoise. The aerodynamic noise sources are inflowturbulence noise (leading edge of the blade), tur-bulent boundary layer noise (interaction with thetrailing edge of the rotor blade) and tip noise (see.Fig. 2)Machinery noise (mainly (gearbox noise, genera-tor noise)) has been reduced significantly so that itis mostly not contributing to the overall soundpower level. On the other hand there are still someturbines radiating an audible tone which isassessed according to IEC 61400-11 ed.2. Insome countries penalties are imposed under thenational code depending on the audibility of thetone.Fig. 3 shows the sound power levels of 49 differenttypes of wind turbines in the range of 80kW to2500kW (see also [2]). These are published data ina catalogue issued annually by the German WindEnergy Association BWE (Market Survey 1997-2005)When looking at the wind speed dependency of the sound power level we have to distinguishbetween stall-regulated turbines and pitch regulat-ed turbines. The power control by the stall-effectcauses an increase of sound power level also athigh wind speeds, while pitch regulated turbinesnot only keep the power constant at high windspeed but also the sound power level. Due to thepitching at rated power the sound power level mayeven decrease at high wind speeds (see Tab. 1). 3.Propagation Meteorological conditions, mainly wind and tem-perature profiles in the boundary layer affect out-door sound propagation [3]. Wind speed and tem-perature are functions of height. They are interre-lated and can be described by the Monin-Obukhovsimilarity theory. The Monin-Obukhov length Lis astability parameter for the turbulent boundary layer.The steepest sound speed gradients causing thehighest sound pressure levels at large distancesoccur for downwind conditions at night-time. Themost pronounced stable atmospheric stratificationcan be expected during clear nights and low windspeeds. For that reason a lot of national codesrequire measurements at low wind speeds atnight-time (often referred to as ‘downwind condi-tion’). As the wind turbines have the highest soundpower levels at high wind speeds these nationalcodes for noise regulations have to be made con-sistent with the international standards of measur-ing the wind turbine noise at wind speeds from 6 to10 m/s at 10 m height. Sprengt Ihre Vorstellungskraft! Der neue Servo-Umrichter „PITCH master  “im Rotorblatt-Verstellsystem optimiert dieEnergieumsetzung. Lust DriveTronics GmbHHeinrich-Hertz-Straße 1859423 UnnaGERMANYTel+ 49 (0) 2303 779-0Fax+ 49 (0) 2303 Schneller und dynamischer ReglerIGBT-EndstufeIntegrierte Pitch- und Positioniersoftware Ethernet, CAN, PROFIBUS, DeviceNet und mehr Schnittstelle zum „Condition Monitoring“Vibrationsfeste Longlife-KomponentenWahlweise Antriebsarten: Asynch./Synch./DCund und … PITCH master  .Zukunftsweisend für Pitchantriebe.
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