Wind turbines - Part 11: Acoustic noise measurement techniques

IEC 61400-11:2012(E) presents measurement procedures that enable noise emissions of a wind turbine to be characterised. This involves using measurement methods appropriate to noise emission assessment at locations close to the machine, in order to avoid errors due to sound propagation, but far away enough to allow for the finite source size. The procedures described are different in some respects from those that would be adopted for noise assessment in community noise studies. They are intended to facilitate characterisation of wind turbine noise with respect to a range of wind speeds and directions. Standardisation of measurement procedures will also facilitate comparisons between different wind turbines. This new edition constitutes a technical revision, introducing new principles for data reduction procedures.

Windenergieanlagen - Teil 11: Schallmessverfahren

Eoliennes - Partie 11: Techniques de mesure du bruit acoustique

l’IEC 61400-11:2012 spécifie les procédures de mesure qui permettent de caractériser les émissions sonores d’une éolienne. Cette démarche implique l’utilisation de méthodes de mesure appropriées à l’évaluation des émissions sonores à des emplacements proches de la machine, afin d’éviter les erreurs dues à la propagation du son, mais toutefois suffisamment éloignés pour permettre de déterminer la dimension des sources finies. Les procédures décrites sont différentes à certains égards de celles qui seraient adoptées pour l’évaluation du bruit dans le cadre des études portant sur le bruit ambiant. Ces procédures sont destinées à faciliter la caractérisation du bruit émis par une éolienne par rapport à une plage donnée de vitesses et de directions du vent. La normalisation des procédures de mesure facilite également les comparaisons entre différentes éoliennes. Cette nouvelle édition constitue une révision technique, introduisant de nouveaux principes concernant les procédures de traitement des données.

Vetrne turbine - 11. del: Tehnike merjenja hrupa

Ta del standarda IEC 61400 predstavlja postopke merjenja, ki omogočajo opredelitev emisij hrupa vetrnih turbin. To vključuje načine merjenja, ki so primerni za ocenjevanje emisij hrupa na lokacijah, ki so dovolj blizu naprave, da bi se izognili napakam zaradi širjenja zvoka na prostem, vendar dovolj daleč, da je mogoče pridobiti končno velikost vzorca. Nekateri vidiki opisanih postopkov so drugačni od postopkov, ki se uporabljajo za ocenjevanje hrupa v študijah hrupa v okolici. Namenjene so opredelitvi hrupa vetrnih turbin, pri čemer upoštevajo razpon hitrosti in smeri vetra. Standardizacija merilnih postopkov bo omogočila tudi primerjave različnih vetrnih turbin. Postopki predstavljajo metodologijo, ki bo omogočila usklajeno in pravilno opredelitev emisij hrupa vetrnih turbin. Ti postopki vključujejo naslednje:
• lokacijo položajev akustičnega merjenja;
• zahteve za pridobitev akustičnih, meteoroloških in drugih povezanih podatkov o delovanju vetrnih turbin;
• analizo pridobljenih podatkov in vsebino za poročila ter
• definicijo posebnih parametrov akustičnih emisij in povezanih deskriptorjev, ki se uporabljajo za izvajanje okoljskih ocen.
Ta mednarodni standard ni omejen na določene velikosti ali vrste vetrnih turbin. Postopki, ki so opisani v tem standardu, omogočajo temeljit opis emisij hrupa iz vetrnih turbin. Metoda za majhne vetrne turbine je opisana v dodatku F.

General Information

Status
Published
Publication Date
28-Feb-2013
Technical Committee
Drafting Committee
Current Stage
6060 - Document made available
Due Date
01-Mar-2013
Completion Date
01-Mar-2013

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2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.Vetrne turbine - 11. del: Tehnike merjenja hrupaWindenergieanlagen - Teil 11: SchallmessverfahrenAérogénérateurs - Partie 11: Techniques de mesure du bruit acoustiqueWind turbines - Part 11: Acoustic noise measurement techniques27.180Sistemi turbin na veter in drugi alternativni viri energijeWind turbine systems and other alternative sources of energy17.140.20Emisija hrupa naprav in opremeNoise emitted by machines and equipmentICS:Ta slovenski standard je istoveten z:EN 61400-11:2013SIST EN 61400-11:2013en01-junij-2013SIST EN 61400-11:2013SLOVENSKI

STANDARD
SIST EN 61400-11:2013
EUROPEAN STANDARD EN 61400-11 NORME EUROPÉENNE
EUROPÄISCHE NORM March 2013

CENELEC European Committee for Electrotechnical Standardization Comité Européen de Normalisation Electrotechnique Europäisches Komitee für Elektrotechnische Normung

Management Centre: Avenue Marnix 17, B - 1000 Brussels
© 2013 CENELEC -

All rights of exploitation in any form and by any means reserved worldwide for CENELEC members.

Ref. No. EN 61400-11:2013 E
ICS 27.180 Supersedes EN 61400-11:2003 + A1:2006
English version
Wind turbines -
Part 11: Acoustic noise measurement techniques (IEC 61400-11:2012)
Eoliennes -
Partie 11: Techniques de mesure
du bruit acoustique (CEI 61400-11:2012)
Windenergieanlagen -
Teil 11: Schallmessverfahren (IEC 61400-11:2012)

This European Standard was approved by CENELEC on 2012-12-12. CENELEC members are bound to comply with the CEN/CENELEC Internal Regulations which stipulate the conditions for giving this European Standard the status of a national standard without any alteration.

Up-to-date lists and bibliographical references concerning such national standards may be obtained on application to the CEN-CENELEC Management Centre or to any CENELEC member.

This European Standard exists in three official versions (English, French, German). A version in any other language made by translation under the responsibility of a CENELEC member into its own language and notified to the CEN-CENELEC Management Centre has the same status as the official versions.

CENELEC members are the national electrotechnical committees of Austria, Belgium, Bulgaria, Croatia, Cyprus, the Czech Republic, Denmark, Estonia, Finland, Former Yugoslav Republic of Macedonia, France, Germany, Greece, Hungary, Iceland, Ireland, Italy, Latvia, Lithuania, Luxembourg, Malta, the Netherlands, Norway, Poland, Portugal, Romania, Slovakia, Slovenia, Spain, Sweden, Switzerland, Turkey and the United Kingdom.

SIST EN 61400-11:2013

EN 61400-11:2013 - 2 - Foreword The text of document 88/436/FDIS, future edition 3 of IEC 61400-11, prepared by IEC/TC 88 "Wind Turbines" was submitted to the IEC-CENELEC parallel vote and approved by CENELEC as EN 61400-11:2013. The following dates are fixed: • latest date by which the document has to be implemented at national level by publication of an identical national standard or by endorsement (dop) 2013-09-12 • latest date by which the national standards conflicting with the document have to be withdrawn (dow) 2015-12-12

This document supersedes EN 61400-11:2003 + A1:2006.

EN 61400-11:2013 includes the following significant technical changes with respect to EN 61400-11:2003 + A1:2006: The technical change is introducing new principles for data reduction procedures.

Attention is drawn to the possibility that some of the elements of this document may be the subject of patent rights. CENELEC [and/or CEN] shall not be held responsible for identifying any or all such patent rights.

Endorsement notice The text of the International Standard IEC 61400-11:2012 was approved by CENELEC as a European Standard without any modification.

SIST EN 61400-11:2013
- 3 - EN 61400-11:2013 Annex ZA
(normative)

Normative references to international publications with their corresponding European publications

The following documents, in whole or in part, are normatively referenced in this document and are indispensable for its application. For dated references, only the edition cited applies. For undated references, the latest edition of the referenced document (including any amendments) applies.

NOTE

When an international publication has been modified by common modifications, indicated by (mod), the relevant EN/HD applies.

Publication Year Title EN/HD Year IEC 60688 - Electrical measuring transducers for converting A.C. and D.C. ectrical quantities to analogue or digital signals EN 60688 -

IEC 60942 2003 Electroacoustics - Sound calibrators EN 60942 2003

IEC 61260 1995 Electroacoustics - Octave-band and fractional-octave-band filters EN 61260 1995

IEC 61400-12-1 2005 Wind turbines -
Part 12-1: Power performance measurements of electricity producing
wind turbines EN 61400-12-1 2006
IEC 61400-12-2 - 1) Wind turbines -

Part 12-2: Power performance of electricity producing wind turbines based on nacelle anemometry EN 61400-12-2 -

IEC 61672 Series Electroacoustics - Sound level meters EN 61672 Series
ISO/IEC Guide 98-3 - Uncertainty of measurement -
Part 3: Guide to the expression of uncertainty in measurement (GUM:1995) - -
1) To be published. SIST EN 61400-11:2013
SIST EN 61400-11:2013
IEC 61400-11 Edition 3.0 2012-11 INTERNATIONAL STANDARD Wind turbines –
Part 11: Acoustic noise measurement techniques

INTERNATIONAL ELECTROTECHNICAL COMMISSION XA ICS 27.180 PRICE CODE ISBN 978-2-83220-463-4

® Registered trademark of the International Electrotechnical Commission ®

Warning! Make sure that you obtained this publication from an authorized distributor. SIST EN 61400-11:2013 colourinside

– 2 – 61400-11 © IEC:2012(E) CONTENTS FOREWORD ........................................................................................................................... 5 INTRODUCTION ..................................................................................................................... 7 1 Scope ............................................................................................................................... 8 2 Normative references ....................................................................................................... 8 3 Terms and definitions ....................................................................................................... 9 4 Symbols and units .......................................................................................................... 12 5 Outline of method ........................................................................................................... 13 6 Instrumentation .............................................................................................................. 14 6.1 Acoustic instruments ............................................................................................. 14 6.1.1 General ..................................................................................................... 14 6.1.2 Equipment for the determination of the equivalent continuous A-weighted sound pressure level................................................................... 14 6.1.3 Equipment for the determination of A-weighted 1/3-octave band spectra ...................................................................................................... 14 6.1.4 Equipment for the determination of narrow band spectra ........................... 14 6.1.5 Microphone with measurement board and windscreen ............................... 14 6.1.6 Acoustical calibrator .................................................................................. 16 6.1.7 Data recording/playback systems .............................................................. 16 6.2 Non-acoustic Instruments ...................................................................................... 16 6.2.1 General ..................................................................................................... 16 6.2.2 Anemometers ............................................................................................ 16 6.2.3 Electric power transducer .......................................................................... 17 6.2.4 Other instrumentation ................................................................................ 17 6.3 Traceable calibration ............................................................................................. 17 7 Acoustic measurements and measurement procedures .................................................. 17 7.1 Acoustic measurement positions ........................................................................... 17 7.2 Acoustic measurements ........................................................................................ 20 7.2.1 General ..................................................................................................... 20 7.2.2 Acoustic measurement requirements ......................................................... 20 7.2.3 A-weighted sound pressure level ............................................................... 21 7.2.4 A-weighted 1/3-octave band measurements ............................................... 21 7.2.5 A-weighted narrow band measurements .................................................... 21 7.2.6 Optional acoustic measurements at positions 2, 3 and 4 ............................ 21 7.2.7 Other optional measurements .................................................................... 21 8 Non-acoustic measurements .......................................................................................... 21 8.1 General ................................................................................................................. 21 8.2 Wind speed measurements ................................................................................... 22 8.2.1 Determination of the wind speed during wind turbine operation.................. 22 8.2.2 Wind speed measurements during background noise measurements ......... 23 8.3 Downwind direction ............................................................................................... 24 8.4 Other atmospheric conditions ................................................................................ 24 8.5 Rotor speed and pitch angle measurement ............................................................ 24 9 Data reduction procedures.............................................................................................. 24 9.1 General methodology for sound power levels and 1/3-octave band levels .............. 24 9.2 Calculation of sound pressure levels ..................................................................... 27 SIST EN 61400-11:2013

61400-11 © IEC:2012(E) – 3 –

9.2.1 General ..................................................................................................... 27 9.2.2 Calculation of average sound spectra and uncertainty per bin ................... 27 9.2.3 Calculation of average wind speed and uncertainty per bin ........................ 29 9.2.4 Calculation of noise levels at bin centres including uncertainty .................. 30 9.3 Apparent sound power levels ................................................................................. 31 9.4 Apparent sound power levels with reference to wind speed in 10 m height ............ 32 9.5 Tonal audibility ...................................................................................................... 33 9.5.1 General methodology for tonality ............................................................... 33 9.5.2 Identifying possible tones .......................................................................... 34 9.5.3 Classification of spectral lines within the critical band ................................ 34 9.5.4 Identified tone ........................................................................................... 37 9.5.5 Determination of the tone level .................................................................. 37 9.5.6 Determination of the masking noise level ................................................... 37 9.5.7 Determination of tonality ............................................................................ 37 9.5.8 Determination of audibility ......................................................................... 38 9.5.9 Background noise ...................................................................................... 38 10 Information to be reported .............................................................................................. 39 10.1 General ................................................................................................................. 39 10.2 Characterisation of the wind turbine ...................................................................... 39 10.3 Physical environment ............................................................................................ 39 10.4 Instrumentation ..................................................................................................... 40 10.5 Acoustic data ........................................................................................................ 40 10.6 Non-acoustic data ................................................................................................. 41 10.7 Uncertainty ............................................................................................................ 41 Annex A (informative)

Other possible characteristics of wind turbine

noise emission and their quantification ......................................................................................................... 42 Annex B (informative)

Assessment of turbulence intensity ................................................... 44 Annex C (informative)

Assessment of measurement uncertainty .......................................... 45 Annex D (informative)

Apparent roughness length ............................................................... 47 Annex E (informative)

Characterization of a secondary wind screen..................................... 49 Annex F (normative)

Small wind turbines ............................................................................. 53 Annex G (informative)

Air absorption ................................................................................... 57 Bibliography .......................................................................................................................... 58

Figure 1 – Mounting of the microphone ................................................................................. 15 Figure 2 – Picture of microphone and measurement board .................................................... 16 Figure 3 – Standard pattern for microphone measurement positions (plan view) ................... 18 Figure 4 – Illustration of the definitions of R0 and slant distance R1 ...................................... 19 Figure 5 – Acceptable meteorological mast position (hatched area) ...................................... 22 Figure 6 – Flowchart showing the data reduction procedure .................................................. 26 Figure 7 – Flowchart for determining tonal audibility for each wind speed bin ....................... 33 Figure 8 – Illustration of L70 % level in the critical band ........................................................ 35 Figure 9 – Illustration of lines below the L70 % + 6 dB criterion ............................................ 36 Figure 10 – Illustration of Lpn,avg level and lines classified as masking................................ 36 Figure 11 – Illustration of classifying all spectral lines ........................................................... 37 Figure E.1 – Example 1 of a secondary wind screen ............................................................. 50 SIST EN 61400-11:2013

– 4 – 61400-11 © IEC:2012(E) Figure E.2 – Example 2 of secondary wind screen ................................................................ 51 Figure E.3 – Example on insertion loss from Table E.1 ......................................................... 52 Figure F.1 – Allowable region for meteorological

mast position as a function of β – Plan view .............................................................................................................................. 54 Figure F.2 – Example immission noise map .......................................................................... 56 Figure G.1 – Example of 1/3-octave spectrum ....................................................................... 57

Table C.1 – Examples of possible values of type B uncertainty components

relevant for apparent sound power spectra ......................................................................................... 46 Table C.2 – Examples of possible values of type B uncertainty components

for wind speed determination relevant for apparent sound power spectra ........................................... 46 Table D.1 – Roughness length .............................................................................................. 47 Table E.1 – Example on reporting of insertion loss................................................................ 51

SIST EN 61400-11:2013
61400-11 © IEC:2012(E) – 5 –
INTERNATIONAL ELECTROTECHNICAL COMMISSION ____________
WIND TURBINES –
Part 11: Acoustic noise measurement techniques

FOREWORD 1) The International Electrotechnical Commission (IEC) is a worldwide organization for standardization comprising all national electrotechnical committees (IEC National Committees). The object of IEC is to promote international co-operation on all questions concerning standardization in the electrical and electronic fields. To this end and in addition to other activities, IEC publishes International Standards, Technical Specifications, Technical Reports, Publicly Available Specifications (PAS) and Guides (hereafter referred to as “IEC Publication(s)”). Their preparation is entrusted to technical committees; any IEC National Committee interested in the subject dealt with may participate in this preparatory work. International, governmental and non-governmental organizations liaising with the IEC also participate in this preparation. IEC collaborates closely with the International Organization for Standardization (ISO) in accordance with conditions determined by agreement between the two organizations. 2) The formal decisions or agreements of IEC on technical matters express, as nearly as possible, an international consensus of opinion on the relevant subjects since each technical committee has representation from all interested IEC National Committees. 3) IEC Publications have the form of recommendations for international use and are accepted by IEC National Committees in that sense. While all reasonable efforts are made to ensure that the technical content of IEC Publications is accurate, IEC cannot be held responsible for the way in which they are used or for any misinterpretation by any end user. 4) In order to promote international uniformity, IEC National Committees undertake to apply IEC Publications transparently to the maximum extent possible in their national and regional publications. Any divergence between any IEC Publication and the corresponding national or regional publication shall be clearly indicated in the latter. 5) IEC itself does not provide any attestation of conformity. Independent certification bodies provide conformity assessment services and, in some areas, access to IEC marks of conformity. IEC is not responsible for any services carried out by independent certification bodies. 6) All users should ensure that they have the latest edition of this publication. 7) No liability shall attach to IEC or its directors, employees, servants or agents including individual experts and members of its technical committees and IEC National Committees for any personal injury, property damage or other damage of any nature whatsoever, whether direct or indirect, or for costs (including legal fees) and expenses arising out of the publication, use of, or reliance upon, this IEC Publication or any other IEC Publications. 8) Attention is drawn to the Normative references cited in this publication. Use of the referenced publications is indispensable for the correct application of this publication. 9) Attention is drawn to the possibility that some of the elements of this IEC Publication may be the subject of patent rights. IEC shall not be held responsible for identifying any or all such patent rights. International Standard IEC 61400-11 has been prepared by IEC technical committee 88: Wind turbines. This third edition of IEC 61400-11 cancels and replaces the second edition published in 2002 and its Amendment 1 (2006). It constitutes a technical revision, introducing new principles for data reduction procedures. The text of this standard is based on the following documents: FDIS Report on voting 88/436/FDIS 88/440/RVD

Full information on the voting for the approval of this standard can be found in the report on voting indicated in the above table. SIST EN 61400-11:2013

– 6 – 61400-11 © IEC:2012(E) This publication has been drafted in accordance with the ISO/IEC Directives, Part 2. A list of all parts of the IEC 61400 series, under the general title Wind turbines, can be found on the IEC website. The committee has decided that the contents of this publication will remain unchanged until the stability date indicated on the IEC web site under "http://webstore.iec.ch" in the data related to the specific publication. At this date, the publication will be • reconfirmed, • withdrawn, • replaced by a revised edition, or • amended. A bilingual edition of this document may be issued at a later date.

IMPORTANT – The 'colour inside' logo on the cover page of this publication indicates that it contains colours which are considered to be useful for the correct understanding of its contents. Users should therefore print this document using a colour printer.

SIST EN 61400-11:2013
61400-11 © IEC:2012(E) – 7 –

INTRODUCTION The purpose of this part of IEC 61400 is to provide a uniform methodology that will ensure consistency and accuracy in the measurement and analysis of acoustical emissions by wind turbine generator systems. This International Standard has been prepared with the anticipation that it would be applied by: • wind turbine manufacturers striving to meet well defined acoustic emission performance requirements and/or a possible declaration system (e.g. IEC/TS 61400-14); • wind turbine purchasers for specifying performance requirements; • wind turbine operators who may be required to verify that stated, or required, acoustic performance specifications are met for new or refurbished units; • wind turbine planners or regulators who must be able to accurately and fairly define acoustical emission characteristics of a wind turbine in response to environmental regulations or permit requirements for new or modified installations. This standard provides guidance in the measurement, analysis and reporting of complex acoustic emissions from wind turbine generator systems. The standard will benefit those parties involved in the manufacture, installation, planning and permitting, operation, utilization, and regulation of wind turbines. The measurement and analysis techniques recommended in this document should be applied by all parties to ensure that continuing development and operation of wind turbines is carried out in an atmosphere of consistent and accurate communication relative to environmental concerns. This standard presents measurement and reporting procedures expected to provide accurate results that can be replicated by others. SIST EN 61400-11:2013

– 8 – 61400-11 © IEC:2012(E) WIND TURBINES –
Part 11: Acoustic noise measurement techniques

1 Scope This part of IEC 61400 presents measurement procedures that enable noise emissions of a wind turbine to be characterised. This involves using measurement methods appropriate to noise emission assessment at locations close to the machine, in order to avoid errors due to sound propagation, but far away enough to allow for the finite source size. The procedures described are different in some respects from those that would be adopted for noise assessment in community noise studies. They are intended to facilitate characterisation of wind turbine noise with respect to a range of wind speeds and directions. Standardisation of measurement procedures will also facilitate comparisons between different wind turbines. The procedures present methodologies that will enable the noise emissions of a single wind turbine to be characterised in a consistent and accurate manner. These procedures include the following: • location of acoustic measurement positions; • requirements for the acquisition of acoustic, meteorological, and associated wind turbine operational data; • analysis of the data obtained and the content for the data report; and • definition of specific acoustic emission parameters, and associated descriptors which are used for making environmental assessments. This International Standard is not restricted to wind turbines of a particular size or type. The procedures described in this standard allow for the thorough description of the noise emission from a wind turbine. A method for small wind turbines is described in Annex F. 2 Normative references The following documents, in whole or in part, are normatively referenced in this document and are indispensable for its application. For dated references, only the edition cited applies. For undated references, the latest edition of the referenced document (including any amendments) applies. IEC 60688, Electrical measuring transducers for converting a.c. electrical quantities to analogue or digital signals

IEC 60942:2003, Electroacoustics – Sound calibrators IEC 61260:1995, Electroacoustics – Octave-band and fractional-octave-band filters IEC 61400-12-1:2005, Wind turbines – Part 12-1: Power performance measurements of electricity producing wind turbines IEC 61400-12-2, Wind turbines – Part 12-2: Power performance verification of electricity producing wind turbines1 ____________ 1 To be published. SIST EN 61400-11:2013

61400-11 © IEC:2012(E) – 9 –

IEC 61672 (all parts), Electroacoustics – Sound level meters ISO/IEC Guide 98-3, Uncertainty of measurement – Part 3: Guide to the expression of uncertainty in measurement (GUM:1995) 3 Terms and definitions For the purposes of this standard, the following terms and definitions apply. 3.1

apparent sound power level LWA A-weighted sound power level re. 1 pW of a point source at the rotor centre with the same emission in the downwind direction as the wind turbine being measured, LWA is determined at bin centre wind speeds at hub height Note 1 to entry: Apparent sound power level is expressed in dB re. 1 pW. 3.2

apparent sound power level with reference to wind speed at 10 m height LWA,10m A-weighted sound power level re. 1 pW of a point source at the rotor centre with the same emission in the downwind direction as the wind turbine being measured, LWA,10m are determined at bin centre wind speeds at 10 m height within the measured wind speed range Note 1 to entry: Apparent sound power level with reference to wind speed at 10 m height is expressed in dB re. 1 pW. 3.3

audibility criterion La frequency dependent criterion curve determined from listening tests, and reflecting the subjective response of a “typical” listener to tones of different frequencies Note 1 to entry: Audibility criterion is expressed in dB re. 20 µPa. 3.4 sound pressure levels 3.4.1 A-weighted sound pressure levels LA sound pressure levels measured with the A frequency weighting networks specified in IEC 61672 Note 1 to entry: A-weighted sound pressure levels are expressed in dB re. 20 µPa. 3.4.2 C-weighted sound pressure levels LC

sound pressure levels measured with the C frequency weighting networks specified in IEC 61672 Note 1 to entry: C-weighted sound pressure levels are expressed in dB re. 20 µPa. 3.5

bin centre centre value of a wind speed bin SIST EN 61400-11:2013
– 10 – 61400-11 © IEC:2012(E) 3.6
inclination angle

φ angle between the plane of the measurement board and a line from the microphone to the rotor centre Note 1 to entry: Inclination angle is expressed in °. 3.7

maximum power maximum value of the binned power curve for the power optimised mode of operation Note 1 to entry: Maximum power is expressed in kW. 3.8

measured wind speed at height Z VZ,m wind speed measured at height Z with a mast mounted anemometer Note 1 to entry: Measured wind speed at height Z is expressed in m/s. 3.9

measured nacelle wind speed at hub height Vnac,m wind speed measured at hub height with a nacelle anemometer Note 1 to entry: Measured nacelle wind speed at hub height is expressed in m/s. 3.10

normalised nacelle wind speed at hub height Vnac,n normalised wind speed measured at hub height with a nacelle anemometer corrected to standard meteorological conditions Note 1 to entry: Normalised nacelle wind speed at hub height is expressed in m/s. 3.11

normalised wind speed derived from power curve VP,n normalised wind speed derived from power curve under standard meteorological conditions Note 1 to entry: Normalised wind speed derived from power curve is expressed in m/s. 3.12

normalised wind speed at hub height during background noise measurements VB,n normalised wind speed at hub height from anemometer Note 1 to entry: Normalised wind speed at hub height during background noise measurements is expressed in m/s. 3.13

normalised wind speed at hub height VH,n normalised wind speed at hub height Note 1 to entry: Normalised wind speed at hub height is expressed in m/s. 3.14

normalised wind speed at height Z VZ,n normalised wind speed at height Z from mast mounted anemometer SIST EN 61400-11:2013

61400-11 © IEC:2012(E) – 11 –
Note
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