EN IEC 61400-15-1:2025

SLOVENSKI STANDARD
01-julij-2025
Sistemi za proizvodnjo energije na veter - 15-1. del: Vhodni pogoji glede
primernosti mesta namestitve vetrnih elektrarn (IEC 61400-15-1:2025)
Wind energy generation systems - Part 15-1: Site suitability input conditions for wind
power plants (IEC 61400-15-1:2025)
Windenergieanlagen - Teil 15-1: Eingangsbedingungen für die Standorteignung von
Windkraftwerken (IEC 61400-15-1:2025)
Systèmes de génération d'énergie éolienne - Partie 15-1: Conditions à remplir pour
l’acceptabilité d’un site de centrale éolienne (IEC 61400-15-1:2025)
Ta slovenski standard je istoveten z: EN IEC 61400-15-1:2025
ICS:
27.180 Vetrne elektrarne Wind turbine energy systems
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

EUROPEAN STANDARD EN IEC 61400-15-1

NORME EUROPÉENNE
EUROPÄISCHE NORM May 2025
ICS 27.180
English Version
Wind energy generation systems - Part 15-1: Site suitability input
conditions for wind power plants
(IEC 61400-15-1:2025)
Systèmes de génération d'énergie éolienne - Partie 15-1: Windenergieanlagen - Teil 15-1: Eingangsbedingungen für
Conditions à remplir pour l'acceptabilité d'un site pour les die Standorteignung von Windkraftwerken
centrales éoliennes (IEC 61400-15-1:2025)
(IEC 61400-15-1:2025)
This European Standard was approved by CENELEC on 2025-04-18. 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, France, Germany, Greece, Hungary, Iceland, Ireland, Italy, Latvia, Lithuania, Luxembourg, Malta, the
Netherlands, Norway, Poland, Portugal, Republic of North Macedonia, Romania, Serbia, Slovakia, Slovenia, Spain, Sweden, Switzerland,
Türkiye and the United Kingdom.

European Committee for Electrotechnical Standardization
Comité Européen de Normalisation Electrotechnique
Europäisches Komitee für Elektrotechnische Normung
CEN-CENELEC Management Centre: Rue de la Science 23, B-1040 Brussels
© 2025 CENELEC All rights of exploitation in any form and by any means reserved worldwide for CENELEC Members.
Ref. No. EN IEC 61400-15-1:2025 E

European foreword
The text of document 88/1041/FDIS, future edition 1 of IEC 61400-15-1, prepared by TC 88 "Wind
energy generation systems" was submitted to the IEC-CENELEC parallel vote and approved by
CENELEC as EN IEC 61400-15-1:2025.
The following dates are fixed:
• latest date by which the document has to be implemented at national (dop) 2026-05-31
level by publication of an identical national standard or by endorsement
• latest date by which the national standards conflicting with the (dow) 2028-05-31
document have to be withdrawn
Attention is drawn to the possibility that some of the elements of this document may be the subject of
patent rights. CENELEC shall not be held responsible for identifying any or all such patent rights.
Any feedback and questions on this document should be directed to the users’ national committee. A
complete listing of these bodies can be found on the CENELEC website.
Endorsement notice
The text of the International Standard IEC 61400-15-1:2025 was approved by CENELEC as a
European Standard without any modification.
Annex ZA
(normative)
Normative references to international publications
with their corresponding European publications
The following documents are referred to in the text in such a way that some or all of their content
constitutes requirements of this document. For dated references, only the edition cited applies. For
undated references, the latest edition of the referenced document (including any amendments)
applies.
NOTE 1  Where an International Publication has been modified by common modifications, indicated by (mod),
the relevant EN/HD applies.
NOTE 2  Up-to-date information on the latest versions of the European Standards listed in this annex is available
here: www.cencenelec.eu.
Publication Year Title EN/HD Year
IEC 61400-1 2019 Wind energy generation systems - Part 1: EN IEC 61400-1 2019
Design requirements
IEC 61400-3-1 2019 Wind energy generation systems - Part 3- EN IEC 61400-3-1 2019
1: Design requirements for fixed offshore
wind turbines
IEC 61400-12-1 2022 Wind energy generation systems - Part 12- EN IEC 61400-12-1 2022
1: Power performance measurement of
electricity producing wind turbines
ISO 2533 1975 Standard Atmosphere - -
ISO/IEC 21778 2017 Information technology - The JSON data - -
interchange syntax
ISO/IEC 10646 2020 Information technology - Universal coded - -
character set (UCS)
ISO 3166 - Country codes - -
IEC 61400-15-1 ®
Edition 1.0 2025-03
INTERNATIONAL
STANDARD
NORME
INTERNATIONALE
Wind energy generation systems –

Part 15-1: Site suitability input conditions for wind power plants

Systèmes de génération d'énergie éolienne –

Partie 15-1: Conditions à remplir pour l'acceptabilité d'un site pour les centrales

éoliennes
INTERNATIONAL
ELECTROTECHNICAL
COMMISSION
COMMISSION
ELECTROTECHNIQUE
INTERNATIONALE
ICS 27.180  ISBN 978-2-8327-0269-7

– 2 – IEC 61400-15-1:2025 © IEC 2025
CONTENTS
FOREWORD . 4
INTRODUCTION . 6
1 Scope . 7
2 Normative references . 7
3 Terms and definitions . 8
4 Symbols, units and abbreviated terms . 10
4.1 Symbols and units . 10
4.2 Abbreviated terms. 10
5 Methods to determine turbine suitability input parameters . 11
5.1 General . 11
5.2 Assessment of wind speed . 12
5.2.1 Wind speed distribution . 12
5.2.2 Extreme wind speed with a recurrence interval of 50 years . 12
5.3 Assessment of turbulence intensity . 14
5.3.1 Ambient turbulence intensity . 14
5.3.2 Extreme ambient turbulence intensity . 16
5.3.3 Turbulence structure correction parameter. 16
5.4 Inflow angle . 16
5.5 Wind shear . 16
5.5.1 General. 16
5.5.2 Spatial extrapolation of wind shear . 17
5.6 Temperature . 18
5.7 Air density . 18
5.8 Site conditions modelling close to significant structures and obstacles . 19
Annex A (normative) Requirements to fill out Site Suitability Input Conditions Form . 20
A.1 Overview . 20
A.2 Turbine layout summary . 20
A.3 Measurement device summary . 21
A.4 Expected annual wind frequency distribution (%) . 21
A.5 Expected annual wind speed Weibull distribution (%) . 22
A.6 Turbulence intensity (TI) . 22
A.7 Standard deviation of turbulence intensity . 23
A.8 Extreme ambient turbulence intensity . 23
A.9 Sector-wise Inflow angle. 23
A.10 Wind shear . 23
A.11 Temperature . 23
Annex B (normative) Turbine suitability input reporting . 24
B.1 General . 24
B.2 Reporting structure . 24
B.2.1 General. 24
B.2.2 General information. 24
B.2.3 Introduction . 24
B.2.4 Summary of site characteristics . 25
B.2.5 Project description . 25
B.2.6 Wind input data. 25
B.2.7 Long-term adjusted wind data . 26

IEC 61400-15-1:2025 © IEC 2025 – 3 –
B.2.8 Flow modelling . 26
B.2.9 Site suitability parameters . 27
B.2.10 References. 27
Annex C (informative) Estimation of extreme wind speed distribution . 28
C.1 General . 28
C.2 Selection of high wind events . 28
C.3 Extreme value distribution fitting . 28
Annex D (informative) Extreme winds long-term adjustment . 29
Annex E (informative) Temporal and spatial resolution correction for mesoscale model
simulation results . 30
E.1 General . 30
E.2 Temporal resolution . 30
E.3 Spatial resolution . 30
Annex F (normative) Data exchange format for site suitability input conditions . 31
F.1 General . 31
F.2 Top level keys . 31
F.3 Description of each object . 32
Bibliography . 38

Figure F.1 – The definition of the wind speed bins . 32

Table F.1 – The contents of the top level keys . 31
Table F.2 – The keys in the object "Meta data" . 32
Table F.3 – The keys in the object "Project Information" . 33
Table F.4 – The keys in the objects of wind turbine IDs in the object "Turbine layout
summary" . 33
Table F.5 – The keys in the objects of measurement device IDs in the object
"Measurement device summary" . 34
Table F.6 – The keys in the objects of IDs of measurement device and wind turbine in
the object "WS frequency" . 35
Table F.7 – The keys in the objects of IDs of measurement device and wind turbine in
the object "WS Weibull". 35
Table F.8 – The keys in the objects of IDs of measurement device and wind turbine in
the object "Ambient mean TI" . 35
Table F.9 – The keys in the objects of IDs of measurement device and wind turbine in
the object "SD TI" . 36
Table F.10 – The key in the objects of IDs of measurement device and wind turbine in
the object "Extreme ambient TI" . 36
Table F.11 – The keys in the objects of IDs of measurement device and wind turbine in
the object "Temperature" . 36
Table F.12 – The keys in the objects of IDs of measurement device and wind turbine in
the object "Shear". 37
Table F.13 – The keys in the objects of IDs of measurement device and wind turbine in
the object "Inflow angle" . 37
Table F.14 – The keys in the objects of IDs of measurement device and wind turbine in
the object "CcT" . 37

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INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
WIND ENERGY GENERATION SYSTEMS –

Part 15-1: Site suitability input conditions for wind power plants

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
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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
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4) In order to promote international uniformity, IEC National Committees undertake to apply IEC Publications
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any IEC Publication and the corresponding national or regional publication shall be clearly indicated in the latter.
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6) All users should ensure that they have the latest edition of this publication.
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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) IEC draws attention to the possibility that the implementation of this document may involve the use of (a)
patent(s). IEC takes no position concerning the evidence, validity or applicability of any claimed patent rights in
respect thereof. As of the date of publication of this document, IEC had not received notice of (a) patent(s), which
may be required to implement this document. However, implementers are cautioned that this may not represent
the latest information, which may be obtained from the patent database available at https://patents.iec.ch. IEC
shall not be held responsible for identifying any or all such patent rights.
IEC 61400-15-1 has been prepared by IEC technical committee 88: Wind energy generation
systems. It is an International Standard.
The text of this International Standard is based on the following documents:
Draft Report on voting
88/1041/FDIS 88/1064/RVD
Full information on the voting for its approval can be found in the report on voting indicated in
the above table.
The language used for the development of this International Standard is English.

IEC 61400-15-1:2025 © IEC 2025 – 5 –
This document was drafted in accordance with ISO/IEC Directives, Part 2, and developed in
accordance with ISO/IEC Directives, Part 1 and ISO/IEC Directives, IEC Supplement, available
at www.iec.ch/members_experts/refdocs. The main document types developed by IEC are
described in greater detail at www.iec.ch/publications.
A list of all parts in the IEC 61400 series, published under the general title Wind energy
generation systems, can be found on the IEC website.
The committee has decided that the contents of this document will remain unchanged until the
stability date indicated on the IEC website under webstore.iec.ch in the data related to the
specific document. At this date, the document will be
• reconfirmed,
• withdrawn, or
• revised.
– 6 – IEC 61400-15-1:2025 © IEC 2025
INTRODUCTION
This part of IEC 61400 defines a framework for assessment and reporting of the site
suitability/turbine suitability input conditions for both onshore and offshore (fixed and floating)
power plants.
IEC 61400-15-1:2025 © IEC 2025 – 7 –
WIND ENERGY GENERATION SYSTEMS –

Part 15-1: Site suitability input conditions for wind power plants

1 Scope
The scope of this part of IEC 61400 is to define a framework for assessment and reporting of
the site suitability/wind turbine suitability conditions for both onshore and offshore (fixed and
floating) wind power plants. This includes:
a) definition, measurement, and prediction of the long-term meteorological and wind flow
characteristics at the site;
b) integration of the long-term meteorological and wind flow characteristics with wind turbine
and balance-of-plant characteristics;
c) characterizing environmental extremes and other relevant plant design drivers;
d) addressing documentation and reporting requirements to help ensure the traceability of the
assessment processes.
The framework is defined such that applicable national norms are considered and industry best
practices are utilized. This framework defines the minimum set of parameters. Additional
parameters may be used if needed.
The meteorological and wind flow characteristics addressed in this document relate to wind
conditions, where parameters such as wind speed, wind direction, turbulence intensity, wind
shear, inflow angle, air density or air temperature are included to the extent that they affect the
structural integrity of a wind turbine.
According to IEC 61400-1, IEC 61400-3-1 and IEC TS 61400-3-2, site specific conditions are
wind conditions, marine conditions, other environmental conditions, soil conditions and
electrical conditions. All of these site-specific conditions other than site specific wind conditions
and related atmospheric variables addressed herein are out of scope for this document.
This document is framed to complement and support the scope of related IEC 61400 series by
defining environmental input conditions. It is not intended to supersede the design and
suitability requirements presented in those documents. Specific analytical and modelling
procedures as described in IEC 61400-1, IEC 61400-2, IEC 61400-3-1 and IEC TS 61400-3-2
are excluded from the scope of this document.
2 Normative references
The following documents are referred to in the text in such a way that some or all of their content
constitutes requirements of this document. For dated references, only the edition cited applies.
For undated references, the latest edition of the referenced document (including any
amendments) applies.
IEC 61400-1:2019, Wind energy generation systems – Part 1: Design requirements
IEC 61400-3-1:2019, Wind energy generation systems – Part 3-1: Design requirements for fixed
offshore wind turbines
IEC 61400-12-1:2022, Wind energy generation systems – Part 12-1: Power performance
measurements of electricity producing wind turbines

– 8 – IEC 61400-15-1:2025 © IEC 2025
ISO 2533:1975, Standard Atmosphere
ISO/IEC 21778:2017, Information technology – The JSON data interchange syntax
ISO/IEC 10646:2020, Information technology – Universal Coded Character Set (UCS)
ISO 3166, Country codes
3 Terms and definitions
For the purposes of this document, the following terms and definitions apply.
ISO and IEC maintain terminology databases for use in standardization at the following
addresses:
• IEC Electropedia: available at https://www.electropedia.org/
• ISO Online browsing platform: available at https://www.iso.org/obp
NOTE All the below parameters are expected to represent the climate conditions over the design lifetime of the
wind turbine and apply at hub height.
3.1
inflow angle
angle between a horizontal plane and the wind velocity vector at hub height
Note 1 to entry: The inflow angle is positive if the wind velocity vector is pointing upwards.
Note 2 to entry: Referred to as flow inclination angle in IEC 61400-1:2019.
3.2
turbulence intensity
TI
ratio of the wind speed standard deviation to the mean wind speed determined from the same
set of measured wind speed data and taken over a period of 10 min and based on at least 1 Hz
sampling frequency
3.3
mean ambient turbulence intensity
average of a subset of the turbulence intensities
Note 1 to entry: The subset typically represents a bin within a wind speed and wind direction matrix.
3.4
standard deviation of turbulence intensity
standard deviation of a subset of the turbulence intensities
Note 1 to entry: The subset typically represents a bin within a wind speed- and wind direction matrix.
3.5
associated data source
primary data source used to derive wind conditions for a given turbine location
Note 1 to entry: This can include, but is not limited to, meteorological towers, remote sensing devices, production
data or model data.
3.6
number of samples
number of data points which form the basis of the associated parameter value

IEC 61400-15-1:2025 © IEC 2025 – 9 –
3.7
mean wind shear exponent
wind shear (or power law) exponent as defined in IEC 61400-1:2019 and estimated across the
rotor swept area
3.8
average turbulence intensity at 15 m/s
mean ambient turbulence intensity over all wind directions in the 15 m/s wind speed bin
Note 1 to entry: Bin width is defined as 14,5 to 15,5 m/s.
3.9
annual mean ambient temperature
annual mean ambient temperature at the site
3.10
annual wind frequency distribution
annual distribution of occurrences as a function of wind direction and/or wind speeds
3.11
Weibull distribution
probability distribution function
3.12
coefficient of variation
standard deviation divided by the mean value
3.13
extreme ambient turbulence intensity
extreme ten minute value of the ambient turbulence intensity with a return period of 50 years
as a function of wind speed
3.14
omni-directional
one value describing all direction sectors
3.15
surface roughness
texture of the surface topography that is irregular (not smooth) due to the soil surface and/or
the presence of vegetation and obstacles such as built-up areas
3.16
site suitability/turbine suitability
given combination of site conditions and turbine properties impacting structural integrity

– 10 – IEC 61400-15-1:2025 © IEC 2025
4 Symbols, units and abbreviated terms
4.1 Symbols and units
ϕ annual mean inflow angle [deg]
TI mean ambient turbulence intensity as defined in 3.8 [-]
σ standard deviation of estimated wind speed standard deviation [m/s]
σ
α mean wind shear (or power law) exponent [-]
α effective wind shear exponent [-]
eff
d displacement height [m]
D rotor diameter [m]
z hub height [m]
hub
V extreme wind speed (averaged over 10 min) with a recurrence interval of 50 years [m/s]
V expected extreme wind speed (averaged over 3 s), with a recurrence time interval of 50 years [m/s]
e50
ρ air density
[kg/m ]
V annual mean wind speed at hub height [m/s]
ave
C scale parameter of the Weibull distribution function [m/s]
k shape parameter of the Weibull distribution function [-]
C turbulence structure correction parameter [-]
CT
P air pressure [hPa]
T air temperature [°C]
V horizontal component of wind speed [m/s]
xy
V vertical component of wind speed [m/s]
z
RH relative humidity [%]
4.2 Abbreviated terms
BOM Byte Order Mark
CFD Computational Fluid Dynamics
COV Coefficient of Variation
GEV Generalized Extreme Value Distribution
GPD Generalized Pareto Distribution
MCP Measure Correlate Predict
MIS Method of Independent Storms
NWP Normal Wind Profile Model
POT Peak-Over-Threshold
TI Turbulence Intensity
WS Wind Speed
IEC 61400-15-1:2025 © IEC 2025 – 11 –
5 Methods to determine turbine suitability input parameters
5.1 General
A site suitability assessment consists of the analysis of the atmospheric, topographical and
obstacle conditions that influence the fatigue and ultimate loads that a wind turbine is required
to withstand throughout its lifetime.
Generally, data from an one or more onsite measurement stations is available as input to
determine the wind turbine suitability parameters. Ideally, measurements are collected at
locations that represent the full range of meteorological conditions expected at the wind turbine
locations. However, since measurements at each wind turbine location are challenging to obtain,
it is common practice to associate clusters of wind turbines to existing onsite measurements in
combination with wind flow modelling. The following list includes the topographic characteristics
that should be considered when determining turbine association.
• Distance between representative measurement device(s) or location and turbine.
• Differences in elevation (relative elevation between measurement station and turbine).
• Exposure (can be estimated as the difference between the elevation/slope at the
measurement location and the average elevation/slope in the surrounding area).
• Surface roughness (land use and/or forestry).
• Relevant obstacles.
The purpose of this clause is to describe both the methodology and best practices to calculate
or estimate such conditions at the wind turbine locations. Note that additional parameters (such
as oceanic) required to evaluate turbine suitability for offshore sites will not be addressed in
this version of the standard.
Wind turbines are structures which incorporate static and dynamic loading. Therefore local
measurements are generally used to assess the site conditions.
For all parameters which are not addressed in this document and for which there are no reliable
data from measurements, the corresponding design values from IEC 61400-1:2019 may be
used. Where different methods can be applied to derive parameters, the chosen method shall
be described, justified and reported as part of the Turbine Suitability Input reporting in Annex B.
Whenever a validated approach is called for in this clause, this means to traceably justify the
model applicability according to the state-of-the-art science and technology. The information
on the state-of-the-art flow models including their potential, limitations and important technical
aspects of flow models are described in [1] .
___________
All methods have limitations when extrapolating wind conditions spatially. Users need to be aware that the
uncertainty in extrapolated parameters may grow significantly with increasing distance from the measurement
point. This is especially relevant at sites with complex terrain, complex surface roughness, and/or complex flow
conditions, or when extrapolating onshore to offshore (or the reverse).
Numbers in square brackets refer to the bibliography.

– 12 – IEC 61400-15-1:2025 © IEC 2025
5.2 Assessment of wind speed
5.2.1 Wind speed distribution
5.2.1.1 General
The wind speed distribution is significant for wind turbine design because it determines the
frequency of occurrence of individual load conditions for the normal design cases. The wind
speed distribution shall be derived from onsite measurement(s) with the format of either wind
speed frequency or Weibull distributions, both omni-directional and sector-wise based. If a
Weibull distribution is used the goodness of fitting shall be checked since a Weibull function
may not be valid to represent the wind speed distribution at some sites, e.g. where binomial
wind distributions may fit better. The data shall be long-term representative and transferred to
each wind turbine's location at corresponding hub heights.
5.2.1.2 Long-term adjustment
The wind speed distribution shall be representative of the operational lifetime of the wind turbine
being evaluated in the site suitability assessment.
If the data is not representative, long-term adjustment shall be carried out by using either
long-term adjustment factor(s), a Measure Correlate and Predict (MCP) method as defined in
for example [2] or a more advanced method, examples of which can be found in [3].
5.2.1.3 Vertical extrapolation of wind speed
The wind distribution shall be adjusted to proposed wind turbine hub height(s). Either power
law, logarithmic law, flow simulation, or combinations of the methods shall be used [4].
5.2.1.4 Horizontal extrapolation of wind speed
The wind speed data shall be extrapolated to the turbine location by using a validated flow
model (e.g. by use of speed up factors that account for the increase in the speed of the wind
flow due to terrain topography).
5.2.2 Extreme wind speed with a recurrence interval of 50 years
5.2.2.1 General
Estimation of the extreme wind speed with a recurrence period of 50 years is necessary for
characterization of wind turbine ultimate loads. Extreme wind speed estimates are accompanied
by high uncertainty and any estimate of extreme wind speed should be considered in the context
and V , 2.3.2 in [5] or guideline [6] can be used.
of its uncertainty. For converting between V
50 e50
At sites that are prone to some atmospheric phenomena that cause extreme winds, such as
severe thunderstorms, tropical cyclones, and strong downwind events, on-site measurements
of any duration could be used, but may need to be adjusted and thus need to be used with
caution. Regional or otherwise representative historical data of extreme wind events may be
used for the estimation of extreme wind speed.
The methods listed in this section may be used. Considering the quality and consistency of all
sources of data and methods, justification of the selected extreme wind speed value shall be
given.
___________
Vertical and horizontal extrapolation can be done simultaneously by using a flow model.

IEC 61400-15-1:2025 © IEC 2025 – 13 –
Alternatively, the local recognized standard or guideline which specifies the design wind speed
with the recurrence period of 50 years may be used to derive the extreme wind speed.
5.2.2.2 Estimation of extreme wind speed using onsite data
5.2.2.2.1 General
The most common approach is to identify a series of extreme wind speed events so that a
mathematical distribution function can be fitted to the measured extreme wind events. Care is
needed to set appropriate thresholds, evaluate the quality of the distribution, and understand
the representativeness of the dataset of the long-term climate. For detail refer to Annex C and
Annex D.
Vertical and spatial extrapolations of extreme wind speeds may be based on the following
approach.
5.2.2.2.2 Vertical extrapolation of extreme winds
Vertical extrapolation of extreme wind speeds observed during tropical cyclones or other unique
storms may employ an event-specific shear value with adequate justification. For example,
see [6] for a hurricane conditions offshore, based upon observed wind profiles[6].
Relevant shear values shall be used for vertical extrapolation of high wind speed events.
Vertical extrapolation may be made using various methods. Shear values for vertical
extrapolation should be derived from representative long-term measured data if available. This
could consider the highest 0,1 % measured wind speeds, or instantaneous profiles at the time
of independent peak storm events. Extrapolation beyond the heights of measurement should
be conducted with caution. Other methods could be using wind shear power law exponent value
of 0,11 as recommended in IEC 61400-1 or the use of flow models.
Offshore extreme wind might face higher wind shear due to marine phenomena (e.g., Gulf
stream, upwelling).
5.2.2.2.3 Horizontal extrapolation of extreme winds
Horizontal extrapolation of extreme winds may be based on appropriate speed up factors that
are representative of extreme wind speeds. Suggested methods are to extrapolate the extreme
wind speed or to extrapolate the individual contributing extreme wind speed events before
estimating the extreme wind speed at each wind turbine.
5.2.2.3 Estimation of extreme wind speed using global or mesoscale models
Global or mesoscale models may be used for the estimation of extreme wind speed distributions.
A sufficient period of simulation shall be carried out to generate virtual measurement data. A
sufficient period shall cover at least the lifetime required for the project. The methods for the
selection of extreme wind events and extreme distribution fitting outlined in 5.2.2.2 also apply
to the analysis of model data and are described in Annex C.
Model data should be validated and if necessary calibrated using representative measured data
whenever possible, considering the methods outlined in Annex D. This is especially important
if extreme wind estimates are intended for engineering design. In offshore regions, long duration
model data can be critical to represent rare storm events, especially if these are of tropical
origin. Short duration measured data are generally required to ensure the model is
representative of the target location (ideally, at least one year of on-site measurements shall
be required to check the seasonal representativeness of global or mesoscale models).

– 14 – IEC 61400-15-1:2025 © IEC 2025
Sophisticated methods applied to mesoscale model data on onshore wind projects are
described in Annex E. Offshore wind projects, often using global rather than mesoscale models,
usually adopt simpler methods. First, the effective averaging period of the model is established.
This is typically longer than one hour for a global model. The extreme wind estimates from the
model are then converted from this long averaging period to the required 10 minutes mean and
3 seconds gust using appropriate factors. Relevant factors may differ from those in Table 1 of
61400-3-1:2019.
Downscaling global models to mesoscale resolution is established best practice onshore and
has proven benefits in nearshore regions. However, global models can be used effectively
without downscaling at some offshore locations.
It is recommended to validate the simulation results by using onsite measurement data.
5.2.2.4 Estimation of extreme wind speed for tropical cyclone regions
In regions prone to tropical cyclones (e.g. cyclones, hurricanes and typhoons), the extreme
wind speed shall be evaluated by appropriate methods, for example as given in
IEC 61400-1:2019, Annex J. If the coefficient of variation of extreme wind distribution
(IEC61400-1:2019, 7.4.7) may exceed 0,15, this value shall be evaluated. A u niform COV value
may be used unless there is a significant difference in the meteorological conditions within the
wind farm.
5.3 Assessment of turbulence intensity
5.3.1 Ambient turbulence intensity
5.3.1.1 General
If measurements have been collected onsite, the ambient turbulence intensity and the standard
deviation of the turbulence intensity shall be calculated according to IEC 61400-1:2019, 3.58.
De-trending of measurement data is not needed. If de-trending methods are used, only a linear
de-trending should be applied. The baseline is assumed to be data measured at a point
(cup/sonic anemometers), unless TI from a remote sensing device can be justified or adjusted
using methods that are clearly demonstrated to closely represent the point measurement.
5.3.1.2 Vertical extrapolation of TI
Whenever measurements are recorded at a different height than the hub height of the wind
turbine and the TI from a height closest to the hub height (preferably lower) is not representative,
the turbulence shall be extrapolated to hub height. The following methods may be considered:
• The wind speed standard deviation can be assumed constant while the wind speed is
extrapolated to hub height. This method is only applicable to an upward extrapolation. Care
should be taken when extrapolating wind speed to hub height since the rate of change of TI
with height may be different from the rate of change of wind speed with height.
• Use validated flow models to predict the TI at appropriate heights. The model prediction
shall be calibrated based on the available TI measurements.
• Extrapolation of standard deviation using similarity theory or observation fitting.

IEC 61400-15-1:2025 © IEC 2025 – 15 –
5.3.1.3 Horizontal extrapolation of TI
The TI from the reference measurement point(s) shall be extrapolated to each wind turbine
location. The method shall be validated against data for the flow complexity of the site under
consideration. Complexity does not only cover orography, but also other impacts such as
roughness, forest or thermal stratification. The following methods may be considered:
• Use empirical formulas based on surface roughness to obtain correction factors between
measurement device location and wind turbine location. This method can only be used for
non-complex or low complexity categories according to IEC 61400-1:2019, 11.2. Care
should be taken when using this method when the roughness at the sites is very
heterogeneous or when obstacles or forests are expected to influence TI at the wind turbine
position.
• Use validated flow models to predict the TI at turbine locations and calibrate the model
prediction based on TI measurements.
• Use TI weighted average of surrounding measurement devices (e.g. inverse distance).
The standard deviation of turbulence intensity at the turbine locations may be assumed to be
the same as the most representative measurement device. Other methods may be applied.
5.3.1.4 Alternative approaches for estim
...