IEC 61400-12-5:2022

Wind energy generation systems - Part 12-5: Power performance - Assessment of obstacles and terrain

IEC 61400-12-5:2022 specifies the procedures for assessing the significance of obstacles and terrain variations on a proposed power performance measurement site and applies to the performance testing of wind turbines of all types and sizes connected to the electrical power network as described in other parts of the IEC 61400 series. The procedure applies to the performance evaluation of specific wind turbines at specific locations.
This first edition of IEC 61400-12-5 is part of a structural revision that cancels and replaces the performance standards IEC 61400-12-1:2017 and IEC 61400-12-2:2013. The structural revision contains no technical changes with respect to IEC 61400-12-1:2017 and IEC 61400‑12‑2:2013, but the parts that relate to wind measurements, measurement of site calibration and assessment of obstacle and terrain have been extracted into separate standards.
The content of the corrigendum 1 (2025-05) has been included in this copy.

Systèmes de génération d'énergie éolienne - Partie 12-5: Performance de puissance - Évaluation des obstacles et du terrain

IEC 61400-12-5:2022 spécifie les procédures d'évaluation de l'importance des obstacles et des variations de terrain sur un site proposé pour le mesurage de la performance de puissance et s'applique aux essais de performance des éoliennes de tous types et de toutes tailles raccordées au réseau électrique comme cela est décrit dans d'autres parties de la série IEC 61400. La procédure s'applique à l’évaluation de la performance d'éoliennes spécifiques sur des sites spécifiques.
La présente première édition de l’IEC 61400-12-5 fait partie d'une révision structurelle qui annule et remplace les normes de performance IEC 61400-12-1:2017 et IEC 61400-12-2:2013. Cette révision structurelle ne contient aucune modification technique par rapport à l'IEC 61400‑12‑1:2017 et l'IEC 61400-12‑2:2013. Toutefois, les parties relatives aux mesurages du vent, au mesurage de l'étalonnage du site et à l'évaluation des obstacles et du terrain ont été extraites vers des normes distinctes.
Le contenu du corrigendum 1 (2025-05) a été pris en considération dans cet exemplaire.

General Information

• Status: Published
• Publication Date: 29-Aug-2022
• ICS: 27.180 - Wind turbine energy systems
• Technical Committee: TC 88 - Wind energy generation systems
• Drafting Committee: MT 12-1 - TC 88/MT 12-1
• Current Stage: PPUB - Publication issued
• Start Date: 30-Aug-2022
• Completion Date: 26-Sep-2022

Relations

• Replaces: IEC 61400-12-1:2017 - Wind energy generation systems - Part 12-1: Power performance measurements of electricity producing wind turbines
• Effective Date: 05-Sep-2023
• Replaces: IEC 61400-12-2:2013 - Wind turbines - Part 12-2: Power performance of electricity-producing wind turbines based on nacelle anemometry
• Effective Date: 05-Sep-2023
• Corrected By: IEC 61400-12-5:2022/COR1:2025 - Corrigendum 1 - Wind energy generation systems - Part 12-5: Power performance - Assessment of obstacles and terrain
• Effective Date: 26-Oct-2025

Standards Content (Sample)

IEC 61400-12-5 ®
Edition 1.0 2022-08
INTERNATIONAL
STANDARD
NORME
INTERNATIONALE
colour
inside
Wind energy generation systems –
Part 12-5: Power performance – Assessment of obstacles and terrain

Systèmes de génération d'énergie éolienne –
Partie 12-5: Performance de puissance – Évaluation des obstacles et du terrain

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IEC 61400-12-5 ®
Edition 1.0 2022-08
INTERNATIONAL
STANDARD
NORME
INTERNATIONALE
colour
inside
Wind energy generation systems –

Part 12-5: Power performance – Assessment of obstacles and terrain

Systèmes de génération d'énergie éolienne –

Partie 12-5: Performance de puissance – Évaluation des obstacles et du terrain

INTERNATIONAL
ELECTROTECHNICAL
COMMISSION
COMMISSION
ELECTROTECHNIQUE
INTERNATIONALE
ICS 27.180 ISBN 978-2-8322-5603-9

– 2 – IEC 61400-12-5:2022 © IEC 2022
CONTENTS
FOREWORD . 4
INTRODUCTION . 6
1 Scope . 7
2 Normative references . 7
3 Terms and definitions . 7
4 Symbols, units and abbreviated terms . 9
5 General . 9
6 Measurement sector procedure to be used for power performance measurements
according to IEC 61400-12-1 . 10
6.1 General . 10
6.2 Requirements regarding neighbouring and operating wind turbines . 11
6.3 Requirements regarding obstacles . 11
6.4 Method for calculation of sectors to exclude . 12
6.5 Special requirements for extended obstacles . 15
7 Measurement sector procedure to be used during the determination of the nacelle
transfer function (NTF) between free wind speed and nacelle anemometer wind
speed . 15
8 Measurement sector procedure to be used during the determination of the nacelle
power curve (NPC) . 15
8.1 General . 15
8.2 Requirements regarding neighbouring and operating wind turbines . 16
8.3 Requirements regarding obstacles . 16
8.4 Calculation of the excluded sector . 17
8.5 Self-consistency check procedure for the measurement sector . 18
9 Assessment of terrain at the test site . 19
10 Terrain classification for evaluation of NTF and NPC . 22
10.1 General . 22
10.2 RIX indices . 22
10.3 Average slope . 22
10.4 Determine terrain class . 23
10.5 Ridge formations . 24

Figure 1 – Sectors to exclude due to wakes of neighbouring and operating wind
turbines and significant obstacles . 13
Figure 2 – An example of sectors to exclude due to wakes of the wind turbine under
test, a neighbouring and operating wind turbine and a significant obstacle . 14
Figure 3 – Example of the result of a sector self-consistency check . 19
Figure 4 – Illustration of area to be assessed, top view . 20
Figure 5 – Example of determination of slope and terrain variation from the best-fit
plane: "2L to 4L" and the case "measurement sector" (Table 5, line 2) . 21
Figure 6 – Determination of slope for the distance "2L to 4L" and "8L to 16L" and the
case "outside measurement sector" (Table 5, line 3 and line 5). 21

Table 1 – Interfaces from other International Standards to this document . 10
Table 2 – Interfaces from this document to other International Standards . 10
Table 3 – Obstacle requirements: relevance of obstacles . 12

Table 4 – Obstacle requirements: relevance of obstacles . 17
Table 5 – Test site requirements: topographical variations . 20
Table 6 – Slope terrain classification . 23
Table 7 – RIX terrain classification . 23
Table 8 – Final terrain class . 23
Table 9 – Maximum ridge step effects on terrain class . 24

– 4 – IEC 61400-12-5:2022 © IEC 2022
INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
WIND ENERGY GENERATION SYSTEMS –

Part 12-5: Power performance –
Assessment of obstacles and terrain

FOREWORD
1) The International Electrotechnical Commission (IEC) is a worldwide organization for standardization comprising
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rights. IEC shall not be held responsible for identifying any or all such patent rights.
IEC 61400-12-5 has been prepared by IEC technical committee 88: Wind energy generation
systems. It is an International Standard.
This first edition of IEC 61400-12-5 is part of a structural revision that cancels and replaces the
performance standards IEC 61400-12-1:2017 and IEC 61400-12-2:2013. The structural revision
contains no technical changes with respect to IEC 61400-12-1:2017 and IEC 61400-12-2:2013,
but the parts that relate to wind measurements, measurement of site calibration and assessment
of obstacle and terrain have been extracted into separate standards.
The purpose of the re-structure was to allow the future management and revision of the power
performance standards to be carried out more efficiently in terms of time and cost and to provide
a more logical division of the wind measurement requirements into a series of separate
standards which could be referred to by other use case standards in the IEC 61400 series and
subsequently maintained and developed by appropriate experts.
The text of this International Standard is based on the following documents:

Draft Report on voting
88/825/CDV 88/870/RVC
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.
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/standardsdev/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,
• replaced by a revised edition, or
• amended.
IMPORTANT – The "colour inside" logo on the cover page of this document 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.

– 6 – IEC 61400-12-5:2022 © IEC 2022
INTRODUCTION
The purpose of this part of IEC 61400 is to describe procedures to determine a measurement
sector that is not influenced by turbines or other obstacles for a specific wind turbine. The
measurement sector is used to assess the terrain and determine if a site calibration is required.
This measurement sector can be used to evaluate wind turbine power performance.
Clause 6 describes a procedure to determine one or more sectors which are not usable for a
power performance measurement because the flow at the wind turbine under test and/or the
flow at the position of the wind measurement equipment (WME: a meteorological mast or a
remote sensing device) might be affected by an operating wind turbine and/or by an obstacle.
Clause 7 describes a procedure for establishing the measurement sector during the
determination of the nacelle transfer function.
Clause 8 describes a procedure for establishing the measurement sector during the
determination of the nacelle power curve.
Clause 9 describes a procedure to assess the terrain surrounding the site and to determine if it
is sufficiently complex to require site calibration. The purpose of site calibration is generally to
measure the change in the boundary layer as it follows the orography, which is generally
attached flow, whereas obstacles often generate more turbulent wakes associated with them,
which are affected by sharp edges and vertical surfaces that can trigger flow separation.
Clause 10 describes a procedure to classify the terrain in order to apply the nacelle transfer
function for a nacelle power curve measurement. The classification is used to estimate the
uncertainty of the nacelle transfer function (NTF) and nacelle power curve (NPC) and also to
determine under what terrain condition the NTF can be used for the NPC.
The creation of this new standard was mandated with the restriction that no technical changes
to the content copied from the source documents (IEC 61400-12-1 and IEC 61400-12-2) would
be allowed. Therefore, in this first edition of the new standard IEC 61400-12-5, there are some
obvious areas of technical disagreement (e.g. assessment of terrain in Clauses 9 and 10) where
a choice needs to be made depending on whether the intended use case is as an input to a
IEC 61400-12-1 or to a IEC 61400-12-2 power curve evaluation. It is recommended that future
revisions of this document aim to harmonise the technical content.

WIND ENERGY GENERATION SYSTEMS –

Part 12-5: Power performance –
Assessment of obstacles and terrain

1 Scope
This part of IEC 61400 specifies the procedures for assessing the significance of obstacles and
terrain variations on a proposed power performance measurement site and applies to the
performance testing of wind turbines of all types and sizes connected to the electrical power
network as described in other parts of the IEC 61400 series. The procedure applies to the
performance evaluation of specific wind turbines at specific locations.
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 61400-12-1, Wind energy generation systems – Part 12-1: Power performance
measurements of electricity producing wind turbines
IEC 61400-12-2, Wind energy generation systems – Part 12-2: Power performance of electricity
producing wind turbines based on nacelle anemometry
IEC 61400-12-3, Wind energy generation systems – Part 12-3: Power performance –
Measurement based site calibration
IEC 61400-12-6, Wind energy generation systems – Part 12-6: Measurement based nacelle
transfer function of electricity producing wind turbines
3 Terms and definitions
For the purposes of this document, the following terms and definitions apply.
ISO and IEC maintain terminological databases for use in standardization at the following
addresses:
• IEC Electropedia: available at http://www.electropedia.org/
• ISO Online browsing platform: available at http://www.iso.org/obp
3.1
complex terrain
terrain surrounding the test site that features significant variations in topography and terrain
obstacles (refer to 3.10) that may cause flow distortion
3.2
cut-in wind speed
lowest wind speed at which a wind turbine will begin to produce power

– 8 – IEC 61400-12-5:2022 © IEC 2022
3.3
data set
collection of data sampled over a continuous period
3.4
flow distortion
change in air flow caused by obstacles, topographical variations, or other wind turbines that
results in the wind speed at the measurement location to be different from the wind speed at
the wind turbine location
3.5
free stream wind speed
horizontal wind speed measured upstream of the rotor of the wind turbine generator that is
unaffected by rotor aerodynamics
3.6
hub height
height of the centre of the swept area of the wind turbine rotor above the
ground at the tower
Note 1 to entry: For a vertical axis wind turbine the hub height is defined as the height of the centroid of the swept
area of the rotor above the ground at the tower.
3.7
measured power curve
table and graph that represent the measured, corrected and normalized net power output of a
wind turbine as a function of measured wind speed, measured under a well-defined
measurement procedure
3.8
measurement sector
sector of wind directions from which data are selected for the measured power curve
3.9
nacelle transfer function
NTF
transfer function applied to estimate the free stream wind speed from the nacelle anemometer
wind speed
3.10
obstacle
obstruction that blocks and distorts the flow of the wind, such as a building or tree
3.11
power performance
measure of the capability of a wind turbine to produce electric power and energy
3.12
test site
location of the wind turbine under test and its surroundings
3.13
uncertainty in measurement
parameter, associated with the result of a measurement, which characterizes the dispersion of
the values that could reasonably be attributed to the measurand
[SOURCE: IEC 60050-415:1999, 415-05-13]

3.14
wind measurement equipment
meteorological mast or remote sensing device
4 Symbols, units and abbreviated terms
Symbol or Description Unit
abbreviated term
D rotor diameter [m]
D equivalent rotor diameter [m]
e
D rotor diameter of neighbouring and operating wind turbine [m]
n
d distance from turbine to point in terrain for worst case scenario of slope [m]
i
H hub height of wind turbine [m]
h height of obstacle [m]
L distance between the wind turbine and the wind measurement equipment [m]
L
distance between the wind turbine or the wind measurement equipment and an [m]
e
obstacle
L distance between the wind turbine or the wind measurement equipment and a [m]
n
neighbouring and operating wind turbine
l height of obstacle [m]
h
l width of obstacle [m]
w
RIX Ruggedness index
VP wind speed evaluated from the power output [m/s]
V measured nacelle wind speed, corrected with nacelle transfer function [m/s]
free
WME wind measurement equipment
z height above ground [m]
z terrain elevation at point of the worst-case scenario for slope [m]
i
5 General
For a power performance measurement carried out according to IEC 61400-12-1 or
IEC 61400-12-2, the wind speed experienced at the measurement location and at the test
turbine location can be influenced both by obstacles (e.g. buildings, forests, other turbines) in
the surroundings and by the terrain (severity of terrain slopes and deviations within the terrain).
The presence of such influences should be checked and the valid direction sector reduced
accordingly in the case of obstacles or a site calibration (according to IEC 61400-12-3) carried
out in the case of sufficiently complex terrain. Clause 6 and Clause 8 describe procedures to
determine one or more sectors which are not usable for the test because the flow at the wind
turbine under test and/or the flow at the position of the wind measurement equipment (WME: a
meteorological mast or a remote sensing device) might be affected by an operating wind turbine
and/or by an obstacle. Clause 9 describes the criteria to assess terrain to determine if a site
can be considered flat (no site calibration required) or complex (site calibration required).
Clause 10 describes the criteria to classify terrain as required for the application of an NTF.
As this method is closely related to both IEC 61400-12-1 and IEC 61400-12-2, both standards
are referred to frequently in this document.
For this document, the interfaces are defined in Table 1 and Table 2; in order to facilitate the
correct interpretation of the interfaces, a short description of the use of the interface has been
added as well.
– 10 – IEC 61400-12-5:2022 © IEC 2022
Table 1 – Other International Standards which refer to this document
References to this document in Short use
Interface description Format
other International Standards description
Filtering the data
Measurement sector IEC 61400-12-1:2022, Clause 6 From [degree] to [degree]
set
Measurement sector Filtering the data
IEC 61400-12-2:2022, Clause 7 From [degree] to [degree]
(NTF) set
Measurement sector Filtering the data
IEC 61400-12-2:2022, Clause 8 From [degree] to [degree]
(NPC) set
Determination of
Terrain assessment IEC 61400-12-1:2022, Clause 9 site calibration Complex or not complex
requirement
Reporting of the
IEC 61400-12-2:2022, Clause 10 Terrain class
Terrain class transfer function
IEC 61400-12-6:2022, Clause 10 [1; 2; 3; 4; 5]
validity
Table 2 – Interfaces from this document to other International Standards
Interface Relevant content Reference to other Short use Format
description in this document standards description
Terrain assessment Clause 9 IEC 61400-12-1:2022 Input to the wind Operational
speed characteristics of
measurement anemometer
uncertainty dependent on
calculation for complexity of
the power curve terrain or
site-specific data
Terrain class Clause 10 IEC 61400-12-2:2022 Input to the wind Terrain class
speed [1; 2; 3; 4; 5]
measurement
uncertainty
calculation for
the nacelle
transfer function
(NTF)
6 Measurement sector procedure to be used for power performance
measurements according to IEC 61400-12-1
6.1 General
Clause 6 describes a procedure to determine one or more sectors which are not usable for the
test because the flow at the wind turbine under test and/or the flow at the position of the wind
measurement equipment (WME: a meteorological mast or a remote sensing device) might be
affected by an operating wind turbine and/or by an obstacle.
The procedure consists of two steps, to be applied in the following order
a) Evaluation of influences caused by operating wind turbines (wind turbine under test, as well
as neighbouring and operating wind turbines), as described in 6.2;
b) Evaluation of influences caused by obstacles, as described in 6.3 (under consideration of
the special requirements for extended obstacles, as described in 6.5).
Step b) requires the preliminary measurement sector, which is the result of step a).
The valid sector which remains after this procedure shall be used for the terrain assessment
according to Clause 9.
The purpose of site calibration (refer to IEC 61400-12-3) is generally to measure the change in
the boundary layer as it follows the orography, which is generally attached flow, whereas
obstacles often generate more turbulent wakes associated with them, which are affected by
sharp edges and vertical surfaces that may trigger flow separation. A site calibration does not
typically work well for correcting the effects of flow separation. This should be considered when
deciding whether to treat an object as an obstacle or as terrain as flow separation and highly
turbulent wakes are to be avoided. With this in mind, it is recommended that an object (including
orographic elements which satisfy the dimensional criteria) whose height is more than half of
its width be treated as an obstacle.
6.2 Requirements regarding neighbouring operating wind turbines
The WME shall not be influenced by the wind turbine under test.
The wind turbine under test and the WME shall not be influenced by neighbouring operating
wind turbines. If a neighbouring wind turbine is operated at any time during the power
performance test, its wake shall be determined and accounted for as described in this document
(using the calculation given in 6.4). Small wind turbines of total height less than (2/3)(H − D/2)
shall be treated as obstacles, and accounted for as described in Clause 8.
If a wind turbine is stopped at all times during the power performance test, it shall be considered
as an obstacle and accounted for as described in Clause 8.
The minimum distance from the wind turbine under test and the neighbouring operating wind
turbines is defined in IEC 61400-12-1. The minimum distance from the WME to any
neighbouring operating wind turbine is also defined in IEC 61400-12-1. The sectors to exclude
due to wakes from neighbouring operating wind turbines shall be taken from Figure 1. The
dimensions to be taken into account are the actual distance L and the rotor diameter D of the
n n
neighbouring operating wind turbine. The sectors to be excluded shall be derived for both the
wind turbine under test and the WME, and they shall be centred on the direction from the
neighbouring operating wind turbine to the wind measurement equipment or the wind turbine.
An example is shown in Figure 2.
6.3 Requirements regarding obstacles
Obstacles near the wind turbine under test or near the WME shall be evaluated. Each obstacle
shall be evaluated either as part of the orography (as described in Clause 9), or – alternatively
– according to the procedure which is described in this Subclause 6.3 as follows.
NOTE The consideration of an obstacle as part of the orography (as described in Clause 9) will typically increase
mainly the terrain variation, whereas the effect on the slope can be very small (except for extended obstacles, e.g.
forests).
No significant obstacles (e.g. buildings, trees, parked wind turbines) shall exist in the
measurement sector within a reasonable distance from the wind turbine or from the WME. Only
small buildings, connected to the wind turbine operation or the wind measurement equipment,
are acceptable. Where significant obstacles are present then the measurement sector shall be
reduced as described in 6.4 and 6.5.
The criterion for the significance of an obstacle (with respect to the wind turbine under test
and/or with respect to the WME) is to exceed one or more of the limits given in Table 3, where
Table 3 shall be applied for all locations:
a) for the evaluation of the surroundings of the wind turbine under test (i.e. using the centre of
the wind turbine under test as centre of the 2L, 4L, 8L, and 16L circles);
b) for the evaluation of the surroundings of the WME (i.e. using the position(s) of the equipment
as centre of the 2L, 4L, 8L, and 16L circles).

– 12 – IEC 61400-12-5:2022 © IEC 2022
Table 3 – Obstacle requirements: relevance of obstacles
Distance* Sector** Maximum obstacle height from
terrain surface***
< 2L 360° < 1/3 (H − 0,5 D)
≥ 2L and < 4L Preliminary measurement sector < 2/3 (H − 0,5 D)
≥ 4 L and < 8L Preliminary measurement sector < (H − 0,5 D)
≥ 8L and < 16L Preliminary measurement sector < 4/3 (H − 0,5 D)
≥ 2L and < 16L Clearly outside preliminary measurement sector by No limit to height
40° or more
*  from obstacle to wind turbine under test, or from obstacle to WME – where L is the horizontal distance between
wind turbine under test and wind measurement equipment.
** "Preliminary measurement sector" shall be understood here as the valid sector which remains after evaluation
of neighbouring operating wind turbines (as described in 6.2, using the calculation described in 6.4), where all
directions which are less than 40° outside shall also be considered.
*** H is the hub height and D is the rotor diameter of the wind turbine under test.

6.4 Method for calculation of sectors to exclude
The wind turbine under test shall always be evaluated according to Figure 1 with respect to its
wake influence on the WME.
A neighbouring operating wind turbine shall always be evaluated according to Figure 1 with
respect to its wake influence on the wind turbine under test and also with respect to its wake
influence on the WME.
With respect to operating wind turbines, the dimensions to be taken into account are the actual
distance L (from centre of wind turbine under test to the position of the WME) and the rotor
n
diameter D of the wind turbine that causes the wake.
n
The influence of the turbine under test on the WME is evaluated by means of L (distance
between turbine under test and wind measurement equipment) and D (rotor diameter of turbine
under test).
An obstacle shall be evaluated according to Figure 1 with respect to its wake influence on the
wind turbine under test if the obstacle is significant with respect to the wind turbine under test
according to Table 3.
An obstacle shall be evaluated according to Figure 1 with respect to its wake influence on the
WME if the obstacle is significant with respect to the WME according to Table 3.
With respect to obstacles, the dimensions to be taken into account are the actual horizontal
distance L (from the centre of wind turbine under test or from the position of the WME as
e
appropriate) and an equivalent rotor diameter D of the obstacle. A stopped neighbouring wind
e
turbine may be treated as a cylinder with a diameter equal to the tower base diameter and a
height equal to the upper tip height. The equivalent rotor diameter of the obstacle shall be
defined as:
2l l
h w
D =
e (1)
l + l
h w
where
D is the equivalent rotor diameter;
e
l is the height of obstacle;
h
l is the width of obstacle as seen from the wind turbine under test or from the WME.
w
Figure 1 – Sectors to exclude due to wakes of neighbouring
and operating wind turbines and significant obstacles

– 14 – IEC 61400-12-5:2022 © IEC 2022

Figure 2 – An example of sectors to exclude due to wakes of the wind turbine under
test, a neighbouring operating wind turbine and a significant obstacle
Figure 1 and Figure 2 show the sectors to exclude if:
a) the WME (meteorological mast or remote sensing device) is in the wake of the wind turbine
under test;
b) the WME is in the wake of the neighbouring operating wind turbine;
c) the wind turbine under test is in the wake of the neighbouring operating wind turbine;
d) the WME is in the wake of the significant obstacle;
e) the wind turbine under test is in the wake of the significant obstacle;

f) all of the above effects a) to e) are combined.
NOTE The example shown in Figure 2 is based on the assumption that the obstacle is significant (according to
Table 3) with respect to the wind turbine under test and also significant with respect to the wind measurement
equipment.
6.5 Special requirements for extended obstacles
Obstacles within a distance of less than 4L (from centre of the wind turbine under test or from
the wind measurement equipment) which extend more than 50 m in any horizontal direction
shall be divided into partial obstacles that do not extend more than 50 m in any horizontal
direction and that extend exactly 50 m in any dimension that is more than 50 m. These partial
obstacles may overlap one another. The combination of these partial obstacles shall at least
cover the original obstacle completely.
For practical purposes, it makes sense to create partial obstacles which all have the same
shape (e.g. squares of 50 m × 50 m). This is a conservative approach and therefore permitted
(in this case, the set union of all partial obstacles covers a larger area than the original obstacle).
Each partial obstacle shall be evaluated separately. The significance of each partial obstacle
shall be assessed, and if it is found to be significant, the sector to be excluded shall be
determined. For example:
a) An obstacle 90 m by 90 m is divided into 4 partial obstacles 50 m by 50 m each. These
partial obstacles are chosen such that they overlap one another by 10 m so that the set
union of the 4 partial obstacles is identical to the original obstacle.
b) An obstacle 70 m by 10 m is divided into 2 partial obstacles 50 m by 10 m each. These
partial obstacles are chosen such that they overlap one another by 30 m so that the set
union of the 2 partial obstacles is identical to the original obstacle.
A grouping of trees or forest shall be treated in this way.
7 Measurement sector procedure to be used during the determination of the
nacelle transfer function (NTF) between free wind speed and nacelle
anemometer wind speed
For the determination of the NTF, the measurement sector is defined according to the
requirements in Clause 8.
8 Measurement sector procedure to be used during the determination of the
nacelle power curve (NPC)
NOTE: NTF has already been determined.
8.1 General
Neighbouring wind turbines, obstacles and terrain can influence the power performance
assessment of a wind turbine. In the case of power performance analysis by means of nacelle
wind speed measurements, the anemometer is positioned at the location where the power is
extracted from the wind, so that the effect of wind turbines, obstacles and terrain lies in a non-
uniform distribution of the wind over the rotor plane.
The choice of suitable measurement sectors for the NPC should be based on the key
requirement that the wind speed, as measured by the nacelle anemometer in combination with
the NTF, results in a wind speed which is representative for the wind speed incident onto the
rotor of the wind turbine.
The wind direction measurement in a nacelle power curve contains two uncertainty components,
namely the uncertainty of the yaw angle and the uncertainty of the wind direction measurement

– 16 – IEC 61400-12-5:2022 © IEC 2022
in the nacelle. This potentially large uncertainty is reflected in the measurement sector
procedure (see also Annex B of IEC 61400-12-2:2022).
The measurement sector procedure requires two steps.
At first, a measurement sector compliant with the requirements described in Clause 6 shall be
derived, based on theoretical assessment.
The requirements in Clause 6 are taken to support measurements according to IEC 61400-12-1,
where they are applied to ensure that the measured wind speed is representative for the wind
turbine rotor, and, to this end, any situations where the mast or the turbine is exposed to wakes
or to the influence of obstacles are excluded, as specified in IEC 61400-12-1.
It is possible that this condition will not be completely appropriate for the NPC as defined in
IEC 61400-12-2. Even if there are closer neighbouring wind turbines or obstacles, the test
turbine has a power curve and the NTF may be valid. Other factors, on the other hand, may
lead to a situation where this is not achieved. For example:
• The NTF may become invalid if the vertical flow inclination is too high and the sector
requirements described in Clause 6 do not avoid sectors with overly steep slopes.
• The effect of the influence of obstacles is more complex and more difficult to interpret
compared to the case considered by Clause 6.
For these reasons, as a second step, the self-consistency check method described in 8.4 shall
be used to check the validity of the measurement sector.
If the self-consistency check method shows that there are problems, the sector shall be reduced
accordingly, as indicated in 6.2. If the measurement sector contains 10° sectors where the
average slopes have both positive and negative signs there is a risk of unsatisfactory results.
It is recommended that the average slope in the measurement sector has the same tendency
(up or down) across all 10° sectors.
8.2 Requirements regarding neighbouring operating wind turbines
The wind turbine under test shall not be influenced by neighbouring wind turbines. If a
neighbouring turbine is operated at any time during the power performance test, its wake shall
be determined and accounted for as described in this document. If the neighbouring turbine is
stopped at all times during the power performance test, it shall be considered as an obstacle.
The minimum distance from the wind turbine under test to neighbouring operating wind turbines
shall be two rotor diameters D of the neighbouring wind turbine or two rotor diameters D of the
n
wind turbine under test if it has a larger diameter. The size of the sectors to exclude due to
wakes from neighbouring operating wind turbines shall be taken from Figure 1 and 6.4. The
dimensions to be taken into account are the actual distance L and the rotor diameter D of the
n n
neighbouring operating wind turbine.
The sectors shall be centred on the direction from the neighbouring operating wind turbine to
the tested wind turbine.
8.3 Requirements regarding obstacles
No significant obstacles (e.g. buildings, trees, parked wind turbines) shall exist in the
measurement sector within a reasonable distance from the wind turbine. Only small buildings
connected to the wind turbine operation or the measurement equipment are acceptable.
An obstacle model is used to predict the influence of obstacles upon the turbine position at hub
height. The criterion for determining a significant obstacle is that the flow at hub height is

affected by 1 % or more for any wind direction in the measurement sector. This criterion shall
be applied to the flow effect on the turbine
...