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.

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.

General Information

Status
Published
Publication Date
29-Aug-2022
Current Stage
PPUB - Publication issued
Completion Date
30-Aug-2022
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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
IEC 61400-12-5:2022-08(en-fr)
---------------------- Page: 1 ----------------------
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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

Warning! Make sure that you obtained this publication from an authorized distributor.

Attention! Veuillez vous assurer que vous avez obtenu cette publication via un distributeur agréé.

® Registered trademark of the International Electrotechnical Commission
Marque déposée de la Commission Electrotechnique Internationale
---------------------- Page: 3 ----------------------
– 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

---------------------- Page: 4 ----------------------
IEC 61400-12-5:2022 © IEC 2022 – 3 –

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

---------------------- Page: 5 ----------------------
– 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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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

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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) 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.

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:
---------------------- Page: 6 ----------------------
IEC 61400-12-5:2022 © IEC 2022 – 5 –
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.

---------------------- Page: 7 ----------------------
– 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.
---------------------- Page: 8 ----------------------
IEC 61400-12-5:2022 © IEC 2022 – 7 –
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
---------------------- Page: 9 ----------------------
– 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]
---------------------- Page: 10 ----------------------
IEC 61400-12-5:2022 © IEC 2022 – 9 –
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]
D rotor diameter of neighbouring and operating wind turbine [m]
d distance from turbine to point in terrain for worst case scenario of slope [m]
H hub height of wind turbine [m]
h height of obstacle [m]
L distance between the wind turbine and the wind measurement equipment [m]
distance between the wind turbine or the wind measurement equipment and an [m]
obstacle
L distance between the wind turbine or the wind measurement equipment and a [m]
neighbouring and operating wind turbine
l height of obstacle [m]
l width of obstacle [m]
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]
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.
---------------------- Page: 11 ----------------------
– 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.
---------------------- Page: 12 ----------------------
IEC 61400-12-5:2022 © IEC 20
...

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