IEC 61400-13:2015

Wind turbines - Part 13: Measurement of mechanical loads

IEC 61400-13:2015(B) describes the measurement of fundamental structural loads on wind turbines for the purpose of the load simulation model validation. The standard prescribes the requirements and recommendations for site selection, signal selection, data acquisition, calibration, data verification, measurement load cases, capture matrix, post-processing, uncertainty determination and reporting. Informative annexes are also provided to improve understanding of testing methods. This standard replaces IEC TS 61400-13 published in 2001; it constitutes a technical revision and transition from technical specification to International Standard.

Éoliennes - Partie 13: Mesurage des charges mécaniques

L'IEC 61400-13:2015(B) décrit le mesurage des charges structurelles fondamentales sur les éoliennes à des fins de validation des modèles de simulation de charges. La norme spécifie les exigences et recommandations concernant le choix du site, la sélection des signaux, l'acquisition des données, l'étalonnage, la vérification des données, les cas de charges de mesure, la matrice de capture, le post-traitement, la détermination de l'incertitude et les rapports d'essai. Des annexes informatives sont également fournies pour améliorer la compréhension des méthodes d'essai. Cette norme remplace l'IEC TS 61400-13 parue en 2001; elle constitue une révision technique, ainsi que la transposition d'une spécification technique en Norme internationale.

General Information

• Status: Published
• Publication Date: 02-Dec-2021
• ICS: 27.180 - Wind turbine energy systems
• Technical Committee: TC 88 - Wind energy generation systems
• Drafting Committee: MT 13 - TC 88/MT 13
• Current Stage: PPUB - Publication issued
• Start Date: 21-Dec-2015
• Completion Date: 15-Feb-2016

Relations

• Replaces: IEC TS 61400-13:2001 - Wind turbine generator systems - Part 13: Measurement of mechanical loads
• Effective Date: 31-May-2024
• Amended By: IEC 61400-13:2015/AMD1:2021 - Amendment 1 - Wind turbines - Part 13: Measurement of mechanical loads
• Effective Date: 05-Sep-2023

Overview

IEC 61400-13:2015 is an international standard from the International Electrotechnical Commission (IEC) that focuses on the measurement of mechanical loads on wind turbines. This standard is essential for professionals involved in wind turbine design, testing, and validation as it provides comprehensive guidelines to measure and analyze fundamental structural loads accurately. Its purpose is to support the validation of load simulation models, which are critical for assessing wind turbine reliability and safety.

Revised from the 2001 technical specification, this 2015 edition transitions IEC 61400-13 into a consolidated International Standard, incorporating updated requirements and recommendations. The standard addresses aspects ranging from site selection and instrumentation to data acquisition, calibration, and uncertainty estimation, ensuring precise and consistent load measurements.

Key Topics

• Measurement Load Cases (MLCs): Defined sets of operational conditions under which loads must be recorded, including steady-state, transient, and dynamic characterization scenarios.
• Site Selection: Criteria for appropriate test site conditions to ensure measurement accuracy and representativeness.
• Signal and Instrumentation Requirements: Guidance on sensor types, placement, and measurement of critical load components like blade root bending moments, rotor torque, tower bending moments, and pitch actuation loads.
• Data Acquisition: Specifications on resolution, anti-aliasing, and system design for accurate capturing of mechanical load signals.
• Calibration Procedures: Methods for determining calibration factors for load and non-load measurement channels to maintain data integrity.
• Data Verification and Post-Processing: Techniques to validate raw data, apply signal processing methods including rainflow counting and power spectral density analysis, and generate damage equivalent loads.
• Uncertainty Estimation: Procedures to evaluate and report measurement uncertainties enhancing the credibility of load data.
• Reporting Requirements: Standardized format and content to document test results and methodologies for transparency and repeatability.
• Informative Annexes: Additional guidance on coordinate systems, evaluation of uncertainties, and testing methodology to aid understanding and practical application.

Applications

IEC 61400-13:2015 is indispensable for:

• Wind Turbine Manufacturers: Helps optimize structural designs by providing reliable mechanical load data validated through standard measurement practices.
• Testing Laboratories: Establishes rigorous methodologies for conducting mechanical load measurements on-site to assure compliance with certification requirements.
• Certification Bodies: Provides a clear framework for verifying mechanical load simulations versus real measured data, crucial for safety and performance certification.
• Research and Development: Assists in developing improved models and control strategies by delivering validated load measurement data.
• Maintenance and Monitoring: Enables ongoing evaluation of turbine structural health by establishing baseline load measurements.

This standard enhances the overall confidence in wind turbine structural integrity assessments, contributing to safer, more efficient wind energy generation.

Related Standards

• IEC 61400 Series: The broader set of standards related to wind turbine design, performance, and testing, including:
  • IEC 61400-1: Design requirements
  • IEC 61400-12: Power performance measurement
  • IEC 61400-22: Conformity testing and certification
• IEC 60068: Environmental testing for electrical equipment, relevant for sensor and instrumentation robustness.
• ISO 2041: Vibration and shock terminology, aiding in understanding mechanical load measurement dynamics.

By aligning with IEC 61400-13:2015, organizations ensure that mechanical load measurements on wind turbines conform to globally recognized best practices, improving load simulation accuracy and contributing to the advancement of wind energy technology.

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Keywords: IEC 61400-13, wind turbine mechanical loads, load measurement, load simulation validation, wind turbine testing standards, load case measurement, load calibration, wind turbine certification, structural load measurement, wind energy standards, turbine load data acquisition, turbine load uncertainty estimation.

Standards Content (Sample)

IEC 61400-13 ®
Edition 1.0 2015-12
INTERNATIONAL
STANDARD
NORME
INTERNATIONALE
colour
inside
Wind turbines –
Part 13: Measurement of mechanical loads

Éoliennes –
Partie 13: Mesurage des charges mécaniques

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IEC 61400-13 ®
Edition 1.0 2015-12
INTERNATIONAL
STANDARD
NORME
INTERNATIONALE
colour
inside
Wind turbines –
Part 13: Measurement of mechanical loads

Éoliennes –
Partie 13: Mesurage des charges mécaniques

INTERNATIONAL
ELECTROTECHNICAL
COMMISSION
COMMISSION
ELECTROTECHNIQUE
INTERNATIONALE
ICS 27.180 ISBN 978-2-8322-3087-9

– 2 – IEC 61400-13:2015 © IEC 2015
CONTENTS
FOREWORD . 8
INTRODUCTION . 10
1 Scope . 11
2 Normative references . 11
3 Terms and definitions . 11
4 Symbols, units and abbreviations . 14
5 General . 16
5.1 Document structure . 16
5.2 Safety during testing . 17
6 Test requirements . 17
6.1 General . 17
6.2 Test site requirements . 17
6.3 Measurement load cases . 17
6.3.1 General . 17
6.3.2 MLCs during steady-state operation . 18
6.3.3 MLCs during transient events . 18
6.3.4 MLCs for dynamic characterization . 19
6.3.5 Capture matrices . 20
6.4 Quantities to be measured . 23
6.4.1 General . 23
6.4.2 Load quantities . 23
6.4.3 Meteorological quantities . 25
6.4.4 Wind turbine operation quantities . 25
6.5 Turbine configuration changes . 26
7 Instrumentation . 27
7.1 Load quantities . 27
7.1.1 Types of sensors . 27
7.1.2 Choice of sensor location . 27
7.1.3 Measurement of blade root bending moments . 27
7.1.4 Blade bending moment distribution . 28
7.1.5 Blade torsion frequency/damping . 28
7.1.6 Measurement of rotor yaw and tilt moment . 28
7.1.7 Measurement of the rotor torque . 28
7.1.8 Measurement of tower base bending . 28
7.1.9 Tower top bending moments . 28
7.1.10 Tower mid bending moments . 29
7.1.11 Tower torque . 29
7.1.12 Tower top acceleration . 29
7.1.13 Pitch actuation loads (on hub side of pitch bearing) . 29
7.2 Meteorological quantities . 29
7.2.1 Measurement and installation requirements . 29
7.2.2 Icing potential . 29
7.2.3 Atmospheric stability . 29
7.3 Wind turbine operation quantities . 30
7.3.1 Electrical power . 30

7.3.2 Rotor speed or generator speed . 30
7.3.3 Yaw misalignment . 30
7.3.4 Rotor azimuth angle. 30
7.3.5 Pitch position . 30
7.3.6 Pitch speed . 30
7.3.7 Brake moment . 30
7.3.8 Wind turbine status . 30
7.3.9 Brake status . 30
7.4 Data acquisition system . 31
7.4.1 General . 31
7.4.2 Resolution . 31
7.4.3 Anti-aliasing. 31
8 Determination of calibration factors . 31
8.1 General . 31
8.2 Calibration of load channels . 32
8.2.1 General . 32
8.2.2 Blade bending moments . 33
8.2.3 Main shaft moments . 33
8.2.4 Tower bending moments . 34
8.2.5 Tower torque . 34
8.3 Calibration of non-load channels . 35
8.3.1 Pitch angle . 35
8.3.2 Rotor azimuth angle. 35
8.3.3 Yaw angle. 35
8.3.4 Wind direction. 35
8.3.5 Pitch actuation loads . 35
8.3.6 Brake moment . 36
9 Data verification . 36
9.1 General . 36
9.2 Verification checks . 36
9.2.1 General . 36
9.2.2 Blade moments . 37
9.2.3 Main shaft . 38
9.2.4 Tower . 38
10 Processing of measured data . 39
10.1 General . 39
10.2 Fundamental load quantities . 39
10.3 Load quantities for larger turbines . 39
10.4 Wind speed trend detection . 39
10.5 Statistics . 40
10.6 Rainflow counting . 40
10.7 Cumulative rainflow spectrum . 40
10.8 Damage equivalent load. 40
10.9 Wind speed binning . 41
10.10 Power spectral density . 42
11 Uncertainty estimation . 42
12 Reporting. 42
Annex A (informative) Example co-ordinate systems . 46

– 4 – IEC 61400-13:2015 © IEC 2015
A.1 General . 46
A.2 Blade co-ordinate system . 46
A.3 Hub co-ordinate system . 46
A.4 Nacelle co-ordinate system . 47
A.5 Tower co-ordinate system . 48
A.6 Yaw misalignment . 49
A.7 Cone angle and tilt angle . 49
A.8 Rotor azimuth angle . 50
A.9 Blade pitch angle . 50
Annex B (informative) Procedure for the evaluation of uncertainties in load
measurements on wind turbines . 51
B.1 List of symbols . 51
B.2 General procedure . 52
B.2.1 Standard uncertainty . 52
B.2.2 Analytical combination of standard uncertainties . 53
B.2.3 Total uncertainty . 54
B.3 Uncertainties of binned averaged values . 55
B.3.1 General . 55
B.3.2 Uncertainty of calibration and signal . 55
B.3.3 Uncertainty of the bin scatter . 55
B.3.4 Uncertainty of the x-axis quantity . 55
B.3.5 Uncertainty of bin averaged mean values . 55
B.4 Standard uncertainty of DEL and load spectra . 56
B.5 Examples of an uncertainty evaluation . 56
B.5.1 Example for analytical shunt calibration of tower torque . 56
B.6 Determination and use of calibration matrix . 63
B.6.1 Determination of the calibration matrix . 63
B.6.2 Use of the calibration matrix . 64
B.6.3 Time series . 65
Annex C (informative) Sample presentation of mechanical load measurements and
analysis . 67
C.1 General . 67
Annex D (informative)  Recommendations for offshore measurements . 79
Annex E (informative) Load model validation . 81
E.1 General . 81
E.2 Methods for loads comparison . 82
E.2.1 Statistical binning . 82
E.2.2 Spectral functions . 83
E.2.3 Fatigue spectra . 84
E.2.4 Point by point . 84
Annex F (informative) Methods for identification of wind speed trends . 86
F.1 List of symbols . 86
F.2 General . 86
F.3 Trend identification methods . 87
F.4 Ongoing procedure . 91
Annex G (informative) Data acquisition considerations . 92
G.1 Data acquisition system . 92
G.1.1 General . 92

G.1.2 Resolution . 92
G.1.3 Sampling model and filtering . 93
G.1.4 Other considerations . 95
Annex H (informative) Load calibration . 96
H.1 General . 96
H.2 Gravity load calibration of the blade bending . 96
H.3 Analytical calibration of the tower bending moments . 97
H.4 External load calibration of the rotor torque . 98
Annex I (informative) Temperature drift . 99
I.1 General . 99
I.2 Known issues . 99
I.3 Recommendations . 100
Annex J (informative) Mechanical load measurements on vertical axis wind turbines . 101
J.1 General . 101
J.2 Terms and definitions . 101
J.3 Coordinate systems . 101
J.4 Quantities to be measured . 102
J.4.1 Fundamental loads . 102
J.5 Measurements . 103
J.5.1 Measurement of blade attachment bending moments . 103
J.5.2 Blade mid-span bending moment . 103
J.5.3 Blade modal frequency/damping . 103
J.5.4 Connecting strut bending moment. 103
J.5.5 Connecting strut axial force . 104
J.5.6 Connecting strut modal frequency/damping . 104
J.5.7 Rotor shaft torque . 104
J.5.8 Tower normal bending . 104
Bibliography . 105

Figure 1 – Fundamental wind turbine loads: tower base, rotor and blade loads . 24
Figure A.1 – Blade co-ordinate system . 46
Figure A.2 – Hub co-ordinate system . 47
Figure A.3 – Nacelle co-ordinate system . 48
Figure A.4 – Tower co-ordinate system . 48
Figure A.5 – Yaw misalignment . 49
Figure A.6 – Cone angle and tilt angle . 49
Figure B.1 – Explanation of used symbols . 61
Figure C.1 – Hub-height wind speed as a function of time . 67
Figure C.2 – Hub-height turbulence intensity as a function of hub-height wind speed . 68
Figure C.3 – Turbulence intensity trending as a function of hub-height wind speed . 68
Figure C.4 – Global capture matrix with all loads channels operating . 69
Figure C.5 – IEC example turbine at 9,1 m/s – Wind turbine operational and
meteorological quantities . 70
Figure C.6 – IEC example turbine at 9,1 m/s – Major load components . 71
Figure C.7 – 10-minute statistics for blade 1 root edge bending . 72
Figure C.8 – Power spectral density of blade 1 root edge bending . 73

– 6 – IEC 61400-13:2015 © IEC 2015
Figure C.9 – Cumulative rainflow spectrum for blade 1 root edge bending during test
period . 75
Figure C.10 – IEC example turbine normal shutdown at 9,5 m/s – Wind turbine
operational and meteorological quantities . 77
Figure C.11 – IEC example turbine normal shutdown at 9,5 m/s – Major load
components . 78
Figure D.1 – Example of wave spectrum and monopile response . 79
Figure D.2 – Example of wave spectrum . 80
Figure E.1 – Measured data . 82
Figure E.2 – Simulated data . 82
Figure E.3 – Comparison of wind speed binned averaged 10 min. statistics . 82
Figure E.4 – Comparison of 1 Hz equivalent loads . 83
Figure E.5 – Comparison of 1 Hz equivalent loads (wind speed binned) . 83
Figure E.6 – Comparison of PSD functions . 83
Figure E.7 – Comparison of fatigue spectra . 84
Figure E.8 – Point by point comparison of wind speed time histories . 85
Figure E.9 – Point by point comparison of load time histories. 85
Figure F.1 – Comparison of measured wind speed (v ), smoothing-filtered wind
meas
speed (v ) and resulting trend-free wind speed (v ) . 87
filt HP
Figure F.2 – Differences of turbulence intensities calculated with un-filtered and filtered
wind speed versus mean measured wind speed . 89
Figure F.3 – Ratio of turbulence intensities calculated with un-filtered and filtered wind
speed versus mean measured wind speed . 90
Figure G.1 – Anti-aliasing check . 93
Figure I.1 – Observed scatter in the original 10-min average values of the blade edge
moment together with the same signal after temperature compensation in dark blue . 99
Figure I.2 – Linear regression through the offsets derived from the different calibration
runs . 100
Figure J.1 – Darrieus style VAWT . 102
Figure J.2 – Helical Darrieus style VAWT . 102

Table 1 – MLCs during steady-state operation related to the DLCs defined in
IEC 61400-1 . 18
Table 2 – Measurement of transient load cases related to the DLCs defined in
IEC 61400-1 . 19
Table 3 – MLCs for dynamic characterization . 19
Table 4 – Capture matrix for normal power production for stall controlled wind turbines . 21
Table 5 – Capture matrix for normal power production for non stall controlled wind
turbines . 22
Table 6 – Capture matrix for parked condition . 22
Table 7 – Capture matrix for normal transient events . 23
Table 8 – Capture matrix for other than normal transient events . 23
Table 9 – Wind turbine fundamental load quantities . 24
Table 10 – Additional load quantities for turbines with a rated power output greater
than 1 500 kW and rotor diameter greater than 75 m . 25
Table 11 – Meteorological quantities . 25

Table 12 – Wind turbine operation quantities . 26
Table 13 – Summary of suitable calibration methods . 32
Table B.1 – Uncertainty components . 56
Table B.2 – Values and uncertainties for the calculation . 60
Table C.1 – Binned data for blade 1 root edge bending . 74
Table C.2 – Transient capture matrix for normal start-up and shutdown . 76
Table C.3 – Brief statistical description for normal shutdown for IEC example turbine
at 9,5 m/s . 76
Table G.1 – Wind turbine significant frequencies . 94
Table G.2 – Sampling ratio . 94
Table J.1 – Minimum recommendations for VAWT fundamental load quantities . 103

– 8 – IEC 61400-13:2015 © IEC 2015
INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
WIND TURBINES –
Part 13: Measurement of mechanical loads

FOREWORD
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This International Standard IEC 61400-13 has been prepared by IEC technical committee 88:
Wind turbines.
This standard replaces IEC TS 61400-13 published in 2001. This first edition constitutes a
technical revision and transition from technical specification to International Standard.
This first edition includes the following changes with respect to the technical specification:
a) scope of the document focused to load measurements for the purpose of model validation;
b) number of measurement load cases to match the new scope reduced;
c) capture matrix requirements to match the new scope reduced;
d) requirements to address the state of the art technology updated.

The text of this standard is based on the following documents:
CDV Report on voting
88/511/CDV 88/554/RVC
Full information on the voting for the approval of this standard can be found in the report on
voting indicated in the above table.
This publication has been drafted in accordance with the ISO/IEC Directives, Part 2.
A list of all parts in the IEC 61400 series, published under the general title Wind turbines, can
be found on the IEC website.
The committee has decided that the contents of this publication will remain unchanged until
the stability date indicated on the IEC website under "http://webstore.iec.ch" in the data
related to the specific publication. At this date, the publication will be
• reconfirmed,
• withdrawn,
• replaced by a revised edition, or
• amended.
IMPORTANT – The 'colour inside' logo on the cover page of this publication indicates
that it contains colours which are considered to be useful for the correct
understanding of its contents. Users should therefore print this document using a
colour printer.
– 10 – IEC 61400-13:2015 © IEC 2015
INTRODUCTION
In the process of structural design of a wind turbine, thorough understanding about, and
accurate quantification of, the loading is of utmost importance.
In the design stage, loads can be predicted with aeroelastic models and codes. However,
such models have their shortcomings and uncertainties, and they always need to be validated
by measurement.
Mechanical load measurements can be used both as the basis for design and as the basis for
certification. Design aspects for wind turbines are covered by IEC 61400-1 whilst certification
procedures are described in IEC 61400-22. This standard is aimed at the test institute, the
turbine manufacturer and the certifying body and clearly defines the minimum requirements
for a mechanical loads test resulting in consistent, high quality reproducible test results.

WIND TURBINES –
Part 13: Measurement of mechanical loads

1 Scope
This part of the IEC 61400 describes the measurement of fundamental structural loads on
wind turbines for the purpose of the load simulation model validation. The standard prescribes
the requirements and recommendations for site selection, signal selection, data acquisition,
calibration, data verification, measurement load cases, capture matrix, post-processing,
uncertainty determination and reporting. Informative annexes are also provided to improve
understanding of testing methods.
The methods described in this document can also be used for mechanical loads
measurements for other purposes such as obtaining a measured statistical representation of
loads, direct measurements of the design loads, safety and function testing, or measurement
of component loads. If these methods are used for an alternative objective or used for an
unconventional wind turbine design, the required signals, measurement load cases, capture
matrix, and post processing methods should be evaluated and if needed adjusted to fit the
objective.
These methods are intended for onshore electricity-generating, horizontal-axis wind turbines
(HAWTs) with rotor swept areas of larger than 200 m . However, the methods described may
be applicable to other wind turbines (for example, small wind turbines, ducted wind turbines,
vertical axis wind turbines).
2 Normative references
The following documents, in whole or in part, are normatively referenced in this document and
are indispensable for its application. For dated references, only the edition cited applies. For
undated references, the latest edition of the referenced document (including any
amendments) applies.
IEC 60050 (all parts), International Electrotechnical Vocabulary (available at
)
IEC 61400-1:2005, Wind turbines – Part 1: Design requirements
IEC 61400-12-1, Wind turbines – Part 12-1: Power performance measurements of electricity
producing wind turbines
ISO/IEC Guide 98-3, Uncertainty of measurement – Part 3: Guide to the expression of
uncertainty in measurement
3 Terms and definitions
For the purposes of this document, the terms and definitions related to wind turbine systems
or wind energy in general of IEC 60050-415 as well as the following apply.
3.1
blade
rotating aerodynamically active part of the rotor

– 12 – IEC 61400-13:2015 © IEC 2015
3.2
blade root
that part of the blade that is connected to the hub of the rotor
3.3
brake status
status indicating if the brake is applied or not
3.4
calibration
determination of the transfer function and its coefficients from sensor output to the physical
value
3.5
capture matrix
organization of the measured time series according to their mean wind speeds and turbulence
intensities
3.6
chord line
imaginary straight line that joins the leading and trailing edges of a blade airfoil cross-section
3.7
cut-in wind speed
lowest wind speed at hub height at which the wind turbine starts to produce power in the case
of ste
...

IEC 61400-13 ®
Edition 1.1 2021-12
CONSOLIDATED VERSION
INTERNATIONAL
STANDARD
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INTERNATIONALE
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Wind turbines –
Part 13: Measurement of mechanical loads

Éoliennes –
Partie 13: Mesurage des charges mécaniques

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IEC 61400-13 ®
Edition 1.1 2021-12
CONSOLIDATED VERSION
INTERNATIONAL
STANDARD
NORME
INTERNATIONALE
colour
inside
Wind turbines –
Part 13: Measurement of mechanical loads
Éoliennes –
Partie 13: Mesurage des charges mécaniques
INTERNATIONAL
ELECTROTECHNICAL
COMMISSION
COMMISSION
ELECTROTECHNIQUE
INTERNATIONALE
ICS 27.180 ISBN 978-2-8322-5001-3

IEC 61400-13 ®
Edition 1.1 2021-12
CONSOLIDATED VERSION
REDLINE VERSION
VERSION REDLINE
colour
inside
Wind turbines –
Part 13: Measurement of mechanical loads

Éoliennes –
Partie 13: Mesurage des charges mécaniques

– 2 – IEC 61400-13:2015+AMD1:2021 CSV
© IEC 2021
CONTENTS
FOREWORD . 8
INTRODUCTION . 10
INTRODUCTION to Amendment 1 . 10
1 Scope . 11
2 Normative references . 11
3 Terms and definitions . 11
4 Symbols, units and abbreviations . 14
5 General . 16
5.1 Document structure . 16
5.2 Safety during testing . 17
6 Test requirements . 17
6.1 General . 17
6.2 Test site requirements . 17
6.3 Measurement load cases . 17
6.3.1 General . 17
6.3.2 MLCs during steady-state operation . 18
6.3.3 MLCs during transient events . 18
6.3.4 MLCs for dynamic characterization . 19
6.3.5 Capture matrices . 20
6.4 Quantities to be measured . 23
6.4.1 General . 23
6.4.2 Load quantities . 23
6.4.3 Meteorological quantities . 25
6.4.4 Wind turbine operation quantities . 25
6.5 Turbine configuration changes . 26
7 Instrumentation . 27
7.1 Load quantities . 27
7.1.1 Types of sensors . 27
7.1.2 Choice of sensor location . 27
7.1.3 Measurement of blade root bending moments . 27
7.1.4 Blade bending moment distribution . 28
7.1.5 Blade torsion frequency/damping . 28
7.1.6 Measurement of rotor yaw and tilt moment . 28
7.1.7 Measurement of the rotor torque . 28
7.1.8 Measurement of tower base bending . 28
7.1.9 Tower top bending moments . 28
7.1.10 Tower mid bending moments . 29
7.1.11 Tower torque . 29
7.1.12 Tower top acceleration . 29
7.1.13 Pitch actuation loads (on hub side of pitch bearing) . 29
7.2 Meteorological quantities . 29
7.2.1 Measurement and installation requirements . 29
7.2.2 Icing potential . 29
7.2.3 Atmospheric stability . 29
7.3 Wind turbine operation quantities . 30

© IEC 2021
7.3.1 Electrical power . 30
7.3.2 Rotor speed or generator speed . 30
7.3.3 Yaw misalignment . 30
7.3.4 Rotor azimuth angle. 30
7.3.5 Pitch position . 30
7.3.6 Pitch speed . 30
7.3.7 Brake moment . 30
7.3.8 Wind turbine status . 30
7.3.9 Brake status . 30
7.4 Data acquisition system . 31
7.4.1 General . 31
7.4.2 Resolution . 31
7.4.3 Anti-aliasing. 31
8 Determination of calibration factors . 31
8.1 General . 31
8.2 Calibration of load channels . 32
8.2.1 General . 32
8.2.2 Blade bending moments . 33
8.2.3 Main shaft moments . 33
8.2.4 Tower bending moments . 34
8.2.5 Tower torque . 34
8.3 Calibration of non-load channels . 35
8.3.1 Pitch angle . 35
8.3.2 Rotor azimuth angle. 35
8.3.3 Yaw angle. 35
8.3.4 Wind direction. 35
8.3.5 Pitch actuation loads . 35
8.3.6 Brake moment . 36
9 Data verification . 36
9.1 General . 36
9.2 Verification checks . 36
9.2.1 General . 36
9.2.2 Blade moments . 37
9.2.3 Main shaft . 38
9.2.4 Tower . 38
10 Processing of measured data . 39
10.1 General . 39
10.2 Fundamental load quantities . 39
10.3 Load quantities for larger turbines . 39
10.4 Wind speed trend detection . 39
10.5 Statistics . 40
10.6 Rainflow counting . 40
10.7 Cumulative rainflow spectrum . 40
10.8 Damage equivalent load. 40
10.9 Wind speed binning . 41
10.10 Power spectral density . 42
11 Uncertainty estimation . 42
12 Reporting. 42

– 4 – IEC 61400-13:2015+AMD1:2021 CSV
© IEC 2021
Annex A (informative) Example co-ordinate systems . 46
A.1 General . 46
A.2 Blade co-ordinate system . 46
A.3 Hub co-ordinate system . 46
A.4 Nacelle co-ordinate system . 47
A.5 Tower co-ordinate system . 48
A.6 Yaw misalignment . 49
A.7 Cone angle and tilt angle . 49
A.8 Rotor azimuth angle . 50
A.9 Blade pitch angle . 50
Annex B (informative) Procedure for the evaluation of uncertainties in load
measurements on wind turbines . 51
B.1 List of symbols . 51
B.2 General procedure . 52
B.2.1 Standard uncertainty . 52
B.2.2 Analytical combination of standard uncertainties . 53
B.2.3 Total uncertainty . 54
B.3 Uncertainties of binned averaged values . 55
B.3.1 General . 55
B.3.2 Uncertainty of calibration and signal . 55
B.3.3 Uncertainty of the bin scatter . 55
B.3.4 Uncertainty of the x-axis quantity . 55
B.3.5 Uncertainty of bin averaged mean values . 55
B.4 Standard uncertainty of DEL and load spectra . 56
B.5 Examples of an uncertainty evaluation . 56
B.5.1 Example for analytical shunt calibration of tower torque . 56
B.6 Determination and use of calibration matrix . 65
B.6.1 Determination of the calibration matrix . 65
B.6.2 Use of the calibration matrix . 66
B.6.3 Time series . 67
Annex C (informative) Sample presentation of mechanical load measurements and
analysis . 69
C.1 General . 69
Annex D (informative) Recommendations for offshore measurements . 81
Annex E (informative) Load model validation . 83
E.1 General . 83
E.2 Methods for loads comparison . 84
E.2.1 Statistical binning . 84
E.2.2 Spectral functions . 85
E.2.3 Fatigue spectra . 86
E.2.4 Point by point . 86
Annex F (informative) Methods for identification of wind speed trends . 88
F.1 List of symbols . 88
F.2 General . 88
F.3 Trend identification methods . 89
F.4 Ongoing procedure . 93
Annex G (informative) Data acquisition considerations . 94
G.1 Data acquisition system . 94

© IEC 2021
G.1.1 General . 94
G.1.2 Resolution . 94
G.1.3 Sampling model and filtering . 95
G.1.4 Other considerations . 97
Annex H (informative) Load calibration . 98
H.1 General . 98
H.2 Gravity load calibration of the blade bending . 98
H.3 Analytical calibration of the tower bending moments . 99
H.4 External load calibration of the rotor torque . 100
Annex I (informative) Temperature drift . 101
I.1 General . 101
I.2 Known issues . 101
I.3 Recommendations . 102
Annex J (informative) Mechanical load measurements on vertical axis wind turbines . 103
J.1 General . 103
J.2 Terms and definitions . 103
J.3 Coordinate systems . 103
J.4 Quantities to be measured . 104
J.4.1 Fundamental loads . 104
J.5 Measurements . 105
J.5.1 Measurement of blade attachment bending moments . 105
J.5.2 Blade mid-span bending moment . 105
J.5.3 Blade modal frequency/damping . 105
J.5.4 Connecting strut bending moment. 105
J.5.5 Connecting strut axial force . 106
J.5.6 Connecting strut modal frequency/damping . 106
J.5.7 Rotor shaft torque . 106
J.5.8 Tower normal bending . 106
Bibliography . 107

Figure 1 – Fundamental wind turbine loads: tower base, rotor and blade loads . 24
Figure A.1 – Blade co-ordinate system . 46
Figure A.2 – Hub co-ordinate system . 47
Figure A.3 – Nacelle co-ordinate system . 48
Figure A.4 – Tower co-ordinate system . 48
Figure A.5 – Yaw misalignment . 49
Figure A.6 – Cone angle and tilt angle . 49
Figure B.1 – Explanation of used symbols . 62
Figure C.1 – Hub-height wind speed as a function of time . 69
Figure C.2 – Hub-height turbulence intensity as a function of hub-height wind speed . 70
Figure C.3 – Turbulence intensity trending as a function of hub-height wind speed . 70
Figure C.4 – Global capture matrix with all loads channels operating . 71
Figure C.5 – IEC example turbine at 9,1 m/s – Wind turbine operational and
meteorological quantities . 72
Figure C.6 – IEC example turbine at 9,1 m/s – Major load components . 73
Figure C.7 – 10-minute statistics for blade 1 root edge bending . 74

– 6 – IEC 61400-13:2015+AMD1:2021 CSV
© IEC 2021
Figure C.8 – Power spectral density of blade 1 root edge bending . 75
Figure C.9 – Cumulative rainflow spectrum for blade 1 root edge bending during test
period . 77
Figure C.10 – IEC example turbine normal shutdown at 9,5 m/s – Wind turbine
operational and meteorological quantities . 79
Figure C.11 – IEC example turbine normal shutdown at 9,5 m/s – Major load
components . 80
Figure D.1 – Example of wave spectrum and monopile response . 81
Figure D.2 – Example of wave spectrum . 82
Figure E.1 – Measured data . 84
Figure E.2 – Simulated data . 84
Figure E.3 – Comparison of wind speed binned averaged 10 min. statistics . 84
Figure E.4 – Comparison of 1 Hz equivalent loads . 85
Figure E.5 – Comparison of 1 Hz equivalent loads (wind speed binned) . 85
Figure E.6 – Comparison of PSD functions . 85
Figure E.7 – Comparison of fatigue spectra . 86
Figure E.8 – Point by point comparison of wind speed time histories . 87
Figure E.9 – Point by point comparison of load time histories. 87
Figure F.1 – Comparison of measured wind speed (v ), smoothing-filtered wind
meas
speed (v ) and resulting trend-free wind speed (v ) . 89
filt HP
Figure F.2 – Differences of turbulence intensities calculated with un-filtered and filtered
wind speed versus mean measured wind speed . 91
Figure F.3 – Ratio of turbulence intensities calculated with un-filtered and filtered wind
speed versus mean measured wind speed . 92
Figure G.1 – Anti-aliasing check . 95
Figure I.1 – Observed scatter in the original 10-min average values of the blade edge
moment together with the same signal after temperature compensation in dark blue . 101
Figure I.2 – Linear regression through the offsets derived from the different calibration
runs . 102
Figure J.1 – Darrieus style VAWT . 104
Figure J.2 – Helical Darrieus style VAWT . 104

Table 1 – MLCs during steady-state operation related to the DLCs defined in
IEC 61400-1 . 18
Table 2 – Measurement of transient load cases related to the DLCs defined in
IEC 61400-1 . 19
Table 3 – MLCs for dynamic characterization . 19
Table 4 – Capture matrix for normal power production for stall controlled wind turbines . 21
Table 5 – Capture matrix for normal power production for non stall controlled wind
turbines . 22
Table 6 – Capture matrix for parked condition . 22
Table 7 – Capture matrix for normal transient events . 23
Table 8 – Capture matrix for other than normal transient events . 23
Table 9 – Wind turbine fundamental load quantities . 24
Table 10 – Additional load quantities for turbines with a rated power output greater
than 1 500 kW and rotor diameter greater than 75 m . 25

© IEC 2021
Table 11 – Meteorological quantities . 25
Table 12 – Wind turbine operation quantities . 26
Table 13 – Summary of suitable calibration methods . 32
Table B.1 – Uncertainty components . 57
Table B.2 – Values and uncertainties for the calculation . 61
Table C.1 – Binned data for blade 1 root edge bending . 76
Table C.2 – Transient capture matrix for normal start-up and shutdown . 78
Table C.3 – Brief statistical description for normal shutdown for IEC example turbine
at 9,5 m/s . 78
Table G.1 – Wind turbine significant frequencies . 96
Table G.2 – Sampling ratio . 96
Table J.1 – Minimum recommendations for VAWT fundamental load quantities . 105

– 8 – IEC 61400-13:2015+AMD1:2021 CSV
© IEC 2021
INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
WIND TURBINES –
Part 13: Measurement of mechanical loads

FOREWORD
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This consolidated version of the official IEC Standard and its amendment has been
prepared for user convenience.
IEC 61400-13 edition 1.1 contains the first edition (2015-12) [documents 88/511/CDV and
88/554/RVC] and its amendment 1 (2021-12) [documents 88/795/CDV and 88/821/RVC].
In this Redline version, a vertical line in the margin shows where the technical content
is modified by amendment 1. Additions are in green text, deletions are in strikethrough
red text. A separate Final version with all changes accepted is available in this
publication.
© IEC 2021
This International Standard IEC 61400-13 has been prepared by IEC technical committee 88:
Wind turbines.
This standard replaces IEC TS 61400-13 published in 2001. This first edition constitutes a
technical revision and transition from technical specification to International Standard.
This first edition includes the following changes with respect to the technical specification:
a) scope of the document focused to load measurements for the purpose of model validation;
b) number of measurement load cases to match the new scope reduced;
c) capture matrix requirements to match the new scope reduced;
d) requirements to address the state of the art technology updated.
This publication has been drafted in accordance with the ISO/IEC Directives, Part 2.
A list of all parts in the IEC 61400 series, published under the general title Wind turbines, can
be found on the IEC website.
The committee has decided that the contents of the base publication and its amendment will
remain unchanged until the stability date indicated on the IEC web site under webstore.iec.ch
in the data related to the specific publication. At this date, the publication will be
• reconfirmed,
• withdrawn,
• replaced by a revised edition, or
• amended.
IMPORTANT – The 'colour inside' logo on the cover page of this publication indicates
that it contains colours which are considered to be useful for the correct
understanding of its contents. Users should therefore print this document using a
colour printer.
– 10 – IEC 61400-13:2015+AMD1:2021 CSV
© IEC 2021
INTRODUCTION
In the process of structural design of a wind turbine, thorough understanding about, and
accurate quantification of, the loading is of utmost importance.
In the design stage, loads can be predicted with aeroelastic models and codes. However,
such models have their shortcomings and uncertainties, and they always need to be validated
by measurement.
Mechanical load measurements can be used both as the basis for design and as the basis for
certification. Design aspects for wind turbines are covered by IEC 61400-1 whilst certification
procedures are described in IEC 61400-22. This standard is aimed at the test institute, the
turbine manufacturer and the certifying body and clearly defines the minimum requirements
for a mechanical loads test resulting in consistent, high quality reproducible test results.

INTRODUCTION to Amendment 1
This amendment to IEC 61400-13:2015 addresses the errors found in Annex B which impact a
significant portion of that annex.

© IEC 2021
WIND TURBINES –
Part 13: Measurement of mechanical loads

1 Scope
This part of the IEC 61400 describes the measurement of fundamental structural loads on
wind turbines for the purpose of the load simulation model validation. The standard prescribes
the requirements and recommendations for site selection, signal selection, data acquisition,
calibration, data verification, measurement load cases, capture matrix, post-processing,
uncertainty determination and reporting. Informative annexes are also provided to improve
understanding of testing methods.
The methods described in this document can also be used for mechanical loads
measurements for other purposes such as obtaining a measured statistical representation of
loads, direct measurements of the design loads, safety and function testing, or measurement
of component loads. If these methods are used for an alternative objective or used for an
unconventional wind turbine design, the required signals, measurement load cases, capture
matrix, and post processing methods should be evaluated and if needed adjusted to fit the
objective.
These methods are intended for onshore electricity-generating, horizontal-axis wind turbines
(HAWTs) with rotor swept areas of larger than 200 m . However, the methods described may
be applicable to other wind turbines (for example, small wind turbines, ducted wind turbines,
vertical axis wind turbines).
2 Normative references
The following documents, in whole or in part, are normatively referenced in this document and
are indispensable for its application. For dated references, only the edition cited applies. For
undated references, the latest edition of the referenced document (including any
amendments) applies.
IEC 60050 (all parts), International Electrotechnical Vocabulary (available at
)
IEC 61400-1:2005, Wind turbines – Part 1: Design requirements
IEC 61400-12-1, Wind turbines – Part 12-1: Power performance measurements of electricity
producing wind turbines
ISO/IEC Guide 98-3, Uncertainty of measurement – Part 3: Guide to the expression of
uncertainty in measurement
3 Terms and definitions
For the purposes of this document, the terms and definitions related to wind turbine systems
or wind energy in general of IEC 60050-415 as well as the following apply.
3.1
blade
rotating aerodynamically active part of the rotor

– 12 – IEC 61400-13:2015+AMD1:2021 CSV
© IEC 2021
3.2
blade root
that part of the blade that is connected to the hub of the rotor
3.3
brake status
status indicating if the brake is applied or not
3.4
calibration
determination of the transfer function and its coefficients from sensor output to the physical
value
3.5
capture matrix
organization of the measured time series accor
...