ISO 20816-21:2025

International
Standard
ISO 20816-21
First edition
Mechanical vibration —
2025-05
Measurement and evaluation of
machine vibration —
Part 21:
Horizontal axis wind turbines
Vibrations mécaniques — Mesurage et évaluation des vibrations
des machines —
Partie 21: Turbines éoliennes à axe horizontal
Reference number
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Contents Page
Foreword .v
Introduction .vi
1 Scope . 1
2 Normative references . 2
3 Terms and definitions . 2
4 Measurement procedures . 2
4.1 General .2
4.2 Measurement positions .3
4.3 Measurement equipment requirements .3
4.4 Vibration transducer mounting and connection .4
4.5 Measurement and assessment of vibration quantities .4
4.5.1 General .4
4.5.2 Bandpass frequency ranges .5
4.5.3 Broad-band assessment of vibration values .6
4.5.4 Evaluation period .6
4.6 Formation of assessment vibration quantities .7
4.7 Operating conditions prevailing when taking measurements .8
5 Measurements and interpretations . 8
5.1 General .8
5.2 Nacelle and tower .9
5.2.1 General .9
5.2.2 Assessment vibration quantities .9
5.2.3 Typical measurement positions .9
5.2.4 Measurement directions for the nacelle .9
5.3 Main rotor (main shaft) .10
5.3.1 Assessment vibration quantities .10
5.3.2 Typical measurement positions .10
5.3.3 Measurement directions .10
5.4 Main gearbox .10
5.4.1 Assessment vibration quantities .10
5.4.2 Measurement positions for wind turbines with separately mounted gearboxes
with integrated rotor bearings .10
5.4.3 Measurement directions .11
5.5 Generators in wind turbines with a gearbox .11
5.5.1 Assessment vibration quantities .11
5.5.2 Typical measurement positions .11
5.5.3 Measurement directions .11
5.6 Generators in direct drive wind turbines.11
5.6.1 Assessment vibration quantities .11
5.6.2 Typical measurement positions . 12
5.6.3 Measurement directions . 12
6 Evaluation criteria .12
6.1 General . 12
6.2 Evaluation method for different wind turbine designs . 12
6.3 Evaluation zones . 13
6.4 Changes in vibration magnitude. 13
6.5 Condition monitoring and diagnostics . 13
6.6 Evaluation zone boundaries .14
7 Setting operational limits . 14
7.1 General .14
7.2 Definition of ALERT limits . 15
7.3 Definition of the ALARM limits . 15

iii
7.4 TRIP limits . . 15
Annex A (informative) Zone boundary evaluation .16
Annex B (informative) Wind turbine working principles.18
Annex C (informative) Diagrams of two typical wind turbine designs with gearbox .20
Annex D (informative) Diagrams of typical direct drive wind turbine designs .22
Annex E (informative) Measurement protocol example .26
Bibliography .28

iv
Foreword
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This document was prepared by Technical Committee ISO/TC 108, Mechanical vibration, shock and condition
monitoring, Subcommittee SC 2, Measurement and evaluation of mechanical vibration and shock as applied to
machines, vehicles and structures.
This first edition of ISO 20816-21 cancels and replaces the first edition of ISO 10816-21:2015, which has
been technically revised taking into account the information contained in VDI 3834-2 for direct drive wind
turbines.
The main changes are as follows:
— scope expansion to include Group 2 wind turbine without gearboxes;
— revision of text and content so that the document can be applied to both groups of wind turbines;
— vibrations of different types of wind turbines are clearly identified and figures show measurement
positions (see Annex C);
— revision and standardization of assessment tables for both types of wind turbines (see Annex A);
— clarification of descriptions of technical functionality of wind turbines (see Annex B);
— added a proposal for protocols for acceptance measurements (see Annex D);
— updated and supplemented information on relevant documents (see Bibliography).
The list of all parts in the ISO 20816 series and ISO 10816 series can be found on the ISO website.
Any feedback or questions on this document should be directed to the user’s national standards body. A
complete listing of these bodies can be found at www.iso.org/members.html.

v
Introduction
The ISO 20816 series of documents provides general guidance relating to the measurement and
evaluation of vibration measured on rotating and non-rotating parts of machines. This document provides
recommendations for the measurement and evaluation criteria for wind turbines. Additional guidance is
given to account for the vibration of the wind turbine tower and nacelle caused by the effects of wind, flow
disturbances due to the tower (tower dam effect) and the natural vibration of the rotor blades and structure
itself (tower and foundation). For offshore wind turbines, guidance is given to take account of the vibration
response due to sea swell, which differs from the response of land-based wind turbines in terms of the
temporal vibration behaviour and spectra.
ISO 20816-1 deals with the measurement and evaluation of machine vibration and serves as the basis for
other documents dealing with specific considerations for different machine types, including wind turbines.
In contrast to the other parts of the ISO 20816 series, this document considers vibration not generated by
the machine itself, but excited by wind and, in case of offshore wind turbines, additionally by sea swell. Due
to the flexibility of the blades and tower and the low rotor rotational speeds, it is necessary to include the
low-frequency vibration in the measurement and evaluation criteria.
The construction methods used and the conditions under which wind turbines are built and operated, entail
that presenting general guidelines for the vibration evaluation of their structures is not possible. However,
the measurement procedures and criteria for evaluating the vibration of their structures explained in
ISO 4866 can be used for guidance. Furthermore, offshore wind turbine structural vibration monitoring is
covered in VDI 4551.
The necessity to measure and evaluate the low-frequency vibration of wind turbine components as a
response to periodic and stochastic excitation sources requires additional evaluation of quantities in
addition to those described in this document (see 4.5). This is complicated by the effects of wind and waves
on the wind turbine structure, which can lead to high-amplitude low-frequency vibration.
Due to the influence of the vibration magnitude on the induced stresses of all wind turbine components and
thus on their operational reliability and service life, it is in the interest of all stakeholders involved in the
manufacture, ownership, operation, service, maintenance, and financing of wind turbines to have a widely
accepted document, which provides clear criteria and recommendations regarding the measurement and
evaluation of their mechanical vibration.
The aim of this document is to standardize the vibration measurements to be performed, to facilitate their
evaluation and to allow a comparative assessment to be made of the measured vibration between different
wind turbines and their components. This document does not address the diagnostic evaluation of the
condition of the wind turbine components themselves. The evaluation criteria described in this document
serve to ensure reliable, safe long-term wind turbine operation.
In addition, evaluation zone boundaries are set, which enable conclusions to be drawn regarding the general
state of wind turbine components or the overall system. Evaluation zone boundary values are not intended
to be used as contractual acceptance values.

vi
International Standard ISO 20816-21:2025(en)
Mechanical vibration — Measurement and evaluation of
machine vibration —
Part 21:
Horizontal axis wind turbines
1 Scope
This document provides information regarding the measurement and evaluation of the mechanical vibration
of wind turbines and their components. The working principles of wind turbines covered by this document
are described in Annex B.
The installation site and type of mechanical drive train of the wind turbine influence the vibration
magnitude, so for the purposes of this document wind turbines have been divided into two groups:
a) Group 1: Horizontal axis wind turbine installations with generators coupled to the rotor via a gearbox; and
b) Group 2: Horizontal axis wind turbine installations with generators coupled to the rotor without a
gearbox (direct drive wind turbines).
The requirements of this document apply to both Group 1 and Group 2 wind turbines with a rated generator
output exceeding 200 kW.
This document recommends zones for evaluating the vibration at continuous load operation. However,
in most cases these evaluation zone boundaries are not suitable for the early detection of faults. Annex A
presents evaluation zone boundaries based on vibration data collected from thousands of wind turbines
with rated generator outputs of 5 MW or less, which can be helpful in facilitating discussion between users
and manufacturers when considering early fault detection.
The evaluation criteria described in this document serve to ensure safe, reliable, long-term operation of the
wind turbine and its components. It is intended to standardize the vibration measurements taken, to assist
in their evaluation and to facilitate a comparative evaluation of the vibration measured in wind turbines and
their components. In addition, recommendations are given for the determination of operational vibration
limit values.
The type and implementation of broad-band vibration monitoring methods to be used for wind turbines are
addressed in this document, along with evaluation criteria for assessing vibration severity. This document
does not address diagnosis or fault detection, although the measurement equipment described can be used
for vibration monitoring.
NOTE 1 For information regarding vibration condition monitoring see the ISO 13373 series. For Information
regarding condition monitoring and diagnostics of wind turbines see the ISO 16079 series.
NOTE 2 IEC 61400-13 describes load measurements that can be taken using strain gauges mounted on the wind
turbine support structure and blades. For procedures to assist the detection of rolling bearing and gearbox defects
see ISO 13373-2. For the measurement and evaluation of structure-borne noise in wind turbines fitted with rolling
bearings see VDI 3832.
NOTE 3 Evaluation of the unbalance of the slowly turning wind turbine rotor requires the use of measurement
techniques and analysis which consider both mass and aerodynamic unbalance. To assess the influence of the rotor
unbalance on vibrations see VDI 3834-1:2015-08, Annex B.

The requirements described in this document do not apply to the acceptance measurements for wind turbine
gearboxes and generators taken in the manufacturer’s test facility.
NOTE 4 Acceptance measurements for wind turbine gearboxes and generators, taken in the manufacturers test
facility, are assessed as described in ISO 20816-9 and/or IEC 60034-14.
This document does not provide evaluation zones for vibration measurements taken on rotating parts (shaft
relative displacement) due to the small number of turbines on which such measurements are/have been taken.
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.
ISO 2041, Mechanical vibration, shock and condition monitoring — Vocabulary
ISO 2954, Mechanical vibration of rotating and reciprocating machinery — Requirements for instruments for
measuring vibration severity
ISO 20816-1:2016, Mechanical vibration — Measurement and evaluation of machine vibration — Part 1: General
guidelines
3 Terms and definitions
For the purposes of this document, the terms and definitions given in ISO 2041 and the following apply.
ISO and IEC maintain terminology databases for use in standardization at the following addresses:
— ISO Online browsing platform: available at https:// www .iso .org/ obp
— IEC Electropedia: available at https:// www .electropedia .org/
3.1
assessment acceleration
a
w0
broad-band root-mean-square (r.m.s.) value of acceleration in a given frequency band, measured over a
fixed period
3.2
assessment velocity
v
w0
broad-band root-mean-square (r.m.s.) value of vibration velocity in a given frequency band, measured over
a fixed period
4 Measurement procedures
4.1 General
The measurement procedures to be followed and the instrumentation which shall be used are specified in
ISO 20816-1, subject to the requirements given in this clause.
Care shall be taken to ensure that the measurement system is not influenced by environmental factors such as
a) temperature variation,
b) magnetic fields, including magnetisation of the wind turbine shaft(s),
c) external sound fields,
d) power source variation,
e) transducer cable runs and faults, and
f) transducer orientation.
Particular attention shall be paid to ensure that the vibration transducers are correctly mounted and that
such mountings do not degrade the accuracy of the measurements taken.
4.2 Measurement positions
For wind turbines it is common to measure vibration only on non-rotating parts. Where vibration
measurements cannot be obtained from sensors mounted on the drive train, selecting sensor positions that
represent the vibration of the entire structure can be necessary.
Measurements shall be taken on the bearings, bearing support housing and structural parts of the wind
turbine which significantly respond to the dynamic forces applied and characterize the overall vibration
of the machine, nacelle and tower. In some wind turbines it is not possible to access the bearing housings
directly. In these cases, the measurements taken shall reasonably represent the vibration of the bearing
housing and do not include any local resonances or amplification. It can be necessary to confirm the
repeatability and validity of the measurements taken from the chosen sensor positions (e.g. by taking
measurements in several different positions and comparing the results). If the manufacturer has identified
suitable positions to mount vibration transducers on their wind turbine, they should be used.
The chosen sensor positions and directions of vibration measurement used on the drive train shall provide
adequate sensitivity to the machine responses and any external dynamic forces present. Two orthogonal
transducers, preferably in the vertical and horizontal directions, can be used. Alternatively, a single sensor
can be used, if it provides adequate information regarding the severity of the machine vibration.
When considering wind turbine structural vibration and aerodynamic effects, it is recommended to
additionally measure in the axial direction of the rotor.
For gearboxes, a single transducer can often be used to detect vibration characteristics from multiple
bearings or shafts. However, the vibration values taken from a single transducer do not always provide a
reasonable approximation to the maximum vibration value for all the components considered (see Clause 5).
For the appropriate measurement positions for measuring bearing housing vibration see Figure C.1 and
Figure C.2 for wind turbines with gearboxes and Figure D.1 to Figure D.4 for direct drive wind turbines.
4.3 Measurement equipment requirements
The equipment used shall be capable of measuring broad-band r.m.s. vibration with a flat response over a
selectable set of frequency ranges. These frequency ranges shall include:
a) 0,1 Hz to 10 Hz;
b) 10 Hz to 1 000 Hz;
c) 10 Hz to 2 000 Hz;
d) 10 Hz to 5 000 Hz.
Measuring instruments used shall meet the requirements of ISO 2954.
Vibration transducers shall be used which enable the entire measurement chain to meet the limits of the
measurement frequency ranges as required in 4.5.
In the lowest frequency range, calibration at 0,1 Hz is not generally available. So it is recommended to use
transducers which can be calibrated using acceleration due to gravity, such as micro-electro-mechanical-
systems (MEMS) acceleration sensors. ISO 16063-16 describes the calibration of the transmission coefficient
at 0 Hz with reference to the local acceleration due to gravity.

If piezoelectric accelerometers are used for the lowest frequency range, the relevant rotor blade and
structural frequencies shall have a sufficiently high separation from the lower 3 dB cut-off frequencies of the
transducer and ideally lie within 10 % of the linear part of the filter curve.
Other vibration transducers (e.g. electrodynamic vibration velocity transducers, piezoresistive
accelerometers, strain gauges or fibre optic transducers) can be used. However, the suitability for use in
the given wind turbine shall be evaluated and the measurement shall be calibrated in accordance with the
requirements of ISO 2954.
Vibration velocity is usually determined using accelerometers. The accelerometer output signal shall be
proportional to acceleration over the entire measurement frequency range and shall be integrated once
to give the vibration velocity. The measurement uncertainty of the vibration velocity shall lie within the
limits specified in this document. Reasons for measurement uncertainties are, e.g. pyroelectrical effects and
insufficient settling periods for the long integration periods that are needed in the low frequency range. All
vibration sensors shall be selected with a working range that encompasses the environmental conditions
expected at the installation position.
The measurements required for vibration evaluation can be taken using on-line or off-line systems. On-
line systems are permanently installed in the wind turbine and automatically carry out data collection and
characteristic quantity calculation and are used for continuous monitoring of the drive train condition as
part of the overall condition monitoring system. Off-line systems are portable for periodic monitoring and
are used for taking short-term measurements, being operated manually.
Both on-line and off-line systems can be used for evaluating wind turbine vibration. If the measurement
results shall be compared with the evaluation zone boundaries, they shall meet the requirements of this
document regarding the characteristic vibration quantities to be measured and the measurement positions,
directions and operating conditions.
4.4 Vibration transducer mounting and connection
The attachment of the transducers shall be appropriate to the wide vibration frequency range, the highest
vibration values and most extreme environmental conditions experienced in the wind turbine. The amplitude
and frequency response of a transducer can be affected by the attachment method. For a description of the
advantages and disadvantages of the various methods used for mounting the transducers see ISO 5348.
Where high frequency vibration is expected, the most rigid mounting methods possible shall be used to
attach the vibration transducer. If brackets are necessary to properly mount the transducer, they shall be
sufficiently rigid, so that their natural frequency shall not be excited within the normal running range of the
wind turbine. If this is uncertain, testing can be performed (e.g. impact tests).
4.5 Measurement and assessment of vibration quantities
4.5.1 General
In all cases, the vibration acceleration and vibration velocity shall be evaluated. During the vibration
measurement period, the wind speed, load and their variation shall be recorded. Acceleration captures the
shock-shaped, high-frequency loads of structure-borne noise and low-frequency force impacts. Velocity
captures the vibration energy of the wind turbine structure and drive train masses, which predominantly
act on the components in the frequency range 0,1 Hz to 1 000 Hz.
Vibration measurement and the monitoring of rotating parts (i.e. shaft vibration measurement) is rare in
wind turbines because the drive trains predominantly have rolling bearings rather than plain bearings. In
some direct drive wind turbines, the air gap between the generator rotor and stator is monitored by taking a
relative displacement measurement. However, this procedure is not within the scope of this document.
Assessment vibration values (see 3.1 and 3.2) can be obtained from the raw measured quantity by carrying
out the procedures described in this document.

4.5.2 Bandpass frequency ranges
The evaluation of assessment vibration quantities first requires an appropriate bandpass filter to be used.
The vibration of the tower, nacelle and blades of a wind turbine are caused and influenced e.g. by
a) the effect of the incident wind,
b) flow disturbances in the incident wind (e.g. due to other wind turbines or topographical features),
c) flexibility of the blades and tower,
d) low natural frequencies of the blades and tower, and
e) mass unbalance and aerodynamic loads (e.g. vertical wind profile for the blades, tower dam and
tower wake).
Consequently, low frequency vibration shall be considered when drafting the measurement and evaluation
procedures.
In addition, the structural response to the excitation frequencies of interest (e.g. due to slowly rotating parts
such as the first planet carrier rotational frequency or the rotation frequency of direct-drive generators) lie
between 0,1 Hz and 10 Hz. Consequently, measurements intended to capture this response shall be bandpass
filtered in the range of 0,1 Hz to 10 Hz for both assessment velocity and assessment acceleration.
At the various gearbox stages the drive shaft speeds and harmonics are likely to lie in the frequency range
0,1 Hz to 10 Hz. However, design-related characteristic frequencies (e.g. tooth meshing) can be over 10 Hz
and the range of interest can extend to 1 000 Hz or even higher when harmonics are considered. Such
measurements shall be bandpass filtered in the range of 10 Hz to 1 000 Hz for assessment velocity and 10 Hz
to 2 000 Hz for assessment acceleration.
For generators with a gearbox, many of the design-related characteristic frequencies lie between 10 Hz and
1 000 Hz, and their harmonics can occur above 1 000 Hz. Vibration frequencies shall therefore be bandpass
filtered in the range of 10 Hz to 1 000 Hz for assessment velocity and 10 Hz to 5 000 Hz for assessment
acceleration.
Table 1 gives an overview of the measurement frequency ranges of the individual assessment vibration
quantities for direct drive wind turbines and for wind turbines with a gearbox. The frequency ranges differ
depending on the wind turbine component under consideration and they are characteristic to the assessment
acceleration or the assessment velocity. A single measurement can require more than one filter to provide
information on different machine components.
NOTE The higher frequency acceleration requirements for the generators in Table 1 are due to the presence of
rolling bearings, frequency converters and other electromagnetic excitations.
Table 1 — Overview of possible assessment value frequency ranges
Frequency band for evaluation
Wind turbine type and component Assessment acceleration, a Assessment velocity, v
w0 w0
Hz Hz
Nacelle and tower 0,1 to 10 0,1 to 10
0,1 to 10 0,1 to 10
Rotor main bearing
10 to 2 000 10 to 1 000
0,1 to 10
Generator in direct drive turbines 10 to 1 000
10 to 2 000
0,1 to 10
Gearbox 10 to 1 000
10 to 2 000
Generator in turbines with gearbox 10 to 5 000 10 to 1 000

4.5.3 Broad-band assessment of vibration values
Wind turbine operating conditions are governed by continual changes in the strength and direction of the
wind, which results in changing vibration excitations and responses. Measurement procedures shall ensure
both repeatability between measurements
...