IEC 61400-1:1999

Wind turbine generator systems - Part 1: Safety requirements

Deals with safety aspects, quality assurance and engineering integrity, and specifies safety requirements for design, installation and operation of wind turbine generator systems.

General Information

• Status: Published
• Publication Date: 18-Feb-1999
• ICS: 27.180 - Wind turbine energy systems
• Technical Committee: TC 88 - Wind energy generation systems
• Current Stage: DELPUB - Deleted Publication
• Start Date: 31-Aug-2005
• Completion Date: 26-Oct-2025

Relations

• Revised: IEC 61400-1:2005 - Wind turbines - Part 1: Design requirements
• Effective Date: 05-Sep-2023

Overview

IEC 61400-1:1999 - "Wind turbine generator systems - Part 1: Safety requirements" (2nd edition) defines minimum safety, quality assurance and engineering-integrity requirements for wind turbine generator systems (WTGS). It applies to WTGS with a rotor swept area ≥ 40 m² and covers design, installation, commissioning, operation and maintenance to provide an appropriate level of protection against hazards throughout the planned lifetime. The standard is intended as a safety framework rather than a complete design manual; most requirements may be waived only when safety is demonstrably uncompromised (note: waivers do not apply to clause 6 on external conditions).

Key topics and requirements

IEC 61400-1 organizes practical safety requirements into clear technical areas:

• Safety philosophy & quality assurance
  • Engineering integrity, quality management and documentation requirements for WTGS projects.
• External conditions & WTGS classes
  • Definition of site wind classes, environmental and electrical network conditions used for design.
• Structural design & loads
  • Design methodology, load calculations, design situations and load cases, and ultimate-limit-state analysis.
• Control and protection systems
  • Functional requirements for turbine controls, protection logic and safety interlocks.
• Mechanical and electrical systems
  • Requirements for mechanical components, fasteners, hydraulic/pneumatic systems and internal electrical systems including grounding, lightning protection, disconnect devices and EMC considerations.
• Assessment, assembly and commissioning
  • Procedures for assessing wind, soil and network conditions; planning, installation, erection and commissioning checks.
• Operation and maintenance
  • Commissioning tests, inspection regimes and maintenance practices to sustain safety over service life.
• Annexes
  • Technical annexes include WTGS class parameters, stochastic and deterministic turbulence models (e.g., Kaimal spectral parameters) and guidance bibliography.

Practical applications and users

IEC 61400-1 is used by:

• Wind turbine OEMs for safety-focused design and verification
• Structural, electrical and controls engineers performing WTGS design and load analysis
• Project developers and site engineers for site-assessment and commissioning procedures
• Certification bodies and testing laboratories assessing compliance with international safety requirements
• Operation & maintenance teams for establishing inspection and safety regimes

Using IEC 61400-1 helps reduce risk, guide component selection, inform structural and control-system validation, and support conformity assessment for wind projects.

Related standards

IEC 61400-1 is intended to be used alongside other IEC/ISO documents referenced in clause 2, such as:

• IEC 60204-1 (machine electrical safety)
• IEC 60364 (electrical installations)
• EMC standards (IEC 61000 series) These related standards address complementary electrical safety, EMC, symbols and terminology used throughout the WTGS safety framework.

Keywords: IEC 61400-1, wind turbine safety requirements, WTGS standard, wind turbine design, structural loads, control and protection, turbulence models, wind turbine commissioning.

Standards Content (Sample)

INTERNATIONAL
IEC
STANDARD
61400-1
Second edition
1999-02
Wind turbine generator systems –
Part 1:
Safety requirements
Aérogénérateurs –
Partie 1:
Spécifications de sécurité
Reference number
Numbering
As from 1 January 1997 all IEC publications are issued with a designation in the
60000 series.
Consolidated publications
Consolidated versions of some IEC publications including amendments are

available. For example, edition numbers 1.0, 1.1 and 1.2 refer, respectively, to the
base publication, the base publication incorporating amendment 1 and the base

publication incorporating amendments 1 and 2.

Validity of this publication
The technical content of IEC publications is kept under constant review by the IEC,
thus ensuring that the content reflects current technology.
Information relating to the date of the reconfirmation of the publication is available
in the IEC catalogue.
Information on the subjects under consideration and work in progress undertaken by
the technical committee which has prepared this publication, as well as the list of
publications issued, is to be found at the following IEC sources:
• IEC web site*
• Catalogue of IEC publications
Published yearly with regular updates
(On-line catalogue)*
• IEC Bulletin
Available both at the IEC web site* and as a printed periodical
Terminology, graphical and letter symbols
For general terminology, readers are referred to IEC 60050: International Electro-
technical Vocabulary (IEV).
For graphical symbols, and letter symbols and signs approved by the IEC for
general use, readers are referred to publications IEC 60027: Letter symbols to be
used in electrical technology, IEC 60417: Graphical symbols for use on equipment.
Index, survey and compilation of the single sheets and IEC 60617: Graphical symbols
for diagrams.
* See web site address on title page.

INTERNATIONAL
IEC
STANDARD
61400-1
Second edition
1999-02
Wind turbine generator systems –
Part 1:
Safety requirements
Aérogénérateurs –
Partie 1:
Spécifications de sécurité
 IEC 1999  Copyright - all rights reserved
No part of this publication may be reproduced or utilized in any form or by any means, electronic or
mechanical, including photocopying and microfilm, without permission in writing from the publisher.
International Electrotechnical Commission 3, rue de Varembé Geneva, Switzerland
Telefax: [phone] e-mail: [e-mail] IEC web site http://www.iec.ch
Commission Electrotechnique Internationale
PRICE CODE
XA
International Electrotechnical Commission
For price, see current catalogue

– 2 – 61400-1 © IEC:1999(E)
CONTENTS
Page
FOREWORD . 5

INTRODUCTION . 6

Clause
1 Scope and object . 7

2 Normative references . 7

3 Terms and definitions. 8

4 Symbols and abbreviated terms. 16
4.1 Symbols and units. 16
4.2 Abbreviations . 17
5 Principal elements. 18
5.1 General. 18
5.2 Design methods . 18
5.3 Safety classes. 18
5.4 Quality assurance . 18
5.5 Wind turbine markings . 19
6 External conditions. 19
6.1 General. 19
6.2 WTGS classes . 19
6.3 Wind conditions . 20
6.4 Other environmental conditions . 28
6.5 Electrical power network conditions . 29
7 Structural design . 29
7.1 General. 29
7.2 Design methodology. 30
7.3 Loads. 30
7.4 Design situations and load cases . 31
7.5 Load calculations . 34
7.6 Ultimate limit state analysis . 34
8 Control and protection system . 39
8.1 General. 39
8.2 Wind turbine control . 40

8.3 Wind turbine protection . 40
8.4 Functional requirements of the control and protection system . 41
9 Mechanical systems . 41
9.1 General. 41
9.2 Errors of fitting . 41
9.3 Hydraulic or pneumatic systems . 41
10 Electrical system . 42
10.1 General. 42
10.2 General requirements for the WTGS electrical system . 42
10.3 Protective devices. 42
10.4 Disconnect devices . 42
10.5 Earth system. 42
10.6 Lightning protection. 43

61400-1  IEC:1999(E) − 3 −
Clause Page
10.7 Electrical cables. 43

10.8 Self-excitation . 43

10.9 Over-voltage protection . 43

10.10 Harmonics and power conditioning equipment . 43

11 Assessment of external conditions. 43

11.1 General. 43

11.2 Assessment of wind conditions. 44

11.3 Assessment of other environmental conditions . 44

11.4 Assessment of electrical network conditions . 45
11.5 Assessment of soil conditions . 45
12 Assembly, installation and erection. 45
12.1 General. 45
12.2 Planning. 46
12.3 Installation conditions. 46
12.4 Site access . 46
12.5 Environmental conditions . 46
12.6 Documentation. 46
12.7 Receiving, handling and storage. 47
12.8 Foundation/anchor systems. 47
12.9 Assembly of WTGS. 47
12.10 Erection of WTGS . 47
12.11 Fasteners and attachments . 47
12.12 Cranes, hoists and lifting equipment . 47
13 Commissioning, operation and maintenance . 48
13.1 General. 48
13.2 Commissioning . 48
13.3 Operations . 49
13.4 Inspection and maintenance. 50
Annex A (normative) Design parameters for describing WTGS class S. 52
Annex B (normative) Stochastic turbulence models . 53
Annex C (normative) Deterministic turbulence description . 55

Annex D (informative) Bibliography . 57
Tables
Table 1 – Basic parameters for WTGS classes . 20
Table 2 – Design load cases. 32
Table 3 – Partial safety factors for loads γ . 37
f
Table 4 – General partial safety factors for materials for inherent variability. 37
Table B.1 – Turbulence spectral parameters for Kaimal model . 53

– 4 – 61400-1 © IEC:1999(E)
Figures
Figure 1 – Characteristic wind turbulence. 22

Figure 2 – Example of extreme operating gust . 24

Figure 3 – Example of extreme direction change magnitude . 25

Figure 4 – Example of extreme direction change. 25

Figure 5 – Extreme coherent gust (ECG) . 25

Figure 6 – The direction change for ECD . 26

Figure 7 – Time development of direction change for V = 25 m/s . 26
hub
Figure 8 – Extreme vertical wind shear, wind profile before onset and at maximum shear. 27
Figure 9 – Wind speeds at rotor top and bottom respectively illustrate the
time development of wind shear. 27

61400-1  IEC:1999(E) − 5 −
INTERNATIONAL ELECTROTECHNICAL COMMISSION

––––––––––
WIND TURBINE GENERATOR SYSTEMS –

Part 1: Safety requirements
FOREWORD
1) The IEC (International Electrotechnical Commission) is a worldwide organization for standardization comprising
all national electrotechnical committees (IEC National Committees). The object of the IEC is to promote
international co-operation on all questions concerning standardization in the electrical and electronic fields. To
this end and in addition to other activities, the IEC publishes International Standards. Their preparation is
entrusted to technical committees; any IEC National Committee interested in the subject dealt with may
participate in this preparatory work. International, governmental and non-governmental organizations liaising
with the IEC also participate in this preparation. The 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 the IEC on technical matters express, as nearly as possible, an
international consensus of opinion on the relevant subjects since each technical committee has representation
from all interested National Committees.
3) The documents produced have the form of recommendations for international use and are published in the form
of standards, technical reports or guides and they are accepted by the National Committees in that sense.
4) In order to promote international unification, IEC National Committees undertake to apply IEC International
Standards transparently to the maximum extent possible in their national and regional standards. Any
divergence between the IEC Standard and the corresponding national or regional standard shall be clearly
indicated in the latter.
5) The IEC provides no marking procedure to indicate its approval and cannot be rendered responsible for any
equipment declared to be in conformity with one of its standards.
6) Attention is drawn to the possibility that some of the elements of this International Standard may be the subject
of patent rights. The IEC shall not be held responsible for identifying any or all such patent rights.
International Standard IEC 61400-1 has been prepared by IEC technical committee 88: Wind
turbine systems.
This second edition of IEC 61400-1 cancels and replaces the first edition published in 1994.
The text of this standard is based on the following documents:
FDIS Report on voting
88/98/FDIS 88/103/RVD
Full information on the voting for the approval of this standard can be found in the report on
voting indicated in the above table.
Annexes A, B and C form an integral part of this standard.
Annex D is for information only.
A bilingual version of this standard may be issued at a later date.

– 6 – 61400-1 © IEC:1999(E)
INTRODUCTION
This International Standard outlines minimum safety requirements for wind turbine generator

systems and is not intended for use as a complete design specification or instruction manual.

Any of the requirements of this standard may be waived if it can be suitably demonstrated that

the safety of the system is not compromised. Nevertheless this waiver does not apply to

clause 6.
Compliance with this standard does not relieve any person, organization, or corporation from
the responsibility of observing other applicable regulations.

61400-1  IEC:1999(E) − 7 −
WIND TURBINE GENERATOR SYSTEMS –

Part 1: Safety requirements
1 Scope and object
This part of IEC 61400 deals with safety philosophy, quality assurance and engineering

integrity, and specifies requirements for the safety of Wind Turbine Generator Systems
(WTGS), including design, installation, maintenance, and operation under specified
environmental conditions. Its purpose is to provide the appropriate level of protection against
damage from all hazards from these systems during their planned lifetime.
This standard is concerned with all subsystems of WTGS such as control and protection
mechanisms, internal electrical systems, mechanical systems, support structures and the
electrical interconnection equipment.
This standard applies to WTGS with a swept area equal to or larger than 40 m .
This standard should be used together with the appropriate IEC/ISO standards identified in
clause 2.
2 Normative references
The following normative documents contain provisions which, through reference in this text,
constitute provisions of this part of IEC 61400. At the time of publication, the editions indicated
were valid. All normative documents are subject to revision, and parties to agreements based
on this part of IEC 61400 are encouraged to investigate the possibility of applying the most
recent editions of the normative documents indicated below. Members of IEC and ISO maintain
registers of currently valid International Standards.
IEC 60204-1:1997, Safety of machinery – Electrical equipment of machines – Part 1: General
requirements
IEC 60364 (all parts), Electrical installations of buildings
IEC 60721-2-1:1982, Classification of environmental conditions – Part 2: Environmental
conditions appearing in nature – Temperature and humidity
IEC 61000-3-2:1998, Electromagnetic compatibility (EMC) – Part 3-2: Limits – Limits for
harmonic current emissions (equipment input current ≤16 A per phase)
IEC 61000-3-3:1994, Electromagnetic compatibility (EMC) – Part 3-3: Limits – Limitation of
voltage fluctuations and flicker in low-voltage supply systems for equipment with rated current
≤16 A
IEC 61000-4-2:1995, Electromagnetic compatibility (EMC) – Part 4-2: Testing and
measurement techniques – Electrostatic discharge immunity test. Basic EMC publication
IEC 61000-4-3:1995, Electromagnetic compatibility (EMC) – Part 4-3: Testing and
measurement techniques – Radiated, radio-frequency, electromagnetic field immunity test

– 8 – 61400-1 © IEC:1999(E)
IEC 61000-4-4:1995, Electromagnetic compatibility (EMC) – Part 4-4: Testing and measurement

techniques – Electrical fast transient/burst immunity test. Basic EMC publication

IEC 61000-4-5:1995, Electromagnetic compatibility (EMC) – Part 4-5: Testing and measurement

techniques – Surge immunity test

IEC 61024-1:1990, Protection of structures against lightning – Part 1: General principles

IEC 61312-1:1995, Protection against lightning electromagnetic impulse – Part 1: General

principles
ISO 2394:1986, General principles on reliability for structures
3 Terms and definitions
For the purpose of this International Standard, the following definitions apply:
3.1
annual average
mean value of a set of measured data of sufficient size and duration to serve as an estimate of
the expected value of the quantity. The averaging time interval shall be a whole number of
years to average out non-stationary effects such as seasonality
3.2
annual average wind speed
wind speed averaged according to the definition of annual average
3.3
auto-reclosing cycle
event with a time period, varying from approximately 0,01 s to a few seconds, during which a
breaker released after a grid fault is automatically reclosed and the line is reconnected to the
network
3.4
blocking (wind turbines)
use of a mechanical pin or other device (other than the ordinary mechanical brake) to prevent
movement, for instance of the rotor shaft or yaw mechanism
3.5
brake (wind turbines)
device capable of reducing the rotor speed or stopping rotation
3.6
catastrophic failure (wind turbines)
disintegration or collapse of a component or structure, that results in loss of vital function which
impairs safety
3.7
characteristic value (of a material property)
value having a prescribed probability of not being attained in a hypothetical unlimited test
series
61400-1  IEC:1999(E) − 9 −
3.8
complex terrain
surrounding terrain that features significant variations in topography and terrain obstacles that

may cause flow distortion
3.9
control system (wind turbines)

subsystem that receives information about the condition of the wind turbine and/or its

environment and adjusts the turbine in order to maintain it within its operating limits

3.10
cut-in wind speed (V )
in
lowest mean wind speed at hub-height at which the wind turbine starts to produce power (see
3.24, hub-height)
3.11
cut-out wind speed (V )
out
highest mean wind speed at hub-height at which the wind turbine is designed to produce power
(see 3.24, hub-height)
3.12
design limits
maximum or minimum values used in a design
3.13
dormant failure (also known as latent fault)
failure of a component or system which remains undetected during normal operation
3.14
downwind
in the direction of the main wind vector
3.15
electrical power network
particular installations, substations, lines or cables for the transmission and distribution of
electricity
NOTE – The boundaries of the different parts of this network are defined by appropriate criteria, such as
geographical situation, ownership, voltage, etc.
3.16
emergency shutdown (wind turbines)
rapid shutdown of the wind turbine triggered by a protection system or by manual intervention
3.17
environmental conditions
characteristics of the environment (altitude, temperature, humidity, etc.) which may affect the
WTGS behaviour
3.18
external conditions (wind turbines)
factors affecting operation of a wind turbine, including the wind regime, the electrical network
conditions, and other climatic factors (temperature, snow, ice, etc.)

– 10 – 61400-1 © IEC:1999(E)
3.19
extreme wind speed
highest average wind speed, averaged over t s, that is likely to be experienced within a

specified time period of N years ("recurrence period": N years)

NOTE – In this standard recurrence periods of N = 50 years and N = 1 year and averaging time intervals of t = 3 s

and t = 10 min are used. In popular language, the less precise term "survival wind speed" is often used. In this
standard, however, the turbine is designed using extreme wind speeds for design load cases.

3.20
fail-safe
design property of an item which prevents its failures from resulting in critical faults

3.21
gust
temporary change in the wind speed
NOTE – A gust may be characterized by its rise-time, its magnitude and its duration.
3.22
horizontal axis wind turbine
wind turbine whose rotor axis is substantially parallel to the wind flow
3.23
hub (wind turbines)
fixture for attaching the blades or blade assembly to the rotor shaft
3.24
hub-height (wind turbines)
height of the centre of the swept area of the wind turbine rotor above the terrain surface (see
3.55, swept area)
3.25
idling (wind turbines)
condition of a wind turbine that is rotating slowly and not producing power
3.26
inertial subrange
frequency interval of the wind turbulence spectrum, where eddies – after attaining isotropy –
undergo successive break-up with negligible energy dissipation
NOTE – At a typical 10 m/s wind speed, the inertial subrange is roughly from 0,02 Hz to 2 kHz.

3.27
isolated operation
stable and temporary operation of a discrete part of a power system after network splitting
3.28
limit state
state of a structure and the loads acting upon it, beyond which the structure no longer satisfies
the design requirement (ISO 2394)
NOTE – The purpose of design calculations (i.e. the design requirement for the limit state) is to keep the probability
of a limit state being reached below a certain value prescribed for the type of structure in question (ISO 2394).
3.29
logarithmic wind shear law
see wind profile
61400-1  IEC:1999(E) − 11 −
3.30
maximum power (wind turbines)
highest level of net electrical power delivered by a wind turbine in normal operation

3.31
mean wind speed
statistical mean of the instantaneous value of the wind speed averaged over a given time

period which can vary from a few seconds to many years

3.32
nacelle
housing which contains the drive-train and other elements on top of a horizontal axis wind
turbine tower
3.33
network connection point (wind turbines)
cable terminals of a single wind turbine or, for a wind power station, the connection point to the
electrical bus of the site power collection system
3.34
normal shutdown (wind turbines)
shutdown in which all stages are under the control of the control system
3.35
operating limits
set of conditions defined by the WTGS designer that govern the activation of the control and
protection system
3.36
parked wind turbine
depending on the construction of the wind turbine, parked refers to the turbine being either in a
stand-still or an idling condition
3.37
power collection system (wind turbines)
electric connection system that collects the power from one or more wind turbines. It includes
all electrical equipment connected between the WTGS terminals and the network connection
point
3.38
power law for wind shear
see wind profile
3.39
power output
power delivered by a device in a specific form and for a specific purpose
NOTE (wind turbines) – The electric power delivered by a WTGS.
3.40
protection system (wind turbine)
system which ensures that a WTGS remains within the design limits

– 12 – 61400-1 © IEC:1999(E)
3.41
rated power
quantity of power assigned, generally by a manufacturer, for a specified operating condition of

a component, device or equipment

NOTE (wind turbines) – Maximum continuous electrical power output which a WTGS is designed to achieve under

normal operating conditions.
3.42
rated wind speed (V )
r
specified wind speed at which a wind turbine's rated power is achieved

3.43
Rayleigh distribution
probability distribution function, see 3.66 (wind speed distribution)
3.44
reference wind speed (V )
ref
basic parameter for wind speed used for defining WTGS classes. Other design related climatic
parameters are derived from the reference wind speed and other basic WTGS class
parameters (see clause 6)
NOTE – A turbine designed for a WTGS class with a reference wind speed V , is designed to withstand climates
ref
for which the extreme 10 min average wind speed with a recurrence period of 50 years at turbine hub-height is
lower than or equal to V .
ref
3.45
resonance
phenomenon appearing in an oscillating system, in which the period of a forced oscillation is
very close to that of free oscillation
3.46
rotationally sampled wind velocity
wind velocity experienced at a fixed point of the rotating wind turbine rotor
NOTE – The turbulence spectrum of a rotationally sampled wind velocity is distinctly different from the normal
turbulence spectrum. While rotating, the blade cuts through a wind flow that varies in space. Therefore, the
resulting turbulence spectrum will contain sizeable amounts of variance at the frequency of rotation and harmonics
of the same.
3.47
rotor speed (wind turbines)
rotational speed of a wind turbine rotor about its axis

3.48
roughness length
extrapolated height at which the mean wind speed becomes zero if the vertical wind profile is
assumed to have a logarithmic variation with height
3.49
safe life
prescribed service life with a declared probability of catastrophic failure
3.50
scheduled maintenance
preventive maintenance carried out in accordance with an established time schedule
3.51
serviceability limit state
limit state which corresponds with criteria governing function related normal use (ISO 2394)

61400-1  IEC:1999(E) − 13 −
3.52
standstill
condition of a WTGS that is stopped

3.53
support structure (wind turbines)

part of a wind turbine comprising the tower and foundation

3.54
survival wind speed
popular name for the maximum wind speed that a construction is designed to withstand

NOTE – In this standard, the expression is not used. Design conditions instead refer to extreme wind speed
(see 3.19).
3.55
swept area
projected area perpendicular to the wind direction that a rotor will describe during one complete
rotation
3.56
turbulence intensity
ratio of the wind speed standard deviation to the mean wind speed, determined from the same
set of measured data samples of wind speed, and taken over a specified period of time
3.57
turbulence scale parameter
wave length where the non-dimensional, longitudinal power spectral density is equal to 0,05
NOTE – The wave length is thus defined as Λ = V /f , where f S (f )/σ = 0,05
1 hub 0 0 1 0 1
3.58
ultimate limit state
limit states which generally correspond to maximum load carrying capacity (ISO 2394)
3.59
unscheduled maintenance
maintenance carried out, not in accordance with an established time schedule, but after
reception of an indication regarding the state of an item
3.60
upwind
in the direction opposite to the main wind vector

3.61
vertical axis wind turbine
wind turbine whose rotor axis is vertical
3.62
Weibull distribution
probability distribution function, see 3.66 (wind speed distribution)
3.63
wind farm
see 3.64 (wind power station)
3.64
wind power station
group or groups of wind turbine generators, commonly called a wind farm

– 14 – 61400-1 © IEC:1999(E)
3.65
wind profile – wind shear law
mathematical expression for assumed wind speed variation with height above ground

NOTE – Commonly used profiles are the logarithmic profile (1) or the power law profile (2).

ln (z/z )
V(z)=V(z ) × (1)
r
ln (z /z )
r 0
α
 
z
V((z) =V(z ) ×   2)
r
 
z
r
 
where
V(z) is the wind speed at height z
z is the height above ground
z is a reference height above ground used for fitting the profile
r
z is the roughness length
α is the wind shear (or power law) exponent
3.66
wind speed distribution
probability distribution function, used to describe the distribution of wind speeds over an
extended period of time
NOTE – Often used distribution functions are the Rayleigh P (V ), and the Weibull P (V ), functions.
R o W o
( ) = 1 − exp [− π ( /2 ]
)
P V V V
R 0 0 ave
(3)
k
( ) = 1 − exp [− ( /C ) ]
P V V
w 0 0
 
 1 
CΓ 1 +
 
 
k
 
 
with V = (4)
ave  
 π 
C , if  k = 2
 
 2 
where
P(V ) is the cumulative probability function, i.e. the probability that V < V
0 0
V is the wind speed (limit)
V is the average value of V
ave
C is the scale parameter of the Weibull function
k is the shape parameter of the Weibull function
Γ is the gamma function
Both C and k can be evaluated from real data. The Rayleigh function is identical to the Weibull
function if k = 2 is chosen and C and V satisfy the condition stated in equation (4) for k = 2.
ave
The distribution functions express the cumulative probability that the wind speed is lower than
V . Thus (P(V ) – P(V )), if evaluated between the specified limits V and V , will indicate the
0 1 2 1 2
fraction of time that the wind speed is within these limits. Differentiating the distribution
functions yields the corresponding probability density functions.

61400-1  IEC:1999(E) − 15 −
3.67
wind shear
variation of wind speed across a plane perpendicular to the wind direction

3.68
wind shear exponent
also commonly known as power law exponent, see 3.65 (wind profile – wind shear law)

3.69
wind speed
at a specified point in space the wind speed is the speed of motion of a minute amount of air
surrounding the specified point
NOTE – The wind speed is also the magnitude of the local wind velocity (vector) (see 3.71, wind velocity).
3.70
wind turbine generator system (WTGS)
system which converts kinetic energy in the wind into electrical energy
3.71
wind velocity
vector pointing in the direction of motion of a minute amount of air surrounding the point of
consideration, the magnitude of the vector being equal to the speed of motion of this air
"parcel" (i.e. the local wind speed)
NOTE – The vector at any point is thus the time derivative of the position vector of the air "parcel" moving through
the point.
3.72
WTGS electrical system
all electrical equipment internal to the WTGS, up to and including the WTGS terminals,
including equipment for earthing, bonding and communications. Conductors local to the WTGS
which are intended to provide an earth termination network specifically for the WTGS are
included
3.73
WTGS terminals
point or points identified by the WTGS supplier at which the WTGS may be connected to the
power collection system. This includes connection for the purposes of transferring energy and
communications
3.74
yawing
rotation of the rotor axis about a vertical axis (for horizontal axis wind turbines only)
3.75
yaw misalignment
horizontal deviation of the wind turbine rotor axis from the wind direction

– 16 – 61400-1 © IEC:1999(E)
4 Symbols and abbreviated terms

4.1 Symbols and units
a slope parameter for turbulence standard deviation model [–]

C scale parameter of the Weibull distribution function [m/s]

Coh coherency function
D rotor diameter [m]
–1
f frequency [s ]
f design value for material strength [–]
d
f characteristic value for material strength [–]
k
F design value for loads [–]
d
F characteristic value for loads [–]
k
I characteristic value of hub-height turbulence intensity at a 10 min
average wind speed of 15 m/s [–]
k shape parameter of the Weibull distribution function [–]
K modified Bessel function [–]
L isotropic turbulence integral scale parameter [m]
L coherency scale parameter [m]
e
L velocity component integral scale parameter [m]
k
n counted number of fatigue cycles in load bin i [–]
i
N( ) is the number of cycles to failure as a function of the stress (or strain)
.
indicated by the argument (i.e. the characteristic S-N curve) [–]
N recurrence period for extreme situations [y]
p survival probability [–]
P (V ) Rayleigh probability distribution, i.e. the probability that V < V [–]
R 0 0
P (V ) Weibull probability distribution [–]
W 0
r magnitude of separation vector projection [m]
s the stress (or strain) level associated with the counted number of cycles in bin i [–]
i
2 2
S (f) power spectral density function [m /s ]
2 2
S single-sided velocity component spectrum [m /s ]
k
T gust characteristic time [s]
t time [s]
V wind speed [m/s]
V(z) wind speed at height z [m/s]
V annual average wind speed at hub-height [m/s]
ave
V extreme coherent gust magnitude over the whole rotor swept area [m/s]
cg
V expected extreme wind speed (averaged over 3 s), with a recurrence time
eN
interval of N years. V and V for 1 year and 50 years, respectively [m/s]
e1 e50
V largest gust magnitude with an expected recurrence period of N years. [m/s]
gustN
V wind speed at hub-height averaged over 10 min [m/s]
hub
V cut-in wind speed [m/s]
in
V limit wind speed in wind speed distribution model [m/s]
V cut-out wind speed [m/s]
out
V rated wind speed [m/s]
r
61400-1  IEC:1999(E) − 17 −
V reference wind speed averaged over 10 min [m/s]
ref
V(y,z,t) longitudinal wind velocity component to describe transient horizontal wind

shear [m/s]
V(z,t) longitudinal wind velocity component to describe transient variation for

extreme gust and shear conditions [m/s]

x, y, z coordinate system used for the wind field description; along wind

(longitudinal), across wind (lateral) and height respectively [m]

z hub-height of the wind turbine [m]
hub
z reference height above ground [m]
r
z roughness length for the logarithmic wind profile [m]
α wind shear power law exponent [–]
β parameter for extreme direction change model [–]
δ coefficient of variation [–]
Γ gamma function [–]
γ partial safety factor for loads [–]
f
γ
partial safety factor for materials [–]
m
γ partial safety factor for consequences of failure [–]
n
θ(t) wind direction change transient [°]
θ angle of maximum deviation from the direction of the average wind speed
cg
under gust conditions [°]
θ extreme direction change with a recurrence period of N years [°]
eN
Λ turbulence scale parameter defined as the wave length where the
non-dimensional, longitudinal power spectral density, fS (f)/σ , is equal to 0,05 [m]
1 1
σ hub-height longitudinal wind velocity standard deviation [m/s]
th
σ
k hub-height component wind velocity standard deviation (k = 1, 2, or 3) [m/s]
k
4.2 Abbreviations
A Abnormal (for partial safety factors)
a.c. Alternating current
C Serviceability constraint
d.c. Direct current
DLC Design load case
ECD Extreme coherent gust with direction change
ECG Extreme coherent gust
EDC Extreme wind direction change
EOG Extreme operating gust
EWM Extreme wind speed model
EWS Extreme wind shear
F Fatigue
HAWT Horizontal axis wind turbine
N Normal and extreme (for partial safety factors)
NWP Normal wind profile model
NTM Normal turbulence model
S Special IEC WTGS class
– 18 – 61400-1 © IEC:1999(E)
T Transport and erection (for partial safety factors)

U Ultimate
VAWT Vertical axis wind turbine

WTGS Wind turbine generator system(s)

5 Principal elements
5.1 General
The engineering and technical requirements to ensure the safety of the structural, mechanical,

electrical and control systems of the WTGS are given in the following clauses. This
specification of requirements applies to the design, manufacture, installation and maintenance
of WTGS and the associated quality management process. In addition, safety procedures
which have been established in the various technologies that are used in the installation,
operation and maintenance of WTGS shall be followed.
5.2 Design methods
This standard requires the use of a structural dynamics model to predict design loads. This
model shall be used to determine the loads over a range of wind speeds, using the turbulence
conditions and other extreme wind conditions defined in clause 6, and design situations defined
in clause 7. All relevant combinations of external conditions and design situations shall be
analyzed. A minimum set of such combinations has been defined as load cases in this
standard.
Data from full scale testing of a WTGS may be used to increase confidence in predicted design
values and to verify structural dynamics models and design situations.
Verification of the adequacy of the design shall be made by calculation and/or by testing. If test
results are used in this verification, the external conditions during the test shall be shown to
reflect the characteristic values and design situations defined in this standard. The selection of
test conditions, including the test loads, shall take account of the relevant safety factors.
5.3 Safety classes
A WTGS shall be designed according to one of the following two safety classes:
– a normal safety class which applies when a failure results in risk of personal injury or
economic and social consequences;
– a special safety class which applies when the safety requirements are determined by local
regulations and/or the safety requirements are agreed between the manufacturer and the
customer.
Partial safety factors, for normal safety class WTGS, are specified in 7.6 of this standard.
Partial safety factors for special safety class WTGS shall be agreed between the manufacturer
and the customer. A WTGS designed according to the special safety class is a WTGS class S
turbine as defined in 6.2.
5.4 Quality assurance
Quality assurance shall be an integral part of the design, procurement, manufacture,
installation, operation and maintenance of the WTGS and all their components.
It is recommended that the quality system complies with the requirements of the relevant ISO
publications (see bibliography in annex D).

61400-1  IEC:1999(E) − 19 −
5.5 Wind turbine markings
The following information shall be as a minimum, prominently and legibly displayed on the

indelibly marked turbine nameplate:

– WTGS manufacturer and country;

– model and serial number;
– production year;
– rated power;
– reference wind speed, V ;
ref
– hub-height operating wind speed range, V – V ;
in out
– operating ambient temperature range;
– IEC WTGS class (see table 1);
– rated voltage at the WTGS terminals;
– frequency at the WTGS terminals or frequency range in the case that the nominal variation
is greater than 2 %.
6 External conditions
6.1 General
The external conditions described in this clause shall be considered in the design of a WTGS.
WTGS are subjected to environmental and electrical conditions which may affect their loading,
durability and operation. To ensure the appropriate level of safety and reliability, the
environmental, electrical and soil parameters shall be taken into account in the design and
shall be explicitly stated in the design documentation.
The environmental conditions are further divided into wind conditions and other environmental
conditions. The electrical conditions refer to the network conditions. Soil properties are relevant
to the design of WTGS foundations.
Each type of external condition may be subdivided into a normal and an extreme external
condition. The normal external conditions generally concern long-term structural loading and
operating conditions, while the extreme external conditions represent the rare but potentially
critical external design conditions. The design load cases shall consist of a combination of
these external conditions with wind turbine operational modes.
Wind conditions are the primary external consideration for structural integrity. Other

environmental conditions also affect design features such as control system function,
durability, corrosion, etc.
The normal and extreme conditions which are to be considered in design according to WTGS
classes are prescribed in the following subclauses.
6.2 WTGS classes
The external conditions to be considered in design are dependent on the intended site or site
type for a WTGS installation. WTGS classes are defined in terms of wind speed and turbulence
parameters. The intention of the classes is to cover most applications. The values of wind
speed and turbulence parameters are intended to represent the characteristic values of many
different sites and do not give a precise representation of any specific site, see clause 11. The
goal is to achieve WTGS classification with clearly varying robustness governed by the wind
speed and turbulence parameters. T
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