IEC 61400-1:2005/AMD1:2010

Amendment 1 - Wind turbines - Part 1: Design requirements

Amendement 1 - Eoliennes - Partie 1: Exigences de conception

General Information

• Status: Published
• Publication Date: 12-Oct-2010
• ICS: 27.180 - Wind turbine energy systems
• Technical Committee: TC 88 - Wind energy generation systems
• Drafting Committee: MT 1 - TC 88/MT 1
• Current Stage: DELPUB - Deleted Publication
• Start Date: 08-Feb-2019
• Completion Date: 26-Oct-2025

Relations

• Amends: IEC 61400-1:2005 - Wind turbines - Part 1: Design requirements
• Effective Date: 05-Sep-2023
• Revised: IEC 61400-1:2019 - Wind energy generation systems - Part 1: Design requirements
• Effective Date: 05-Sep-2023

Standards Content (Sample)

IEC 61400-1 ®
Edition 3.0 2010-10
INTERNATIONAL
STANDARD
AMENDMENT 1
Wind turbines –
Part 1: Design requirements
IEC 61400-1:2005/A1:2010(E)
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IEC 61400-1 ®
Edition 3.0 2010-10
INTERNATIONAL
STANDARD
AMENDMENT 1
Wind turbines –
Part 1: Design requirements
INTERNATIONAL
ELECTROTECHNICAL
COMMISSION
PRICE CODE
U
ICS 27.180 ISBN 978-2-88912-201-1
– 2 – 61400-1 Amend.1 © IEC:2010(E)

FOREWORD
This amendment has been prepared by IEC technical committee 88: Wind turbines.

The text of this amendment is based on the following documents:

FDIS Report on voting
88/374/FDIS 88/378/RVD
Full information on the voting for the approval of this amendment can be found in the report
on voting indicated in the above table.
The committee has decided that the contents of this amendment and the base publication will
remain unchanged until the stability date indicated on the IEC web site 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• withdrawn,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.
A bilingual version may be issued at a later date.
_____________
61400-1 Amend.1 © IEC:2010(E) – 3 –

2 Normative references
Replace the existing list of normative references by the following new list:

IEC 60204-1, Safety of machinery – Electrical equipment of machines – Part 1: General

requirements
IEC 60204-11, Safety of machinery – Electrical equipment of machines – Part 11:

Requirements for HV equipment for voltages above 1 000 V a.c. or 1 500 V d.c. and not

exceeding 36 kV
IEC 60364 (all parts), Low-voltage electrical installations
IEC 60364-5-54, Electrical installations of buildings – Part 5-54: Selection and erection of
electrical equipment – Earthing arrangements, protective conductors and protective bonding
conductors
IEC 60721-2-1, Classification of environmental conditions – Part 2: Environmental conditions
appearing in nature – Temperature and humidity
IEC 61000-6-1, Electromagnetic compatibility (EMC) – Part 6-1: Generic standards –
Immunity for residential, commercial and light-industrial environments
IEC 61000-6-2, Electromagnetic compatibility (EMC) – Part 6-2: Generic standards –
Immunity for industrial environments
IEC 61000-6-4, Electromagnetic compatibility (EMC) – Part 6-4: Generic standards –
Emission standard for industrial environments
IEC 61400-2, Wind turbines – Part 2: Design requirements for small wind turbines
IEC 61400-21, Wind turbines – Part 21: Measurement and assessment of power quality
characteristics of grid connected wind turbines
IEC 61400-24, Wind turbines – Part 24: Lightning protection
IEC 62305-3, Protection against lightning – Part 3: Physical damage to structures and life
hazard
IEC 62305-4, Protection against lightning – Part 4: Electrical and electronic systems within
structures
ISO 76:2006, Rolling bearings – Static load ratings
ISO 281, Rolling bearings – Dynamic load ratings and rating life
ISO 2394:1998, General principles on reliability for structures
ISO 2533:1975, Standard atmosphere
ISO 4354, Wind actions on structures
ISO 6336-2, Calculation of load capacity of spur and helical gears – Part 2: Calculation of
surface durability (pitting)
– 4 – 61400-1 Amend.1 © IEC:2010(E)

ISO 6336-3:2006, Calculation of load capacity of spur and helical gears – Part 3: Calculation

of tooth bending strength
ISO 81400-4, Wind turbines – Part 4: Design and specification of gearboxes

3 Terms and definitions
3.26 – limit state
Replace ISO 2394 by 2.2.9 of ISO 2394.

3.55 – ultimate limit state
Replace ISO 2394 by 2.2.10 of ISO 2394.
4 Symbols and abbreviated terms
4.1 Symbols and units
Switch the definitions of σ and σ . The vertical wind velocity standard deviation should be σ ,
2 3 3
not σ .
6 External conditions
6.3.1.3 Normal turbulence model (NTM)
Replace the existing Figures 1a and 1b by the following new figures:

Category A
4,5
Category B
Category C
3,5
2,5
1,5
0,5
0 5 10 15 20 25 30
V  (m/s)
IEC  2236/10
hub
Figure 1a –Turbulence standard deviation for the normal turbulence model (NTM)

σ  (m/s)
61400-1 Amend.1 © IEC:2010(E) – 5 –

0,5
Category A
Category B
0,4
Category C
0,3
0,2
0,1
0 5 10 15 20 25 30
V  (m/s)
hub
IEC  2237/10
Figure 1b – Turbulence intensity for the normal turbulence model (NTM)
6.3.2.6 Extreme wind shear (EWS)
Replace the number 2,5 in equations (26) and (27) to 2,5 [m/s]. (The number 2,5 in equations
(26) and (27) is not dimensionless.)
7 Structural design
7.4.2 Power production plus occurrence of fault or loss of electrical network
connection (DLC 2.1 – 2.4)
nd
paragraph, the following new text:
Add, as 2
As an alternative to the specification of DLC 2.3 above and in Table 2, DLC 2.3 may instead
be considered as a normal event (i.e. a partial safety factor for load of 1,35) to be analyzed
using stochastic wind simulations (NTM - V in hub out
electrical system fault (including loss of electrical network connection). In this case, 12
response simulations shall be carried out for each considered mean wind speed. For each
response simulation, the extreme response after the electrical fault has occurred is sampled.
The fault must be introduced after the effect of initial conditions has become negligible. For
each mean wind speed, a nominal extreme response is evaluated as the mean of the 12
sampled extreme responses plus three times the standard deviation of the 12 samples. The
characteristic response value for DLC 2.3 is determined as the extreme value among the
nominal extreme responses.
7.5 Load calculations
Add, after second paragraph, the following new text:
When turbulent winds are used for dynamic simulations, attention should be given to the grid
resolution regarding the spatial and time resolution.
—————————
Concerning the spatial resolution, the maximum distance between adjacent points should be smaller than 25 %
of Λ1 (Equation (5)) and no larger than 15 % of the rotor diameter. This distance is meant to be the diagonal
distance between points in each grid cell defined by four points. In the case of a non-uniform grid, an average
value over the rotor surface of the distance between grid points can be considered as the representative spatial
resolution, but this distance should always decrease towards the blade tip.
Turbulence intensity
– 6 – 61400-1 Amend.1 © IEC:2010(E)

Replace the last paragraph by the following new text:

Ultimate load components may also be combined in a conservative manner assuming the
extreme component values occur simultaneously. In case this option is pursued, both
minimum and maximum extreme component values shall be applied in all possible

combinations to avoid introducing non-conservatism.

Guidance for the derivation of extreme design loads from contemporaneous loads taken from

a number of stochastic realisations is given in Annex H.

7.6.1.2 Partial safety factor for consequence of failure and component classes

Add, after the bullets defining the component classes, the following new text:
The consequences of failure factor shall be included in the test load when performing tests
as for example full scale blade testing.
7.6.2 Ultimate strength analysis
Replace equation (31) by the following new equation:
1 1
γ F ≤ ⋅ f (31)
f k k
γ γ
n m
Add the following new paragraph after equation (31):
Note that γ is a consequence of failure factor and shall not be treated as a safety factor on
n
materials.
th
Delete the last sentence in 5 paragraph (“For guidance see Annex F”) and insert, after the
th
5 paragraph, the following two paragraphs:
Data used in extrapolation methods shall be extracted from time series of turbine simulations
of at least 10 min in length over the operating range of the turbine for DLC 1.1. A minimum of
15 simulations is required for each wind speed from (V – 2 m/s) to cut-out and six
rated
simulations are required for each wind speed below (V – 2 m/s). When extracting data,
rated
the designer must consider the effect of independence between peaks on the extrapolation
and minimize dependence when possible. The designer shall aggregate data and probability
distributions to form a consistent long-term distribution. To ensure stable estimation of long-
term loads, a convergence criterion shall be applied to a probability fractile less than the

mode of the data for either the short-term or long-term exceedance distributions. For
guidance, see Annex F.
The characteristic value for blade root in-plane and out-of-plane moments and tip deflection
may be determined by a simplified procedure . The characteristic value may then be
determined by calculating the mean of the extremes for each 10-min bin and using the largest
value, multiplied by an extrapolation factor of 1,5, while maintaining the partial load factor for
statistical load extrapolation, see Table 3.
—————————
This approach is considered conservative for 3-bladed upwind wind turbines. Caution should be exercised for
other wind turbine concepts.
61400-1 Amend.1 © IEC:2010(E) – 7 –

7.6.2.1 Partial safety factor for loads

Replace the existing formula in the footnote of Table 3 by the following new formula:

⎧ F
gravity
1− ; F ≤ F
⎪
gravity k
⎪
F
ς = k
⎨
⎪
0; F > F
⎪ gravity k
⎩
Add the following new text after Table 3:

The approach in 7.6.1.1, where the partial safety factor for loads is applied to the load
response, assumes that a proper representation of the dynamic response is of prime concern.
For foundations or where a proper representation of non-linear material behaviour or
geometrical non-linearities or both are of primary concern, the design load response S shall
d
be obtained from a structural analysis for the combination of the design loads F , where the
d
design load is obtained by multiplication of the characteristic loads F by the specified partial

k
load factor γ for favourable and unfavourable loads,
f
F = γ F
d f k
The load responses in the tower at the interface (shear forces and bending moments) factored
with γ from Table 3 shall be applied as boundary conditions.
f
For gravity foundations, the limit states considering overall stability (rigid body motion with no
failure in soil) and bearing capacity of soil and foundation shall be regarded and calculated
according to a recognized standard. In general, a partial safety factor of γ = 1,1 for
f
unfavourable permanent loads and γ = 0,9 for favourable permanent loads shall be applied
f
for foundation load, backfilling and buoyancy. If it can be demonstrated by respective quality
management and surveillance that the foundation material densities specified in the design
documentation are met on site, a partial safety factor for permanent foundation load γ = 1,0
f
can be used for the limit states regarding bearing capacity of soil and foundation. If buoyancy
is calculated equal to a terrain water level, a partial safety factor for buoyancy γ = 1,0 can be
f
applied.
Alternatively, the check of capacity of soil and foundation can be based on a partial safety
factor γ = 1,0 for both favourable and unfavourable permanent loads and the check of overall
f
stability can be based on a partial safety factor of γ = 1,1 for unfavourable permanent loads
f
and γ = 0,9 for favourable permanent loads, using in all cases conservative estimates of
f
weights or densities defined as 5 % / 95 % fractiles. The lower fractile is to be used when the
load is favourable. Otherwise, the upper fractile is to be used.

7.6.5 Critical deflection analysis
Replace the existing text by the following new text:
7.6.5.1 General
It shall be verified that no deflections affecting structural integrity occur in the design
conditions detailed in Table 2.
The maximum elastic deflection in the unfavourable direction shall be determined for the load
cases detailed in Table 2 using the characteristic loads. The resulting deflection is then
multiplied by the combined partial safety factor for loads, materials and consequences of
failure.
• Partial safety factor for loads
The values of γ shall be chosen from Table 3.
f
– 8 – 61400-1 Amend.1 © IEC:2010(E)

• Partial safety factor for the elastic properties of materials

The value of γ shall be 1,1 except when the elastic properties of the component in question
m
have been determined by testing and monitoring in which case it may be reduced. Particular

attention shall be paid to geometrical uncertainties and the accuracy of the deflection

calculation method.
• Partial safety factor for consequences of failure

Component class 1: γ = 1,0
n
Component class 2: γ = 1,0
n
Component class 3: γ = 1,3.
n
The elastic deflection shall then be added to the un-deflected position in the most
unfavourable direction and the resulting position compared to the requirement for non-
interference.
7.6.5.2 Blade (tip) deflection
One of the most important considerations is to verify that no mechanical interference between
blade and tower will occur.
In general, blade deflections have to be calculated for the ultimate load cases as well as for
the fatigue load cases. The deflections caused by the ultimate load cases can be calculated
based on beam models, FE models or the like. All relevant load cases from Table 2 have to
be taken into account with the relevant partial load safety factors.
Moreover, for load case 1.1 extrapolation of tip deflection is mandatory according to 7.4.1.
Here direct dynamic deflection analysis can be used. The exceedance probability in the most
unfavourable direction shall be the same for the characteristic deflection as for the
characteristic load. The characteristic deflection is then to be multiplied by the combined
safety factor for loads, materials and consequences of failure and be added to the un-
deflected position in the most unfavourable direction and the resulting position compared to
the requirement for non-interference.
9 Mechanical systems
9.4 Main gearbox
Replace the existing text by the following new text:
The main gearbox shall be designed according to ISO 81400-4, until a similar document is

published in the IEC 61400 series.
9.5 Yaw system
Replace the second paragraph by the following new text:
Any motors shall comply with relevant parts of Clause 10.
Non-redundant parts of the gear system such as the final yaw gear shall be considered as
component class 2. When multiple yaw drives ensure sufficient redundancy in the yaw gear
system, and easy replacement is possible, the reduction gearbox and the final drive pinion
may be considered to be in component class 1.
The safety against pitting shall be determined in accordance with ISO 6336-2. The application
of the upper limit curve (1) for life factor Z , which allows limited pitting, is permissible.
NT
Sufficient tooth bending strength shall be proven in accordance with ISO 6336-3. The reverse

61400-1 Amend.1 © IEC:2010(E) – 9 –

bending loads on gear teeth shall be considered in accordance with ISO 6336-3 Annex B.

Minimum values for S and S are specified in Table 5. These values must be achieved by
F H
using characteristic loads F Hence they include the partial safety factor for consequences, γ ,

k n
materials, γ and loads, γ .
m f
Table 5 – Minimum required safety factor S and S for the yaw gear system
H F
Component class 1 Component class 2

Surface durability (pitting) s ≥ 1,0 s ≥ 1,1

H H
Tooth bending fatigue strength
s ≥ 1,1 s ≥ 1,25
F F
Static bending strength s ≥ 1,0 s ≥ 1,2
F F
Lower safety factors may be applicable in cases where efficient monitoring is implemented. If
safety factors below 1,0 are applied, then the maintenance manual must reflect anticipated
replacement intervals.
10 Electrical system
10.5 Earth system
Replace, in the first paragraph, IEC 61024-1 by IEC 62305-3.
10.6 Lightning protection
Replace IEC 61024-1 by IEC 62305-3.
10.9 Protection against lightning electromagnetic fields
Replace, in the first paragraph, IEC 61312-1 by IEC 62305-4.
11 Assessment of a wind turbine for site-specific conditions
11.2 Assessment of the topographical complexity of a site
Replace the text of this subclause by the following new text:
The complexity of the site is characterised by the slope of the terrain and variations of the
terrain topography from a plane.

To obtain the slope of the terrain, planes are defined that fit the terrain within specific
distances and sector amplitudes for all wind direction sectors around the wind turbine and
pass through the tower base. The slope, used in Table 4, denotes the slopes of the different
mean lines of sectors passing through the tower bases and contained in the fitted planes.
Accordingly, the terrain variation from the fitted plane denotes the distance, along a vertical
line, between the fitted plane and the terrain at the surface points.

– 10 – 61400-1 Amend.1 © IEC:2010(E)

Table 4 – Terrain complexity indicators

Maximum terrain
Distance range from Maximum slope of fitted

Sector amplitude
wind turbine plane
variation
< 5 z 360º < 0,3 z
hub hub
30º < 10º
< 10 z < 0,6 z
hub hub
30º
< 20 z < 1,2 z
hub hub
The resolution of surface grids used for terrain complexity assessment must not exceed the

smallest of 1,5 z and 100 m.
hub
The site shall be considered complex, if 15 % of the energy in the wind comes from sectors
that fail to conform to the criteria in Table 4 and homogeneous, if less than 5 % of the energy
in the wind comes from sectors that fail to conform.
A complexity index i is defined, such that i = 0 when less than 5 % of the energy comes
c c
from complex sectors, and i = 1 when more than 15 % of the energy comes from complex
c
sectors. In between, i varies linearly.
c
11.4 Assessment of wake effects from neighbouring wind turbines
Add the following new text after the 3rd paragraph:
Generally, the effective turbulence for fatigue and various ultimate loads cannot be assumed
to be the same.
th
Delete the 4 paragraph to the end of the subclause.
11.9 Assessment of structural integrity by reference to wind data
Replace the existing footnote 18 by the following new footnote:
The effect of complex terrain may be included by additional multiplication with a turbulence
structure correction parameter C defined as
CT
2 2
1+()σˆ / σˆ +()σˆ / σˆ
2 1 3 1
C =
CT
1,375
where ratios of the estimated standard deviations, σˆ , correspond to hub height values. Where

i
there are no site data for the components of turbulence and the terrain is complex, results of
modelling or C = 1+0,15 i , where i is the complexity index defined in Subclause 11.2, may
CT c c
be used.
th
Replace the 5 paragraph to the end of the subclause by the following new text:
An adequate assessment of wake effects can be performed by verifying that the turbulence
standard deviation σ from the normal turbulence model is greater or equal to the estimated
90 % fractile of the turbulence standard deviation (including both ambient and wake
—————————
3 The check criteria is considered fulfilled if the requisite fails over a surface less than 5 z .
hub
This approach can also be used for the assessment of sector-wise varying turbulence, alone or in combination
with wake turbulence. The standard deviation ˆ of σˆ may be determined as the average of the sector-wise
σ
σ
values.
61400-1 Amend.1 © IEC:2010(E) – 11 –

turbulence) between the wind speeds 0,2 V and 0,4 V (or when the turbine properties are
ref ref
known, between 0,6 V and V ), i.e.:
r out
σ≥⋅I V
(35)
1 eff hub
Guidance for calculating I can be found in Annex D.
eff
Furthermore, it shall be demonstrated that the site specific horizontal shear due to partial

wakes does not exceed EWS in 6.3.2.6 and that the site specific extreme turbulence ,

including the wake effects, does not exceed the ETM model in 6.3.2.3. For determination of

the site specific turbulence, the site specific conditions, the frequency of the wake situations
and wind farm layout shall be accounted for.
11.10 Assessment of structural integrity by load calculations with reference to site
specific conditions
nd
Replace the 2 paragraph to the end of the subclause by the following new text:
Where there are no site data for the components of turbulence and the terrain is complex, it
shall be assumed that the lateral and upward turbulence standard deviations relative to the
longitudinal component are equal to 1,0 and 0,7, respectively.
In the case of wake effects, it shall be verified that structural integrity is not compromised for
ultimate and fatigue limit states. For fatigue limit state in DLC 1.2 σ in the normal turbulence,
model is replaced by an appropriate wake turbulence model, e.g. I , found in Annex D.
eff
For ultimate limit state analysis, DLC 1.1 or DLC 1.3, as well as DLC 1.5, shall be applied with
site specific conditions including wake effects represented by appropriate models. NTM for
ULS loads can be set to characteristic ambient turbulence inside large farms as defined in
Annex D, Equation (D.4).
Since for fatigue load calculations, I as defined in Annex D depends on the Wöhler curve
eff
exponent m of the material of the considered component, the loads on structural components
with other material properties shall either be recalculated or assessed with the appropriate
value of m.
Annex B – Turbulence models
B.1 Mann (1994) uniform shear turbulence model

Replace the equation defining C by the following new equation:
⎛ 2 2 ⎞
β()k k k + k
k k ⎜ ⎟
2 0 1 1 2
C = arctan
⎜ ⎟
2 2 2 ⎜ k −()k + β()k k k β(k)⎟
0 3 1 1
()k + k
⎝ ⎠
1 2
—————————
The site specific extreme turbulence may be represented by the maximum centre wake turbulence in the most
severe direction.
– 12 – 61400-1 Amend.1 © IEC:2010(E)

Annex D – Wake and wind farm turbulence

Replace the existing text of Annex D by the following new text:

D.1 Wake effects
Wake effects from neighbouring wind turbines may be taken into account during normal

operation for fatigue calculation by an effective turbulence intensity I , Frandsen (2007). The
eff,
effective turbulence intensity – conditioned on hub height mean wind speed - may be defined
as
2π
⎧ ⎫ m
⎪ ⎪
m
I (V ) = p(θV )I (θV )dθ (D.1)
⎨ ⎬
eff hub hub
hub
∫
⎪ ⎪
⎩ ⎭
where
V is the wind speed at hub height;
hub
p is the probability density function of wind direction;
I is the turbulence intensity of the combined ambient and wake flows from wind direction θ,
and
m is the Wöhler (SN-curve) exponent for the considered material.
In the following, a uniform distribution pV()θ is assumed. It is also acceptable to adjust
hub
the formulas for other than uniform distribution . No reduction in mean wind speed inside the
wind farm shall be assumed.
If min{d } ≥ 10 D:
I
ˆ
σ
c
I = (D.2)
eff
V
hub
If min{d } < 10 D:
I
N
⎡ ⎤ m
ˆ
σ
m m
eff
ˆ ˆ
I = = ⎢()1– N p σ + p σ(d)⎥ ; p = 0,06 (D.3)
eff w c w i w
∑
T
V V
⎢ ⎥
hub hub
i =1
⎣ ⎦
where
σσˆˆ=+1, 28σˆ is the characteristic ambient turbulence standard deviation;
c σ
ˆ
σ is the estimated ambient turbulence standard deviation;
—————————
In the case of non-uniform distribution or non-grid wind farm layout, the formula must be modified accordingly, maintaining
the concept implied in the more general formula D.1, it must be taken into consideration for each neighbor affecting wind
turbine, the sector disturbed and their associated probability of occurrence conditioned on hub height mean wind speed.

61400-1 Amend.1 © IEC:2010(E) – 13 –

σˆ is the estimated standard deviation of the ambient turbulence standard deviation;

σ
V
hub
σ = + σ
ˆ ˆ is the characteristic value of the maximum center-wake, hub height
T c
⎛ ⎞
0,8d
i
⎜ ⎟
1,5 +
⎜ ⎟
C
T
⎝ ⎠
ˆ
turbulence standard deviation (σ shall not account for farm generated ambient
c
turbulence);
C is the characteristic value of the wind turbine thrust coefficient for the corresponding

T
hub height wind velocity. If the thrust coefficient for the neighbouring wind turbines are

not known, a generic value C = 7 c /V can be used;
T hub
d is the distance, normalised by rotor diameter, to neighbouring wind turbine no. i;
i
c is a constant equal to 1 m/s;
I is the effective turbulence intensity;

eff
N is the number of neighbouring wind turbines; and
m is the Wöhler curve exponent corresponding to the material of the considered structural
component.
Wake effects from wind turbines “hidden” behind other machines need not be considered, for
example in a row, only wakes from the two units closest to the machine in question are to be
taken into account. Dependent on wind farm configuration, the number of nearest wind
turbines to be included in the calculation of I is as given in Table D.1.
eff
The wind farm configurations are illustrated in Figure D.1 for the case “Inside a wind farm with
more than 2 rows”.
Table D.1 – Number of nearest wind turbine to be considered
Wind farm configuration N
2 wind turbines 1
1 row 2
2 rows 5
Inside a wind farm with more than 2 rows 8

Inside large wind farms, wind turbines tend to generate their own ambient turbulence. Thus,
when
a) the number of wind turbines from the considered unit to the “edge” of the wind farm is
more than 5, or
b) the spacing in the rows perpendicular to the predominant wind direction is less than 3D,
then the following characteristic ambient turbulence shall be assumed instead of σˆ except in
c
the expression for σ :
ˆ
T
ˆˆ′ ˆˆ ˆ
σσ=+σ+σ+1, 28σ (D.4)
cw2() σ
where
0,36V
hub
σˆ = (D.5)
w
d d
r f
1+ 0,2
C
T
– 14 – 61400-1 Amend.1 © IEC:2010(E)

In which d and d are separations in rotor diameters in rows and separation between rows,
r f
respec
...

IEC 61400-1 ®
Edition 3.0 2010-10
INTERNATIONAL
STANDARD
NORME
INTERNATIONALE
A MENDMENT 1
AM ENDEMENT 1
Wind turbines –
Part 1: Design requirements
Eoliennes –
Partie 1: Exigences de conception

IEC 61400-1:2005-08/AMD1:2010-10(en-fr)

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IEC 61400-1 ®
Edition 3.0 2010-10
INTERNATIONAL
STANDARD
NORME
INTERNATIONALE
A MENDMENT 1
AM ENDEMENT 1
Wind turbines –
Part 1: Design requirements
Eoliennes –
Partie 1: Exigences de conception

INTERNATIONAL
ELECTROTECHNICAL
COMMISSION
COMMISSION
ELECTROTECHNIQUE
INTERNATIONALE
ICS 27.180 ISBN 978-2-8322-1971-3

– 2 – IEC 61400-1:2005/AMD1:2010

© IEC 2010
FOREWORD
This amendment has been prepared by IEC technical committee 88: Wind turbines.

This bilingual version (2015-02) corresponds to the English version, published in 2010-10.

The text of this amendment is based on the following documents:

FDIS Report on voting
88/374/FDIS 88/378/RVD
Full information on the voting for the approval of this amendment can be found in the report
on voting indicated in the above table.
The French version of this amendment has not been voted upon.
The committee has decided that the contents of this amendment and the base publication will
remain unchanged until the stability date indicated on the IEC web site 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.
_____________
© IEC 2010
2 Normative references
Replace the existing list of normative references by the following new list:

IEC 60204-1, Safety of machinery – Electrical equipment of machines – Part 1: General

requirements
IEC 60204-11, Safety of machinery – Electrical equipment of machines – Part 11:

Requirements for HV equipment for voltages above 1 000 V a.c. or 1 500 V d.c. and not

exceeding 36 kV
IEC 60364 (all parts), Low-voltage electrical installations
IEC 60364-5-54, Electrical installations of buildings – Part 5-54: Selection and erection of
electrical equipment – Earthing arrangements, protective conductors and protective bonding
conductors
IEC 60721-2-1, Classification of environmental conditions – Part 2: Environmental conditions
appearing in nature – Temperature and humidity
IEC 61000-6-1, Electromagnetic compatibility (EMC) – Part 6-1: Generic standards –
Immunity for residential, commercial and light-industrial environments
IEC 61000-6-2, Electromagnetic compatibility (EMC) – Part 6-2: Generic standards –
Immunity for industrial environments
IEC 61000-6-4, Electromagnetic compatibility (EMC) – Part 6-4: Generic standards –
Emission standard for industrial environments
IEC 61400-2, Wind turbines – Part 2: Design requirements for small wind turbines
IEC 61400-21, Wind turbines – Part 21: Measurement and assessment of power quality
characteristics of grid connected wind turbines
IEC 61400-24, Wind turbines – Part 24: Lightning protection
IEC 62305-3, Protection against lightning – Part 3: Physical damage to structures and life
hazard
IEC 62305-4, Protection against lightning – Part 4: Electrical and electronic systems within
structures
ISO 76:2006, Rolling bearings – Static load ratings
ISO 281, Rolling bearings – Dynamic load ratings and rating life
ISO 2394:1998, General principles on reliability for structures
ISO 2533:1975, Standard atmosphere
ISO 4354, Wind actions on structures
ISO 6336-2, Calculation of load capacity of spur and helical gears – Part 2: Calculation of
surface durability (pitting)
– 4 – IEC 61400-1:2005/AMD1:2010

© IEC 2010
ISO 6336-3:2006, Calculation of load capacity of spur and helical gears – Part 3: Calculation

of tooth bending strength
ISO 81400-4, Wind turbines – Part 4: Design and specification of gearboxes

3 Terms and definitions
3.26 – limit state
Replace ISO 2394 by 2.2.9 of ISO 2394.

3.55 – ultimate limit state
Replace ISO 2394 by 2.2.10 of ISO 2394.
4 Symbols and abbreviated terms
4.1 Symbols and units
Switch the definitions of σ and σ . The vertical wind velocity standard deviation should be σ ,
2 3 3
not σ .
6 External conditions
6.3.1.3 Normal turbulence model (NTM)
Replace the existing Figures 1a and 1b by the following new figures:

Category A
4,5
Category B
Category C
3,5
2,5
1,5
0,5
0 5 10 15 20 25 30
V  (m/s)
hub IEC  2236/10
Figure 1a –Turbulence standard deviation for the normal turbulence model (NTM)

s  (m/s)
© IEC 2010
0,5
Category A
Category B
0,4
Category C
0,3
0,2
0,1
0 5 10 15 20 25 30
V  (m/s)
hub
IEC  2237/10
Figure 1b – Turbulence intensity for the normal turbulence model (NTM)
6.3.2.6 Extreme wind shear (EWS)
Replace the number 2,5 in equations (26) and (27) to 2,5 [m/s]. (The number 2,5 in equations
(26) and (27) is not dimensionless.)
7 Structural design
7.4.2 Power production plus occurrence of fault or loss of electrical network
connection (DLC 2.1 – 2.4)
nd
Add, as 2 paragraph, the following new text:
As an alternative to the specification of DLC 2.3 above and in Table 2, DLC 2.3 may instead
be considered as a normal event (i.e. a partial safety factor for load of 1,35) to be analyzed
using stochastic wind simulations (NTM - V in hub out
electrical system fault (including loss of electrical network connection). In this case, 12
response simulations shall be carried out for each considered mean wind speed. For each
response simulation, the extreme response after the electrical fault has occurred is sampled.
The fault must be introduced after the effect of initial conditions has become negligible. For
each mean wind speed, a nominal extreme response is evaluated as the mean of the 12
sampled extreme responses plus three times the standard deviation of the 12 samples. The
characteristic response value for DLC 2.3 is determined as the extreme value among the
nominal extreme responses.
7.5 Load calculations
Add, after second paragraph, the following new text:
When turbulent winds are used for dynamic simulations, attention should be given to the grid
resolution regarding the spatial and time resolution.
—————————
Concerning the spatial resolution, the maximum distance between adjacent points should be smaller than 25 %
of Λ1 (Equation (5)) and no larger than 15 % of the rotor diameter. This distance is meant to be the diagonal
distance between points in each grid cell defined by four points. In the case of a non-uniform grid, an average
value over the rotor surface of the distance between grid points can be considered as the representative spatial
resolution, but this distance should always decrease towards the blade tip.
Turbulence intensity
– 6 – IEC 61400-1:2005/AMD1:2010

© IEC 2010
Replace the last paragraph by the following new text:

Ultimate load components may also be combined in a conservative manner assuming the
extreme component values occur simultaneously. In case this option is pursued, both
minimum and maximum extreme component values shall be applied in all possible

combinations to avoid introducing non-conservatism.

Guidance for the derivation of extreme design loads from contemporaneous loads taken from

a number of stochastic realisations is given in Annex H.

7.6.1.2 Partial safety factor for consequence of failure and component classes

Add, after the bullets defining the component classes, the following new text:
The consequences of failure factor shall be included in the test load when performing tests
as for example full scale blade testing.
7.6.2 Ultimate strength analysis
Replace equation (31) by the following new equation:
1 1
γ F ≤ ⋅ f (31)
f k k
γ γ
n m
Add the following new paragraph after equation (31):
Note that γ is a consequence of failure factor and shall not be treated as a safety factor on
n
materials.
th
Delete the last sentence in 5 paragraph (“For guidance see Annex F”) and insert, after the
th
5 paragraph, the following two paragraphs:
Data used in extrapolation methods shall be extracted from time series of turbine simulations
of at least 10 min in length over the operating range of the turbine for DLC 1.1. A minimum of
15 simulations is required for each wind speed from (V – 2 m/s) to cut-out and six
rated
simulations are required for each wind speed below (V – 2 m/s). When extracting data,
rated
the designer must consider the effect of independence between peaks on the extrapolation
and minimize dependence when possible. The designer shall aggregate data and probability
distributions to form a consistent long-term distribution. To ensure stable estimation of long-
term loads, a convergence criterion shall be applied to a probability fractile less than the

mode of the data for either the short-term or long-term exceedance distributions. For
guidance, see Annex F.
The characteristic value for blade root in-plane and out-of-plane moments and tip deflection
may be determined by a simplified procedure . The characteristic value may then be
determined by calculating the mean of the extremes for each 10-min bin and using the largest
value, multiplied by an extrapolation factor of 1,5, while maintaining the partial load factor for
statistical load extrapolation, see Table 3.
—————————
This approach is considered conservative for 3-bladed upwind wind turbines. Caution should be exercised for
other wind turbine concepts.
© IEC 2010
7.6.2.1 Partial safety factor for loads

Replace the existing formula in the footnote of Table 3 by the following new formula:


F
gravity
1− ; F ≤ F

gravity k

F
ς = k


0; F > F
 gravity k

Add the following new text after Table 3:

The approach in 7.6.1.1, where the partial safety factor for loads is applied to the load
response, assumes that a proper representation of the dynamic response is of prime concern.
For foundations or where a proper representation of non-linear material behaviour or
geometrical non-linearities or both are of primary concern, the design load response S shall
d
be obtained from a structural analysis for the combination of the design loads F , where the
d
design load is obtained by multiplication of the characteristic loads F by the specified partial

k
load factor γ for favourable and unfavourable loads,
f
F = γ F
d f k
The load responses in the tower at the interface (shear forces and bending moments) factored
with γ from Table 3 shall be applied as boundary conditions.
f
For gravity foundations, the limit states considering overall stability (rigid body motion with no
failure in soil) and bearing capacity of soil and foundation shall be regarded and calculated
according to a recognized standard. In general, a partial safety factor of γ = 1,1 for
f
unfavourable permanent loads and γ = 0,9 for favourable permanent loads shall be applied
f
for foundation load, backfilling and buoyancy. If it can be demonstrated by respective quality
management and surveillance that the foundation material densities specified in the design
documentation are met on site, a partial safety factor for permanent foundation load γ = 1,0
f
can be used for the limit states regarding bearing capacity of soil and foundation. If buoyancy
is calculated equal to a terrain water level, a partial safety factor for buoyancy γ = 1,0 can be
f
applied.
Alternatively, the check of capacity of soil and foundation can be based on a partial safety
factor γ = 1,0 for both favourable and unfavourable permanent loads and the check of overall
f
stability can be based on a partial safety factor of γ = 1,1 for unfavourable permanent loads
f
and γ = 0,9 for favourable permanent loads, using in all cases conservative estimates of
f
weights or densities defined as 5 % / 95 % fractiles. The lower fractile is to be used when the
load is favourable. Otherwise, the upper fractile is to be used.

7.6.5 Critical deflection analysis
Replace the existing text by the following new text:
7.6.5.1 General
It shall be verified that no deflections affecting structural integrity occur in the design
conditions detailed in Table 2.
The maximum elastic deflection in the unfavourable direction shall be determined for the load
cases detailed in Table 2 using the characteristic loads. The resulting deflection is then
multiplied by the combined partial safety factor for loads, materials and consequences of
failure.
• Partial safety factor for loads
The values of γ shall be chosen from Table 3.
f
– 8 – IEC 61400-1:2005/AMD1:2010

© IEC 2010
• Partial safety factor for the elastic properties of materials

The value of γ shall be 1,1 except when the elastic properties of the component in question
m
have been determined by testing and monitoring in which case it may be reduced. Particular

attention shall be paid to geometrical uncertainties and the accuracy of the deflection

calculation method.
• Partial safety factor for consequences of failure

Component class 1: γ = 1,0
n
= 1,0
Component class 2: γ
n
Component class 3: γ = 1,3.
n
The elastic deflection shall then be added to the un-deflected position in the most
unfavourable direction and the resulting position compared to the requirement for non-
interference.
7.6.5.2 Blade (tip) deflection
One of the most important considerations is to verify that no mechanical interference between
blade and tower will occur.
In general, blade deflections have to be calculated for the ultimate load cases as well as for
the fatigue load cases. The deflections caused by the ultimate load cases can be calculated
based on beam models, FE models or the like. All relevant load cases from Table 2 have to
be taken into account with the relevant partial load safety factors.
Moreover, for load case 1.1 extrapolation of tip deflection is mandatory according to 7.4.1.
Here direct dynamic deflection analysis can be used. The exceedance probability in the most
unfavourable direction shall be the same for the characteristic deflection as for the
characteristic load. The characteristic deflection is then to be multiplied by the combined
safety factor for loads, materials and consequences of failure and be added to the un-
deflected position in the most unfavourable direction and the resulting position compared to
the requirement for non-interference.
9 Mechanical systems
9.4 Main gearbox
Replace the existing text by the following new text:
The main gearbox shall be designed according to ISO 81400-4, until a similar document is

published in the IEC 61400 series.
9.5 Yaw system
Replace the second paragraph by the following new text:
Any motors shall comply with relevant parts of Clause 10.
Non-redundant parts of the gear system such as the final yaw gear shall be considered as
component class 2. When multiple yaw drives ensure sufficient redundancy in the yaw gear
system, and easy replacement is possible, the reduction gearbox and the final drive pinion
may be considered to be in component class 1.
The safety against pitting shall be determined in accordance with ISO 6336-2. The application
of the upper limit curve (1) for life factor Z , which allows limited pitting, is permissible.
NT
Sufficient tooth bending strength shall be proven in accordance with ISO 6336-3. The reverse

© IEC 2010
bending loads on gear teeth shall be considered in accordance with ISO 6336-3 Annex B.

Minimum values for S and S are specified in Table 5. These values must be achieved by
F H
using characteristic loads F Hence they include the partial safety factor for consequences, γ ,

k n
materials, γ and loads, γ .
m f
Table 5 – Minimum required safety factor S and S for the yaw gear system
H F
Component class 1 Component class 2

Surface durability (pitting) s ≥ 1,0 s ≥ 1,1

H H
Tooth bending fatigue strength
s ≥ 1,1 s ≥ 1,25
F F
Static bending strength s ≥ 1,0 s ≥ 1,2
F F
Lower safety factors may be applicable in cases where efficient monitoring is implemented. If
safety factors below 1,0 are applied, then the maintenance manual must reflect anticipated
replacement intervals.
10 Electrical system
10.5 Earth system
Replace, in the first paragraph, IEC 61024-1 by IEC 62305-3.
10.6 Lightning protection
Replace IEC 61024-1 by IEC 62305-3.
10.9 Protection against lightning electromagnetic fields
Replace, in the first paragraph, IEC 61312-1 by IEC 62305-4.
11 Assessment of a wind turbine for site-specific conditions
11.2 Assessment of the topographical complexity of a site
Replace the text of this subclause by the following new text:
The complexity of the site is characterised by the slope of the terrain and variations of the
terrain topography from a plane.

To obtain the slope of the terrain, planes are defined that fit the terrain within specific
distances and sector amplitudes for all wind direction sectors around the wind turbine and
pass through the tower base. The slope, used in Table 4, denotes the slopes of the different
mean lines of sectors passing through the tower bases and contained in the fitted planes.
Accordingly, the terrain variation from the fitted plane denotes the distance, along a vertical
line, between the fitted plane and the terrain at the surface points.

– 10 – IEC 61400-1:2005/AMD1:2010

© IEC 2010
Table 4 – Terrain complexity indicators

Maximum terrain
Distance range from Maximum slope of fitted

Sector amplitude
wind turbine plane
variation
< 5 z 360º < 0,3 z
hub hub
30º
< 10 z < 10º < 0,6 z
hub hub
30º
< 20 z < 1,2 z
hub hub
The resolution of surface grids used for terrain complexity assessment must not exceed the

smallest of 1,5 z and 100 m.
hub
The site shall be considered complex, if 15 % of the energy in the wind comes from sectors
that fail to conform to the criteria in Table 4 and homogeneous, if less than 5 % of the energy
in the wind comes from sectors that fail to conform.
A complexity index i is defined, such that i = 0 when less than 5 % of the energy comes
c c
from complex sectors, and i = 1 when more than 15 % of the energy comes from complex
c
sectors. In between, i varies linearly.
c
11.4 Assessment of wake effects from neighbouring wind turbines
Add the following new text after the 3rd paragraph:
Generally, the effective turbulence for fatigue and various ultimate loads cannot be assumed
to be the same.
th
Delete the 4 paragraph to the end of the subclause.
11.9 Assessment of structural integrity by reference to wind data
Replace the existing footnote 18 by the following new footnote:
The effect of complex terrain may be included by additional multiplication with a turbulence
structure correction parameter C defined as
CT
2 2
1+ (sˆ / sˆ ) + (sˆ / sˆ )
2 1 3 1
C =
CT
1,375
where ratios of the estimated standard deviations, sˆ , correspond to hub height values. Where

i
there are no site data for the components of turbulence and the terrain is complex, results of
modelling or C = 1+0,15 i , where i is the complexity index defined in Subclause 11.2, may
CT c c
be used.
th
Replace the 5 paragraph to the end of the subclause by the following new text:
An adequate assessment of wake effects can be performed by verifying that the turbulence
standard deviation s from the normal turbulence model is greater or equal to the estimated
90 % fractile of the turbulence standard deviation (including both ambient and wake
—————————
3 The check criteria is considered fulfilled if the requisite fails over a surface less than 5 z .
hub
This approach can also be used for the assessment of sector-wise varying turbulence, alone or in combination
with wake turbulence. The standard deviation of sˆ may be determined as the average of the sector-wise
sˆ
s
values.
© IEC 2010
turbulence) between the wind speeds 0,2 V and 0,4 V (or when the turbine properties are
ref ref
known, between 0,6 V and V ), i.e.:
r out
s ≥⋅IV
(35)
1 eff hub
Guidance for calculating I can be found in Annex D.
eff
Furthermore, it shall be demonstrated that the site specific horizontal shear due to partial

wakes does not exceed EWS in 6.3.2.6 and that the site specific extreme turbulence ,

including the wake effects, does not exceed the ETM model in 6.3.2.3. For determination of

the site specific turbulence, the site specific conditions, the frequency of the wake situations
and wind farm layout shall be accounted for.
11.10 Assessment of structural integrity by load calculations with reference to site
specific conditions
nd
Replace the 2 paragraph to the end of the subclause by the following new text:
Where there are no site data for the components of turbulence and the terrain is complex, it
shall be assumed that the lateral and upward turbulence standard deviations relative to the
longitudinal component are equal to 1,0 and 0,7, respectively.
In the case of wake effects, it shall be verified that structural integrity is not compromised for
ultimate and fatigue limit states. For fatigue limit state in DLC 1.2 s in the normal turbulence,
model is replaced by an appropriate wake turbulence model, e.g. I , found in Annex D.
eff
For ultimate limit state analysis, DLC 1.1 or DLC 1.3, as well as DLC 1.5, shall be applied with
site specific conditions including wake effects represented by appropriate models. NTM for
ULS loads can be set to characteristic ambient turbulence inside large farms as defined in
Annex D, Equation (D.4).
Since for fatigue load calculations, I as defined in Annex D depends on the Wöhler curve
eff
exponent m of the material of the considered component, the loads on structural components
with other material properties shall either be recalculated or assessed with the appropriate
value of m.
Annex B – Turbulence models
B.1 Mann (1994) uniform shear turbulence model

Replace the equation defining C by the following new equation:
 2 2 
β (k)k k + k
k k  
1 1 2
2 0
C = arctan
 
2 2 2  k − (k + β (k)k )k β (k) 
0 3 1 1
(k + k )
 
1 2
—————————
The site specific extreme turbulence may be represented by the maximum centre wake turbulence in the most
severe direction.
– 12 – IEC 61400-1:2005/AMD1:2010

© IEC 2010
Annex D – Wake and wind farm turbulence

Replace the existing text of Annex D by the following new text:

D.1 Wake effects
Wake effects from neighbouring wind turbines may be taken into account during normal

operation for fatigue calculation by an effective turbulence intensity I , Frandsen (2007). The
eff,
effective turbulence intensity – conditioned on hub height mean wind speed - may be defined
as
2p
 
m
 
m
(D.1)
I (V ) = p(θV )I (θV )dθ
 
eff hub hub
hub
∫
 
 
where
V is the wind speed at hub height;
hub
p is the probability density function of wind direction;
I is the turbulence intensity of the combined ambient and wake flows from wind direction θ,
and
m is the Wöhler (SN-curve) exponent for the considered material.
In the following, a uniform distribution pV()θ is assumed. It is also acceptable to adjust
hub
the formulas for other than uniform distribution . No reduction in mean wind speed inside the
wind farm shall be assumed.
If min{d } ≥ 10 D:
I
ˆ
σ
c
I = (D.2)
eff
V
hub
If min{d } < 10 D:
I
N
  m
ˆ
s
eff m m
ˆ ˆ
I = = (1– N p )s + p s (d ) ; p = 0,06 (D.3)
eff w c w∑ i w
T
V V
 
hub hub
 i =1 
where
ssˆˆ+1,28sˆ is the characteristic ambient turbulence standard deviation;
c s
ˆ
s is the estimated ambient turbulence standard deviation;
—————————
In the case of non-uniform distribution or non-grid wind farm layout, the formula must be modified accordingly, maintaining
the concept implied in the more general formula D.1, it must be taken into consideration for each neighbor affecting wind
turbine, the sector disturbed and their associated probability of occurrence conditioned on hub height mean wind speed.
=
© IEC 2010
ˆ
s is the estimated standard deviation of the ambient turbulence standard deviation;

s
V
hub
sˆ = + sˆ is the characteristic value of the maximum center-wake, hub height
T c
 
0,8d
i
 
1,5 +
 
C
T
 
ˆ
turbulence standard deviation ( s shall not account for farm generated ambient
c
turbulence);
C is the characteristic value of the wind turbine thrust coefficient for the corresponding
T
hub height wind velocity. If the thrust coefficient for the neighbouring wind turbines are

not known, a generic value C = 7 c /V can be used;
T hub
d is the distance, normalised by rotor diameter, to neighbouring wind turbine no. i;
i
c is a constant equal to 1 m/s;
I is the effective turbulence intensity;

eff
N is the number of neighbouring wind turbines; and
m is the Wöhler curve exponent corresponding to the material of the considered structural
component.
Wake effects from wind turbines “hidden” behind other machines need not be considered, for
example in a row, only wakes from the two units closest to the machine in question are to be
taken into account. Dependent on wind farm configuration, the number of nearest wind
turbines to be included in the calculation of I is as given in Table D.1.
eff
The wind farm configurations are illustrated in Figure D.1 for the case “Inside a wind farm with
more than 2 rows”.
Table D.1 – Number of nearest wind turbine to be considered
Wind farm configuration N
2 wind turbines 1
1 row 2
2 rows 5
Inside a wind farm with more than 2 rows 8

Inside large wind farms, wind turbines tend to generate their own ambient turbulence. Thus,
when
a) the number of wind turbines from the considered unit to the “edge” of the wind farm is
more than 5, or
b) the spacing in the rows perpendicular to the predominant wind direction is less than 3D,
then the following characteristic ambient turbulence shall be assumed instead of sˆ except in
c
the expression for :
sˆ
T
2 2
ˆ′ ˆˆ ˆ ˆ
s ss++s+1,28s (D.4)
cw2 ( ) s
where
0,36V
hub
sˆ = (D.5)
w
d d
r f
1+ 0,2
C
T
=
– 14 – IEC 61400-1:2005/AMD1:2010

© IEC 2010
In which d and d are separations in rotor diameters in rows and separation between rows,
r f
respectively.
IEC  2238/10
Figure D.1 – Configuration – Inside a wind farm with more than 2 rows
D.2 Reference documents
FRANDSEN S. (2007) Turbulence and turbulence generated loading in wind turbine clusters,
Risø report R-1188.
© IEC 2010
Annex F – Statistical extrapolation of loads for ultimate strength analysis

Replace the existing text of Annex F by the following new text:

F.1 General
Failure of a structure occurs when the stress at a critical location exceeds the resistance

capacity of the component material. Assuming that local stresses are related to the loading so

that the stress progressively increases with increased loading, the strength of a structural

component can be defined in terms of an ultimate load that causes failure. Given the service

loading, the adequacy of the structure can be assessed by comparing the extreme values of
the loading with the ultimate load resistance, applying suitable factors of safety.
For wind turbines, loading depends on the turbulent wind inflow for a variety of wind
conditions. Thus, it is necessary to analyze the extreme values of the loading on a statistical
basis in order to determine a suitable characteristic load.
For a given wind condition, it is reasonable to model the short-term load response as a
stationary random process. Given that loads can be represented as such processes, methods
are described in the following for the extraction of data for extrapolation and load
extrapolation. Convergence criteria also are proposed and an alternative for estimating the
long-term loads using the Inverse first-order reliability method (IFORM) is given.
The methods have been tested for a 3-bladed horizontal-axis upwind turbine. Special
attention may be necessary for other wind turbine concepts and/or control schemes including
load feedback. More information and guidance can be found in [1] .
F.2 Data extraction for extrapolation
Data used in extrapolation methods are extracted from time series of turbine simulation over
the operating range of the turbine in specified wind conditions. Data may be extracted by
choosing the global individual response extremes from each simulation or some subset
created by breaking the simulation into blocks of equal time or ensuring a minimum time
separation between extremes.
Establishing independence among the individual load response extremes is important for
some methods of extrapolation. When extracting, the designer must consider the effect of
independence between peaks on the extrapolation and minimize dependence when possible.
If the method chosen for extrapolation is sensitive to independence assumption (e.g. the
method involves transforming probability functions between time bases), the designer should

attempt to statistically test for independence.
A simple approach to ensure independence is to assume that the global extreme in each ten-
minute simulation or local extremes from intervals no shorter than three response cycles are
independent and thus require a minimum time separation between individual response
extremes of three response cycles (defined by three mean crossings over the block size). If a
systematic statistical approach is desired, the designer may test for independence using
standard estimation techniques (e.g. [5],[6]) and then minimize dependence in a controlled
manner.
Peak over threshold methods may also be employed, but the designer must be careful that
truncation errors and correlation introduced by the threshold do not influence the shape of the
empirical distribution dramatically.
—————————
Figures in square brackets refer to Clause F.6.

– 16 – IEC 61400-1:2005/AMD1:2010

© IEC 2010
F.3 Load extrapolation methods

F.3.1 General
The suggested approaches of extrapolation of extreme events for determination of the 50-

year load of a wind turbine can be divided into the following procedures.

a) Parametric fitting and aggregation afterwards

Subdivision of the operational range of the turbine into discrete wind speeds and

performance of time domain simulations at the normal turbulence (NTM) level. Estimation

of an extreme value (parametric) distribution [2] for every wind speed realization.

Aggregation of all distributions according to the long-term distribution function of the mean
wind speed. Prediction of the 50-year value of the aggregated distribution function. For
global extreme from ten-minute simulations, the probability of the 50-year load is
–7
3,8 × 10 .
b) Data aggregation first and fitting afterwards
Subdivision of the operational range of the turbine into discrete wind speeds and
performance of time domain simulations at normal turbulence (NTM) level. Aggregation of
all relevant extremes from all time series according to the long-term distribution function of
the mean wind speed within the operational range of the turbine. Estimation of one
(aggregated) distribution function for all extremes. Prediction of the 50-year value from the
resulting distribution function.
Two different cases are regarded for aggregation of simulated short-term distributions of
extremes for a specific observation period T into an empirical distribution of the long-term
extremes for the same period: extrapolation from global extremes, and from local extremes.
F.3.2 Global extremes
The short-term distribution of global extremes in the observation period, T, is denoted
F (|s V;T ) (F.1)
short−term
where s stands for load response. From this, and by use of the long-term distribution of the
mean wind speeds, the long-term distribution of extreme values is obtained:
V
out
F (s;)T = F (s |V;)T f (V )dV (F.2)
long −−term short term
∫
V
in
The extreme load response, s , of the desired return period, T , is obtained from the following
r r
equation:
1 T
r
F (sT; )=1−=,  N (F.3)
long −term r
NT
The practical implementation of these formulas would typically be to use discrete wind speed
values. Then one has
M
F (sT; )≈ F (s |V ;T )p ,  p f (V )∆V ,V≤ V<<. V≤ V
long −−term ∑ short term k k k k k in 1 M out
(F.4)
k =1
=
© IEC 2010
The distribution F is obtained by fitting to the empirical distribution:
short-term
r
i
F (S V ) = ,i = 1,.,n (F.5)
short −term ki k k
n + 1
k
th
where s denotes the i extreme value sample from wind speed k and r is s ’s rank among
ki i ki
the n extremes arising from wind speed k. For the following developments, it is worth noting
k
that an equivalent expression for the empirical distribution by use of a summation is

n
k
F (S V ) = I (S – S ),i = 1,.,n (F.6)
short−term ki k ∑ kj ki k
n +1
k
j=1
where an indicator function I(x) has the expression:
10 for x ≤

Ix() = (F.7)

0 for x > 0

The task of the indicator function is to pick out all values less than or equal s in order that
ki
they can contribute to the empirical probability of having values less than or equal s . Note
ki
that the specific definition of the indicator function ensures that the event that identical
extreme values should be realized is accounted for.
F.3.3 Local extremes
Now the short-term distribution of global extremes in the observation period, T, is obtained
from n(V) independent local extreme values in the period (assuming the extremes are positive,
otherwise a change of sign may be made):
nV()
F (|s V;T ) = F (|s V;T ) (F.8)
short−term local
The long-term distribution, defined in (F.9), and the extreme load response, s , of the desired
r
return period, T , are established as described in the previous subclause. Strictly, n should be
r
a random number for which a distribution (dependent on V) must be assumed. However, n has
for wind turbine applications limited variation compared to its mean value. Consequently,
replacing n by its mean value (conditional on V), as implicitly done above, is sufficiently
accurate. The approximation may be accepted if, when applying the formulas proposed in the
following, one uses an s-value representative of the wind speeds that contribute most to the

specific load response under consideration. Based on the approximation one has the following
expressions:
V
out
n
(F.9)
F (s;)T = F (s |V;)T f (V )dV
long −term local
∫
V
in
1 T
r
(F.10)
F (sT; )=1−=, N
long −term r
NT
– 18 – IEC 61400-1:2005/AMD1:2010

© IEC 2010
F.3.4 Long-term empirical distributions

There are advantages to aggregating data from all wind speeds and then fitting a distribution

to the combined data. One method for accomplishing this is to compute a number of

simulations, where the number of simulations per bin is determined by the Weibull (or

appropriate) distribution of wind speed.

N (V )≈ N p ,  p fV( )∆V ,V≤ V<<. V≤ V (F.11)
sims k total k k k k in 1 M out

Once simulations are completed and maxima are extracted, all maxima from all wind speeds

are combined into a single distribution and ranked such that
r
i
F (S ) = ,i = 1,.,n (F.12)
long−term i total
n + 1
k
th
where s denotes the i extreme value sample over all wind speeds and r is s ’s rank among
i i i
the n extremes arising from the combined distribution.
total
One potential disadvantage of this method is that loads that are dominated by high wind
speeds may have very few simulations from which to extract large extreme values in the tail of
the empirical distribution. To address this issue, additional long-term distributions can be
calculated using additional simulations for the low probability wind speed bins. The total
simulation time per bin must follow the original wind speed distribution. But, a number of new
long-term empirical distributions can be formed using randomly bootstrapped data from all
bins, in which a large number of simulations are available. Once a number of long-term
distributions are formed, they can be averaged to form a single aggregate long-term
distribution that can be used for extrapolation to lower probability levels.
F.4 Convergence criteria
F.4.1 General
In the context of turbine extreme loads, the importance of different wind speeds varies
depending on the load that is being extrapolated. Some loads are dominated by wind speeds
near rated while others are dominated near cut-out or other wind speeds. It is important that
the designer examines the dominant wind speeds closely to ensure that a sufficient number of
simulations are carried out to ensure stability of the method. A minimum of 15 simulations is
– 2 m/s) to cut-out and six simulations are
necessary for each wind speed from (V
rated
necessary for each wind speed with V below (V – 2 m/s).
rated
In addition to a minimum number of simulations for the wind speeds (V – 2 m/s) to cut-
rated
out, an additional convergence criterion shall also be applied according to 7.6.2. The
recommended number of simulations is determined by calculating a confidence interval for the
resulting empirical distribution. The number of simulations deemed sufficient is
...