IEC 61400-12-6:2022

Wind energy generation systems - Part 12-6: Measurement based nacelle transfer function of electricity producing wind turbines

IEC 61400-12-6:2022 specifies a procedure for measuring the nacelle transfer function of a single electricity-producing, horizontal axis wind turbine, which is not considered to be a small wind turbine in accordance with IEC 61400-2. It is expected that this document be used when a valid nacelle transfer function is needed to execute a power performance measurement according to IEC 61400-12-2. This document specifies how to characterise a wind turbine's nacelle transfer function. The nacelle transfer function is determined by collecting simultaneous measurements of nacelle‑measured wind speed and free stream wind speed (as measured on a meteorological mast) for a period that is long enough to establish a statistically significant database over a range of wind speeds and under varying wind and atmospheric conditions. The procedure also provides guidance on determination of measurement uncertainty including assessment of uncertainty sources and recommendations for combining them.
This first edition of IEC 61400-12-6 is part of a structural revision that cancels and replaces the performance standards IEC 61400-12-1:2017 and IEC 61400-12-2:2013. The structural revision contains no technical changes with respect to IEC 61400-12-1:2017 and IEC 61400‑12‑2:2013, but the parts that relate to wind measurements, measurement of site calibration and assessment of obstacle and terrain have been extracted into separate standards.

Systèmes de génération d'énergie éolienne - Partie 12-6: Fonction de transfert de la nacelle fondée sur le mesurage des éoliennes de production d'électricité

IEC 61400-12-6:2022 spécifie une procédure de mesure de la fonction de transfert de la nacelle d’une éolienne simple de production d’électricité à axe horizontal, qui n’est pas considérée comme un petit aérogénérateur selon l’IEC 61400-2. Il est attendu que le présent document soit utilisé lorsqu’il est nécessaire d'employer une fonction de transfert de la nacelle valide pour mesurer la performance de puissance conformément à l’IEC 61400-12-2. Le présent document décrit la façon de déterminer la fonction de transfert de la nacelle d’une éolienne. La fonction de transfert de la nacelle est déterminée en collectant simultanément des valeurs de mesure de la vitesse du vent mesurée à la nacelle et de la vitesse du vent en écoulement libre (mesurée sur un mât météorologique) pendant une période suffisamment longue pour créer une base de données statistiquement significative sur une certaine plage de vitesses du vent et dans des conditions de vent et des conditions atmosphériques variables. Cette procédure fournit également des recommandations relatives à la détermination de l’incertitude de mesure, incluant l’évaluation des sources d’incertitude et des recommandations qui permettent de les combiner.
La présente première édition de l’IEC 61400-12-6 fait partie d'une révision structurelle qui annule et remplace les normes de performance IEC 61400-12-1:2017 et IEC 61400-12-2:2013. Cette révision structurelle ne contient aucune modification technique par rapport à l'IEC 61400‑12‑1:2017 et l'IEC 61400-12‑2:2013. Toutefois, les parties relatives aux mesurages du vent, au mesurage de l'étalonnage du site et à l'évaluation des obstacles et du terrain ont été extraites vers des normes distinctes.

General Information

• Status: Published
• Publication Date: 29-Aug-2022
• ICS: 27.180 - Wind turbine energy systems
• Technical Committee: TC 88 - Wind energy generation systems
• Drafting Committee: MT 12-1 - TC 88/MT 12-1
• Current Stage: PPUB - Publication issued
• Start Date: 30-Aug-2022
• Completion Date: 26-Sep-2022

Relations

• Replaces: IEC 61400-12-1:2017 - Wind energy generation systems - Part 12-1: Power performance measurements of electricity producing wind turbines
• Effective Date: 05-Sep-2023
• Replaces: IEC 61400-12-2:2013 - Wind turbines - Part 12-2: Power performance of electricity-producing wind turbines based on nacelle anemometry
• Effective Date: 05-Sep-2023

Standards Content (Sample)

IEC 61400-12-6 ®
Edition 1.0 2022-08
INTERNATIONAL
STANDARD
NORME
INTERNATIONALE
colour
inside
Wind energy generation systems –
Part 12-6: Measurement based nacelle transfer function of electricity producing
wind turbines
Systèmes de génération d'énergie éolienne –
Partie 12-6: Fonction de transfert de la nacelle fondée sur le mesurage des
éoliennes de production d'électricité

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IEC 61400-12-6 ®
Edition 1.0 2022-08
INTERNATIONAL
STANDARD
NORME
INTERNATIONALE
colour
inside
Wind energy generation systems –

Part 12-6: Measurement based nacelle transfer function of electricity producing

wind turbines
Systèmes de génération d'énergie éolienne –

Partie 12-6: Fonction de transfert de la nacelle fondée sur le mesurage des

éoliennes de production d'électricité

INTERNATIONAL
ELECTROTECHNICAL
COMMISSION
COMMISSION
ELECTROTECHNIQUE
INTERNATIONALE
ICS 27.180 ISBN 978-2-8322-5604-6

– 2 – IEC 61400-12-6:2022 © IEC 2022
CONTENTS
FOREWORD . 5
INTRODUCTION . 7
1 Scope . 8
2 Normative references . 8
3 Terms and definitions . 9
4 Symbols, units and abbreviated terms . 11
5 Overview of test method . 15
6 Preparation for measurement of nacelle transfer function . 15
6.1 General . 15
6.2 Wind turbine . 15
6.3 Test site . 16
6.3.1 General . 16
6.3.2 Terrain classification . 17
6.4 Test plan . 17
7 Test equipment . 17
7.1 General . 17
7.2 Data acquisition . 18
8 Measurement procedure . 19
8.1 General . 19
8.2 Data system(s) synchronisation . 19
8.3 Data collection . 20
8.4 Data quality check . 20
8.4.1 General . 20
8.4.2 Measured signals are in range and available . 20
8.4.3 Sensors are operating properly . 20
8.4.4 Ensure data acquisition system(s) is(are) operating properly . 21
8.4.5 Sector self-consistency check . 21
8.5 Data rejection . 21
8.6 Data correction . 21
8.7 Database . 22
9 Derived results . 22
9.1 Overview of derived results . 22
9.2 Determination of measured nacelle transfer function . 23
9.3 Data quality check . 24
9.3.1 General . 24
9.3.2 Directional stability check . 24
9.3.3 Self-consistency check for NTF, using the NPC . 25
9.4 Uncertainty analysis . 25
10 Reporting format . 25
Annex A (informative) Nacelle instrument mounting . 29
A.1 General . 29
A.2 Preferred method of anemometer's mounting . 29
A.3 Preferred position of anemometer . 29
Annex B (normative) Evaluation of uncertainty in measurement . 31
B.1 General . 31

B.2 The measurands . 31
B.3 Uncertainty components. 31
B.4 Wind direction uncertainty . 33
Annex C (normative) Theoretical basis for determining the uncertainty of
measurement using the method of bins . 34
C.1 General . 34
C.2 Propagation of uncertainty through the stages of NTF/NPC measurement . 35
C.3 Category A uncertainties . 38
C.3.1 General . 38
C.3.2 Category A uncertainty in electric power . 38
C.4 Category B uncertainties . 39
C.4.1 General . 39
C.4.2 Category B uncertainties in climatic variations . 40
C.5 Expanded uncertainty . 40
Annex D (normative) NTF uncertainty estimates and calculation . 41
D.1 Methods and assumptions. 41
D.1.1 General . 41
D.1.2 Site calibration . 41
D.1.3 Nacelle transfer function uncertainty component estimates . 43
D.1.4 Nacelle power curve uncertainty component estimates . 45
D.1.5 Wind direction uncertainty . 45
D.1.6 Contribution factors . 47
D.2 Uncertainty example calculations . 48
D.2.1 Example description . 48
D.2.2 Example case – NTF uncertainty . 49
D.2.3 Example case – NPC uncertainty . 49
Annex E (normative) Allowable anemometry instrument types. 51
E.1 General . 51
E.2 Recalibration of sonic anemometers . 51
E.3 Uncertainty of sonic and propeller anemometers . 51
Annex F (informative) Organisation of test, safety and communication . 52
F.1 General . 52
F.2 Responsibility for test . 52
F.3 Safety during test . 52
F.4 Communication . 52
F.5 Prior to test . 52
F.6 During test . 52
F.7 After test . 53
Annex G (informative) NTF flowchart . 54
Bibliography . 55

Figure 1 – Presentation of example data: measured transfer function . 27
Figure A.1 – Mounting of anemometer on top of nacelle . 30
Figure G.1 – NTF flowchart . 54

Table 1 – Example of presentation of a measured power curve based on data from the
meteorological mast, for consistency check . 28
Table B.1 – Uncertainty components in nacelle transfer function evaluation . 32

– 4 – IEC 61400-12-6:2022 © IEC 2022
Table B.2 – Uncertainty components in nacelle based absolute wind direction . 33
Table C.1 – Example cancellation sources . 36
Table C.2 – List of category A and B uncertainties for NTF . 39
Table C.3 – Expanded uncertainties. 40
Table D.1 – Estimates for uncertainty components from site calibration . 42
Table D.2 – Estimates for uncertainty components from NTF measurement . 44
Table D.3 – Estimates for uncertainty components for wind direction . 46
Table D.4 – Estimates for contribution factors for site calibration . 47
Table D.5 – Estimates for contribution factors for NTF . 48

INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
WIND ENERGY GENERATION SYSTEMS –

Part 12-6: Measurement based nacelle transfer
function of electricity producing wind turbines

FOREWORD
1) The International Electrotechnical Commission (IEC) is a worldwide organization for standardization comprising
all national electrotechnical committees (IEC National Committees). The object of IEC is to promote international
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8) Attention is drawn to the Normative references cited in this publication. Use of the referenced publications is
indispensable for the correct application of this publication.
9) Attention is drawn to the possibility that some of the elements of this IEC Publication may be the subject of patent
rights. IEC shall not be held responsible for identifying any or all such patent rights.
IEC 61400-12-6 has been prepared by IEC technical committee 88: Wind energy generation
systems. It is an International Standard.
This first edition of IEC 61400-12-6 is part of a structural revision that cancels and replaces the
performance standards IEC 61400-12-1:2017 and IEC 61400-12-2:2013. The structural revision
contains no technical changes with respect to IEC 61400-12-1:2017 and IEC 61400-12-2:2013,
but the parts that relate to wind measurements, measurement of site calibration and assessment
of obstacle and terrain have been extracted into separate standards.
The purpose of the re-structure was to allow the future management and revision of the power
performance standards to be carried out more efficiently in terms of time and cost and to provide
a more logical division of the wind measurement requirements into a series of separate
standards which could be referred to by other use case standards in the IEC 61400 series and
subsequently maintained and developed by appropriate experts.

– 6 – IEC 61400-12-6:2022 © IEC 2022
The text of this International Standard is based on the following documents:
Draft Report on voting
88/826/CDV 88/871/RVC
Full information on the voting for its approval can be found in the report on voting indicated in
the above table.
The language used for the development of this International Standard is English.
This document was drafted in accordance with ISO/IEC Directives, Part 2, and developed in
accordance with ISO/IEC Directives, Part 1 and ISO/IEC Directives, IEC Supplement, available
at www.iec.ch/members_experts/refdocs. The main document types developed by IEC are
described in greater detail at www.iec.ch/standardsdev/publications.
A list of all parts in the IEC 61400 series, published under the general title Wind energy
generation systems, can be found on the IEC website.
The committee has decided that the contents of this document will remain unchanged until the
stability date indicated on the IEC website under webstore.iec.ch in the data related to the
specific document. At this date, the document will be
• reconfirmed,
• withdrawn,
• replaced by a revised edition, or
• amended.
IMPORTANT – The "colour inside" logo on the cover page of this document indicates
that it contains colours which are considered to be useful for the correct understanding
of its contents. Users should therefore print this document using a colour printer.

INTRODUCTION
The IEC 61400-12 series consists of the following parts, under the general title Wind energy
generation systems:
IEC 61400-12: Power performance measurements of electricity producing wind
turbines – Overview
IEC 61400-12-1: Power performance measurement of electricity producing wind
turbines
IEC 61400-12-2: Power performance of electricity producing wind turbines based on
nacelle anemometry
IEC 61400-12-3: Power performance – Measurement based site calibration
IEC 61400-12-4: Numerical site calibration
IEC 61400-12-5: Power performance – Assessment of obstacles and terrain
IEC 61400-12-6: Measurement based nacelle transfer function of electricity
producing wind turbines
The purpose of this document is to provide a uniform methodology of measurement, analysis,
and reporting for the determination of a nacelle transfer function of electricity producing wind
turbines utilising nacelle-anemometry methods. This document is intended to be applied only
to horizontal axis wind turbines of sufficient size that the nacelle-mounted anemometer does
not significantly affect the flow through the turbine's rotor and around the nacelle and hence
does not affect the wind turbine's performance. The intent of this document is that the methods
presented herein be utilised when applying the methodology described in IEC 61400-12-2 to
determine the power performance of individual wind turbines. This will ensure that the results
are as consistent, accurate, and reproducible as possible within the current state of the art for
instrumentation and measurement techniques.
This procedure describes how to characterise a wind turbine's nacelle transfer function in terms
of wind speeds measured on a meteorological mast as well as a wind speed measured on the
hub or nacelle of a wind turbine. The anemometer that is placed on the turbine is measuring a
wind speed that is strongly affected by the test turbine's rotor. This procedure includes methods
for determining and applying appropriate corrections for this interference. Such a correction is
termed a nacelle transfer function which relates the wind speed measured on the turbine to a
free-stream wind speed as measured on a meteorological mast. The procedure also provides
guidance on determination of measurement uncertainty including assessment of uncertainty
sources and recommendations for combining them into uncertainties.
Even when anemometers are carefully calibrated in a quality wind tunnel, fluctuations in
magnitude and direction of the wind vector can cause different anemometers to perform
differently in the field. Further, the flow conditions close to a turbine nacelle are complex and
variable. Therefore, special care should be taken in the selection and installation of the
anemometer. These issues are addressed in this document.
This document will benefit those parties interested in power performance testing of wind
turbines using IEC 61400-12-2 as well as parties involved in the installation, planning and
execution of such tests. When and where appropriate, the technically accurate measurement
and analysis techniques recommended in this document should be applied by all parties to
ensure that continuing development and operation of wind turbines is carried out in an
atmosphere of consistent and accurate communication relative to environmental concerns. This
document presents measurement and reporting procedures expected to provide accurate
results that can be replicated by others.
Meanwhile, a user of this document should be aware of differences that arise from large
variations in wind shear and turbulence intensity, and from the chosen criteria for data selection.
Therefore, a user should consider the influence of these differences and the data selection
criteria in relation to the purpose of the test before engaging in nacelle transfer function
measurements.
– 8 – IEC 61400-12-6:2022 © IEC 2022
WIND ENERGY GENERATION SYSTEMS –

Part 12-6: Measurement based nacelle transfer
function of electricity producing wind turbines

1 Scope
This part of IEC 61400-12 specifies a procedure for measuring the nacelle transfer function of
a single electricity-producing, horizontal axis wind turbine, which is not considered to be a small
wind turbine in accordance with IEC 61400-2. It is expected that this document be used when
a valid nacelle transfer function is needed to execute a power performance measurement
according to IEC 61400-12-2.
A wind speed measured on the nacelle or hub of a wind turbine is affected by the turbine rotor
(i.e. speeded up or slowed down wind speed). In IEC 61400-12-1, an anemometer is located on
a meteorological tower that is located between two and four rotor diameters upwind of the test
turbine. This location allows direct measurement of the "free" wind with minimum interference
from the test turbine's rotor. In the procedure of this document, the anemometer is located on
or near the test turbine's nacelle. In this location, the anemometer is measuring a wind speed
that is strongly affected by the test turbine's rotor and the nacelle. The procedure in this
document includes methods for determining and applying appropriate corrections for this
interference. However, note that these corrections inherently increase the measurement
uncertainty compared to a properly configured test conducted in accordance with
IEC 61400-12-1.
This document specifies how to characterise a wind turbine's nacelle transfer function. The
nacelle transfer function is determined by collecting simultaneous measurements of
nacelle-measured wind speed and free stream wind speed (as measured on a meteorological
mast) for a period that is long enough to establish a statistically significant database over a
range of wind speeds and under varying wind and atmospheric conditions. The procedure also
provides guidance on determination of measurement uncertainty including assessment of
uncertainty sources and recommendations for combining them.
2 Normative references
The following documents are referred to in the text in such a way that some or all of their content
constitutes requirements of this document. For dated references, only the edition cited applies.
For undated references, the latest edition of the referenced document (including any
amendments) applies.
IEC 60688, Electrical measuring transducers for converting AC and DC electrical quantities to
analogue or digital signals
IEC 61400-12-1, Wind energy generation systems – Part 12-1: Power performance
measurement of electricity producing wind turbines
IEC 61400-12-2:2022, Wind energy generation systems – Part 12-2: Power performance of
electricity producing wind turbines based on nacelle anemometry
IEC 61400-12-3, Wind energy generation systems – Part 12-3: Power performance –
Measurement based site calibration
IEC 61400-12-5:2022, Wind energy generation systems – Part 12-5: Power performance –
Assessment of obstacles and terrain

IEC 61400-50-1, Wind energy generation systems – Part 50-1: Wind measurement –
Application of meteorological mast, nacelle and spinner mounted instruments
ISO/IEC GUIDE 98-3:2008, Uncertainty of measurement – Part 3: Guide to the expression of
uncertainty in measurement (GUM:1995)
3 Terms and definitions
For the purposes of this document, the following terms and definitions apply.
ISO and IEC maintain terminological databases for use in standardization at the following
addresses:
• IEC Electropedia: available at http://www.electropedia.org/
• ISO Online browsing platform: available at http://www.iso.org/obp
3.1
accuracy
closeness of the agreement between the result of a measurement and a true value of the
measurand
3.2
complex terrain
terrain surrounding the test site that features significant variations in topography and terrain
obstacles that may cause flow distortion
3.3
data set
collection of data sampled over a contiguous period
3.4
documentation
any information regarding the test which is kept in files or data, or both, but which is not
necessarily presented in the final report
3.5
flow distortion
change in air flow caused by obstacles, topographical variations, turbine's rotor, turbine's
nacelle or other wind turbines that results in a significant deviation of the measured wind speed
from the free stream wind speed
3.6
free stream wind speed
horizontal wind speed measured upstream of the rotor of the wind turbine generator that is
unaffected by rotor aerodynamics
3.7
turbulence intensity
ratio of the wind speed standard deviation to the mean wind speed, determined from the same
set of measured data samples of horizontal wind speed, and taken over a specific period of
time
3.8
hub height
height of the centre of the swept area of the wind turbine rotor above the ground
level at the tower base
– 10 – IEC 61400-12-6:2022 © IEC 2022
3.9
measurement period
period during which a statistically significant database has been collected for the power
performance test
3.10
measurement sector
sector of wind directions from which data are selected for the measured power curve or during
the determination of the nacelle transfer function
3.11
measurement uncertainty
parameter, associated with the result of a measurement, which characterises the dispersion of
the values that could reasonably be attributed to the measurand
3.12
method of bins
data reduction procedure that groups test data for a certain parameter into intervals (bins)
Note 1 to entry: The method of bins is normally used for wind speed bins but is also applicable to other parameters.
Note 2 to entry: For each bin, the number of data sets or samples and their sum are recorded, and the average
parameter value within each bin is calculated.
3.13
nacelle
housing which contains the drive train and other elements on top of a horizontal axis wind
turbine generator
3.14
NPC
nacelle power curve
measured power performance of a wind turbine expressed as net active electric power output
from the wind turbine as a function of free stream wind speed
Note 1 to entry: For the NPC, the free stream wind speed is not directly measured, but rather the nacelle wind
speed is measured and a nacelle transfer function is applied to arrive at the free stream wind speed.
3.15
nacelle wind speed
horizontal wind speed measured on top of or in front of the nacelle of a wind turbine
3.16
obstacle
object that blocks the wind and distorts the flow, such as a building or tree
3.17
pitch angle
angle between the chord line at a defined blade radial location (usually 100 % of the blade
radius) and the rotor plane of rotation
3.18
report
any information regarding the test which is stated in the final report
3.19
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

3.20
ruggedness index
RIX
xx
measure of terrain, the ruggedness index of which is calculated as the percentage of altitude
differences within a given direction sector that exceed an altitude difference of xx × (D + H)
3.21
site calibration
procedure that quantifies and potentially reduces the effects of terrain and obstacles by
measuring the correlation over wind direction between the wind speed measured at a reference
meteorological mast and the wind speed measured at the wind turbine position
3.22
standard uncertainty
uncertainty of the result of a measurement expressed as a standard deviation
3.23
swept area
for a horizontal-axis turbine, the projected area of the moving rotor upon a plane normal to axis
of rotation
Note 1 to entry: For teetering rotors, it should be assumed that the rotor remains normal to the low-speed shaft.
3.24
test site
location of the wind turbine under test and its surroundings
3.25
turbine online
status of the wind turbine, during normal operation excluding cut-in or cut-out, but including any
operation at rotor speed in normal operating range where the turbine briefly disconnects from
the grid, e.g. switching between generators, generator stages, star/delta or similar
3.26
wind shear
variation of wind speed across a plane perpendicular to the wind direction
4 Symbols, units and abbreviated terms
Symbol or Description Unit
abbreviated term
A swept area of the wind turbine rotor 2
[m ]
AEP annual energy production
ASL above sea level [m]
B barometric pressure [Pa]
B measured air pressure averaged over 10 minutes [Pa]
10min
c sensitivity factor on a parameter (the partial differential)
c
sensitivity factor of air pressure in bin i [W/Pa]
B,i
c
sensitivity factor of data acquisition system in bin i
d,i
c
sensitivity factor of component k in bin i
k,i
c sensitivity factor of component l in bin j
l,j
c sensitivity factor of air density correction in bin i 3
m,i [W/m kg]
– 12 – IEC 61400-12-6:2022 © IEC 2022
Symbol or Description Unit
abbreviated term
c sensitivity factor of component k in bin i on turbine m
m,k,i
c sensitivity factor of air temperature in bin i [W/K]]
T,i
c sensitivity factor of wind speed in bin i -1
V,i [W / ms ]
D rotor diameter [m]
D equivalent rotor diameter [m]
e
D rotor diameter of neighbouring and operating wind turbine [m]
n
D
blade rotor diameter [m]
r
Elevation elevation above sea level [m]
F(V) the Rayleigh cumulative probability distribution function for wind speed
fi relative occurrence of wind speed between V − 1 and V (F(V ) – F(V − 1))
i i i i
within bin i
H hub height of wind turbine [m]
K von Karman constant, 0,4
NT number of turbines
L
distance between turbine and met mast (2,5D) in terms of rotor diameters
Lx contribution factor related to source x
M number of uncertainty components in each bin
M number of category A uncertainty components
A
M number of category B uncertainty components
B
N number of bins
N number of hours in one year ≈ 8 760
h
N number of 10-min data sets in wind speed bin i
i
N number of 10-min data sets in wind direction bin j
j
N is the number of 10-min data sets in bin k
k
N number of bins on turbine m
m
N
number of bins on turbine n
n
n number of turbines tested
Test
N velocity profile exponent (n = 0,14)
NPC nacelle power curve
NTF nacelle transfer function
P normalised and averaged power output in bin i [W]
i
P normalised power output [W]
n
P normalised power output of data set j in bin i [W]
n,i,j
P measured power averaged over 10 min [W]
10min
P water vapour pressure [Pa]
w
R
gas constant (= 287,05) [J/(kg × K)]
R
gas constant of water vapour (= 461,5) [J/(kg × K)]
w
RIX percentage of calculated slopes within a given direction sector that exceed 20 %
S standard deviation of the wind speed ratios in bin i
sc,i
S uncertainty component of category A [W]

Symbol or Description Unit
abbreviated term
s category A standard uncertainty of component k in bin i [W]
k,i
s combined category A uncertainties in bin i [W]
i
s category A standard uncertainty of power in bin i [W]
P,i
s statistical uncertainty in captured dataset
NTF,i
s category A standard uncertainty of wind speed ratios in bin j [W]
α,j
Slope slope between adjacent elevation points [°]
i
T absolute temperature [K]
TI turbulence intensity
T measured absolute air temperature averaged over 10 min [K]
10min
T time [s]
U uncertainty component of category B
U Combined category B uncertainties in bin i
i
u combined standard uncertainty in the estimated annual energy production [Wh]
AEP
u uncertainty in the average AEP [Wh]
AEP,AVG
u uncertainty in AEP from category B component k [Wh]
AEP,k
u uncertainty in AEP from category B component k on turbine m [Wh]
AEP,m,k
u
ratio of the uncertainty in the AEP [Wh]
AEPRATIO
u
uncertainty related to anemometer class [W]
ano_class
u
category B standard uncertainty of air pressure in bin i [W]
B,i
u combined standard uncertainty of the power in bin i [W]
c,i
u combined uncertainty in power in bin i on turbine m [W]
c,m,i
u uncertainty component for data acquisition system
dFS,i
u uncertainty component for data acquisition of power in bin i
dP,i
u uncertainty component for data acquisition of temperature in bin i
dT,i
u uncertainty component for data acquisition of wind speed in bin i
dV,i
u uncertainty component for data acquisition of wind direction in bin i
dWD,i
u uncertainty component for free stream wind speed [W]
FS
U combined category B uncertainties in bin i
i
u
category B standard uncertainty of component k in bin i [W]
k,i
u standard uncertainty of component k in bin i on turbine m 3
m,k,i [W][kg/m ]
u standard uncertainty of component l in bin j [W]
l,j
u category B standard uncertainty of air density correction in bin i 3
m,i [kg/m ]
u uncertainty component for nacelle wind speed
N
u category B standard uncertainty of power in bin i [W]
P,i
u category B standard uncertainty of power in bin i [W]
sc
u uncertainty component for site calibration in wind speed bin i and wind direction [W]
sc,i,j
bin j
u category B standard uncertainty of air temperature in bin i [K]
T,i
u category B standard uncertainty of wind speed in bin i [W]
V,i
– 14 – IEC 61400-12-6:2022 © IEC 2022
Symbol or Description Unit
abbreviated term
u standard uncertainty of wind speed component k in bin i
V,k,i
u wind direction uncertainty [°]
WD
u wind direction uncertainty, nacelle measured [°]
WD,SENSOR
u wind direction uncertainty, yaw component [°]
YAW
u total uncertainty in wind farm AEP [Wh]
wind farm_AEP
u combined standard uncertainty of site calibration in wind speed bin i and wind [m/s]
α,i,j
direction bin j
u combined uncertainty in power in bin i on turbine m [W]
c,m,i
V wind speed [m/s]
V annual average wind speed at hub height [m/s]
ave
V measured nacelle wind speed, corrected with the nacelle transfer function [m/s]
free
V normalised and averaged wind speed in bin i [m/s]
i
V , bin averages of the met-mast wind speed in bin i wind speed determined with [m/s]
met i
the nacelle anemometer
V
normalised wind speed [m/s]
n
V
normalised wind speed of data set j in bin i [m/s]
n,i,j
V normalised wind speed of data set j in bin k
n,k,j
V measured value of the nacelle anemometer for which we want to estimate the [m/s]
nacelle
free stream wind speed
V bin average value of the nacelle anemometer for bin i [m/s]
nacelle,i
V measured wind speed averaged over 10 min [m/s]
10min
v wind speed evaluated from the power output [m/s]
P
X distance downstream obstacle to met mast or wind turbine [m]
Z
height above ground [m]
z roughness length [m]
Α ratio of wind speeds in wind direction bin j (wind turbine position to
j
meteorological mast position)
∆U influence of an obstacle in wind speed difference [m/s]
z
∆z Vertical distance between adjacent elevation points [m]
i
Ρ correlation coefficient
ρ correlation coefficient between uncertainty component k in bin i and uncertainty
k,l,i,j
component l in bin j
ρ correlation coefficient between turbine m and turbine n for component k
k,m,n
Ρ correlation coefficient between uncertainty component k in bin i on turbine m and
k,m,i,l,j,n
uncertainty component l in bin j on turbine n
ρ reference air density 3
0 [kg/m ]
ρ derived air density averaged over 10 min 3
10min [kg/m ]
ρ
correlation coefficient for pressure
ubi,m,n
ρ
correlation coefficient for method
umi,m,n
ρ
correlation coefficient for electric power
upi,m,n
ρ statistical correlation coefficient
sp,m,n
Symbol or Description Unit
abbreviated term
ρ correlation coefficient for temperat
...