EN 61400-12-2:2013

SLOVENSKI STANDARD
01-september-2013
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Wind turbines - Part 12-2: Power performance of electricity producing wind turbines
based on nacelle anemometry
Windturbinen - Teil 12-2: Leistungsverhalten von Elektrizität erzeugenden Windturbinen
mit Gondelanemometer
Eoliennes - Partie 12-2: Performance de puissance des éoliennes de production
d'électricité basée sur l'anémométrie de nacelle
Ta slovenski standard je istoveten z: EN 61400-12-2:2013
ICS:
27.180 Sistemi turbin na veter in Wind turbine systems and
drugi alternativni viri energije other alternative sources of
energy
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

EUROPEAN STANDARD
EN 61400-12-2
NORME EUROPÉENNE
July 2013
EUROPÄISCHE NORM
ICS 27.180
English version
Wind turbines -
Part 12-2: Power performance of electricity-producing wind turbines
based on nacelle anemometry
(IEC 61400-12-2:2013)
Eoliennes -  Windenergieanlagen -
Partie 12-2: Performance de puissance Teil 12-2: Messung des
des éoliennes de production d'électricité Leistungsverhaltens von Elektrizität
basée sur l'anémométrie de nacelle erzeugenden Windturbinen basierend auf
(CEI 61400-12-2:2013) Gondelanemometrie
(IEC 61400-12-2:2013)
This European Standard was approved by CENELEC on 2013-05-02. CENELEC members are bound to comply
with the CEN/CENELEC Internal Regulations which stipulate the conditions for giving this European Standard
the status of a national standard without any alteration.

Up-to-date lists and bibliographical references concerning such national standards may be obtained on
application to the CEN-CENELEC Management Centre or to any CENELEC member.

This European Standard exists in three official versions (English, French, German). A version in any other
language made by translation under the responsibility of a CENELEC member into its own language and notified
to the CEN-CENELEC Management Centre has the same status as the official versions.

CENELEC members are the national electrotechnical committees of Austria, Belgium, Bulgaria, Croatia, Cyprus,
the Czech Republic, Denmark, Estonia, Finland, Former Yugoslav Republic of Macedonia, France, Germany,
Greece, Hungary, Iceland, Ireland, Italy, Latvia, Lithuania, Luxembourg, Malta, the Netherlands, Norway, Poland,
Portugal, Romania, Slovakia, Slovenia, Spain, Sweden, Switzerland, Turkey and the United Kingdom.

CENELEC
European Committee for Electrotechnical Standardization
Comité Européen de Normalisation Electrotechnique
Europäisches Komitee für Elektrotechnische Normung

Management Centre: Avenue Marnix 17, B - 1000 Brussels

© 2013 CENELEC - All rights of exploitation in any form and by any means reserved worldwide for CENELEC members.
Ref. No. EN 61400-12-2:2013 E
Foreword
The text of document 88/442/FDIS, future edition 1 of IEC 61400-12-2, prepared by IEC/TC 88 "Wind
turbines" was submitted to the IEC-CENELEC parallel vote and approved by CENELEC as
The following dates are fixed:
(dop) 2014-02-02
• latest date by which the document has to be
implemented at national level by
publication of an identical national
standard or by endorsement
• latest date by which the national (dow) 2016-05-02
standards conflicting with the
document have to be withdrawn
Attention is drawn to the possibility that some of the elements of this document may be the subject of
patent rights. CENELEC [and/or CEN] shall not be held responsible for identifying any or all such
patent rights.
Endorsement notice
The text of the International Standard IEC 61400-12-2:2013 was approved by CENELEC as a
European Standard without any modification.

- 3 - EN 61400-12-2:2013
Annex ZA
(normative)
Normative references to international publications
with their corresponding European publications

The following documents, in whole or in part, are normatively referenced in this document and are
indispensable for its application. For dated references, only the edition cited applies. For undated
references, the latest edition of the referenced document (including any amendments) applies.

NOTE  When an international publication has been modified by common modifications, indicated by (mod), the relevant EN/HD
applies.
Publication Year Title EN/HD Year

IEC 60688 1992 Electrical measuring transducers for EN 60688 1992
+ A1 (mod) 1997 converting a.c. electrical quantities to + A1 1999
+ A2 2001 analogue or digital signals + A2 2001

IEC 61400-12-1 2005 Wind turbines - EN 61400-12-1 2006
Part 12-1: Power performance
measurements of electricity producing
wind turbines
IEC 61869-2 - Instrument transformers - EN 61869-2 -
Part 2: Additional requirements for current
transformers
IEC 61869-3 - Instrument transformers - EN 61869-3 -
Part 3: Additional requirements for inductive
voltage transformers
ISO/IEC 17025 - General requirements for the competence of EN ISO/IEC 17025 -
testing and calibration laboratories

ISO/IEC - Uncertainty of measurement - - -
Guide 98-3 Part 3: Guide to the expression of
uncertainty in measurement (GUM:1995)

ISO 2533 - Standard atmosphere - -

IEC 61400-12-2 ®
Edition 1.0 2013-03
INTERNATIONAL
STANDARD
NORME
INTERNATIONALE
colour
inside
Wind turbines –
Part 12-2: Power performance of electricity-producing wind turbines based on

nacelle anemometry
Eoliennes –
Partie 12-2: Performance de puissance des éoliennes de production d'électricité

basée sur l'anémométrie de nacelle

INTERNATIONAL
ELECTROTECHNICAL
COMMISSION
COMMISSION
ELECTROTECHNIQUE
PRICE CODE
INTERNATIONALE
CODE PRIX XE
ICS 27.180 ISBN 978-2-83220-658-4

– 2 – 61400-12-2 © IEC:2013
CONTENTS
FOREWORD . 5
INTRODUCTION . 7
1 Scope . 8
2 Normative references . 8
3 Terms and definitions . 9
4 Symbols and units . 13
5 Overview of test method . 16
6 Preparation for performance test . 19
6.1 General . 19
6.2 Wind turbine . 19
6.3 Test site . 19
6.3.1 Terrain classification. 20
6.3.2 RIX indices . 20
6.3.3 Average slope . 21
6.3.4 Determine terrain class . 21
6.3.5 Ridge formations . 22
6.4 Nacelle wind speed transfer function . 23
6.5 Test plan . 23
7 Test equipment . 23
7.1 Electric power . 23
7.2 Wind speed . 24
7.3 Wind direction . 24
7.3.1 Nacelle yaw position sensor . 24
7.3.2 Nacelle wind direction sensor . 25
7.3.3 Wind direction . 25
7.4 Air density . 25
7.5 Rotor speed . 26
7.6 Pitch angle . 26
7.7 Wind turbine status . 26
7.8 Data acquisition. 26
8 Measurement procedure . 27
8.1 General . 27
8.2 Wind turbine operation . 27
8.3 Data system(s) synchronisation . 27
8.4 Data collection . 28
8.5 Data quality check . 28
8.6 Data rejection . 29
8.7 Data correction . 30
8.8 Database . 30
9 Derived results . 31
9.1 Data normalisation . 31
9.1.1 Density correction . 31
9.2 Determination of measured power curve . 32
9.3 Annual energy production (AEP) . 32
9.4 Power coefficient . 33
9.5 Uncertainty analysis . 34

61400-12-2 © IEC:2013 – 3 –
10 Reporting format . 34
Annex A (informative) Nacelle instrument mounting . 42
Annex B (normative) Measurement sector procedure . 44
Annex C (normative) Nacelle wind speed transfer function validity procedure . 49
Annex D (normative) Nacelle wind speed transfer function measurement procedure . 51
Annex E (normative) Evaluation of uncertainty in measurement . 58
Annex F (normative) Theoretical basis for determining the uncertainty of
measurement using the method of bins . 62
Annex G (normative) NTF/NPC uncertainty estimates and calculation . 70
Annex H (normative) Allowable anemometry instrument types . 83
Annex I (informative) Results and uncertainty considerations . 85
Annex J (informative) Example multi-turbine NTF/NPC uncertainty calculation . 90
Annex K (informative) Organisation of test, safety and communication . 98
Annex L (informative) NPC and NTF flowchart . 100

Figure 1 – Procedural overview . 18
Figure 2 – Presentation of example data: transfer function resulting from Annex D . 37
Figure 3 – Presentation of example data: nacelle power performance test scatter plots . 38
Figure 4 – Presentation of example data: binned power curve with uncertainty bands . 38
Figure 5 – Presentation of example data: measured power curve and C curve . 39
p
Figure A.1 – Mounting of anemometer on top of nacelle . 43
Figure B.1 – Sectors to exclude due to wakes of neighbouring and operating wind
turbines and significant obstacles . 46
Figure B.2 – Example of the result of a sector self-consistency check . 48
Figure D.1 – Nacelle transfer function for wind speed . 56
Figure J.1 – Impact of multiple turbine testing on measurement uncertainty . 97
Figure J.2 – Impact of multiple turbine testing on sampling uncertainty . 97
Figure L.1 – NPC flowchart . 100
Figure L.2 – NTF flowchart. 101

Table 1 – Slope terrain classification . 21
Table 2 – RIX terrain classification . 22
Table 3 – Final terrain class . 22
Table 4 – Maximum ridge step effects on terrain class . 22
Table 5 – Example of a measured power curve . 40
Table 6 – Example of estimated annual energy production . 41
Table B.1 – Obstacle requirements: relevance of obstacles . 45
Table D.1 – Example of presentation of a measured power curve based on data from
the meteorological mast, for consistency check . 57
Table E.1 – Uncertainty components in nacelle transfer function evaluation . 59
Table E.2 – Uncertainty components in nacelle power curve evaluation . 60
Table E.3 – Uncertainty components in nacelle based absolute wind direction . 61
Table F.1 – Example cancellation sources . 64
Table F.2 – List of category A and B uncertainties for NTF . 64

– 4 – 61400-12-2 © IEC:2013
Table F.3 – List of category A and B uncertainties for NPC . 66
Table F.4 – Expanded uncertainties . 69
Table G.1 – Estimates for uncertainty components from site calibration . 70
Table G.2 – Estimates for uncertainty components from NTF measurement . 72
Table G.3 – Estimates for uncertainty components from NPC measurement . 74
Table G.4 – Estimates for u for NPC terrain class . 76
V5,i
Table G.5 – Estimates for uncertainty components for wind direction . 77
Table G.6 – Estimates for contribution factors for site calibration . 78
Table G.7 – Estimates for contribution factors for NTF . 79
Table G.8 – Estimates for contribution factors for NPC . 80
Table J.1 – List of correlated uncertainty components . 91
Table J.2 – Sample AEP and uncertainty data from 3 turbines . 93
Table J.3 – Component uncertainty contribution to AEP uncertainty on turbine 1 . 93
Table J.4 – Combination of uncertainty components across turbines. . 95

61400-12-2 © IEC:2013 – 5 –
INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
WIND TURBINES –
Part 12-2: Power performance of electricity-producing
wind turbines based on nacelle anemometry

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 co-operation on all questions concerning standardization in the electrical and electronic fields. To
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agreement between the two organizations.
2) The formal decisions or agreements of IEC on technical matters express, as nearly as possible, an international
consensus of opinion on the relevant subjects since each technical committee has representation from all
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3) IEC Publications have the form of recommendations for international use and are accepted by IEC National
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4) In order to promote international uniformity, IEC National Committees undertake to apply IEC Publications
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between any IEC Publication and the corresponding national or regional publication shall be clearly indicated in
the latter.
5) IEC itself does not provide any attestation of conformity. Independent certification bodies provide conformity
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6) All users should ensure that they have the latest edition of this publication.
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expenses arising out of the publication, use of, or reliance upon, this IEC Publication or any other IEC
Publications.
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.
International Standard IEC 61400-12-2 has been prepared by IEC technical committee 88:
Wind turbines.
The text of this standard is based on the following documents:
FDIS Report on voting
88/442/FDIS 88/445/RVD
Full information on the voting for the approval of this standard can be found in the report on
voting indicated in the above table.
This publication has been drafted in accordance with the ISO/IEC Directives, Part 2.
A list of all parts in the IEC 61400 series, published under the general title Wind turbines, can
be found on the IEC website.
– 6 – 61400-12-2 © IEC:2013
The committee has decided that the contents of this 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.
IMPORTANT – The 'colour inside' logo on the cover page of this publication
indicates that it contains colours which are considered to be useful for the correct
understanding of its contents. Users should therefore print this document using a
colour printer.
61400-12-2 © IEC:2013 – 7 –
INTRODUCTION
The purpose of this part of IEC 61400-12 is to provide a uniform methodology of
measurement, analysis, and reporting of power performance characteristics for individual
electricity-producing wind turbines utilising nacelle-anemometry methods. This standard 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 standard is that the methods presented herein be utilised when the requirements set forth
in IEC 61400-12-1:2005 are not feasible. 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 power performance
characteristics in terms of a measured power curve and the estimated annual energy
production (AEP) based on nacelle-anemometry. In this procedure, the anemometer is located
on or near the test turbine’s nacelle. In this location, the anemometer is measuring 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. However, it must be
noted that these corrections inherently increase the measurement uncertainty compared to a
properly-configured test conducted in accordance with IEC 61400-12-1:2005. The procedure
also provides guidance on determination of measurement uncertainty including assessment of
uncertainty sources and recommendations for combining them into uncertainties in reported
power and AEP.
A key element of power performance testing is the measurement of wind speed. 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 standard.
The standard will benefit those parties involved in the manufacture, installation, planning and
permitting, operation, utilisation and regulation of wind turbines. When appropriate, the
technically accurate measurement and analysis techniques recommended in this standard
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 standard presents measurement and reporting procedures
expected to provide accurate results that can be replicated by others.
Meanwhile, a user of the standard 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 contracting power performance
measurements.
– 8 – 61400-12-2 © IEC:2013
WIND TURBINES –
Part 12-2: Power performance of electricity-producing
wind turbines based on nacelle anemometry

1 Scope
This part of IEC 61400-12 specifies a procedure for verifying the power performance
characteristics of a single electricity-producing, horizontal axis wind turbine, which is not
considered to be a small wind turbine per IEC 61400-2. It is expected that this standard will
be used when the specific operational or contractual specifications may not comply with the
requirements set forth in IEC 61400-12-1:2005. The procedure can be used for power
performance evaluation of specific turbines at specific locations, but equally the methodology
can be used to make generic comparisons between different turbine models or different
turbine settings.
The wind turbine power performance characterised by the measured power curve and the
estimated AEP based on nacelle-measured wind speed will be affected by the turbine rotor
(i.e. speeded up or slowed down wind speed). The nacelle-measured wind speed shall be
corrected for this flow distortion effect. Procedures for determining that correction will be
included in the methodology. In IEC 61400-12-1:2005, 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 this IEC 61400-12-2 procedure, the anemometer is located on
or near the test turbine’s nacelle. In this location, the anemometer is measuring wind speed
that is strongly affected by the test turbine’s rotor and the nacelle. This procedure includes
methods for determining and applying appropriate corrections for this interference. However,
it should be noted that these corrections inherently increase the measurement uncertainty
compared to a properly-configured test conducted in accordance with IEC 61400-12-1:2005.
This IEC 61400-12-2 standard describes how to characterise a wind turbine’s power
performance in terms of a measured power curve and the estimated AEP. The measured
power curve is determined by collecting simultaneous measurements of nacelle-measured
wind speed and power output 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. In order to accurately measure the power curve, the nacelle-measured wind speed
is adjusted using a transfer function to estimate the free stream wind speed. The procedure to
measure and validate such a transfer function is presented herein. The AEP is calculated by
applying the measured power curve to the reference wind speed frequency distributions,
assuming 100 % availability. The procedure also provides guidance on determination of
measurement uncertainty including assessment of uncertainty sources and recommendations
for combining them into uncertainties in reported power and AEP.
2 Normative references
The following documents, in whole or in part, are normatively referenced in this document and
are indispensable for its application. For dated references, only the edition cited applies. For
undated references, the latest edition of the referenced document (including any
amendments) applies.
IEC/TR 60688, Electrical measuring transducers for converting a.c. electrical quantities to
analogue or digital signals
Amendment 1 (1997)
Amendment 2 (2001)
61400-12-2 © IEC:2013 – 9 –
IEC 61400-12-1:2005, Wind turbines – Part 12-1: Power performance measurements of
electricity producing wind turbines
IEC 61869-2, Instrument transformers – Part 2: Additional requirements for current
transformers
IEC 61869-3, Instrument transformers – Part 3: Additional requirements for inductive voltage
transformers
ISO/IEC 17025, General requirements for the competence of testing and calibration
laboratories
ISO/IEC Guide 98-3, Uncertainty of measurement – Part 3: Guide to the expression of
uncertainty in measurement (GUM:1995)
ISO 2533, Standard atmosphere
3 Terms and definitions
For the purposes of this document, the following terms and definitions apply.
3.1
accuracy
closeness of the agreement between the result of a measurement and a true value of the
measurand
3.2
annual energy production (AEP)
estimate of the total energy production of a wind turbine during a one-year period by applying
the measured power curve to different reference wind speed frequency distributions at hub
height, assuming 100 % availability
3.3
annual energy production − measured (AEP-measured)
estimate of the total energy production of a wind turbine during a one-year period by applying
the measured power curve to different reference wind speed frequency distributions at hub
height, assuming 100 % availability, without power curve extrapolation to higher wind speeds
3.4
annual energy production − extrapolated (AEP-extrapolated)
estimate of the total energy production of a wind turbine during a one-year period by applying
the measured power curve to different reference wind speed frequency distributions at hub
height, assuming 100 % availability, with power curve extrapolation to cut-out wind speed of
the turbine
3.5
complex terrain
terrain surrounding the test site that features significant variations in topography and terrain
obstacles that may cause flow distortion
3.6
data set
collection of data that was sampled over a contiguous period

– 10 – 61400-12-2 © IEC:2013
3.7
documentation
any information regarding the test which is kept in files and/or data, but which may not
necessarily be presented in the final report
3.8
extrapolated power curve
extension of the measured power curve by estimating power output from the maximum
measured wind speed to cut-out wind speed
3.9
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
3.10
free stream wind speed
horizontal wind speed measured upstream of the rotor of the wind turbine generator that is
unaffected by rotor aerodynamics
3.11
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
3.12
hub height (wind turbines)
height of the centre of the swept area of the wind turbine rotor above the ground level at the
tower base
3.13
machine configuration change
a change to the turbine or intervention in the turbine operation which causes a significant
change in the power performance of the turbine and which is not normal maintenance
EXAMPLE Replacements of hardware components, especially rotor blade, gearbox or generator; a change or
update of the turbine software or its parameters, unplanned blade washing.
3.14
measured power curve
table and graph that represents the measured, corrected and normalised net power output of
a wind turbine as a function of measured free stream wind speed, measured under a well-
defined measurement procedure
3.15
measurement period
period during which a statistically significant database has been collected for the power
performance test
3.16
measurement sector
a sector of wind directions from which data are selected for the measured power curve or
during the determination of the nacelle transfer function

61400-12-2 © IEC:2013 – 11 –
3.17
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
3.18
method of bins
data reduction procedure that groups test data for a certain parameter into intervals (bins).
Normally used for wind speed bins but also applicable to other parameters.
Note 1 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.
3.19
nacelle
housing which contains the drive train and other elements on top of a horizontal axis wind
turbine generator
3.20
nacelle power curve (NPC)
the 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; 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
3.21
nacelle wind speed
horizontal wind speed measured on top of or in front of the nacelle of a wind turbine
3.22
net active electric power
measure of the wind turbine electric power output that is delivered to the electrical power
network
3.23
normal maintenance
any intervention which is done according to a defined regular maintenance program,
independent from the fact that a power performance test is being done, e.g. oil change, blade
washing (if due anyway, independent from the power performance test)
3.24
obstacles
objects that block the wind and create distortion of the flow, such as buildings and trees
3.25
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
3.26
power coefficient
ratio of the net electric power output of a wind turbine to the power available in the free
stream wind over the rotor swept area
3.27
power performance
measure of the capability of a wind turbine to produce electric power and energy

– 12 – 61400-12-2 © IEC:2013
3.28
rated power
quantity of power assigned, generally by a manufacturer, for a specified operating condition of
a component, device or equipment
3.29
report
any information regarding the test which is stated in the final report
3.30
roughness length
extrapolated height at which the mean speed becomes zero if the vertical wind profile is
assumed to have a logarithmic variation with height
3.31
ruggedness index
RIX
xx
a measure of terrain, the ruggedness index is calculated as the percentage of altitude
differences within a given direction sector that exceed an altitude difference of xx × (D+H)
3.32
site calibration
a 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
3.33
standard uncertainty
uncertainty of the result of a measurement expressed as a standard deviation
3.34
swept area
for a horizontal-axis turbine, the projected area of the moving rotor upon a plane normal to
axis of rotation; for teetering rotors, it should be assumed that the rotor remains normal to the
low-speed shaft
3.35
test site
location of the wind turbine under test and its surroundings
3.36
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
3.37
wind shear
variation of wind speed across a plane perpendicular to the wind direction

61400-12-2 © IEC:2013 – 13 –
4 Symbols and units
Symbol Description Unit
A swept area of the wind turbine rotor [m ]
AEP annual energy production [Wh]
AEP the measured AEP on turbine m [Wh]
m
AEP Sum of annual energy production [Wh]
s
ASL Above Sea Level [m]
B barometric pressure [Pa]
B measured air pressure averaged over 10 minutes [Pa]
10min
C power coefficient
p
C
power coefficient in bin i
p,i
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 [W/m kg]
m,i
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
–1
c sensitivity factor of wind speed in bin i [W / ms ]
V,i
D
rotor diameter [m]
D equivalent rotor diameter [m]
e
D rotor diameter of neighbouring and operating wind turbine [m]
n
D blade root diameter [m]
r
Δz Vertical distance between adjacent elevation points [m]
i
elevation
elevation above sea level [m]
the Rayleigh cumulative probability distribution function for wind
F(V)
speed
H hub height of wind turbine [m]
h height of obstacle minus zero displacement [m]
K von Karman constant, 0,4
NT
number of turbines
distance between the turbine and met mast (2,5D) in terms of
L
rotor diameters
distance between the wind turbine or the meteorological mast
Le [m]
and an obstacle
actual distance between neighbouring and operating wind
Ln turbine or the meteorological mast and a neighbouring and [m]
operating wind turbine
Lh actual height of obstacle [m]
distance between the wind turbine or the meteorological mast
Lw [m]
and a neighbouring and operating wind turbine
M number of uncertainty components in each bin

– 14 – 61400-12-2 © IEC:2013
Symbol Description Unit
MA number of category A uncertainty components
MB number of category B uncertainty components
N
number of bins
Nh number of hours in one year ≈ 8760
N number of 10-minute data sets in wind speed bin i
i
N number of 10-minute data sets in wind direction bin j
j
N number of bins on turbine m
m
N number of bins on turbine n
n
N number of samples within sampling interval
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 porosity of obstacle (0: solid, 1: no obstacle)
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 minutes [W]
10min
P the water vapour pressure [Pa]
w
R distance to mast centre [m]
R the gas constant (=287,05 ) [J/(kg × K)]
Rw the gas constant of water vapour (=461,5 ) [J/(kg × K)]
the percentage of calculated slopes within a given direction
RIX
sector that exceed 20 %
S standard deviation of the wind speed ratios in bin i
sc,i
S uncertainty component of category A [W]
s uncertainty in AEP from category A component k [W]
AEP,k
s uncertainty in AEP from category A component k on turbine m [W]
AEP,m,k
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 category A standard uncertainty of wind speed ratios in bin j [W]
α,j
se standard error in the mean AEP of the sample [Wh]
AEP
slope slope between adjacent elevation points  [°]
i
T
absolute temperature [K]
TI turbulence intensity
T measured absolute air temperature averaged over 10 minutes [K]
10min
T time [s]
U horizontal wind speed component [m/s]
U uncertainty component of category B

61400-12-2 © IEC:2013 – 15 –
Symbol Description Unit
combined standard uncertainty in the estimated annual energy
u [Wh]
AEP
production
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 the ratio of the uncertainty in the AEP [Wh]
AEP,RATIO
u uncertainty related to anemometer class [m/s]
ano_class
u category B standard uncertainty of air pressure in bin i [Pa]
B,i
u
combined standard uncertainty of the power in bin i [W]
c,i
u uncertainty component for free stream wind speed [m/s]
FS
u combined category B uncertainties in bin i
i
u category B standard uncertainty of component k in bin i
k,i
category B standard uncertainty of component k in bin i on
u
m,k,i
turbine 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 [kg/m ]
m,i
u uncertainty component for nacelle wind speed [m/s]
N
a guess / estimate of the magnitude of the variation in results
u from a NTF measured at different times of the year using the [W] [m/s]
NTFM,i
same equipment
u total uncertainty in wind farm AEP [Wh]
wind farm_AEP
u category B standard uncertainty of power i
...