IEC 61400-12-0:1998

INTERNATIONAL IEC
STANDARD
61400-12
First edition
1998-02
Wind turbine generator systems –
Part 12:
Wind turbine power performance testing
Aérogénérateurs –
Partie 12:
Techniques de mesure des performances de puissance

Reference number
IEC 61400-12:1998(E)
Numbering
As from 1 January 1997 all IEC publications are issued with a designation in the 60000 series.

Consolidated publications
Consolidated versions of some IEC publications including amendments are available. For example, edition numbers
1.0, 1.1 and 1.2 refer, respectively, to the base publication, the base publication incorporating amendment 1 and the

base publication incorporating amendments 1 and 2.

Validity of this publication
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reflects current technology.
Information relating to the date of the reconfirmation of the publication is available in the IEC catalogue.
Information on the revision work, the issue of revised editions and amendments may be obtained from IEC National
Committees and from the following IEC sources:
• IEC Bulletin
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On-line access*
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Published yearly with regular updates
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Terminology, graphical and letter
symbols
For general terminology, readers are referred to IEC 60050: International Electrotechnical Vocabulary (IEV).
For graphical symbols, and letter symbols and signs approved by the IEC for general use, readers are referred to
publications IEC 60027: Letter symbols to be used in electrical technology, IEC 60417: Graphical symbols for use on
equipment. Index, survey and compilation of the single sheets and IEC 60617: Graphical symbols for diagrams.
IEC publications prepared by the same
technical committee
The attention of readers is drawn to the end pages of this publication which list the IEC publications issued by the
technical committee which has prepared the present publication.
* See web site address on title page.

INTERNATIONAL IEC
STANDARD
61400-12
First edition
1998-02
Wind turbine generator systems –
Part 12:
Wind turbine power performance testing
Aérogénérateurs –
Partie 12:
Techniques de mesure des performances de puissance

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– 2 – 61400-12 © IEC:1998(E)
CONTENTS
Page
FOREWORD . 4

INTRODUCTION . 5

Clause
1 General. 6

1.1 Scope. 6
1.2 Normative references. 6
1.3 Definitions . 7
1.4 Symbols and units. 9
1.5 Abbreviations. 10
2 Test conditions. 11
2.1 Wind turbine generator system. 11
2.2 Test site. 11
3 Test equipment. 13
3.1 Electric power. 13
3.2 Wind speed. 13
3.3 Wind direction. 14
3.4 Air density. 14
3.5 Precipitation. 14
3.6 Wind turbine generator system status . 14
3.7 Data acquisition system . 14
4 Measurement procedure. 15
4.1 Introduction. 15
4.2 Wind turbine generator system operation . 15
4.3 Data collection. 15
4.4 Data selection. 15

4.5 Data correction. 16
4.6 Database. 16
5 Derived results. 16
5.1 Data normalization. 16
5.2 Determination of measured power curve . 17
5.3 Annual energy production (AEP). 18
5.4 Power coefficient. 19
6 Reporting format. 19
Tables
1 Example of presentation of a measured power curve. 22
2 Example of presentation of estimated annual energy production. 23

61400-12  IEC:1998(E) – 3 –
Page
Figures
1 Requirements as to distance of the meteorological mast and maximum allowed

measurement sectors . 12

2 Presentation of example data: power performance test scatter plots. 20

3 Presentation of example measured power curve. 21

Annexes
A Assessment of test site. 24
B Calibration of test site . 28
C Evaluation of uncertainty in measurement . 29
D Theoretical basis for determining the uncertainty of measurement using the method
of bins . 31
E Bibliography. 44

– 4 – 61400-12 © IEC:1998(E)
INTERNATIONAL ELECTROTECHNICAL COMMISSION

___________
WIND TURBINE GENERATOR SYSTEMS –

Part 12: Wind turbine power performance testing

FOREWORD
1) The IEC (International Electrotechnical Commission) is a worldwide organization for standardization comprising
all national electrotechnical committees (IEC National Committees). The object of the IEC is to promote
international co-operation on all questions concerning standardization in the electrical and electronic fields. To
this end and in addition to other activities, the IEC publishes International Standards. Their preparation is
entrusted to technical committees; any IEC National Committee interested in the subject dealt with may
participate in this preparatory work. International, governmental and non-governmental organizations liaising
with the IEC also participate in this preparation. The IEC collaborates closely with the International Organization
for Standardization (ISO) in accordance with conditions determined by agreement between the two
organizations.
2) The formal decisions or agreements of the IEC on technical matters express, as nearly as possible, an
international consensus of opinion on the relevant subjects since each technical committee has representation
from all interested National Committees.
3) The documents produced have the form of recommendations for international use and are published in the form
of standards, technical reports or guides and they are accepted by the National Committees in that sense.
4) In order to promote international unification, IEC National Committees undertake to apply IEC International
Standards transparently to the maximum extent possible in their national and regional standards. Any
divergence between the IEC Standard and the corresponding national or regional standard shall be clearly
indicated in the latter.
5) The IEC provides no marking procedure to indicate its approval and cannot be rendered responsible for any
equipment declared to be in conformity with one of its standards.
6) Attention is drawn to the possibility that some of the elements of this International Standard may be the subject
of patent rights. The IEC shall not be held responsible for identifying any or all such patent rights.
International Standard IEC 61400-12 has been prepared by IEC technical committee 88: Wind
turbine generator systems.
The text of this standard is based on the following documents:
FDIS Report on voting
88/85/FDIS 88/89/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.
A bilingual version of this standard may be issued at a later date.
Annexes A and C form an integral part of this standard.
Annexes B, D and E are for information only.

61400-12  IEC:1998(E) – 5 –
INTRODUCTION
The purpose of this part of IEC 61400 is to provide a uniform methodology that will ensure

consistency and accuracy in the measurement and analysis of power performance by wind

turbine generator systems (WTGS). The standard has been prepared with the anticipation that
it would be applied by:
– the WTGS manufacturer striving to meet well-defined power performance requirements

and/or a possible declaration system;

– the WTGS purchaser in specifying such performance requirements;

– the WTGS operator who may be required to verify that stated, or required, power
performance specifications are met for new or refurbished units;
– the WTGS planner or regulator who must be able to accurately and fairly define power
performance characteristics of WTGS in response to regulations or permit requirements for
new or modified installations.
This standard provides guidance in the measurement, analysis, and reporting of power
performance testing for wind turbine generator systems (WTGS). The standard will benefit
those parties involved in the manufacture, installation planning and permitting, operation,
utilization, and regulation of WTGS. 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 WTGS 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.
However, readers should be warned that the site calibration procedure is quite new. As yet
there is no substantial evidence that it can provide accurate results for all sites, especially sites
in complex terrain. Part of the procedure is based on applying uncertainty calculations on the
measurements. In complex terrain situations it is not adequate to state that results are
accurate since uncertainties might be 10 % to 15 % in standard deviation. A new measurement
standard, accounting for these problems, will be developed in future.

– 6 – 61400-12 © IEC:1998(E)
WIND TURBINE GENERATOR SYSTEMS –

Part 12: Wind turbine power performance testing

1 General
1.1 Scope
This part of IEC 61400 specifies a procedure for measuring the power performance
characteristics of a single wind turbine generator system (WTGS) and applies to the testing of
WTGS of all types and sizes connected to the electrical power network. It is applicable for the
determination of both the absolute power performance characteristics of a WTGS and of
differences between the power performance characteristics of various WTGS configurations.
The WTGS power performance characteristics are determined by the measured power curve
and the estimated annual energy production (AEP). The measured power curve is determined
by collecting simultaneous measurements of wind speed and power output at the test site for a
period that is long enough to establish a statistically significant database over a range of wind
speeds and under varying wind conditions. The AEP is calculated by applying the measured
power curve to reference wind speed frequency distributions, assuming 100 % availability.
The standard describes a measurement methodology that requires the measured power curve
and derived energy production figures to be supplemented by an assessment of uncertainty
sources and their combined effects.
1.2 Normative references
The following normative documents, through reference in this text, constitute provisions of this
part of IEC 61400. At the time of publication, the editions indicated were valid. All normative
documents are subject to revision, and parties to agreements based on this part of IEC 61400
are encouraged to investigate the possibility of applying the most recent editions of the
standards indicated below. Members of IEC and ISO maintain registers of currently valid
International Standards.
IEC 60044-1:1996, Instrument transformers – Part 1: Current transformers
IEC 60186:1987, Voltage transformers
Amendment 1 (1988).
Amendment 2 (1995).
IEC 60688:1992, Electrical measuring transducers for converting a.c. electrical quantities to
analogue or digital signals
ISO 2533:1975, Standard atmosphere
Guide to the expression of uncertainty in measurement, ISO information publications, 1995,
110 p. ISBN 92-67-10188-9
61400-12  IEC:1998(E) – 7 –
1.3 Definitions
For the purposes of this part of IEC 61400, the following definitions apply.

1.3.1
accuracy
closeness of the agreement between the result of a measurement and a true value of the
measurand
1.3.2
annual energy production
estimate of the total energy production of a WTGS during a one-year period by applying the
measured power curve to different reference wind speed frequency distributions at hub height,
assuming 100 % availability
1.3.3
availability
ratio of the total number of hours during a certain period, excluding the number of hours that
the WTGS could not be operated due to maintenance or fault situations, to the total number of
hours in the period, expressed as a percentage
1.3.4
complex terrain
terrain surrounding the test site that features significant variations in topography and terrain
obstacles that may cause flow distortion
1.3.5
data set
collection of data that was sampled over a continuous period
1.3.6
distance constant
indication of the response time of an anemometer, defined as the length of air that must pass
the instrument for it to indicate 63 % of the final value for a step input in wind speed
1.3.7
extrapolated power curve
extension of the measured power curve by estimating power output from the maximum
measured wind speed to cut-out wind speed
1.3.8
flow distortion
change in air flow caused by obstacles, topographical variations, or other wind turbines that
results in a deviation of the measured wind speed from the free stream wind speed and in a
significant uncertainty
1.3.9
free stream wind speed
speed of the undisturbed natural air flow, usually at hub height
1.3.10
hub height (wind turbines)
height of the center of the swept area of the wind turbine rotor above the terrain surface
NOTE – For a vertical axis wind turbine the hub height is the height of the equator plane.

– 8 – 61400-12 © IEC:1998(E)
1.3.11
measured power curve
table and graph that represents the measured, corrected and normalized net power output of a

WTGS as a function of measured wind speed, measured under a well-defined measurement

procedure
1.3.12
measurement period
period during which a statistically significant database has been collected for the power

performance test
1.3.13
measurement sector
a sector of wind directions from which data are selected for the measured power curve
1.3.14
method of bins
data reduction procedure that groups test data for a certain parameter into wind speed
intervals (bins)
NOTE – 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.
1.3.15
net electric power output
measure of the WTGS electric power output that is delivered to the electrical power network
1.3.16
obstacles
stationary obstacles, such as buildings and trees, neighboring the WTGS that cause wind flow
distortion
1.3.17
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
1.3.18
power coefficient
ratio of the net electric power output of a WTGS to the power available in the free stream wind
over the rotor swept area
1.3.19
power performance
measure of the capability of a WTGS to produce electric power and energy
1.3.20
rated power
quantity of power assigned, generally by a manufacturer, for a specified operating condition of
a component, device or equipment
NOTE – (Wind turbines) Maximum continuous electrical power output which a WTGS is designed to achieve under
normal operating conditions.
1.3.21
standard uncertainty
uncertainty of the result of a measurement expressed as a standard deviation

61400-12  IEC:1998(E) – 9 –
1.3.22
swept area
area of the projection, upon a plane perpendicular to the wind velocity vector, of the circle

along which the rotor blade tips move during rotation

1.3.23
test site
location of the WTGS under test and its surroundings

1.3.24
uncertainty in measurement
parameter, associated with the result of a measurement, which characterizes the dispersion of
the values that could reasonably be attributed to the measurand
1.4 Symbols and units
A swept area of the WTGS rotor [m ]
AEP annual energy production [kWh]
B measured air pressure averaged over 10 min [Pa]
10min
c sensitivity factor on a parameter (the partial differential)
C power coefficient in bin i
P
,i
D rotor diameter [m]
D equivalent rotor diameter [m]
e
D rotor diameter of neighbouring and operating wind turbine [m]
n
f the relative occurrence of wind speed in a wind speed interval
i
F(V) the Rayleigh cumulative probability distribution function for wind speed
l height of obstacle [m]
h
l width of obstacle [m]
w
L distance between the WTGS and the meteorology mast [m]
L distance between the WTGS or the meteorology mast and an obstacle [m]
e
L distance between the WTGS or the meteorology mast and
n
a neighbouring and operating wind turbine [m]
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 ≈ 8760 [h]
h
N number of 10 min data sets in bin i
i
N number of pre-processed data sets within a 10 min period
k
N number of data samples of pre-processed data sets
s
P normalized and averaged power output in bin i [kW]
i
– 10 – 61400-12 © IEC:1998(E)
P normalized power output [kW]
n
P normalized power output of data set j in bin i
n,i,j
P measured power averaged over 10 min [kW]
10min
R gas constant [J/(kg×K)]
s uncertainty component of category A

T measured absolute air temperature averaged over 10 min [K]
10min
u uncertainty component of category B

u combined standard uncertainty in the estimated annual energy production [kWh]
AEP
u combined standard uncertainty of the power in bin i [kW]
c,i
V wind speed [m/s]
V annual average wind speed at hub height [m/s]
ave
V normalized and averaged wind speed in bin i [m/s]
i
V normalized wind speed [m/s]
n
V normalized wind speed of data set j in bin i [m/s]
n,i,j
V measured wind speed averaged over 10 min [m/s]
10min
X parameter averaged over pre-processing time period
k
X parameter averaged over 10 min
10min
ρ correlation coefficient
ρ reference air density [kg/m ]
ρ derived air density averaged over 10 min [kg/m ]
10min
σ standard deviation of pre-processed parameter
k
σ standard deviation of the normalized power data in bin i [kW]
P,i
σ standard deviation of parameter averaged over 10 min
10min
1.5 Abbreviations
WTGS wind turbine generator system

61400-12  IEC:1998(E) – 11 –
2 Test conditions
The specific test conditions related to the power performance measurement of the WTGS shall

be well defined and documented in the test report, as detailed in clause 6.

2.1 Wind turbine generator system

As detailed in clause 6, the WTGS shall be described and documented to identify uniquely the

specific machine configuration that is tested.

2.2 Test site
At the test site a meteorological mast shall be set up in the neighbourhood of the WTGS to
determine the speed of the wind that drives the wind turbine. The test site may have significant
influence on the measured power performance of the WTGS. In particular, flow distortion
effects may cause the wind speed at the meteorological mast and at the WTGS to be different,
though correlated.
The test site shall be assessed for sources of wind flow distortion in order to:
– choose the position of the meteorological mast;
– define a suitable measurement sector;
– estimate appropriate flow distortion correction factors;
– evaluate the uncertainty due to wind flow distortion.
The following factors shall be considered in particular:
– topographical variations;
– other wind turbines;
– obstacles (buildings, trees, etc.).
The test site shall be documented as detailed in clause 6.
2.2.1 Distance of meteorological mast
Care shall be taken in locating the meteorological mast. It shall not be located too close to the
WTGS, since the wind speed will be slowed down in front of the WTGS. Also, it shall not be
located too far from the WTGS, since the correlation between wind speed and electric power
output will be reduced. The meteorological mast shall be positioned at a distance from the
WTGS of between 2 and 4 times the rotor diameter D of the WTGS. A distance of 2,5 times the

rotor diameter D is recommended. The meteorological mast should be positioned within the
selected measurement sector. In the case of a vertical axis WTGS, D should be selected as
1,5 times the maximum horizontal rotor diameter.
Figure 1 shows the separation requirements between the meteorological mast and the WTGS.
It also shows the recommended separation distance of 2,5 times the rotor diameter of the
WTGS between the meteorological mast and the WTGS.

– 12 – 61400-12 © IEC:1998(E)
Meteorology mast at 4 D
Distance of meteorology
mast to WTGS between
2 D and 4 D; 2,5 D is
2,5 D
recommended
2 D
Wind
D
WTGS
Maximum measurement sector:
Disturbed sector due
257° at 2 D
to wake of of WTGS
267° at 2,5 D
on meteorology
286° at 4 D
mast; sector angle
taken from annex A:
103° at 2 D
93° at 2,5 D
74° at 4 D
IEC  145/98
Figure 1 – Requirements as to distance of the meteorological mast
and maximum allowed measurement sectors
2.2.2 Measurement sector
The measurement sector shall exclude directions having significant obstacles, significant
variations in topography or other wind turbines, as seen from both the WTGS under test and
the meteorological mast.
The disturbed sectors to be excluded due to the meteorological mast being in the wake of the
WTGS under test are for distances of 2, 2,5 and 4 times the rotor diameter of the WTGS as
shown in figure 1. For all other distances between the WTGS under test and the meteorological
mast, and for all neighboring wind turbines and obstacles, the directions to be excluded due to
wake effects shall be determined using the procedure in annex A.

2.2.3 Correction factors and uncertainty due to flow distortion at the test site
If the test site meets the requirements defined in annex A, then no further site analysis is
required, and no flow distortion correction factors are necessary. The applied standard
uncertainty due to flow distortion of the test site shall be taken to be 2 % or greater of the
measured wind speed if the meteorological mast is positioned at a distance between 2 and
3 times the rotor diameter of the WTGS and 3 % or greater if the distance is 3 to 4 times the
rotor diameter.
If the test site does not meet the requirements defined in annex A, or a smaller uncertainty due
to flow distortion of the test site is required, then either an experimental test site calibration or
a test site analysis with a three-dimensional flow model, which is validated for the relevant type
of terrain, shall be undertaken.

61400-12  IEC:1998(E) – 13 –
If an experimental test site calibration is undertaken, it is recommended that the procedure in
annex B be used. The measured flow distortion correction factors for each sector should be

used. The standard uncertainty assigned to the site correction shall be no less than one-third of

the maximum correction found within the entire measurement sector and the 60° sector centred

on the predominant test wind direction.

If a theoretical assessment of the correction factors for the test site is undertaken, using a valid

three-dimensional flow model, then sectors less than or equal to 30° should be used. The

standard uncertainty assigned to the site correction shall be no less than half of the maximum

correction found within the entire measurement sector and the 60° sector centred on the

predominant test wind direction.

Although the site calibration procedure (annex B) can be used for determination of the
performance characteristics of individual wind turbines within a wind power station, it is
important to evaluate the consistency of the results in very complex terrain.
3 Test equipment
3.1 Electric power
The net electric power of the WTGS shall be measured using a power measurement device
(e.g. power transducer) and be based on measurements of current and voltage on each phase.
The class of the current transformers shall meet the requirements of IEC 60044-1 and the
class of the voltage transformers, if used, shall meet the requirements of IEC 60186. They are
all recommended to be of class 0,5 or better.
The accuracy of the power measurement device, if it is a power transducer, shall meet the
requirements of IEC 60688 and is recommended to be class 0,5 or better. If the power
measurement device is not a power transducer then the accuracy should be equivalent to class
0,5 power transducers. The operating range of the power measurement device shall be set to
measure all positive and negative instantaneous power peaks generated by the WTGS. As a
guide, the full-scale range of the power measurement device should be set to –50 % to 200 %
of the WTGS rated power. All data shall be periodically reviewed during the test to ensure that
the range limits of the power measurement device have not been exceeded. The power
measurement device shall be mounted at the network connection point to ensure that only the
net active power output, delivered to the electrical power network, is measured.
3.2 Wind speed
Wind speed measurements shall be made with a cup anemometer that is properly installed at

hub height on a meteorological mast, at a point that represents the free stream wind flow that
drives the WTGS.
The wind speed shall be measured with a cup anemometer that has a distance constant of less
than 5 m and maintains its calibration over the duration of the measurement period. Calibration
of the anemometer shall have been undertaken before and after the completion of the power
performance test to a traceable standard. The second calibration can be replaced by an in situ
comparison against another calibrated reference anemometer, mounted at a distance of 1,5 m
to 2 m from the hub height anemometer, during the measurement period. During calibration,
the anemometer should be mounted on a configuration similar to the one to be used during the
power performance test. The measurement uncertainty of the anemometer shall be stated.
The anemometer shall be mounted within ±2,5 % of hub height, preferably on the top of a
vertical circular tube standing clear of the top of the meteorological mast. As an alternative, the
anemometer may be mounted on a boom clamped to the side of the mast and pointing in the
predominant wind direction.
– 14 – 61400-12 © IEC:1998(E)
Care shall be taken to minimize flow disturbance experienced in the vicinity of the anemometer.

To reduce flow effects, the anemometer shall be mounted so that its vertical separation from

any mounting boom is at least 7 times the boom diameter and its horizontal separation from the

mast at the anemometer height is at least 7 times the maximum mast diameter; the mast being

of a tube, cone, or lattice type. No other instrument shall be mounted so that the flow, incident

upon the anemometer, could be disturbed.

Any corrections, which are applied to the indicated wind speed to take account of factors such

as flow distortion due to the site, shall be reported clearly. The uncertainty in the correction
shall also be assessed and reported, and typically shall be no less than half the difference
between the corrected and uncorrected value.

3.3 Wind direction
Wind direction measurements shall be made with a wind vane that is mounted on the
meteorological mast within 10 % of the hub height. Proper attention shall be paid to the
positioning of the wind vane to avoid wind flow distortion between the anemometer and the
vane. The absolute accuracy of the wind direction measurement should be better than 5°.
3.4 Air density
Air density shall be derived from the measurement of air temperature and air pressure using
equation (3). At high temperatures it is recommended also to measure relative humidity and to
correct for it.
The air temperature sensor shall be mounted at least 10 m above ground level. It should be
mounted on the meteorological mast close to hub height to give a good representation of the
air temperature at the WTGS rotor centre.
The air pressure sensor should be mounted on the meteorological mast close to hub height to
give a good representation of the air pressure at the WTGS rotor centre. If the air pressure
sensor is not mounted close to the hub height, air pressure measurements shall be corrected
to the hub height according to ISO 2533.
3.5 Precipitation
To distinguish measurements from dry and wet periods, precipitation should be monitored
during the measurement period and documented in the test report.
3.6 Wind turbine generator system status
At least one parameter that indicates the operational status of the WTGS shall be monitored.
The status information shall be used in the process of determining WTGS availability.
3.7 Data acquisition system
A digital data acquisition system having a sampling rate per channel of at least 0,5 Hz shall be
used to collect measurements and store pre-processed data.
End-to-end calibration of the installed data acquisition system shall be performed for each
signal. As a guideline, the uncertainty of the data acquisition system should be negligible
compared with the uncertainty of the sensors.

61400-12  IEC:1998(E) – 15 –
4 Measurement procedure
4.1 Introduction
The objective of the measurement procedure is to collect data that meet a set of clearly

defined criteria to ensure that the data are of sufficient quantity and quality to determine the

power performance characteristics of the WTGS accurately. The measurement procedure shall

be documented, as detailed in clause 6, so that every procedural step and test condition can be
reviewed and, if necessary, repeated.

Accuracy of the measurements shall be expressed in terms of measurement uncertainty, as
described in annex C. During the measurement period, data should be periodically checked to
ensure high quality and repeatability of the test results. Test logs shall be maintained to
document all important events during the power performance test.
4.2 Wind turbine generator system operation
During the measurement period, the WTGS shall be in normal operation, as prescribed in the
WTGS operations manual, and the machine configuration shall not be changed. All data
collected while the WTGS is unavailable shall be discarded.
4.3 Data collection
Data shall be collected continuously at a sampling rate of 0,5 Hz or faster. Air temperature, air
pressure and precipitation, and WTGS status may be sampled at a slower rate, but at least
once per minute.
The data acquisition system shall store either sampled data or pre-processed data sets as
described below, or both. The pre-processed data sets shall comprise the following information
on the sampled data:
– mean value;
– standard deviation;
– maximum value;
– minimum value.
The total duration of each pre-processed data set shall be between 30 s and 10 min and shall
be 10 min divided by an integer number. Furthermore, if the data sets have a duration of less
than 10 min, then adjacent data sets shall not be separated by a time delay. Data shall be
collected until the requirements defined in 4.6 are satisfied.

4.4 Data selection
Selected data sets shall be based on 10 min periods derived from contiguous measured data.
The mean and standard deviation values for each 10 min period shall, when derived from pre-
processed data sets, be calculated according to the following equations:
N
k
= ∑ X (1)
X
k
10min
N
k
N 2
k
σ = ( ∑ N()XX− + (N −1)) (2)
σ
10min k
10min s s
1 k
−
NN 1
ks
where
N is the number of pre-processed data sets within a 10 min period;
k
X is the parameter averaged over pre-processing time period;
k
X is the parameter averaged over 10 min;
10min
– 16 – 61400-12 © IEC:1998(E)
N is the number of data samples of pre-processed data sets;
s
σ is the standard deviation of pre-processed parameter;
k
σ is the standard deviation of pre-processed parameter averaged over 10 min.
10min
Data sets shall be excluded from the database under the following circumstances:

– WTGS unavailable;
– failure of test equipment;
– wind directions outside the measurement sector.

Data sets collected under special operational conditions (e.g. high blade roughness due to
dust, salt, insects, ice) or atmospheric conditions (e.g. precipitation, wind shear) that occur
during the measurement period may be selected as a special database, and the selection
criteria shall be stated in the measurement report.
4.5 Data correction
Selected data sets shall be corrected for flow distortion (see 2.2) and for air pressure if
measured at a height other than close to hub height (see 3.4). Corrections may be applied to
measurements if it can be shown that better accuracy can be obtained (for example,
anemometer corrections for errors due to over-speeding at high turbulence sites).
4.6 Database
After data normalization (see 5.1) the selected data sets shall be sorted using the “method of
bins” procedure (see 5.2). The selected data sets shall cover a wind speed range extending
from 1 m/s below cut-in to 1,5 times the wind speed at 85 % of the rated power of the WTGS.
Alternatively, the wind speed range shall extend from 1m/s below cut-in to a wind speed at
which "AEP-measured" is greater than or equal to 95 % of "AEP-extrapolated" (see 5.3). The
wind speed range shall be divided into 0,5 m/s contiguous bins centred on integer multiples of
0,5 m/s.
The database shall be considered complete when it has met the following criteria:
– each bin includes a minimum of 30 min of sampled data;
– the total duration of the measurement period includes a minimum of 180 h with the WTGS
available within the wind speed range.
The database shall be presented in the test report as detailed in clause 6.

5 Derived results
5.1 Data normalization
The selected data sets shall be normalized to two reference air densities. One shall be the
average of the measured air density data at the test site rounded to the nearest 0,05 kg/m .
The other shall be the sea level air density, referring to ISO standard atmosphere
(1,225 kg/m ). No air density normalization to actual average air density is needed when the
actual average air density is within 1,225 ± 0,05 kg/m . The air density is determined from
measured air temperature and air pressure according to the equation:
B
10min
ρ
= (3)
10min
⋅
RT
10min
61400-12  IEC:1998(E) – 17 –
where
ρ is the derived air density averaged over 10 min;
10min
T is the measured absolute air temperature averaged over 10 min;
10min
B is the measured air pressure averaged over 10 min;
10min
R is the gas constant 287,05 J/(kg × K).

For a stall-regulated WTGS with constant pitch and constant rotational speed, data

normalization shall be applied to the measured power output according to the equation:

ρ
PP=⋅ (4)
n 10min
ρ
10min
where
P is the normalized power output;
n
P is the measured power averaged over 10 min;
10min
ρ is the reference air density;
ρ is the measured air density averaged over 10 min.
10min
For a WTGS with active power control, the normalization shall be applied to the wind speed
according to the equation:
13/
 
ρ
10min
VV= (5)
 
n 10min
ρ
 
where
V is the normalized wind speed;
n
V is the measured wind speed averaged over 10 min;
10min
ρ is the reference air density;
ρ is the measured air density averaged over 10 min.
10min
5.2 Determination of the measured power curve
The measured power curve is determined by applying the "method of bins" for the normalized
data sets, using 0,5 m/s bins and by calculation of the mean values of the normalized wind
speed and normalized power output for each wind speed bin according to the equations:

N
i
V = V (6)
i ∑ n,i, j
N
j =1
i
N
i
P = P (7)
i ∑ n,i, j
N
j =1
i
where
V is the normalized and averaged wind speed in bin i;
i
V is the normalized wind speed of data set j in bin i;
n,i,j
P is the normalized and averaged power output in bin i;
i
P is the normalized power output of data set j in bin i;
n,i,j
N is the number of 10 min data sets in bin i.
i
The measured power curve shall be presented as detailed in clause 6.

– 18 – 61400-12 © IEC:1998(E)
5.3 Annual energy production (AEP)

The AEP is estimated by applying the measured power curve to different reference wind speed

frequency distributions. A Rayleigh distribution, which is identical to a Weibull distribution with

a shape factor of 2, shall be used as the reference wind speed frequency distribution. AEP

calculations shall be made for annual average wind speeds of 4, 5, 6, 7, 8, 9, 10 and 11 m/s
according to the equation:
PP+ 
N i1− i
∑
AEP=−N[]F()V F(V )   (8)
hi i−1
i=1
 2 
where
AEP is the annual energy production;
N is the number of hours in one year ≈ 8760;
h
N is the number of bins;
V is the normalized and averaged wind speed V in bin i;
i
P is the normalized and averaged power output in bin i.
i
 
 
π V
 
and FV =−1exp − (9)
()
 
 
4 V
 
ave
 
where
F(V) is the Rayleigh cumulative probability distribution function for wind speed;
V is the annual average wind speed at hub height;
ave
V is the wind speed.
The summation is initiated by setting V equal to V – 0,5 m/s and P equal to 0,0 kW.
i–1 i i–1
The AEP shall be calculated in two ways, one designated “AEP-measured”, the other “AEP-
extrapolated”. If the measured power curve does not include data up to cut-out wind speed, the
power curve shall be extrapolated from the maximum measured wind speed up to cut-out wind
speed.
AEP-measured shall be obtained from the measured power curve by assuming zero power for
all wind speeds above and below the range of the measured power curve.
AEP-extrapolated shall be obtained from the measured power curve by assuming zero power

for all wind speeds below the lowest wind speed in the measured power curve and constant
power for wind between the highest wind speed in the measured power curve and the cut-out
wind speed. The constant power used for the extrapolated AEP shall be the power value from
the bin at the highest wind speed in the measured power curve.
AEP-measured and AEP-extrapolated shall be presented in the test report, as detailed in
clause 6. For all AEP calculations, the availability of the WTGS shall be set to 100 %. For given
annual average wind speeds, estimations of AEP-measured shall be labelled as "incomplete"
when calculations show that the AEP-measured is less than 95 % of the AEP-extrapolated.
Estimations of measurement uncertainty in terms of standard uncertainty of the AEP according
to annex C, shall be reported for the AEP-measured for all given annual average wind speeds.

61400-12  IEC:1998(E) – 19 –
The uncertainties in AEP, described above, only deal with uncertainties originating from the
power performance test and do not take into account uncertainties due to other important

factors. Practical AEP forecasting should account for additional uncertainties, including those

concerning: local wind distribution, local air density, high atmospheric turbulence, severe wind

shear, variations in the WTGS performance within a wind power station, availability of the

WTGS and WTGS performance variations due to blade roughness effects.

5.4 Power coefficient
The power coefficient, C , of the WTGS may be added to the test results and presented as
P
detailed in clause 6. C shall be determined from the measured power curve accordin
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