EN 61400-21:2008

SLOVENSKI STANDARD
01-januar-2009
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SIST EN 61400-21:2002
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Wind turbines -- Part 21: Measurement and assessment of power quality characteristics
of grid connected wind turbines
Windenergieanlagen --Teil 21: Messung und Bewertung der Netzverträglichkeit von
netzgekoppelten Windenergieanlagen
Eoliennes -- Partie 21: Mesure et évaluation des caractéristiques de qualité de puissance
des éoliennes connectées au réseau
Ta slovenski standard je istoveten z: EN 61400-21:2008
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 61400-21
NORME EUROPÉENNE
November 2008
EUROPÄISCHE NORM
ICS 27.180 Supersedes EN 61400-21:2002

English version
Wind turbines -
Part 21: Measurement and assessment
of power quality characteristics of grid connected wind turbines
(IEC 61400-21:2008)
Eoliennes -  Windenergieanlagen -
Partie 21: Mesurage et évaluation Teil 21: Messung und Bewertung
des caractéristiques de qualité der Netzverträglichkeit
de puissance des éoliennes von netzgekoppelten
connectées au réseau Windenergieanlagen
(CEI 61400-21:2008) (IEC 61400-21:2008)

This European Standard was approved by CENELEC on 2008-10-01. 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 Central Secretariat 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 Central Secretariat has the same status as the official versions.

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

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

Central Secretariat: rue de Stassart 35, B - 1050 Brussels

© 2008 CENELEC - All rights of exploitation in any form and by any means reserved worldwide for CENELEC members.
Ref. No. EN 61400-21:2008 E
Foreword
The text of document 88/317/FDIS, future edition 2 of IEC 61400-21, prepared by IEC TC 88, Wind
turbines, was submitted to the IEC-CENELEC parallel vote and was approved by CENELEC as
EN 61400-21 on 2008-10-01.
This European Standard supersedes EN 61400-21:2002.
– interharmonics and current distortions (< 9 kHz),
– response to voltage dips,
– active power ramp rate limitation and set-point control,
– reactive power capabilities and set-point control,
– grid protection and reconnection time after grid faults.
The following dates were fixed:
– latest date by which the EN has to be implemented
at national level by publication of an identical
national standard or by endorsement (dop) 2009-07-01
– latest date by which the national standards conflicting
with the EN have to be withdrawn (dow) 2011-10-01
Annex ZA has been added by CENELEC.
__________
Endorsement notice
The text of the International Standard IEC 61400-21:2008 was approved by CENELEC as a European
Standard without any modification.
In the official version, for Bibliography, the following notes have to be added for the standards indicated:
IEC 61000-3-3 NOTE  Harmonized as EN 61000-3-3:2008 (not modified).
IEC 61000-4-30 NOTE  Harmonized as EN 61000-4-30:2003 (not modified).
IEC 61400-1 NOTE  Harmonized as EN 61400-1:2005 (not modified).
__________
– 3 – EN 61400-21:2008
Annex ZA
(normative)
Normative references to international publications
with their corresponding European publications

The following referenced documents are indispensable for the application 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.

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

1) 2)
IEC 60044-1 - Instrument transformers - EN 60044-1 1999
(mod) Part 1: Current transformers

1) 2)
IEC 60044-2 - Instrument transformers - EN 60044-2 1999
(mod) Part 2: Inductive voltage transformers

1)
IEC 60050-161 - International Electrotechnical Vocabulary - -
(IEV) -
Chapter 161: Electromagnetic compatibility

1)
IEC 60050-415 - International Electrotechnical Vocabulary - -
(IEV) -
Part 415: Wind turbine generator systems

IEC 61000-4-7 2002 Electromagnetic compatibility (EMC) - EN 61000-4-7 2002
Part 4-7: Testing and measurement
techniques - General guide on harmonics and
interharmonics measurements and
instrumentation, for power supply systems
and equipment connected thereto

1) 2)
IEC 61000-4-15 - Electromagnetic compatibility (EMC) - EN 61000-4-15 1998
Part 4-15: Testing and measurement
techniques - Flickermeter - Functional and
design specifications
1) 2)
IEC 61400-12-1 - Wind turbines - EN 61400-12-1 2006
Part 12-1: Power performance measurements
of electricity producing wind turbines

IEC 61800-3 2004 Adjustable speed electrical power drive EN 61800-3 2004
systems -
Part 3: EMC requirements and specific test
methods
1) 2)
IEC 62008 - Performance characteristics and calibration EN 62008 2005
methods for digital data acquisition systems
and relevant software
1)
Undated reference.
2)
Valid edition at date of issue.

IEC 61400-21
Edition 2.0 2008-08
INTERNATIONAL
STANDARD
NORME
INTERNATIONALE
Wind turbines –
Part 21: Measurement and assessment of power quality characteristics of grid
connected wind turbines
Eoliennes –
Partie 21: Mesurage et évaluation des caractéristiques de qualité de puissance
des éoliennes connectées au réseau

INTERNATIONAL
ELECTROTECHNICAL
COMMISSION
COMMISSION
ELECTROTECHNIQUE
PRICE CODE
INTERNATIONALE
XA
CODE PRIX
ICS 27.180 ISBN 2-8318-9938-9
– 2 – 61400-21 © IEC:2008
CONTENTS
FOREWORD.4
INTRODUCTION.6
1 Scope.7
2 Normative references .7
3 Terms and definitions .8
4 Symbols and units .11
5 Abbreviations .13
6 Wind turbine power quality characteristic parameters .14
6.1 General .14
6.2 Wind turbine specification .14
6.3 Voltage fluctuations.14
6.3.1 General .14
6.3.2 Continuous operation .14
6.3.3 Switching operations .15
6.4 Current harmonics, interharmonics and higher frequency components .15
6.5 Response to voltage drops .16
6.6 Active power .16
6.6.1 Maximum measured power .16
6.6.2 Ramp rate limitation .16
6.6.3 Set-point control .17
6.7 Reactive power .17
6.7.1 Reactive power capability .17
6.7.2 Set-point control .17
6.8 Grid protection .18
6.9 Reconnection time.18
7 Test procedures .18
7.1 General .18
7.1.1 Test validity.19
7.1.2 Test conditions .19
7.1.3 Test equipment.20
7.2 Wind turbine specification .21
7.3 Voltage fluctuations.21
7.3.1 General .21
7.3.2 Fictitious grid.21
7.3.3 Continuous operation .23
7.3.4 Switching operations .26
7.4 Current harmonics, interharmonics and higher frequency components .27
7.5 Response to temporary voltage drop .28
7.6 Active power .30
7.6.1 Maximum measured power .30
7.6.2 Ramp rate limitation .31
7.6.3 Set point control .31
7.7 Reactive power .31
7.7.1 Reactive power capability .31
7.7.2 Set point control .32
7.8 Grid protection .32

61400-21 © IEC:2008 – 3 –
7.9 Reconnection time.33
8 Assessment of power quality .34
8.1 General .34
8.2 Voltage fluctuations.34
8.2.1 General .34
8.2.2 Continuous operation .35
8.2.3 Switching operations .35
8.3 Current harmonics, interharmonics and higher frequency components .36
Annex A (informative) Sample report format.38
Annex B (informative) Voltage fluctuations and flicker.48
Annex C (informative) Measurement of active power, reactive power and voltage.56
Bibliography.58

Figure 1 – Adjustment of active power set-point.17
Figure 2 – Adjustment of reactive power set-point.18
Figure 3 – Assumed elements of measurement system .20
Figure 4 – Fictitious grid for simulation of fictitious voltage .22
Figure 5 – System with short circuit emulator for testing wind turbine response to
temporary voltage drop. .29
Figure 6 – Tolerance of voltage drop .30
Figure B.1 – Measurement and assessment procedures for flicker during continuous
operation of the wind turbine.48
Figure B.2 – Measurement and assessment procedures for voltage changes and flicker
during switching operations of the wind turbine.49
Figure B.3 – Flicker coefficient as a function of wind speed .50

Table 1 – Specification of voltage drops. The specified magnitudes, duration and
shape are for the voltage drop occurring when the wind turbine under test is not
connected.16
Table 2 – Specification of requirements for measurement equipment .21
Table 3 – Specification of exponents according to IEC 61000-3-6 .37
Table B.1 – Number of measurements N and frequency of occurrence of f and f
m,i m,i y,i
for each wind speed bin in the range from cut-in wind speed to 15 m/s .51
Table B.2 – Weighting factor w for each wind speed bin.51
i
Table B.3 – Total sum of weighting factor multiplied by number of measurements for
all wind speed bins .52
Table B.4 – Weighted accumulated distribution of the flicker coefficients Pr(c each wind speed distribution.52
Table B.5 – Resulting flicker coefficient in continuous operation .53
Table B.6 – Probabilities and percentiles for different wind speeds .53

– 4 – 61400-21 © IEC:2008
INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
WIND TURBINES –
Part 21: Measurement and assessment of power quality
characteristics of grid connected 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 co-operation on all questions concerning standardization in the electrical and electronic fields. To
this end and in addition to other activities, IEC publishes International Standards, Technical Specifications,
Technical Reports, Publicly Available Specifications (PAS) and Guidances (hereafter referred to as “IEC
Publication(s)”). 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. 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 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 IEC National Committees.
3) IEC Publications have the form of recommendations for international use and are accepted by IEC National
Committees in that sense. While all reasonable efforts are made to ensure that the technical content of IEC
Publications is accurate, IEC cannot be held responsible for the way in which they are used or for any
misinterpretation by any end user.
4) In order to promote international uniformity, IEC National Committees undertake to apply IEC Publications
transparently to the maximum extent possible in their national and regional publications. Any divergence
between any IEC Publication and the corresponding national or regional publication shall be clearly indicated in
the latter.
5) IEC provides no marking procedure to indicate its approval and cannot be rendered responsible for any
equipment declared to be in conformity with an IEC Publication.
6) All users should ensure that they have the latest edition of this publication.
7) No liability shall attach to IEC or its directors, employees, servants or agents including individual experts and
members of its technical committees and IEC National Committees for any personal injury, property damage or
other damage of any nature whatsoever, whether direct or indirect, or for costs (including legal fees) and
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 61400-21 has been prepared by IEC technical committee 88: Wind
turbines.
This second edition cancels and replaces the first edition published in 2001. This edition
constitutes a technical revision.
This edition includes the following new items with respect to the previous edition:
– Interharmonics and current distortions (<9 kHz)
– Response to voltage dips
– Active power ramp rate limitation and set-point control
– Reactive power capabilities and set-point control
– Grid protection and reconnection time after grid faults

61400-21 © IEC:2008 – 5 –
The text of this standard is based on the following documents:
FDIS Report on voting
88/317/FDIS 88/326/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 of the IEC 61400 series, under the general title Wind turbines, can be found
on the IEC website.
The committee has decided that the contents of this publication will remain unchanged until
the maintenance result 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.
– 6 – 61400-21 © IEC:2008
INTRODUCTION
The purpose of this part of IEC 61400 is to provide a uniform methodology that will ensure
consistency and accuracy in the presentation, testing and assessment of power quality
characteristics of grid connected wind turbines (WTs). The power quality characteristics here
include wind turbine specifications, voltage quality (emissions of flicker and harmonics),
voltage drop response, power control (control of active and reactive power), grid protection
and reconnection time.
This part of IEC 61400 has been prepared with the anticipation that it would be applied by:
• the WT manufacturer striving to meet well-defined power quality characteristics;
• the WT purchaser in specifying such power quality characteristics;
• the WT operator who may be required to verify that stated, or required power quality
characteristics are met;
• the WT planner or regulator who has to be able to accurately and fairly determine the
impact of a WT on the voltage quality to ensure that the installation is designed so that
voltage quality requirements are respected;
• the WT certification authority or component testing organization in evaluating the power
quality characteristics of the wind turbine type;
• the planner or regulator of the electric network who has to be able to determine the grid
connection required for a WT.
This part of IEC 61400 provides recommendations for preparing the measurements and
assessment of power quality characteristics of grid connected WTs. This part of IEC 61400
will benefit those parties involved in the manufacture, installation planning, obtaining of
permission, operation, utilization, testing and regulation of WTs. The measurement and
analysis techniques recommended in this part of IEC 61400 should be applied by all parties to
ensure that the continuing development and operation of WTs are carried out in an
atmosphere of consistent and accurate communication.
This part of IEC 61400 presents measurement and analysis procedures expected to provide
consistent results that can be replicated by others.

61400-21 © IEC:2008 – 7 –
WIND TURBINES –
Part 21: Measurement and assessment of power quality
characteristics of grid connected wind turbines

1 Scope
This part of IEC 61400 includes:
• definition and specification of the quantities to be determined for characterizing the power
quality of a grid connected wind turbine;
• measurement procedures for quantifying the characteristics;
• procedures for assessing compliance with power quality requirements, including
estimation of the power quality expected from the wind turbine type when deployed at a
specific site, possibly in groups.
The measurement procedures are valid for single wind turbines with a three-phase grid
connection. The measurement procedures are valid for any size of wind turbine, though this
part of IEC 61400 only requires wind turbine types intended for PCC (Point of Common
Coupling) at MV or HV to be tested and characterized as specified in this part of IEC 61400.
The measured characteristics are valid for the specific configuration and operational mode of
the assessed wind turbine type only. Other configurations, including altered control
parameters that cause the wind turbine to behave differently with respect to power quality,
require separate assessment.
The measurement procedures are designed to be as non-site-specific as possible, so that
power quality characteristics measured at for example a test site can be considered valid also
at other sites.
The procedures for assessing compliance with power quality requirements are valid for wind
turbines with PCC at MV or HV in power systems with fixed frequency within ±1 Hz, and
sufficient active and reactive power regulation capabilities. In other cases, the principles for
assessing compliance with power quality requirements may still be used as a guidance.
This part of IEC 61400 is for testing of wind turbines, though it contains information that may
also be useful for testing of wind farms.
NOTE This part of IEC 61400 uses the following terms for system voltage:
– low voltage (LV) refers to U ≤ 1 kV;
n
– medium voltage (MV) refers to 1 kV < U ≤ 35 kV;
n
– high voltage (HV) refers to U > 35 kV.
n
2 Normative references
The following referenced documents are indispensable for the application 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 60044-1, Instrument transformers – Part 1: Current transformers
IEC 60044-2, Instrument transformers – Part 2: Inductive voltage transformers

– 8 – 61400-21 © IEC:2008
IEC 60050-161, International Electrotechnical Vocabulary – Part 161: Electromagnetic
compatibility
IEC 60050-415, International Electrotechnical Vocabulary – Part 415: Wind turbine generator
systems
IEC 61000-4-7:2002, Electromagnetic compatibility (EMC) – Part 4-7: Testing and
measurement techniques – General guide on harmonics and interharmonics measurements
and instrumentation, for power supply systems and equipment connected thereto
IEC 61000-4-15, Electromagnetic compatibility (EMC) – Part 4: Testing and measurement
techniques – Section 15: Flickermeter – Functional and design specifications
IEC 61400-12-1, Wind turbines – Part 12-1: Power performance measurements of electricity
producing wind turbines
IEC 61800-3:2004, Adjustable speed electrical power drive systems – Part 3: EMC
requirements and specific test methods
IEC 62008, Performance characteristics and calibration methods for digital data acquisition
systems and relevant software
3 Terms and definitions
For purposes of this document, the following terms and definitions apply.
3.1
continuous operation (for wind turbines)
normal operation of the wind turbine excluding start-up and shutdown operations
3.2
cut-in wind speed (for wind turbines)
lowest wind speed at hub height at which the wind turbine starts to produce power
[IEV 415-03-05]
3.3
flicker coefficient for continuous operation (for wind turbines)
normalized measure of the flicker emission during continuous operation of the wind turbine:
S
k,fic
c()ψ = P ×
k st,fic
S
n
where
P is the flicker emission from the wind turbine on the fictitious grid;
st,fic
S is the rated apparent power of the wind turbine;
n
S is the short-circuit apparent power of the fictitious grid
k,fic
NOTE The flicker coefficient for continuous operation is the same for a short-term (10 min) and long-term period
(2 h).
3.4
flicker step factor (for wind turbines)
a normalized measure of the flicker emission due to a single switching operation of the wind
turbine:
61400-21 © IEC:2008 – 9 –
S
k,fic
0,31
k (ψ ) = × × P × T
f k st,fic p
130 S
n
where
T is the measurement period, long enough to ensure that the transient of the switching
p
operation has abated, though limited to exclude possible power fluctuations due to
turbulence;
P is the flicker emission from the wind turbine on the fictitious grid;
st,fic
S is the rated apparent power of the wind turbine;
n
S is the short-circuit apparent power of the fictitious grid
k,fic
NOTE The flicker coefficient P is here evaluated over the time period T .
st,fic p
3.5
maximum measured power (for wind turbines)
power (with a specified averaging time) which is observed during continuous operation of the
wind turbine
3.6
network impedance phase angle
phase angle of network short-circuit impedance:
ψ = arctan (X R )
k k
k
where
X is the network short-circuit reactance;
k
R is the network short-circuit resistance
k
3.7
normal operation (for wind turbines)
fault free operation complying with the description in the wind turbine manual
3.8
operational mode (for wind turbines)
operation according to control setting, for example voltage control mode, frequency control
mode, reactive power control mode, active power control mode, etc.
3.9
output power (for wind turbines)
electric active power delivered by the wind turbine at its terminals
[IEV 415-04-02, modified]
3.10
point of common coupling
PCC
point of a power supply network, electrically nearest to a particular load, at which other loads
are, or may be, connected
NOTE 1 These loads can be either devices, equipment or systems, or distinct customer's installations.
NOTE 2 In some applications, the term “point of common coupling” is restricted to public networks.
[IEV 161-07-15, modified]
3.11
power collection system (for wind turbines)
electrical system that collects the power from a wind turbine and feeds it into an electrical
supply network
– 10 – 61400-21 © IEC:2008
[IEV 415-04-06, modified]
3.12
rated apparent power (for wind turbines)
apparent power from the wind turbine while operating at rated current and nominal voltage
and frequency:
S = 3U I
n n n
where
U is the nominal voltage;
n
I is the rated current
n
3.13
rated current (for wind turbines)
maximum continuous electric output current which a wind turbine is designed to achieve
under normal operating conditions
3.14
rated power (for wind turbines)
maximum continuous electric output power which a wind turbine is designed to achieve under
normal operating conditions
[IEV 415-04-03, modified]
3.15
rated wind speed (for wind turbines)
wind speed at which a wind turbine’s rated power is achieved
[IEV 415-03-04, modified]
3.16
standstill (for wind turbines)
condition of a wind turbine that is stopped
[IEV 415-01-15, modified]
3.17
start-up (for wind turbines)
transitional state of a wind turbine between standstill and power production
3.18
switching operation (for wind turbines)
start-up or switching between generators
3.19
turbulence intensity
ratio of the wind speed standard deviation to the mean wind speed, determined from the same
set of measured data samples of wind speed, and taken over a specified period of time
[IEV 415-03-25]
3.20
voltage change factor (for wind turbines)
a normalized measure of the voltage change due to a switching operation of the wind turbine:
U − U S
fic,max fic,min k,fic
k (ψ ) = 3 × ×
u k
U S
n n
61400-21 © IEC:2008 – 11 –
where
U and U are the minimum and maximum one period RMS value of the phase-to-
fic,min fic,max
neutral voltage on the fictitious grid during the switching operation;
U is the nominal phase-to-phase voltage;
n
S is the rated apparent power of the wind turbine;
n
S is the short-circuit apparent power of the fictitious grid.
k,fic
NOTE The voltage change factor k is similar to k being the ratio between the maximum inrush current and the
u i
rated current, though k is a function of the network impedance phase angle. The highest value of k will be
u u
numerically close to k .
i
3.21
wind turbine
WT
system which converts kinetic wind energy into electric energy
3.22
wind turbine terminals
point being a part of the WT and identified by the WT supplier at which the WT may be
connected to the power collection system
4 Symbols and units
In this part of IEC 61400, the following symbols and units are used.
ΔU
dyn
maximum permitted voltage change (%)

U
n
ψ
network impedance phase angle (°)
k
α (t)
m electrical angle of the fundamental of the measured voltage (°)
β
exponent associated with summation of harmonics
c(ψ )
flicker coefficient for continuous operation
k
d
relative voltage change (%)
E
Plti long-term flicker emission limit
E
Psti short-term flicker emission limit
f
nominal grid frequency (50 Hz or 60 Hz)
g
f
m,i frequency of occurrence of flicker coefficient values within the i’th wind speed bin
f
over
over-frequency protection level
f
under
under-frequency protection level
f
y,i frequency of occurrence of wind speeds within the i’th wind speed bin
h
harmonic order
– 12 – 61400-21 © IEC:2008
I
h’th order harmonic current distortion of i’th wind turbine (A)
h,i
i (t)
m measured instantaneous current (A)
I
rated current (A)
n
k (ψ )
flicker step factor
f k
k
i ratio of maximum inrush current and rated current
k (ψ )
voltage change factor
u k
L
fic inductance of fictitious grid (H)
N
maximum number of one type of switching operations within a 10 min period
10m
N
120m maximum number of one type of switching operations within a 120 min period
N
total number of wind speed bins between v and 15 m/s
bin
cut-in
n
i ratio of the transformer at the i’th wind turbine
N
total number of measured flicker coefficient values
m
N
m,i number of measured flicker coefficient values within the i’th wind speed bin
N
m,i,c N
wt number of wind turbines
P
active power (W)
P
0,2 maximum measured active power (0,2 s average value) (W)
P
maximum measured active power (60 s average value) (W)
P
600 maximum measured active power (600 s average value) (W)
P
lt long-term flicker disturbance factor
P
n rated active power of wind turbine (W)
Pr(c accumulated distribution of c
P
st short-term flicker disturbance factor
P
st,fic short-term flicker disturbance factor at fictitious grid
Q
reactive power (var)
R
fic
resistance of fictitious grid (Ω)

61400-21 © IEC:2008 – 13 –
S
short-circuit apparent power of grid (VA)
k
S
,
k fic short-circuit apparent power of the fictitious grid (VA)
S
rated apparent power of wind turbine (VA)
n
THC
total harmonic current distortion (% of I )
n
T
transient time period of a switching operation (s)
p
U
phase-to-phase voltage (V)
u (t)
instantaneous phase-to-neutral voltage of an ideal voltage source (V)
u (t)
fic instantaneous phase-to-neutral voltage simulated at fictitious grid (V)
U
maximum phase-to-neutral voltage at fictitious grid (V)
fic,max
U
fic,min minimum phase-to-neutral voltage at fictitious grid (V)
U
nominal phase-to-phase voltage (V)
n
U
under under-voltage protection level
U
over-voltage protection level
over
v annual average wind speed (m/s)
a
v
cut-in wind speed (m/s)
cut-in
v
i
mid-point of the i’th wind speed bin
w
weighting factor for the i’th wind speed bin
i
X
fic reactance of fictitious grid (Ω)
Z
impedance for limiting the effect of the short-circuit on the upstream grid (Ω)
Z
2 impedance between phases or to ground during short-circuit (Ω)
5 Abbreviations
The following abbreviations are used in this part of IEC 61400.
A/D converter analogue to digital converter
DFT discrete Fourier transform
HV high voltage
LV low voltage
– 14 – 61400-21 © IEC:2008
MV medium voltage
PCC point of common coupling
RMS root mean square
SCADA supervisory control and data acquisition
THC total harmonic current distortion
WT wind turbine
6 Wind turbine power quality characteristic parameters
6.1 General
This clause gives the quantities that shall be stated for characterizing the power quality of a
wind turbine, i.e. wind turbine specifications (6.2), voltage quality (6.3 to 6.4), voltage drop
response (6.5), power control (6.6 to 6.7), grid protection and reconnection (6.8 to 6.9). a
sample report format is given in Annex A.
Generator sign convention shall be used, i.e. the positive direction of the power flow is
defined to be from the generator to the grid. If the wind turbine is replaced with a resistor and
an inductor, both active and reactive power will be negative.
6.2 Wind turbine specification
The rated data of the wind turbine (referred to the wind turbine terminals) shall be specified,
including P , S , U and I .
n n n n
NOTE The rated data are used only for normalizing purposes in this part of IEC 61400.
6.3 Voltage fluctuations
6.3.1 General
The voltage fluctuations (flicker and voltage changes) imposed by the wind turbine shall be
characterized as described in 6.3.2 and 6.3.3.
6.3.2 Continuous operation
The wind turbine flicker coefficient for continuous operation, c(ψ ,v ) shall be stated as the
k a
99th percentile for the network impedance phase angles ψ = 30°, 50°, 70° and 85° in a Table
k
for four different wind speed distributions with annual average wind speed v = 6 m/s, 7,5 m/s,
a
8,5 m/s and 10 m/s respectively. The 10 min average values of the wind speed shall be
assumed to be Rayleigh distributed (see Note). The annual average wind speed refers to the
hub height of the wind turbine.
The characteristics shall be stated for the wind turbine operating with reactive power as close
as possible to zero, i.e. if applicable, the reactive set-point control shall be set to Q=0. If any
other operational mode is used, this shall be clearly stated.
NOTE The Rayleigh distribution is a probability distribution that commonly fits the annual wind speed distribution.
The Rayleigh distribution may be described by:
⎛ ⎞
π ⎛ v ⎞
⎜ ⎟
= − − ⎜ ⎟
F(v) 1 exp
⎜ ⎜ ⎟ ⎟
⎜ 4 v ⎟
a
⎝ ⎠
⎝ ⎠
61400-21 © IEC:2008 – 15 –
where
F(v) is the Rayleigh cumulative probability distribution function for the wind speed;
v is the annual average wind speed at hub height;
a
v is the wind speed.
6.3.3 Switching operations
The characteristics shall be stated for the following types of switching operations:
a) Wind turbine start-up at cut-in wind speed.
b) Wind turbine start-up at rated wind speed or higher wind speed.
c) The worst case of switching between generators (applicable only to wind turbines with
more than one generator or a generator with multiple windings). See also Note 1.
For each of the above types of switching operations, the values of the parameters below shall
be stated (see also Notes 2 and 3):
• The maximum number N of the switching operation within a 10 min period.
10m
• The maximum number N of the switching operation within a 2 h period.
120m
• The flicker step factor k (ψ ) for the network impedance phase angles ψ = 30°, 50°, 70°
f k k
and 85°.
• The voltage change factor k (ψ ) for the network impedance phase angles ψ = 30°, 50°,
u k k
70° and 85°.
The characteristics shall be stated for the wind turbine operating with reactive power as close
as possible to zero, i.e. if applicable the reactive set-point control shall be set to Q=0. If other
operational mode is used, this shall be clearly stated.
NOTE 1 The worst case of switching between generators is in the context of flicker step factor defined as the
switching operation that gives the highest flicker step factor, and in the context of voltage change factor defined as
the switching operation that gives the highest voltage change factor.
NOTE 2 The parameters N and N may be based on manufacturers information, whereas k (ψ ) and k (ψ )
10m 120m f k u k
should be measured and computed.
NOTE 3 Depending on the control system of the wind turbine, the maximum number of the switching operation
within a 2 h period may be less than twelve times the maximum number of the switching operation within a 10 min
period.
6.4 Current harmonics, interharmonics and higher frequency components
The emission of current harmonics, interharmonics and higher frequency components during
continuous operation shall be stated (see Note).
The values of the individual current components (harmonics, interharmonics and higher
frequency components) and the total harmonic current distortion shall be given in tables in
percentage of I and for operation of the wind turbine within the active power bins 0, 10,
n
20, … , 100 % of P . 0, 10, 20, … , 100 % are the bin midpoints.
n
The individual harmonic current components shall be specified as subgrouped values for
frequencies up to 50 times the fundamental grid frequency, and the total harmonic current
distortion shall be stated as derived from these.
The interharmonic current components shall be specified as subgrouped values for
frequencies up to 2 kHz in accordance to Annex A of IEC 61000-4-7:2002.
The higher frequency current components shall be specified as subgrouped values for
frequencies between 2 kHz and 9 kHz in accordance to Annex B of IEC 61000-4-7:2002.

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The current harmonics, interharmonics and higher frequency components shall be stated for
the wind turbine operating with reactive power as close as possible to zero, i.e. if applicable
the reactive set-point control shall be set to Q=0. If other operational mode is used, this shall
be clearly stated.
NOTE Harmonics are considered harmless as long as the duration is limited to a short period of time. Hence, this
part of IEC 61400 does not require specification of short-duration harmonics caused by wind turbine start-up or
other switching operations.
6.5 Response to voltage drops
The response of the wind turbine to the voltage drops specified in Table 1 shall be stated for
the wind turbine operating at a) between 0,1 P and 0,3 P and b) above 0,9 P . The stated
n n n
response shall include results from 2 consecutive tests of each case (VD1-VD6) by time-
series of active power, reactive power, active current, reactive current and voltage at the wind
turbine terminals for the time shortly prior to the voltage drop and until the effect of the
voltage drop has abated, but also the wind turbine operational mode shall be specified.
The test is basically for verifying wind turbine response to voltage drops (due to grid faults)
and providing a basis for wind turbine numerical simulation model
...