EN 61400-21:2002

SLOVENSKI STANDARD
01-september-2002
Wind turbine generator systems - Part 21: Measurement and assessment of power
quality characteristics of grid connected wind turbines
Wind turbine generator systems -- Part 21: Measurement and assessment of power
quality characteristics of grid connected wind turbines
Windenergieanlagen -- Teil 21: Anforderungen an die Netzverträglichkeit für an ein
Verbundnetz angeschlossene Windenergieanlagen
Aérogénérateurs -- Partie 21: Mesurage et évaluation des caractéristiques de qualité de
puissance des éoliennes connectées au réseau
Ta slovenski standard je istoveten z: EN 61400-21:2002
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
EUROPÄISCHE NORM February 2002
ICS 27.180
English version
Wind turbine generator systems
Part 21: Measurement and assesment of power quality
characteristics of grid connected wind turbines
(IEC 61400-21:2001)
Aérogénérateurs Windenergieanlagen
Partie 21: Mesurage et évaluation Teil 21: Anforderungen an die
des caractéristiques de qualité Netzverträglichkeit für an ein
de puissance des éoliennes Verbundnetz angeschlossene
connectées au réseau Windenergieanlagen
(CEI 61400-21:2001) (IEC 61400-21:2001)
This European Standard was approved by CENELEC on 2002-02-01. 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, Czech Republic,
Denmark, Finland, France, Germany, Greece, Iceland, Ireland, Italy, Luxembourg, Malta, Netherlands,
Norway, Portugal, Spain, Sweden, Switzerland and 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
© 2002 CENELEC - All rights of exploitation in any form and by any means reserved worldwide for CENELEC members.
Ref. No. EN 61400-21:2002 E
Foreword
The text of document 88/144/FDIS, future edition 1 of IEC 61400-21, prepared by IEC TC 88, Wind
turbine systems, was submitted to the IEC-CENELEC parallel vote and was approved by CENELEC
as EN 61400-21 on 2002-02-01.
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) 2002-11-01
– latest date by which the national standards conflicting
with the EN have to be withdrawn (dow) 2005-02-01
Annexes designated "normative" are part of the body of the standard.
Annexes designated "informative" are given for information only.
In this standard, annex ZA is normative and annexes A and B are informative.
Annex ZA has been added by CENELEC.
__________
Endorsement notice
The text of the International Standard IEC 61400:2001 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 61000-3-3:1995 (not modified).
IEC 61400-12 NOTE  Harmonized as EN 61400-12:1998 (not modified).
__________
- 3 - EN 61400-21:2002
Annex ZA
(normative)
Normative references to international publications
with their corresponding European publications
This European Standard incorporates by dated or undated reference, provisions from other
publications. These normative references are cited at the appropriate places in the text and the
publications are listed hereafter. For dated references, subsequent amendments to or revisions of any
of these publications apply to this European Standard only when incorporated in it by amendment or
revision. For undated references the latest edition of the publication referred to applies (including
amendments).
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 60034-1 - Rotating electrical machines EN 60034-1 1998
(mod) Part 1: Rating and performance
1) 2)
IEC 60044-1 - Instrument transformers EN 60044-1 1999
(mod) Part 1: Current transformers
1)
IEC 60050-161 - International Electrotechnical --
Vocabulary (IEV) - Chapter 161:
Electromagnetic compatibility
1)
IEC 60050-393 - Chapter 393: Nuclear instrumentation:--
Physical phenomena and basic
concepts
1)
IEC 60050-415 - Part 415: Wind turbine generator --
systems
IEC 60186 (mod) 1987 Voltage transformers - -
2)
+ A1 (mod) 1988 HD 554 S1 1992
A2 1995 - -
1) 2)
IEC 60688 - Electrical measuring transducers for EN 60688 1992
converting a.c. electrical quantities to
analogue or digital signals
1) 2)
IEC 61000-4-7 - Electromagnetic compatibility (EMC) EN 61000-4-7 1993
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)
Undated reference.
2)
Valid edition at date of issue.

Publication Year Title EN/HD Year
1) 2)
IEC 61000-4-15 - Part 4-15: Testing and measurement EN 61000-4-15 1998
techniques - Flickermeter - Functional
and design specifications
1) 2)
IEC 61800-3 - Adjustable speed electrical power drive EN 61800-3 1996
systems
Part 3: EMC product standard including
specific test methods
NORME
CEI
INTERNATIONALE IEC
61400-21
INTERNATIONAL
Première édition
STANDARD
First edition
2001-12
Aérogénérateurs –
Partie 21:
Mesurage et évaluation des caractéristiques
de qualité de puissance des éoliennes
connectées au réseau
Wind turbine generator systems –
Part 21:
Measurement and assessment of power quality
characteristics of grid connected wind turbines
© IEC 2001 Droits de reproduction réservés ⎯ Copyright - all rights reserved
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CODE PRIX
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PRICE CODE
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Pour prix, voir catalogue en vigueur
For price, see current catalogue

61400-21 © IEC:2001 − 3 −
CONTENTS
FOREWORD.7
INTRODUCTION.9
1 Scope.11
2 Normative references.13
3 Definitions .13
4 Symbols and units .21
5 Abbreviations.23
6 Wind turbine power quality characteristic parameters .25
6.1 General .25
6.2 Rated data.25
6.3 Maximum permitted power .25
6.4 Maximum measured power .25
6.5 Reactive power.25
6.6 Voltage fluctuations .25
6.7 Harmonics .27
7 Measurement procedures .29
7.1 General .29
7.2 Rated data.35
7.3 Maximum permitted power .35
7.4 Maximum measured power .35
7.5 Reactive power.35
7.6 Voltage fluctuations .37
7.7 Harmonics .47
8 Assessment of power quality.49
8.1 General .49
8.2 Steady-state voltage .51
8.3 Voltage fluctuations .51
8.4 Harmonics .57
Annex A (informative) Sample report format .59
Annex B (informative) Voltage fluctuations and flicker.69
Bibliography .85
Figure 1 – Assumed elements of measurement system.33
Figure 2 – Fictitious grid for simulation of fictitious voltage .37
Figure B.1 – Measurement and assessment procedures for flicker during continuous
operation of the wind turbine .69
Figure B.2 – Measurement and assessment procedures for voltage changes and flicker
during switching operations of the wind turbine.71
Figure B.3 – Flicker coefficient as a function of wind speed .73

61400-21 © IEC:2001 − 5 −
Table 1 – Specification of requirements for measurement equipment.33
Table 2 – Specification of exponents according to IEC 61000-3-6 .57
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 .75
Table B.2 – Weighting factor w for each wind speed bin .75
i
Table B.3 – Total sum of weighting factor multiplied by number of measurements for all
wind speed bins .77
Table B.4 – Weighted accumulated distribution of the flicker coefficients Pr(c each wind speed distribution .77
Table B.5 – Resulting flicker coefficient in continuous operation .79
Table B.6 – Probabilities and percentiles for different wind speeds .79

61400-21 © IEC:2001 − 7 −
INTERNATIONAL ELECTROTECHNICAL COMMISSION
___________
WIND TURBINE GENERATOR SYSTEMS –
Part 21: Measurement and assessment of power quality characteristics
of grid connected wind turbines
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 specifications, 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 61400-21 has been prepared by IEC technical committee 88: Wind
turbine systems.
The text of this standard is based on the following documents:
FDIS Report on voting
88/144/FDIS 88/150/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 3.
Annexes A and B are for information only.
The committee has decided that the contents of this publication will remain unchanged until
2005. At this date, the publication will be
• reconfirmed;
• withdrawn;
• replaced by a revised edition, or
• amended.
61400-21 © IEC:2001 − 9 −
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 assessment of power quality characteristics
of grid connected wind turbines (WTs). In this respect the term power quality includes those
electric characteristics of the WT that influence the voltage quality of the grid to which the WT
is connected. The standard 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 must 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 must be able to determine the grid
connection required for a WT.
This standard provides recommendations for preparing the measurements and assessment
of power quality characteristics of grid connected WTs. The standard 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 standard 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 standard presents measurement and analysis procedures expected to provide consistent
results that can be replicated by others.

61400-21 © IEC:2001 − 11 −
WIND TURBINE GENERATOR SYSTEMS –
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, and as long as the wind turbine is not operated to actively control the frequency or
voltage at any location in the network. The measurement procedures are valid for any size of
wind turbine, though this standard only requires wind turbine types intended for PCC at MV or
HV to be tested and characterized as specified in this standard.
The measured characteristics are valid for the specific configuration of the assessed wind
turbine 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 and sufficient load to absorb the
wind power production. In other cases, the principles for assessing compliance with power
quality requirements may still be used as a guide.
NOTE 1 This standard 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
NOTE 2 The issue of interharmonics is not addressed in this standard, though it is under consideration awaiting
proper measurement and assessment procedures to be established by the appropriate IEC committee.

61400-21 © IEC:2001 − 13 −
2 Normative references
The following normative documents contain provisions which, through reference in this text,
constitute provisions of this part of IEC 61400. For dated references, subsequent amendments
to, or revisions of, any of these publications do not apply. However, parties to agreements
based on this part of IEC 61400 are encouraged to investigate the possibility of applying the
most recent editions of the normative documents indicated below. For undated references, the
latest edition of the normative document referred to applies. Members of IEC and ISO maintain
registers of currently valid International Standards.
IEC 60034-1, Rotating electrical machines – Part 1: Rating and performance
IEC 60044-1, Instrument transformers – Part 1: Current transformers
IEC 60050(161), International Electrotechnical Vocabulary (IEV) – Chapter 161: Electro-
magnetic compatibility
IEC 60050(393), International Electrotechnical Vocabulary (IEV) – Chapter 393: Nuclear
instrumentation: Physical phenomena and basic concepts
IEC 60050(415), International Electrotechnical Vocabulary (IEV) – Chapter 415: Wind turbine
generator systems
IEC 60186, Voltage transformers
Amendment 1 (1988)
Amendment 2 (1995)
IEC 60688, Electrical measuring transducers for converting a.c. electrical quantities to
analogue or digital signals
IEC 61000-4-7, Electromagnetic compatibility (EMC) – Part 4: Testing and measurement
techniques – Section 7: 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 61800-3, Adjustable speed electrical power drive systems – Part 3: EMC product standard
including specific test methods
3 Definitions
For the purpose of this part of IEC 61400, the following 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]
61400-21 © IEC:2001 − 15 −
3.3
flicker coefficient for continuous operation (for wind turbines)
a 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:
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
3.5
maximum permitted power (for wind turbines)
the 10 min average power from the wind turbine that must not be exceeded irrespective of
weather and grid conditions
3.6
maximum measured power (for wind turbines)
that power (with a specified averaging time) which is observed during continuous operation of
the wind turbine
3.7
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
3.8
normal operation (for wind turbines)
fault free operation complying with the description in the wind turbine manual
[IEV 393-08-12, modified]
61400-21 © IEC:2001 − 17 −
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
[IEV 415-04-06, modified]
3.12
rated apparent power (for wind turbines)
the apparent power from the wind turbine while operating at rated power and nominal voltage
and frequency:
2 2
S = P + Q
n n n
where
P is the rated power;
n
Q is the corresponding reactive power.
n
3.13
rated current (for wind turbines)
the current from the wind turbine while operating at rated power and nominal voltage and
frequency
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
rated reactive power (for wind turbines)
the reactive power from the wind turbine while operating at rated power and nominal voltage
and frequency
61400-21 © IEC:2001 − 19 −
3.17
standstill (for wind turbines)
condition of a wind turbine that is stopped
[IEV 415-01-15, modified]
3.18
start-up (for wind turbines)
transitional state of a wind turbine between standstill and power production
3.19
switching operation (for wind turbines)
start-up or switching between generators
3.20
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]
3.21
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
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
3.22
wind turbine (WT)
system which converts kinetic wind energy into electric energy
3.23
wind turbine terminals
a 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

61400-21 © IEC:2001 − 21 −
4 Symbols and units
In this standard, the following symbols and units are used.
∆U
dyn
maximum permitted voltage change (%)
U
n
ψ network impedance phase angle (degrees)
k
electrical angle of the fundamental of the measured voltage (degrees)
α (t)
m
β exponent associated with summation of harmonics
c(ψ ) flicker coefficient for continuous operation
k
d relative voltage change (%)
E long-term flicker emission limit
Plti
E short-term flicker emission limit
Psti
f fundamental grid frequency (Hz)
g
f frequency of occurrence of flicker coefficient values within the i’th wind speed bin
m,i
f frequency of occurrence of wind speeds within the i’th wind speed bin
y,i
h harmonic order
I h’th order harmonic current distortion of i’th wind turbine (A)
h,i
i (t) measured instantaneous current (A)
m
I rated current (A)
n
k (ψ ) flicker step factor
f k
k ratio of maximum inrush current and rated current
i
k (ψ ) voltage change factor
u k
L inductance of fictitious grid (H)
fic
N maximum number of one type of switching operations within a 10 min period
N 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 ratio of the transformer at the i’th wind turbine
i
N total number of measured flicker coefficient values
m
N number of measured flicker coefficient values within the i’th wind speed bin
m,i
N number of flicker coefficient values less than x within the i’th wind speed bin
m,i,c N number of wind turbines
wt
P maximum measured power (0,2-second-average value) (W)
0,2
P maximum measured power (60-second-average value) (W)
P long-term flicker disturbance factor
lt
P maximum permitted power (W)
mc
61400-21 © IEC:2001 − 23 −
P rated power of wind turbine (W)
n
Pr(c P short-term flicker disturbance factor
st
P short-term flicker disturbance factor at fictitious grid
st,fic
Q reactive power (0,2-second-average value) at P (var)
0,2 0,2
Q reactive power (60-second-average value) at P (var)
60 60
Q reactive power at P (var)
mc mc
Q rated reactive power of wind turbine (var)
n
R resistance of fictitious grid (Ω)
fic
S apparent power (0,2-second-average value) at P (VA)
0,2 0,2
S apparent power (60-second-average value) at P (VA)
60 60
S short-circuit apparent power of grid (VA)
k
S short-circuit apparent power of the fictitious grid (VA)
,
k fic
S apparent power at P (VA)
mc mc
S rated apparent power of wind turbine (VA)
n
T transient time period of a switching operation (s)
p
u (t) instantaneous phase-to-neutral voltage of an ideal voltage source (V)
u (t) instantaneous phase-to-neutral voltage simulated at fictitious grid (V)
fic
U maximum phase-to-neutral voltage at fictitious grid (V)
fic,max
U minimum phase-to-neutral voltage at fictitious grid (V)
fic,min
U nominal phase-to-phase voltage (V)
n
v annual average wind speed (m/s)
a
v cut-in wind speed (m/s)
cut-in
v mid-point of the i’th wind speed bin
i
w weighting factor for the i’th wind speed bin
i
X
reactance of fictitious grid (Ω)
fic
5 Abbreviations
The following abbreviations are used in this standard.
A/D converter analogue to digital converter
HV high voltage
LV low voltage
MV medium voltage
PCC point of common coupling
RMS root mean square
WT wind turbine
61400-21 © IEC:2001 − 25 −
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. 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
assumed to be from the wind turbine and to the grid.
6.2 Rated data
The rated data of the wind turbine shall be specified, including P , Q , S , U and I .
n n n n n
NOTE The rated data are used only for normalizing purposes in this standard.
6.3 Maximum permitted power
The maximum permitted power P of the wind turbine (permitted by the control system) shall
mc
be specified.
6.4 Maximum measured power
The maximum measured power of the wind turbine shall be specified both as a 60 s average
value, P and as a 0,2 s average value, P .
60 0,2
6.5 Reactive power
The reactive power of the wind turbine shall be specified in a table as 10 min average values
as a function of the 10 min average output power for 0, 10, . 90, 100 % of the rated power.
Also the reactive power at P , P and P shall be specified.
mc 60 0,2
6.6 Voltage fluctuations
The voltage fluctuations (flicker and voltage changes) imposed by the wind turbine shall be
characterized as described in 6.6.1 and 6.6.2.
6.6.1 Continuous operation
The wind turbine flicker coefficient for continuous operation, c(ψ ,v ) shall be stated as the 99th
k a
percentile for the network impedance phase angles ψ = 30°, 50°, 70° and 85° in a table for
k
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.
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 ⎠
⎝ ⎠
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.
61400-21 © IEC:2001 − 27 −
6.6.2 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.
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):
1) The maximum number N of the switching operation within a 10 min period.
2) The maximum number N of the switching operation within a 2 h period.
3) The flicker step factor k (ψ ) for the network impedance phase angles ψ = 30°, 50°, 70°
f k k
and 85°.
4) The voltage change factor k (ψ ) for the network impedance phase angles ψ = 30°, 50°,
u k k
70° and 85°.
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 )
10 120 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.
6.7 Harmonics
For a wind turbine with a power electronic converter (see notes 1, 2, 3 and 4), the wind
turbine’s emission of harmonic currents during continuous operation shall be stated. These
shall be stated for frequencies up to 50 times the fundamental grid frequency (see note 5), as
the individual harmonic currents and the maximum total harmonic current distortion. The
individual harmonic currents shall be given as 10 min average data for each harmonic order at
the output power giving the maximum individual harmonic current. The values shall be
specified in a table as a percentage of the rated current. Harmonic currents below 0,1 % of the
rated current for any of the harmonic orders need not be specified.
NOTE 1 Harmonic emissions have been reported from a few installations of wind turbines with induction
generators but without power electronic converters. There is however no agreed procedure for measurement of
harmonic emissions from induction machines. Further, there is no known instance of customer annoyance or
damage to equipment due to harmonic emissions from such wind turbines. This standard therefore does not require
measurement of harmonic emissions from such wind turbines.
NOTE 2 The synchronous generator generates a voltage with a waveform depending on the shape of the magnetic
field in the air-gap and the regularity of its stator winding. For a wind turbine with a synchronous generator directly
connected to the grid, the waveform should according to 8.4 comply with the requirements in 8.9 of IEC 60034-1.
Then the wind turbine will only emit very limited harmonic and interharmonic currents, and hence this standard
therefore does not require specification of these.
NOTE 3 Harmonics are considered harmless as long as the duration is limited to a short period of time.
Experience with “soft-start” power electronic units in wind turbines has not generally shown that the short-duration
harmonic emissions cause problems. Hence, this standard does not require specification of short-duration
harmonics caused by wind turbine start-up or other switching operations.
NOTE 4 There has been a reported problem with unnecessary operation of earth-leakage protection on a low-
voltage circuit possibly due to harmonic current emissions during starting of a wind turbine. The issue may be
considered in a future issue of this standard.

61400-21 © IEC:2001 − 29 −
NOTE 5 Power electronic converters operating with switching frequencies in the kHz range may emit harmonics
above 50 times the fundamental grid frequency. The issue of harmonics above 50 times the fundamental grid
frequency is under consideration awaiting more experience as well as proper measurement and assessment
procedures to be established by the appropriate IEC committee.
7 Measurement procedures
Subclause 7.1 gives general information about the validity of the measurements, required test
conditions and equipment. Subclauses 7.2 to 7.7 state the required measurements to be taken
to determine the power quality characteristic parameters of the assessed wind turbine.
7.1 General
The measurement procedures are valid for single wind turbines with a three-phase grid
connection, and as long as the wind turbine is not operated to actively control the frequency or
voltage at any location in the network.
The measurements aim in general to verify the characteristic power quality parameters for the
full operational range of the assessed wind turbine. Measurements are however not required
for wind speeds above 15 m/s (see note 1). This is because requiring measurements at higher
wind speeds would normally give a significant longer measurement period due to the rare
appearance of higher wind speeds, and are not expected to give significantly better verification
of the characteristic power quality parameters of the assessed wind turbine. See also note 2.
The measured characteristics are valid for the specific configuration of the assessed wind
turbine only. Other configurations, including altered control parameters, that cause the wind
turbine to behave differently with respect to power quality, require separate assessment. See
also note 3.
NOTE 1 If measurements are taken above 15 m/s, they can be omitted. If they are included however, the applied
wind speed range should be stated in the test report.
NOTE 2 Inclusion of measurements above 15 m/s may improve the accuracy of the determined flicker coefficient,
and for some wind turbine design give greater maximum measured power (0,2 s average). Aiming for a balance
between cost and accuracy however, inclusion of measurements above 15 m/s is not required. If measurements
above 15 m/s are included, this will improve confidence in the results of the procedures of 8.3 for high-wind speed
sites.
NOTE 3 Some wind turbine designs include a built-in transformer. The measurements of the electrical
characteristics should be made at the wind turbine terminals. It is up to the WT supplier to define the wind turbine
terminals to be at the lower-voltage or higher-voltage side of the transformer. Changing the transformer from one
output voltage to another is not expected to cause the wind turbine to behave differently with respect to power
quality. Thus, separate assessment is not required if the transformer output voltage is changed, except that rated
voltage and current must be updated.
7.1.1 Test conditions
The following test conditions are required (see note 1).
• The wind turbine shall be connected directly to the MV-network through a standard
transformer with rated power at least corresponding to the apparent power at P of the
mc
assessed wind turbine.
• The short-circuit apparent power at the point of connection to the MV-network shall be at
least 50 times the apparent power at P of the assessed wind turbine. The short-circuit
mc
apparent power of the network may be determined by calculation or by reference to the
network operator prior to testing the wind turbine. See also note 2.
• The total harmonic distortion of the voltage including all harmonics up to the order of 50
shall be less than 5 % measured as 10 min average data at the wind turbine terminals while
the wind turbine is not generating. The total harmonic distortion of the voltage may be
determined by measurement prior to testing the wind turbine.

61400-21 © IEC:2001 − 31 −
• The grid frequency measured as 0,2 s average data shall be within ±1 % of the nominal
frequency, and the rate of change of the grid frequency measured as 0,2 s average data
shall be less than 0,2 % of the nominal frequency per 0,2 s. If the grid frequency is known to
be very stable and well within the above requirements, which would commonly be the case
in a large interconnected power system, this need not be assessed any further. Otherwise,
the grid frequency must be measured during the test, and test data possibly sampled during
periods with inappropriate grid frequency must be excluded.
• The voltage shall be within ±5 % of its nominal value measured as 10 min average data at
the wind turbine terminals. If the voltage is known to be very stable and well within the
above requirement, which would commonly be the case if the wind turbine is connected to a
very strong grid, this need not be assessed any further. Otherwise, the voltage must be
measured during the test, and test data possibly sampled during periods with inappropriate
voltage must be excluded.
• The voltage unbalance factor shall be less than 2 % measured as 10 min data at the wind
turbine terminals. The voltage unbalance factor may be determined as described in
IEC 61800-3, clause B.3. If the voltage unbalance factor is known to be well within the
above requirement, it need not be assessed any further. Otherwise, the voltage unbalance
factor must be measured during the test, and test data possibly sampled during periods with
inappropriate voltage unbalance factor must be excluded.
• The turbulence intensity taken over a 10 min period shall be between 8 % and 16 %. The
turbulence intensity shall be assessed based on sector-wise identification of obstacles and
terrain variations or based on wind speed measurements. Either way, test data possibly
sampled during periods with turbulence intensity outside the above range must be excluded.
See al
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