IEC TR 61400-21-3:2019

Wind energy generation systems - Part 21-3: Measurement and assessment of electrical characteristics - Wind turbine harmonic model and its application

IEC TR 61400-21-3:2019 provides guidance on principles which can be used as the basis for determining the application, structure and recommendations for the WT harmonic model. For the purpose of this Technical Report, a harmonic model means a model that represents harmonic emissions of different WT types interacting with the connected network.
This document is focused on providing technical guidance concerning the WT harmonic model. It describes the harmonic model in detail, covering such aspects as application, structure, as well as validation. By introducing a common understanding of the WT representation from a harmonic performance perspective, this document aims to bring the overall concept of the harmonic model closer to the industry (e.g. suppliers, developers, system operators, academia, etc.).
A standardized approach of WT harmonic model representation is presented in this document. The harmonic model will find a broad application in many areas of electrical engineering related to design, analysis, and optimisation of electrical infrastructure of onshore as well as offshore WPPs.

General Information

• Status: Published
• Publication Date: 12-Sep-2019
• ICS: 27.180 - Wind turbine energy systems
• Technical Committee: TC 88 - Wind energy generation systems
• Current Stage: PPUB - Publication issued
• Start Date: 10-Sep-2019
• Completion Date: 13-Sep-2019

Overview

IEC TR 61400-21-3:2019 provides technical guidance for a standardized wind turbine (WT) harmonic model - a model that represents harmonic emissions of different WT types interacting with the connected power network. The Technical Report defines principles for the model’s application, structure, input/output requirements and validation, and introduces a common WT representation for harmonic performance assessment. Its aim is to harmonize modelling practice across suppliers, developers, system operators and academia for onshore and offshore wind power plants (WPPs).

Key topics and requirements

• Scope and purpose: Guidance on creating a WT harmonic model used to represent harmonic emissions and interactions with the network.
• Model structure: Recommended representation using equivalent circuits (e.g., Thévenin/Norton equivalents), harmonic voltage/current sources and harmonic equivalent impedances.
• WT type considerations: Different approaches for common WT types (Type 1–4) acknowledging converter behaviour and generator/converter interactions.
• Minimal requirements: Suggested input parameters, terminal definitions, and output variables needed for consistent harmonic studies and data exchange.
• Validation: Procedures and principles for model validation, including use of measured data, power bins and fictitious grid concepts to ensure model fidelity across operating conditions.
• Interfaces: Guidance on links to other IEC documents and harmonization with Annexes of IEC 61400-21-1 for harmonic evaluation and power-quality assessment.
• Limitations and recommended use: Statements on the intended domain of applicability and known modelling boundaries.

Practical applications and users

IEC TR 61400-21-3:2019 is intended for stakeholders involved in the design, assessment and operation of wind generation systems and grid infrastructure. Typical users and applications include:

• Transmission and distribution system operators (TSOs/DSOs) - assessing network harmonic impact and grid-connection studies.
• Wind power plant (WPP) developers and owners - electrical infrastructure design and optimization for onshore and offshore projects.
• WT manufacturers and component suppliers - providing standardized harmonic data and validated WT harmonic models.
• Consulting engineers and simulation software developers - building consistent harmonic study models and interfaces for system-level analysis.
• Academia and research bodies - benchmarking, comparative studies and development of mitigation strategies.

Common practical uses: system-level harmonic studies, filter design and resonance mitigation, component sizing for harmonic losses, assessing background distortion impact, and standardized data exchange for performance contracts.

Related standards

• IEC 61400-21-1 (harmonic evaluation and power-quality assessment) - referenced for interfaces and assessment criteria.
• Other IEC 61400 series documents covering measurement and grid interaction topics.

Keywords: IEC TR 61400-21-3:2019, wind turbine harmonic model, harmonic emissions, harmonic assessment, wind power plant, WT harmonic performance, grid-connection studies.

Standards Content (Sample)

IEC TR 61400-21-3 ®
Edition 1.0 2019-09
TECHNICAL
REPORT
colour
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Wind energy generation systems –
Part 21-3: Measurement and assessment of electrical characteristics – Wind
turbine harmonic model and its application
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IEC TR 61400-21-3 ®
Edition 1.0 2019-09
TECHNICAL
REPORT
colour
inside
Wind energy generation systems –

Part 21-3: Measurement and assessment of electrical characteristics – Wind

turbine harmonic model and its application

INTERNATIONAL
ELECTROTECHNICAL
COMMISSION
ICS 27.180 ISBN 978-2-8322-7288-6

– 2 – IEC TR 61400-21-3:2019 © IEC 2019
CONTENTS
FOREWORD . 4
INTRODUCTION . 6
1 Scope . 7
2 Normative references . 8
3 Terms, definitions and abbreviations . 8
3.1 Terms and definitions . 8
3.2 Abbreviations . 12
4 General description . 13
4.1 Overview. 13
4.2 Background. 14
5 Recommendations of minimal requirements . 17
5.1 General . 17
5.2 Application . 18
5.3 Input parameters . 18
5.4 Harmonic model terminal . 18
5.5 Output variables. 19
5.6 Structure . 19
6 Interfaces to other IEC documents . 20
6.1 IEC 61400-21-1:2019, Annex D – Harmonic evaluation . 20
6.2 IEC 61400-21-1:2019, Annex E – Assessment of power quality of wind
turbines and wind power plants . 21
7 Harmonic model . 21
7.1 General . 21
7.2 Thévenin/Norton equivalent circuit . 22
7.3 Equivalent harmonic voltage/current sources . 22
7.3.1 General . 22
7.3.2 Harmonic equivalent impedance . 23
7.4 Wind turbine types . 24
7.4.1 General . 24
7.4.2 Type 1 and Type 2 . 24
7.4.3 Type 3 . 25
7.4.4 Type 4 . 26
8 Validation . 28
8.1 General . 28
8.2 Overview. 28
8.3 Model validation . 29
8.4 Fictitious grid . 30
9 Limitations . 30
Bibliography . 32

Figure 1 – Example of a phase angle between the harmonic current and the harmonic
voltage component as well as the fundamental voltage . 9
Figure 2 – Example of wind power plant typical components relevant for the harmonic
studies and potential challenges in harmonic performance . 14
Figure 3 – Example of a WPP complex structure . 15
Figure 4 – Example of a WPP complex electrical infrastructure with many WTs . 16

Figure 5 – Harmonic impedance estimated at the point of connection specified in
Figure 4 . 17
Figure 6 – Generic harmonic model structure represented as Norton/Thévenin

equivalent circuit . 20
Figure 7 – Main electrical and mechanical components of Type 3 WTs [6] . 25
Figure 8 – Example of a structure of a DFAG harmonic model (from [13]) . 26
Figure 9 – Main electrical and mechanical components of Type 4 WTs [6] . 26
Figure 10 – Example of a converter harmonic model as Thévenin equivalent circuit
together with an example of a WT power circuit (from [9]) . 27
Figure 11 – Harmonic voltage comparison for respective power bins . 28

Table 1 – Example of a representation/template of the harmonic voltage source . 23
Table 2 – Example of a representation/template of the harmonic current source . 23
Table 3 – Example of a representation/template of the harmonic equivalent impedance . 24

– 4 – IEC TR 61400-21-3:2019 © IEC 2019
INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
WIND ENERGY GENERATION SYSTEMS –

Part 21-3: Measurement and assessment of electrical characteristics –
Wind turbine harmonic model and its application

FOREWORD
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The main task of IEC technical committees is to prepare International Standards. However, a
technical committee may propose the publication of a Technical Report when it has collected
data of a different kind from that which is normally published as an International Standard, for
example "state of the art".
IEC TR 61400-21-3, which is a Technical Report, has been prepared by IEC Technical
Committee 88: Wind energy generation systems.
The text of this Technical Report is based on the following documents:
DTR Report on voting
88/698/DTR 88/717/RVDTR
Full information on the voting for the approval of this Technical Report can be found in the
report on voting indicated in the above table.

This document has been drafted in accordance with the ISO/IEC Directives, Part 2.
A list of all parts in the IEC 61400 series, published under the general title Wind energy
generation systems, can be found on the IEC website.
Future standards in this series will carry the new general title as cited above. Titles of existing
standards in this series will be updated at the time of the next edition.
The committee has decided that the contents of this document will remain unchanged until the
stability date indicated on the IEC website under "http://webstore.iec.ch" in the data related to
the specific document. At this date, the document will be
• reconfirmed,
• withdrawn,
• replaced by a revised edition, or
• amended.
A bilingual version of this publication may be issued at a later date.

IMPORTANT – The 'colour inside' logo on the cover page of this publication indicates
that it contains colours which are considered to be useful for the correct
understanding of its contents. Users should therefore print this document using a
colour printer.
– 6 – IEC TR 61400-21-3:2019 © IEC 2019
INTRODUCTION
The purpose of this IEC Technical Report (TR) is to provide a methodology that will ensure
understanding, consistency and accuracy in application, structure and validation of the
harmonic model of grid connected wind turbines (WTs).
There is an understandable requirement from wind power industry shareholders, e.g.
transmission system operators (TSOs) and distribution system operators (DSOs), wind power
plant (WPP) developers, WT manufacturers, WT component suppliers, academic units,
research institutions, certifying bodies and standardization groups (e.g. TC88 MT21), for
having a standardized WT harmonic model.
The standardized harmonic model would find a broad application in many areas of electrical
engineering related to design, analysis, and optimisation of electrical infrastructure of onshore
as well as offshore WPPs. Among others, this could be the evaluation of the WT harmonic
performance, system-level harmonic studies, electrical infrastructure design and proposal of
harmonic mitigation measures.
Standardized WT harmonic models as a performance measure starts to be important in such
multi stakeholder systems as large offshore WPPs where TSOs, WPP developers and
operators as well as WT manufacturers need to have a common understanding about
harmonic modelling of WTs and harmonic studies in WPPs.

WIND ENERGY GENERATION SYSTEMS –

Part 21-3: Measurement and assessment of electrical characteristics –
Wind turbine harmonic model and its application

1 Scope
This part of IEC 61400 provides guidance on principles which can be used as the basis for
determining the application, structure and recommendations for the WT harmonic model. For
the purpose of this Technical Report, a harmonic model means a model that represents
harmonic emissions of different WT types interacting with the connected network.
This document is focused on providing technical guidance concerning the WT harmonic
model. It describes the harmonic model in detail, covering such aspects as application,
structure, as well as validation. By introducing a common understanding of the WT
representation from a harmonic performance perspective, this document aims to bring the
overall concept of the harmonic model closer to the industry (e.g. suppliers, developers,
system operators, academia, etc.).
A standardized approach of WT harmonic model representation is presented in this document.
The harmonic model will find a broad application in many areas of electrical engineering
related to design, analysis, and optimisation of electrical infrastructure of onshore as well as
offshore WPPs.
The structure of the harmonic model presented in this document will find an application in the
following potential areas:
– evaluation of the WT harmonic performance during the design of electrical
infrastructure and grid-connection studies;
– harmonic studies/analysis of modern power systems incorporating a number of WTs
with line side converters;
– active or passive harmonic filter design to optimize electrical infrastructure (e.g.
resonance characteristic shaping) as well as meet requirements in various grid codes;
– sizing of electrical components (e.g. harmonic losses, static reactive power
compensation, noise emission, harmonic compatibility levels, etc.) within WPP
electrical infrastructure;
– evaluation of external network background distortion impact on WT harmonic
assessment;
– standardised communication interfaces in relation to WT harmonic data exchange
between different stakeholders (e.g. system operators, generators, developers, etc.);
– universal interface for harmonic studies for engineering software developers;
– possible benchmark of WT introduced to the academia and the industry.
The advantage of having standardized WT harmonic performance assessment by means of
the harmonic model is getting more and more crucial in case of large systems with different
types of WTs connected to them, e.g. multi-cluster wind power plants incorporating different
types of WTs connected to the same offshore or onshore substation.
th th th
The WT harmonic model can cover the integer harmonic range up to the 40 , 50 , or 100 .
And can be expanded, depending on requirements and application, to higher harmonic range
as well as can also cover interharmonic components.

– 8 – IEC TR 61400-21-3:2019 © IEC 2019
2 Normative references
The following documents are referred to in the text in such a way that some or all of their
content constitutes requirements of this document. For dated references, only the edition
cited applies. For undated references, the latest edition of the referenced document (including
any amendments) applies.
IEC 60050-415:1999, International Electrotechnical Vocabulary – Part 415: Wind turbine
generator systems (available at IEC TR 61000-3-6:2008, Electromagnetic compatibility (EMC) – Part 3-6: Limits – Assessment
of emission limits for the connection of distorting installations to MV, HV and EHV power
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 61400-21-1:2019, Wind energy generation systems – Part 21-1: Measurement and
assessment of electrical characteristics – Part 1 – Wind turbines
3 Terms, definitions and abbreviations
3.1 Terms and definitions
For the purposes of this document, the terms and definitions given in IEC 60050-415 and the
following apply.
ISO and IEC maintain terminological databases for use in standardization at the following
addresses:
• IEC Electropedia: available at http://www.electropedia.org/
• ISO Online browsing platform: available at http://www.iso.org/obp
3.1.1
compatibility levels
reference levels of a particular disturbance in a particular environment defined for co-
ordinating the emission and immunity of equipment which is part of, or supplied by, a supply
system in order to ensure the EMC in the whole system (including system and connected
equipment)
Note 1 to entry: Compatibility levels are generally based on the 95 % probability levels of entire systems, using
statistical distributions which represent both time and space variations of disturbances.
Note 2 to entry: There is allowance for the fact that the system operator or owner cannot control all points of a
system at all times. Therefore, evaluation with respect to compatibility levels should be made on a system-wide
basis and no assessment method is provided for evaluation at a specific location.
3.1.2
factor K
indicator of the ability of a transformer to be loaded with non-sinusoidal currents
Note 1 to entry: The equivalent power rating is equal to the power based on the RMS value of the non-sinusoidal
current multiplied by the factor K.
[SOURCE: EN 50464-3:2007, modified – additional elaboration, creation of a note to entry
and deletion of the formula]
3.1.3
harmonic phase or angle
phase (angle) α of the spectral component y , that is, the phase between the harmonic
h h
current component or harmonic voltage component and the fundamental component voltage
defined in Figure 1 and equation below
y = c sin htω+α
h h  1h
where c is the spectral component magnitude
h
Figure 1 – Example of a phase angle between the harmonic current and the harmonic
voltage component as well as the fundamental voltage
Note 1 to entry: The sign convention used for the voltages and currents is the generator convention as defined in
IEC 61400-21-1:2019, Annex C.
Note 2 to entry: Please check IEC 61400-21-1:2019, Annex D for more details.
3.1.4
harmonic distortion
cyclic departure of a waveform from the sinusoidal shape
Note 1 to entry: This can be described by the addition of one or more harmonics to the fundamental.
3.1.5
harmonic model
model that represents harmonic emissions of a WT interacting with the connected network
Note 1 to entry: Different WT types may be modelled by changing the model parameters.
3.1.6
harmonic model terminals
reference point on the electric power system where here the harmonic model is connected
3.1.7
negative-sequence component of 3-phase voltages (or currents)
symmetrical vector system derived by application of the Fortescue’s transformation matrix,
and that rotates in the opposite direction to the power frequency voltage (or current)
[SOURCE: IEC TR 61000-3-13:2008, 3.26.4, modified – the formula has been deleted]
3.1.8
operational mode
operation according to control setting, for example voltage control mode,
frequency control mode, reactive power control mode, active power control mode, etc.
[SOURCE: IEC 61400-21-1:2019, 3.9]

– 10 – IEC TR 61400-21-3:2019 © IEC 2019
3.1.9
percentile
the value of a variable below which a certain percent of observations fall
3.1.10
planning level
level of a particular disturbance in a particular environment, adopted as a reference value for
the limits to be set for the emissions from the installations in a particular system, in order to
co-ordinate those limits with all the limits adopted for equipment and installations intended to
be connected to the power supply system
Note 1 to entry: Planning levels are considered internal quality objectives to be specified at a local level by those
responsible for planning and operating the power supply system in the relevant area.
[SOURCE: IEC TR 61000-3-6:2008, 3.16]
3.1.11
point of connection
reference point on the electric power system where here the WPP is connected
[SOURCE: IEC 60050-617:2009, 617-04-01, modified – "user’s electrical facility" has been
replaced by WPP]
3.1.12
positive-sequence component of 3-phase voltages (or currents)
symmetrical vector system derived by application of the Fortescue’s transformation matrix,
and that rotates in the same direction as the power frequency voltage (or current)
[SOURCE: IEC TR 61000-3-13:2008, 3.26.3, modified – the formula has been deleted]
3.1.13
power bin
consecutive, non-overlapping intervals of WT active power measured at WT terminals
Note 1 to entry: The bins (intervals) shall be adjacent, and are usually equal size, e.g. 0, 10, 20, … , 100 % of P .
n
0, 10, 20, … , 100 % are the bin midpoints.
[SOURCE: IEC 61400-21-1:2019, 3.62, modified – "active" has been deleted from the term
defined; in the note, "shall be adjacent" has been added and the text has been slightly
modified]
3.1.14
prevailing angle
phase of the spectral component is described by
n
 
n
Im C
( )
∑ h,i
i=1
 
α = arctan ,     if Re C ≥ 0
( )
h,avg ∑ h,i
n
 
Re C
i=1
( )
∑ h,i
 i=1 
n
 
n
Im C
( )
∑
h,i
i=1
 
α = π + arctan ,     if Re C <0
( )
h,avg ∑ h,i
n
 
Re C i=1
( )
∑ h,i
 i=1 
where
n  is the number of DFT windows;

C is the complex value of the h-th harmonic from the estimated spectrum from each of i-th
h,i
10-cycle or 12-cycle window, and
C is the h-th harmonic magnitude.
h
Note 1 to entry: Definition of rectangular window as in IEC 61000-4-7:2002.
3.1.15
prevailing angle ratio
ratio describing the phase randomness of spectral component and expressed by
nn
C a +jb
( )
∑∑h,i h,i h,i
ii==11
PAR==
nn
C a +jb
∑∑h,i h,i h,i
ii==11
where
C is the complex spectral component from DFT;
h,i
a and b are the real and imaginary components of the complex spectral component of
h,i h,i
the i-th window, respectively.
3.1.16
short-circuit power
product of the current in the short-circuit I at a point of a system and a nominal voltage U ,
k n
generally the operating voltage
S = 3IU
k kn
Note 1 to entry: Using physical units for line current (A) and nominal phase-to-phase voltage (V), the product
should also include the factor 3.
√
[SOURCE: IEC 60050-601:1985, 601-01-14, modified – "conventional" has been replaced by
"nominal"]
3.1.17
short-circuit ratio
ratio of the short-circuit power S at the point of connection to the nominal power S of the
k n
WPP or WT
S
k
SCR=
S
n
[SOURCE: IEC 61400-27-1:2015, 3.1.18, modified – "active" has been deleted and the
equation has been added]
3.1.18
system operator or responsible
entity responsible for making technical connection agreements with customers who are
seeking connection of load or generation to a distribution or transmission system
[SOURCE: IEC TR 61000-3-6:2008, 3.23, modified – “owner” has been changed to
responsible]
3.1.19
total harmonic distortion
ratio of the RMS value of the sum of all the harmonic components up to a specified order to
the RMS value of the fundamental component

– 12 – IEC TR 61400-21-3:2019 © IEC 2019
H
 
Q
h
THD=
 
∑
Q
h=2
 1
where
Q represents either current or voltage;
Q is the RMS value of the fundamental component;
h is the harmonic order;
Q is the RMS value of the harmonic component of order h;
h
H is generally 40, 50 or 100 depending on the application.
[SOURCE: IEC TR 61000-3-6:2008, 3.26.7, modified – H is defined differently]
3.1.20
wind power plant
power station comprising one or more WTs, auxiliary equipment and plant control
[SOURCE: IEC 61400-27-1:2015, 3.1.25]
3.1.21
wind turbine
rotating machinery in which the kinetic wind energy is transformed into another form of energy
[SOURCE: IEC 60050-415:1999, 415-01-01]
3.1.22
wind turbine terminals
point being a part of the WT and identified by the WT supplier at which the WT is connected
to the power system
Note 1 to entry: Same definition as in IEC 61400-21 defining the measurement point of the tests.
3.2 Abbreviations
The following abbreviations are used in this document.
AUX auxiliary equipment
CB circuit breaker
DC direct current
DCL DC link
DFAG doubly fed asynchronous generator
DFT discrete Fourier transform
GSC generator (machine) side converter
HD harmonic distortion
HIL hardware in the loop
HMT harmonic model terminals
HV high voltage
___________
Often referred to as a doubly-fed induction generator (DFIG), but it is not operated as an induction generator
when the rotor current is controlled.

HVAC high voltage alternating current
HVDC high voltage direct current
LSC line (grid) side converter
LV low voltage
MV medium voltage
PA prevailing angle
PAR prevailing angle ratio
POC point of connection
PWM pulse width modulation
RMS root mean square
SCR short circuit ratio
SIL software in the loop
STATCOM static synchronous compensator
TR transformer
THD total harmonic distortion
VSC voltage source converter
WPP wind power plant
WT wind turbine
WTT wind turbine terminals
4 General description
4.1 Overview
Harmonics are of special concern in power system studies. In the past, the power system
comprised mainly passive components with a relatively linear operating range as well as
synchronous generators.
Renewable energy sources, e.g. WTs, are becoming more prevalent in many power systems.
Power electronic equipment in modern power systems is obviously a source of additional
harmonic components not seen previously. On the other hand, the application of advanced
and fast control in grid-connected voltage-source power converters (VSCs) introduces the
possibility of controlling higher frequency components than the fundamental. Appropriately
used power electronics can definitely improve power quality.
Therefore, due to the modern power converters complexity, there is a great necessity to
perform a careful power quality evaluation including harmonic measurements, data
processing, data analysis, and harmonic modelling of WTs.
Measurements are an important part in the WPP and WT evaluation process. In order to
validate theoretical analysis and numerical simulations, measurements are required.
Appropriate measurements as well as data processing techniques are crucial in the WT
analysis and evaluation. Figure 2 is presenting how WPP main components relevant for the
harmonic studies are contributing to potential challenges in harmonic performance. WT
harmonic model is one of the most important parts required in overall system modelling and
behaviour estimation.
– 14 – IEC TR 61400-21-3:2019 © IEC 2019

Figure 2 – Example of wind power plant typical components relevant for the harmonic
studies and potential challenges in harmonic performance
4.2 Background
Nowadays, large offshore WPPs with complex structures including WT, array cable systems,
and HVAC or HVDC offshore/onshore transmission systems are being introduced (see
Figure 3 and Figure 4). This represents new challenges to the industry in relation to prediction
and mitigation of harmonic emission and propagation [7] . Due to increasing complexity of
WPPs, it is more and more important to appropriately address harmonic analysis of WTs as
well as WPP on a system level by means of modelling during the design stage as well as
harmonic evaluation during operation.
nd
The measurement procedure and assessment of harmonics in the 2 edition of IEC 61400-21
[5] is based on a harmonic current evaluation dependent on local grid conditions. Moreover,
the direct application of the harmonic current measurements to other grid scenarios has been
reported by the industry to be potentially inaccurate, causing incorrect design and
dimensioning of passive filters. This is mainly due to the fact that the existing standard
provides only current spectrum of a WT and as a consequence can be considered as an ideal
harmonic current source neglecting the internal impedance. This approach also neglects any
grid impedance impact on the generated harmonic currents. More accurate evaluation
methods are described in IEC 61400-21-1:2019, Annex D.
___________
Figures in square brackets refer to the Bibliography.

Figure 3 – Example of a WPP complex structure
Harmonic current emissions from the WT are strongly dependent on the WT internal
impedances as well as the external network frequency-dependent short circuit impedance. To
enable a more accurate assessment procedure, IEC 61400-21-1:2019, Annex D recommends
besides the harmonic currents also harmonic voltage measurement procedures, including
phase information and aggregation techniques [8]. Additionally, a number of
recommendations and guidance is provided to exclude the impact of the external network
during the measurement process. Afterwards, such extensive measurement dataset can be
used either for WT harmonic model validation or even development as shown for example in
[9].
– 16 – IEC TR 61400-21-3:2019 © IEC 2019

Figure 4 – Example of a WPP complex electrical infrastructure with many WTs
Furthermore, it also addresses the evaluation of uncertainties of the measurements and the
data analysis. IEC 61400-21-1 provides guidelines on how to detect which harmonic currents
are affected by the background harmonic distortion which is specified in Annex D.
In terms of harmonic evaluation, IEC 61400-21 [5] specifies a standard approach on how to
take into account the impacts of the grid. The state-of-the-art approach is to report current
based power quality characteristics like current harmonics in the test report. This is based on
the assumption that the current emission is independent on the grid voltage, i.e. the emission
can be described as a current source which is characteristic to the specific unit type.
However, such an assumption is not valid for WTs and complex WPP systems (see
for example Figure 3 and Figure 4) comprising many WTs and characterised by various
resonance phenomena (see Figure 5).
Unfortunately, until now there has been no systematic approach of representing WTs from a
harmonic performance perspective. This brings inconsistency in WT harmonic performance
assessment, background distortion evaluation in grid-connected WTs, harmonic analysis of
WPPs, etc.
Figure 5 – Harmonic impedance estimated at the point
of connection specified in Figure 4
Having a standardized WT harmonic model definition allows consistent and fluent
communication between parties within the wind power industry. The WT harmonic model
could be used as follows:
– provide a more comprehensive characterization of WT harmonic performance;
– supplement the harmonic measurements report from IEC 61400-21-1;
– introduce a standardised way of performing harmonic analysis in WPPs;
– assess the influence of the external network on the harmonic distortion at POC;
– introduce common interfaces to various engineering tools for harmonic analysis;
– define a common basis for dialog with manufacturers, developers, system operators or
owners;
– provide a benchmark for the academia and industry.
Desired properties of the WT harmonic model are as follows:
– allows estimating the influence of the grid where the WT under test is connected;
– correctly represents the WT reaction to background harmonic voltages in the
connection grid;
– provides a universal measure of WT harmonic performance;
– can be applied in harmonic assessment studies comprising various grid conditions,
e.g. contingency scenarios or outage conditions, whereas the harmonic current
measurements from a single scenario cannot;
– represent all possible WT operational modes affecting harmonic performance;
– has a standard and commonly recognized engineering structure as well as can be
widely used for harmonic analysis/studies on a system level.
5 Recommendations of minimal requirements
5.1 General
The WT harmonic model, in order to be broadly used by the industry, needs to have a
standardised and universal structure. This would allow WT manufacturers, WPP developers,
system operators or owners, universities and other potential stakeholders to have a common
understanding and to more easily establish a dialog between each other. Therefore, the
minimal requirements need to be defined. It would be:

– 18 – IEC TR 61400-21-3:2019 © IEC 2019
– application,
– input parameters,
– harmonic model terminal,
– output variables,
– structure.
5.2 Application
The WT harmonic model in the basic form is expected to be applicable to broadly understood
harmonic analysis of WPPs. This will include harmonic emission studies as well as harmonic
propagation/resonance studies. Harmonic analysis of WPPs is focused on the following
aspects:
– grid code requirements,
– harmonic filter design,
– WPP components sizing (e.g. planning levels, compatibility levels, factor K, etc.),
– overall electrical infrastructure optimisation.
Therefore, the WT harmonic model should reflect the WT electrical behaviour including
harmonic emission and impedance characteristic.
Typically, the electrical infrastructure of large WPPs is developed and designed based on the
design guidance available in applicable recommendations and standards [10]. Planning levels
are applied to determine harmonic distortion limits, taking into consideration all distorting
installations. These are levels of a particular disturbance in a particular environment, adopted
as a reference value for the limits to be set for the emissions from the installations in a
particular system, in order to co-ordinate those limits with all the limits adopted for equipment
and installations intended to be connected to the power supply system. Planning levels are
considered as internal quality objectives to be specified at a local level by those responsible
for planning and operating the power supply system in the relevant area.
5.3 Input parameters
The input parameters characterising the WT harmonic behaviour need to be considered in the
model development. Such parameters are dependent on the model application, i.e. simplified
model for basic studies and detailed to evaluate more precisely WT harmonic behaviour.
Depending on the application, it should be decided and defined by the model developer
whether such parameters, as for example active and reactive power setpoints, generator RPM
speed, converter modulation index, fundamental frequency phase, etc., can affect the model
harmonic behaviour. Any limitations and uncertainties should be addressed and described.
In IEC 61400-21, the WT harmonic performance is evaluated depending on active power bins.
Active power is one of the recommended input parameters to be taken during the harmonic
model development. Of course, for the sake of simplicity, the worst case harmonic magnitude
for each harmonic component from all active power bins can be taken. However, this leads to
too conservative results. If the application of a harmonic model reflecting the worst-case
magnitude allows, for example, fulfilling the requirements of the TSO or design a feasible
harmonic filter, then the simplification can be justified. If this is not the case, a detailed
analysis based on power bins is recommended for analysing in which situation the limits are
violated or filter oversizing is obtained. The more extensive harmonic model representation is
usually a good information for the WPP owner/developer or utility.
5.4 Harmonic model terminal
It is important that the WT harmonic model reflects the harmonic behaviour of the whole WT
as one of the components in modern power systems (e.g. WPPs). Therefore, it is important to
define which part of the WT the harmonic model reflects. Based on the model, it should be

possible to estimate the level of harmonic distortion at the WT terminals (WTT), i.e. LV or HV
side of the WT transformer (TR).
For models intended to represent a WT in power systems analysis, the harmonic model
terminals (MHTs) should be defined as the WTT. Therefore, any relevant components which
are part of the WT internal power circuit (e.g. filters or auxiliary circuits) should be considered
in model development and included in the model. It is the model developer’s responsibility to
evaluate which component properties (e.g. frequency dependent inductor losses) need to be
considered in order to achieve the desired level of accuracy.
5.5 Output variables
To perform typical harmonic analysis in frequency/harmonic domain and evaluate the WT
harmonic performance, there is a need to express WT as harmonic source covering all
th th
relevant harmonic components (typically up to the 40 , 50 order), [10] and frequency-
dependent impedance incorporating the WT active (e.g. converter controller, etc.) and passive
(e.g. filter, reactor, etc.) components. The harmonic model is devoted to allow estimating the
harmonic distortion level (i.e. harmonic magnitude and phase) as well as frequency-
dependent impedance at HMT. The harmonic range needs to be adjusted accordingly to
specific studies and grid code requirements.
Assessment of harmonic disturbance in offshore WPPs is becoming an increasingly important
task as they are increasing in size. It has been customary in the past to base all compliance
and design studies on positive sequence simulation models. However, the use of long high
voltage cables (for example in WPPs) gives rise to the need for more sophisticated modelling.
It is justified that on unsymmetrical cable systems, a decoupled sequence model (i.e. no
coupling in the sequence impedance matrix or only simple positive sequence representation)
can lead to underestimation of the harmonic distortion in the system [11].
However, for systems with strong unbalanced impedance profile (e.g. long underground HVAC
cables with flat formation), it starts to be important to also address the sequence of the
harmonic components as well as the 3-phase WT harmonic model representation. The
structure of the model should be specified depending on the level of details in modelling as
well as electrical infrastructure of the investigated system. In classical power systems,
harmonic analysis, the sequence decomposition typically shows that the harmonics in general
follow their natural
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