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
Current Stage
PPUB - Publication issued
Completion Date
13-Sep-2019
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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
IEC TR 61400-21-3:2019-09(en)
---------------------- Page: 1 ----------------------
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---------------------- Page: 2 ----------------------
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

Warning! Make sure that you obtained this publication from an authorized distributor.

® Registered trademark of the International Electrotechnical Commission
---------------------- Page: 3 ----------------------
– 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

---------------------- Page: 4 ----------------------
IEC TR 61400-21-3:2019 © IEC 2019 – 3 –
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

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– 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.
---------------------- Page: 6 ----------------------
IEC TR 61400-21-3:2019 © IEC 2019 – 5 –

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.
---------------------- Page: 7 ----------------------
– 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.
---------------------- Page: 8 ----------------------
IEC TR 61400-21-3:2019 © IEC 2019 – 7 –
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.
---------------------- Page: 9 ----------------------
– 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]
---------------------- Page: 10 ----------------------
IEC TR 61400-21-3:2019 © IEC 2019 – 9 –
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

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]
---------------------- Page: 11 ----------------------
– 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 .

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
 
Im C
( )
∑ h,i
i=1
 
α = arctan ,     if Re C ≥ 0
( )
h,avg ∑ h,i
 
Re C
i=1
( )
∑ h,i
 i=1 
 
Im C
( )
h,i
i=1
 
α = π + arctan ,     if Re C <0
( )
h,avg ∑ h,i
 
Re C i=1
( )
∑ h,i
 i=1 
where
n  is the number of DFT windows;
---------------------- Page: 12 ----------------------
IEC TR 61400-21-3:2019 © IEC 2019 – 11 –

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.
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
C a +jb
( )
∑∑h,i h,i h,i
ii==11
PAR==
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
SCR=

[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
---------------------- Page: 13 ----------------------
– 12 – IEC TR 61400-21-3:2019 © IEC 2019
 
THD=
 
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 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 connecte

...

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