IEC TR 63401-1:2022

IEC TR 63401-1
®

Edition 1.0 2022-11
TECHNICAL
REPORT

colour
inside


Dynamic characteristics of inverter-based resources in bulk power systems –
Part 1: Interconnecting inverter-based resources to low short circuit ratio AC
networks

IEC TR 63401-1:2022-11(en)

---------------------- Page: 1 ----------------------
THIS PUBLICATION IS COPYRIGHT PROTECTED
Copyright © 2022 IEC, Geneva, Switzerland

All rights reserved. Unless otherwise specified, no part of this publication may be reproduced or utilized in any form
or by any means, electronic or mechanical, including photocopying and microfilm, without permission in writing from
either IEC or IEC's member National Committee in the country of the requester. If you have any questions about IEC
copyright or have an enquiry about obtaining additional rights to this publication, please contact the address below or
your local IEC member National Committee for further information.


IEC Secretariat Tel.: +41 22 919 02 11
3, rue de Varembé info@iec.ch
CH-1211 Geneva 20 www.iec.ch
Switzerland

About the IEC
The International Electrotechnical Commission (IEC) is the leading global organization that prepares and publishes
International Standards for all electrical, electronic and related technologies.

About IEC publications
The technical content of IEC publications is kept under constant review by the IEC. Please make sure that you have the
latest edition, a corrigendum or an amendment might have been published.

IEC publications search - webstore.iec.ch/advsearchform IEC Products & Services Portal - products.iec.ch
The advanced search enables to find IEC publications by a Discover our powerful search engine and read freely all the
variety of criteria (reference number, text, technical publications previews. With a subscription you will always have
committee, …). It also gives information on projects, replaced access to up to date content tailored to your needs.
and withdrawn publications.

Electropedia - www.electropedia.org
IEC Just Published - webstore.iec.ch/justpublished
The world's leading online dictionary on electrotechnology,
Stay up to date on all new IEC publications. Just Published
containing more than 22 300 terminological entries in English
details all new publications released. Available online and once
and French, with equivalent terms in 19 additional languages.
a month by email.
Also known as the International Electrotechnical Vocabulary

(IEV) online.
IEC Customer Service Centre - webstore.iec.ch/csc

If you wish to give us your feedback on this publication or need
further assistance, please contact the Customer Service
Centre: sales@iec.ch.

---------------------- Page: 2 ----------------------
IEC TR 63401-1

®


Edition 1.0 2022-11




TECHNICAL



REPORT








colour

inside










Dynamic characteristics of inverter-based resources in bulk power systems –

Part 1: Interconnecting inverter-based resources to low short circuit ratio AC

networks

























INTERNATIONAL

ELECTROTECHNICAL


COMMISSION





ICS 29.020 ISBN 978-2-8322-6143-9




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


® Registered trademark of the International Electrotechnical Commission

---------------------- Page: 3 ----------------------
– 2 – IEC TR 63401-1:2022  IEC 2022
CONTENTS
FOREWORD . 6
INTRODUCTION . 8
1 Scope . 9
2 Normative references . 10
3 Terms and definitions . 10
4 Characteristics of low short circuit ratio AC networks . 12
4.1 Definition of low short circuit ratio . 12
4.1.1 General . 12
4.1.2 Low SCR in IEEE Std 1204-1997 . 13
4.1.3 Low SCR in CIGRE B4.62 TB671 . 13
4.2 Stability issues posed by inverter-based resources . 15
4.2.1 General . 15
4.2.2 Static voltage control . 16
4.2.3 Fault ride-through . 16
4.2.4 Multi-frequency oscillation . 16
4.3 Summary . 17
5 Identification of low short circuit ratio AC networks . 17
5.1 Problem statement . 17
5.2 Short circuit ratio for a single-connected REPP system . 18
5.2.1 SCR calculation with fault current . 18
5.2.2 SCR calculation with equivalent circuit . 19
5.3 Short circuit ratio for multi grid-connected WPP system . 26
5.3.1 General . 26
5.3.2 Modal decoupling method . 27
5.3.3 Circuit aggregation method . 38
5.4 Summary . 45
6 Steady state voltage stability issue for low short circuit ratio AC networks . 47
6.1 Problem statements . 47
6.2 Steady state stability analysis method . 47
6.2.1 P-V curve . 47
6.2.2 Q-V curve . 48
6.2.3 Voltage sensitivity analysis . 49
6.2.4 Relation to short circuit ratio . 54
6.3 Control strategy for inverter-based resource . 56
6.3.1 Active power and reactive power control . 56
6.3.2 Voltage control . 58
6.4 Case study . 59
6.4.1 Steady state voltage stability problem – China . 59
6.4.2 Low SCR interconnection experience – Vestas . 62
6.5 Summary . 63
7 Transient issue for low short circuit ratio AC networks . 64
7.1 Problem statement . 64
7.2 Transient characteristic modelling and analysis . 65
7.2.1 Transient stability analysis tools and limitations . 65
7.2.2 Electromagnetic transient (EMT) type models . 66
7.2.3 Transient stability analysis model requirements . 67

---------------------- Page: 4 ----------------------
IEC TR 63401-1:2022  IEC 2022 – 3 –
7.3 Fault ride-through protection and control issue. 67
7.3.1 General . 67
7.3.2 Hardware protection of inverter-based resource during fault . 68
7.3.3 Unbalancing-voltage ride-through issue . 71
7.3.4 Overvoltage ride-through control strategy . 73
7.3.5 Multiple fault ride-through . 75
7.3.6 Under and over -voltage ride-through in time sequence . 78
7.3.7 Active/reactive current support of inverter-based resource during fault . 79
7.4 Operating experiences . 80
7.4.1 Operating experience – China . 80
7.4.2 Operating experience . 81
7.5 Summary . 83
8 Oscillatory instability issue for low short circuit ratio AC networks. 83
8.1 Problem statement . 83
8.2 Modelling and stability analysis . 86
8.2.1 Analysis and modelling of the inverter in the time-domain . 86
8.2.2 Analysis and modelling of the inverter in the frequency-domain . 86
8.3 Mitigation of the oscillation issues by active damping control . 93
8.4 Cases study based on the benchmark model . 94
8.5 Summary . 99
9 Conclusions . 100
Bibliography . 101

Figure 1 – Measured voltage and current curves of sub-synchronous oscillation . 15
Figure 2 – Schematic diagram of a WPP with no static or dynamic reactive support . 19
Figure 3 – Equivalent circuit representation of the WPP shown in Figure 2 . 20
Figure 4 – A typical SIPES . 24
Figure 5 – Changes of system eigenvalues, and the weakest system eigenvalue’s
damping ratio with SCR in a SIPES. 24
Figure 6 – Schematic diagram of a WPP with static reactive support plant (capacitor
banks) . 25
Figure 7 – Equivalent circuit representation of the WPP shown in Figure 6 . 25
Figure 8 – Schematic diagram of a WPP with dynamic reactive support plant
(synchronous condensers) . 26
Figure 9 – Equivalent circuit representation of the WPP shown in Figure 8 . 26
Figure 10 – Mechanism illustration of decoupling a MIPES into a set of equivalent
SIPESs . 28
Figure 11 – A typical MIPES . 29
Figure 12 – A test wind farm system that contains nine wind turbines . 33
Figure 13 – One-line diagram of 5-infeed PES . 35
Figure 14 – Eigenvalue comparison of 5-infeed PES and its 5 equivalent SIPESs . 36
Figure 15 – The 9-converter heterogeneous system with a IEEE 39-bus network
topology . 37
Figure 16 – The dominant eigenvalues and the damping ratios . 38
Figure 17 – Nearby WPP connected to the same region in a power system. 39
Figure 18 – Equivalent representation of multiple windfarms connecting to a power
system with its Z matrix . 40

---------------------- Page: 5 ----------------------
– 4 – IEC TR 63401-1:2022  IEC 2022
Figure 19 – Equivalent circuit representation of two WPPs connected to the same
connection point-configuration 2 . 41
Figure 20 – Four WPPs integrated into the system with weak connections . 42
Figure 21 – Multiple WPPs connecting to the same HV bus or HV buses in close
proximity . 43
Figure 22 – Equivalent circuit representation of WPPs connecting to the same HV bus . 43
Figure 23 – Approximate equivalent representation assumed for CSCR method . 43
Figure 24 – System topology . 47
Figure 25 – Typical P-V curve . 47
Figure 26 – System topology . 48
Figure 27 – Typical Q-V curve . 48
Figure 28 – Simplified equivalent circuit of large-scale wind power integration system . 49
Figure 29 – Voltage sensitivity at PCC of large-scale wind power integration system . 51
Figure 30 – Single generator connected to an infinite bus via grid impedance . 52
Figure 31 – P-V curves for a typical generator in a weak grid . 53
Figure 32 – Power limit curve of DFIG . 58
Figure 33 – Voltage control block diagram of the doubly-fed wind turbine . 59
Figure 34 – Network structure of Baicheng grid . 59
Figure 35 – Short circuit capacity of Baicheng network . 60
Figure 36 – P-V curves and V-Q curves . 61
Figure 37 – Reactive power of the wind farm and voltage level at the PCC . 62
Figure 38 – Schematic representation of the study system . 63
Figure 39 – Fault characteristics . 64
Figure 40 – Comparison of VER fault response between transient stability and EMT
models . 66
Figure 41 – Doubly-fed wind turbine rotor-side crowbar protection circuit topology . 69
Figure 42 – Schematic diagram of positive and negative sequence current control of
DFIG converter under grid unbalanced fault . 72
Figure 43 – Comparative analysis of simulation results . 73
Figure 44 – Overvoltage ride-through control flow diagram . 74
Figure 45 – Multiple fault conditions . 75
Figure 46 – Pitch angle control strategy . 76
Figure 47 – Typical characteristics of P and P under multiple fault ride-through . 77
m e
Figure 48 – Characteristics of P and P under multiple fault ride-through . 78
m e
Figure 49 – Under/overvoltage ride-through curve . 78
Figure 50 – Circuit diagram in Jiuquan . 80
Figure 51 – Analysis of wind power disconnection incident . 81
Figure 52 – Demonstration of voltage regulation performance during variable power
output conditions . 82
Figure 53 – Configuration of a system of multiple grid-tied VSIs . 85
Figure 54 – Control schematic diagram and structure of inverter . 87
Figure 55 – Frequency coupling in different frequency range . 90
Figure 56 – Negative resistor caused by PLL . 91
Figure 57 – Negative resistor caused by DVC . 92

---------------------- Page: 6 ----------------------
IEC TR 63401-1:2022  IEC 2022 – 5 –
Figure 58 – Equivalent circuits of the LC-filter considering the virtual resistor . 93
Figure 59 – Active damping control methods . 94
Figure 60 – Impact of virtual resistance control on the stability . 95
Figure 61 – Impact of line length on the stability . 97
Figure 62 – Impact of PLL on the stability . 98
Figure 63 – Impact of current control loop on the stability . 99

Table 1 – Rated capacity of PEDs in 5-infeed PES in p.u. . 35
Table 2 – Network parameters of 5-infeed PES in p.u. . 36
Table 3 – Relationship between equivalent SIPESs and eigenvalues of Y in 5-infeed PES 36
eq
Table 4 – Control parameters of converters . 37
Table 5 – Wind capacity and SCR values assuming no interaction . 42
Table 6 – The definition of different MISCRs . 45
Table 7 – Comparison of SCR methods . 46
Table 8 – Wind farm’s maximum power under different conditions . 61
Table 9 – New oscillation issues of power systems in the world . 83
Table 10 – Detailed influence frequency ranges of every loop . 89
Table 11 – Approximate distribution of high frequency negative damping range . 93
Table 12 – Typical cases of weak grid parameters . 96

---------------------- Page: 7 ----------------------
– 6 – IEC TR 63401-1:2022  IEC 2022
INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________

DYNAMIC CHARACTERISTICS OF INVERTER-BASED
RESOURCES IN BULK POWER SYSTEMS –

Part 1: Interconnecting inverter-based resources
to low short circuit ratio AC networks

FOREWORD
1) The International Electrotechnical Commission (IEC) is a worldwide organization for standardization comprising
all national electrotechnical committees (IEC National Committees). The object of IEC is to promote international
co-operation on all questions concerning standardization in the electrical and electronic fields. To this end and
in addition to other activities, IEC publishes International Standards, Technical Specifications, Technical Reports,
Publicly Available Specifications (PAS) and Guides (hereafter referred to as “IEC Publication(s)”). Their
preparation is entrusted to technical committees; any IEC National Committee interested in the subject dealt with
may participate in this preparatory work. International, governmental and non-governmental organizations liaising
with the IEC also participate in this preparation. IEC collaborates closely with the International Organization for
Standardization (ISO) in accordance with conditions determined by agreement between the two organizations.
2) The formal decisions or agreements of IEC on technical matters express, as nearly as possible, an international
consensus of opinion on the relevant subjects since each technical committee has representation from all
interested IEC National Committees.
3) IEC Publications have the form of recommendations for international use and are accepted by IEC National
Committees in that sense. While all reasonable efforts are made to ensure that the technical content of IEC
Publications is accurate, IEC cannot be held responsible for the way in which they are used or for any
misinterpretation by any end user.
4) In order to promote international uniformity, IEC National Committees undertake to apply IEC Publications
transparently to the maximum extent possible in their national and regional publications. Any divergence between
any IEC Publication and the corresponding national or regional publication shall be clearly indicated in the latter.
5) IEC itself does not provide any attestation of conformity. Independent certification bodies provide conformity
assessment services and, in some areas, access to IEC marks of conformity. IEC is not responsible for any
services carried out by independent certification bodies.
6) All users should ensure that they have the latest edition of this publication.
7) No liability shall attach to IEC or its directors, employees, servants or agents including individual experts and
members of its technical committees and IEC National Committees for any personal injury, property damage or
other damage of any nature whatsoever, whether direct or indirect, or for costs (including legal fees) and
expenses arising out of the publication, use of, or reliance upon, this IEC Publication or any other IEC
Publications.
8) Attention is drawn to the Normative references cited in this publication. Use of the referenced publications is
indispensable for the correct application of this publication.
9) Attention is drawn to the possibility that some of the elements of this IEC Publication may be the subject of patent
rights. IEC shall not be held responsible for identifying any or all such patent rights.
IEC TR 63401-1 has been prepared by subcommittee SC 8A: Grid integration of renewable
energy generation, of IEC technical committee TC 8: Systems aspects of electrical energy
supply. It is a Technical Report.
The text of this Technical Report is based on the following documents:
Draft TR Report on voting
8A/109/DTR 8A/113/RVDTR

Full information on the voting for its approval can be found in the report on voting indicated in
the above table.
The language used for the development of this Technical Report is English.

---------------------- Page: 8 ----------------------
IEC TR 63401-1:2022  IEC 2022 – 7 –
This document was drafted in accordance with ISO/IEC Directives, Part 2, and developed in
accordance with ISO/IEC Directives, Part 1 and ISO/IEC Directives, IEC Supplement, available
at www.iec.ch/members_experts/refdocs. The main document types developed by IEC are
described in greater detail at www.iec.ch/standardsdev/publications.
A list of all parts in the IEC 63401 series, published under the general title Dynamic
characteristics of inverter-based resources in bulk power systems, can be found on the IEC
website.
The committee has decided that the contents of this document will remain unchanged until the
stability date indicated on the IEC website under 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.

IMPORTANT – The "colour inside" logo on the cover page of this document 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: 9 ----------------------
– 8 – IEC TR 63401-1:2022  IEC 2022
INTRODUCTION
As the penetration of inverter-based energy generating resources increases, huge challenges
to all sections of the power system including planning, operation, control, etc. have been
created. The impact on the power grid extends from local to the whole power system. New
technical solutions are needed to address the different challenges. The solutions will include
the new technologies, methods and practices, to provide more flexibility and improve the
efficiency of power systems, constantly balancing generation and load.
The purpose of this document (TR) is to specifically focus on information collection from
regulatory agencies, including specifying low short circuit ratio AC networks and the challenges
they pose for inverter-based resources, and methods, indexes, and characteristics of low short
circuit ratio AC networks. This TR addresses renewable energy (RE) integration in low short
circuit ratio AC networks, mainly focusing on the technology development trends, best practices
of RE grid integration, and future standardization activities.
The aim of this TR is to create a strategic, technically oriented and referenced document, which
presents the core and key issues of interconnecting inverter-based resources to low short circuit
ratio AC networks. Renewable energy station developers and owners, transmission systems
operators need to have a common understanding of the key issues based on practices and
challenges between inverter-based resources and AC networks.

---------------------- Page: 10 ----------------------
IEC TR 63401-1:2022  IEC 2022 – 9 –
DYNAMIC CHARACTERISTICS OF INVERTER-BASED
RESOURCES IN BULK POWER SYSTEMS –

Part 1: Interconnecting inverter-based resources
to low short circuit ratio AC networks



1 Scope
As the use of inverter-based RE power generation resources increases, the use of low short
circuit ratio AC networks is becoming more common. Considering the advantages of short circuit
ratio in stability analysis, the low short circuit ratio is an important indication for describing weak
AC networks. This document focuses on technologies and standardization aspects of
interconnecting inverter-based resources to low short circuit ratio AC networks. A clear
definition of low short circuit ratio AC networks with or without a high proportion of inverter-
based resources and the calculation method is described. The adaptability of traditional
modelling and analytical method for low short circuit ratio AC networks are discussed. Some
new characteristics and challenges will be re-examined, and some adapted control strategies
will be studied. This document covers the following major aspects.
In terms of defining a weak AC network, for example the (X/R) ratio, voltage sensitivity, system
inertia and the short circuit ratio (SCR) are important characteristics. The definition of low short
TM 1
circuit ratio AC networks in IEEE Std 1204 -1997 [1] and in CIGRE B4.62 TB671 [2] is used.
Some stability challenges for inverter-based resources in a low short circuit ratio AC network
(SCR AC) will be analyzed. There are stability challenges in a low short circuit ratio (SCR) AC
network, typically complex static voltage control, risk of failure in
...