IEC TR 61850-90-20:2025

IEC TR 61850-90-20 ®
Edition 1.0 2025-08
TECHNICAL
REPORT
Communication networks and systems for power utility automation -
Part 90-20: Use cases of redundancy in systems
ICS 33.200  ISBN 978-2-8327-0614-5

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 - IEC Products & Services Portal - products.iec.ch
webstore.iec.ch/advsearchform Discover our powerful search engine and read freely all the
The advanced search enables to find IEC publications by a
publications previews, graphical symbols and the glossary.
variety of criteria (reference number, text, technical With a subscription you will always have access to up to date
committee, …). It also gives information on projects, content tailored to your needs.
replaced and withdrawn publications.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 22 500 terminological entries in English
details all new publications released. Available online and and French, with equivalent terms in 25 additional languages.
once 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.
CONTENTS
FOREWORD . 3
INTRODUCTION . 5
1 Scope . 7
2 Normative references . 7
3 Terms and definitions . 7
4 Abbreviations . 8
5 Introduction to redundancy concepts. 8
5.1 General . 8
5.2 Standby redundancy . 9
5.2.1 General. 9
5.2.2 Cold standby redundancy . 9
5.2.3 Warm standby redundancy . 10
5.2.4 Hot standby redundancy . 10
5.2.5 1:N redundancy . 11
5.3 Active-active redundancy . 11
5.3.1 Active-active (hot-hot) redundancy . 11
5.3.2 Active-active redundancy with load balancing . 12
5.4 Modular redundancy . 13
5.4.1 General. 13
5.4.2 Dual modular redundancy . 13
5.4.3 Triple modular redundancy . 13
5.5 Partial redundancy . 14
6 Use cases . 14
6.1 General . 14
6.1.1 Overview . 14
6.1.2 Client redundancy . 15
6.1.3 Server redundancy . 15
6.1.4 Actors . 15
6.1.5 State machine . 15
6.2 Use case 1: Startup of redundant clients . 16
6.2.1 Overview . 16
6.2.2 Use case description . 17
6.3 Use case 2: State change of redundant clients . 18
6.3.1 Overview . 18
6.3.2 Use case description . 18
6.4 Use case 3: Redundant reporting . 20
6.4.1 Overview . 20
6.4.2 Use case description . 20
6.5 Use case 4: Startup of redundant servers . 22
6.5.1 Overview . 22
6.5.2 Use case description . 22
6.6 Use case 5: State change of redundant servers . 23
6.6.1 Overview . 23
6.6.2 Use case description . 23
6.7 Service on demand use cases . 24
6.7.1 Overview . 24
6.7.2 Use case 6: Redundant clients perform service on demand on single
server . 25
6.7.3 Use case 7: Single client performs service on demand on redundant
servers . 26
6.7.4 Use case 8: Redundant clients perform service on demand on
redundant servers . 28
Bibliography . 30

Figure 1 – Basic redundancy scheme, Redundant Client . 5
Figure 2 – Basic redundancy scheme, Redundant server . 5
Figure 3 – Cold standby redundancy . 10
Figure 4 – Warm standby redundancy . 10
Figure 5 – Hot standby redundancy . 11
Figure 6 – 1:N redundancy . 11
Figure 7 – Active – Active redundancy . 12
Figure 8 – Active-aActive redundancy with load balancing . 12
Figure 9 – Dual modular redundancy . 13
Figure 10 – Triple modular redundancy . 14
Figure 11 – Simplified state diagram for Initialization and change of state for one IED . 16
Figure 12 – Collaboration diagram for use case 1 – Startup of redundant clients . 16
Figure 13 – Collaboration diagram for use case 2 – State change of redundant clients . 18
Figure 14 – Collaboration diagram for use case 3 – Redundant reporting . 20
Figure 15 – Collaboration diagram for use case 4 – Startup of redundant servers. 22
Figure 16 – Collaboration diagram for use case 5 – State change of redundant servers . 23
Figure 17 – Collaboration diagram for use case 6 – Service on demand from redundant
clients on single server . 25
Figure 18 – Collaboration diagram for use case 7 – Service on demand from single
client on redundant servers . 27
Figure 19 – Collaboration diagram for use case 8 – Service on demand from redundant
clients to redundant servers . 28

Table 1 – Actors . 15

INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
Communication networks and systems for power utility automation -
Part 90-20: Use cases of redundancy in systems

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) IEC draws attention to the possibility that the implementation of this document may involve the use of (a)
patent(s). IEC takes no position concerning the evidence, validity or applicability of any claimed patent rights in
respect thereof. As of the date of publ
...