IEC 60071-11:2022

IEC 60071-11 ®
Edition 1.0 2022-11
INTERNATIONAL
STANDARD
NORME
INTERNATIONALE
Insulation co-ordination –
Part 11: Definitions, principles and rules for HVDC system

Coordination de l'isolement –
Partie 11: Définitions, principes et règles relatifs au réseau CCHT

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.

Droits de reproduction réservés. Sauf indication contraire, aucune partie de cette publication ne peut être reproduite
ni utilisée sous quelque forme que ce soit et par aucun procédé, électronique ou mécanique, y compris la photocopie
et les microfilms, sans l'accord écrit de l'IEC ou du Comité national de l'IEC du pays du demandeur. Si vous avez des
questions sur le copyright de l'IEC ou si vous désirez obtenir des droits supplémentaires sur cette publication, utilisez
les coordonnées ci-après ou contactez le Comité national de l'IEC de votre pays de résidence.

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
committee, …). It also gives information on projects, replaced have 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and withdrawn publications.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
and French, with equivalent terms in 19 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.
A propos de l'IEC
La Commission Electrotechnique Internationale (IEC) est la première organisation mondiale qui élabore et publie des
Normes internationales pour tout ce qui a trait à l'électricité, à l'électronique et aux technologies apparentées.

A propos des publications IEC
Le contenu technique des publications IEC est constamment revu. Veuillez vous assurer que vous possédez l’édition la
plus récente, un corrigendum ou amendement peut avoir été publié.

Recherche de publications IEC - IEC Products & Services Portal - products.iec.ch
webstore.iec.ch/advsearchform Découvrez notre puissant moteur de recherche et consultez
La recherche avancée permet de trouver des publications IEC gratuitement tous les aperçus des publications. Avec un
en utilisant différents critères (numéro de référence, texte, abonnement, vous aurez toujours accès à un contenu à jour
comité d’études, …). Elle donne aussi des informations sur adapté à vos besoins.
les projets et les publications remplacées ou retirées.

Electropedia - www.electropedia.org
IEC Just Published - webstore.iec.ch/justpublished
Le premier dictionnaire d'électrotechnologie en ligne au
Restez informé sur les nouvelles publications IEC. Just
monde, avec plus de 22 300 articles terminologiques en
Published détaille les nouvelles publications parues.
anglais et en français, ainsi que les termes équivalents dans
Disponible en ligne et une fois par mois par email.
19 langues additionnelles. Egalement appelé Vocabulaire

Electrotechnique International (IEV) en ligne.
Service Clients - webstore.iec.ch/csc

Si vous désirez nous donner des commentaires sur cette
publication ou si vous avez des questions contactez-nous:
sales@iec.ch.
IEC 60071-11 ®
Edition 1.0 2022-11
INTERNATIONAL
STANDARD
NORME
INTERNATIONALE
Insulation co-ordination –
Part 11: Definitions, principles and rules for HVDC system

Coordination de l'isolement –
Partie 11: Définitions, principes et règles relatifs au réseau CCHT

INTERNATIONAL
ELECTROTECHNICAL
COMMISSION
COMMISSION
ELECTROTECHNIQUE
INTERNATIONALE
ICS 29.080.30 ISBN 978-2-8322-6026-5

– 2 – IEC 60071-11:2022 © IEC 2022
CONTENTS
FOREWORD . 4
INTRODUCTION . 6
1 Scope . 7
2 Normative references . 7
3 Terms and definitions . 8
4 Symbols and abbreviations . 13
4.1 General . 13
4.2 Subscripts . 14
4.3 Letter symbols . 14
4.4 Abbreviations . 15
5 Principles of insulation co-ordination . 15
5.1 General . 15
5.2 Essential differences between AC and DC systems . 15
5.3 Insulation co-ordination procedure . 16
5.4 Differences of withstand voltage selection in AC and DC systems . 16
6 Design procedure of insulation co-ordination . 18
6.1 General . 18
6.2 Arrester characteristics . 18
6.3 Insulation characteristics . 18
6.4 Determination of the representative overvoltages (U ) . 18
rp
6.5 Determination of the co-ordination withstand voltages (U ) . 19
cw
6.6 Determination of the required withstand voltages (U ) . 19
rw
6.7 Determination of the specified withstand voltage (U ) . 21
w
7 Requirements for withstand voltage tests. 21
8 Creepage distances . 21
8.1 General . 21
8.2 Base voltage for creepage distance . 22
8.3 Creepage distance for outdoor insulation under DC voltage . 22
8.4 Creepage distance for indoor insulation under DC or mixed voltage . 22
8.5 Creepage distance of AC insulators . 22
9 Clearances in air . 23
Annex A (informative) Typical HVDC converter station schemes . 24
Annex B (informative) Example of air clearances calculation . 28
B.1 Introductory remarks . 28
B.2 Calculated minimum air clearance for switching impulse stress . 28
B.2.1 General . 28
B.2.2 Example calculation . 29
B.3 Calculated minimum air clearance for lightning impulse stress . 29
B.3.1 General . 29
B.3.2 Example calculation . 30
Annex C (normative) Example of typical DC voltages with possible insulation levels
and corresponding air clearances . 31
C.1 Introductory remarks . 31
C.2 List of typical DC voltagesand possible insulation levels . 31

C.3 Example of presumed switching impulse insulation levels and minimum air
clearances . 31
C.4 Example of presumed lightning impulse insulation levels and minimum air
clearances . 33
C.5 Possible/Presumed specified DC withstand voltages . 33
C.5.1 General . 33
C.5.2 Specified DC withstand voltages . 34
C.5.3 List of specified power frequency withstand voltages . 34
Annex D (informative) Typical arrester characteristics . 35
Annex E (informative) The Correlation of clauses between IEC 60071-11 and
IEC 60071-5:2014 . 36
Bibliography . 37

Figure 1 – Comparison of the selection between withstand voltages for AC equipment
and for HVDC converter station equipment . 17
Figure A.1 – Possible arrester locations in one pole of bipole LCC converter station
with 12-pulse converters in series . 25
Figure A.2 – Possible arrester locations in one pole of bipolar of VSC converter
stations . 26
Figure A.3 – Possible arrester locations in symmetrical monopole VSC converter
stations . 26
Figure D.1 – Typical arrester V-I characteristics . 35

Table 1 – Classes and shapes of overvoltages, standard voltage shapes and standard
withstand voltage tests . 9
Table 2 – Comparison of the insulation co-ordination procedure of AC and DC systems . 16
Table 3 – Indicative values of ratios of required impulse withstand voltage to impulse
protective level . 20
Table A.1 – Symbol description . 27
Table C.1 – Typical DC voltages and switching/lightning impulse withstand voltage . 32
Table C.2 – Correlation between presumed rated switching impulse withstand voltages
and minimum phase-to-earth air clearances . 33
Table C.3 – Correlation between presumed rated lightning impulse withstand voltages

and minimum phase-to-earth air clearances . 34

– 4 – IEC 60071-11:2022 © IEC 2022
INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
INSULATION CO-ORDINATION –
Part 11: Definitions, principles and rules for HVDC system

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 60071-11 has been prepared by IEC technical committee 99: Insulation co-ordination and
system engineering of high voltage electrical power installations above 1,0 kV AC and
1,5 kV DC. It is an International Standard.
This international standard replaces, in conjunction with IEC 60071-12, IEC 60071-5 published
in 2014.
This edition includes the following significant technical changes with respect to
IEC 60071-5:2014:
a) This standard applies to both LCC and VSC HVDC systems whereas IEC 60071-5 only dealt
with LCC HVDC system;
b) Annex C (normative) gives the recommended specified withstand voltage (LI and SI);
c) Annex C (normative) gives the minimum air clearances;
d) Annex E shows the correlation of clauses between this standard and IEC 60071-5:2014.

The text of this International Standard is based on the following documents:
Draft Report on voting
99/374/FDIS 99/394/RVD
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 International Standard is English.
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 60071 series, published under the general title Insulation
co-ordination, 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.
– 6 – IEC 60071-11:2022 © IEC 2022
INTRODUCTION
As the demand for electrical energy is growing, more and more HVDC projects have appeared,
and the voltage up to ±1 100 kV so far. However, the nominal voltage, nominal current and
insulation levels for HVDC system are not yet as standardized as the AC system.
In October 2016, IEC Technical Committee 28 (Insulation co-ordination) established AHG 8
(Ad hoc group 8) to make the roadmap for HVDC system insulation co-ordination standards.
After IEC TC 28 was merged into IEC TC 99 in 2017, JWG 13 (Joint working group 13) was
built by IEC TC 99 and TC 115 and was responsible for making the series standards for HVDC
system according to the approved roadmap, as follows:
a) Part 11: Definitions, principles and rules for HVDC system;
b) Part 12: Application guidelines for LCC HVDC converter stations;
c) Part 13: Application guidelines for VSC HVDC converter stations;
d) Part 14: Insulation co-ordination for AC/DC filters;
e) Part 15: Insulation co-ordination for DC transmission lines.

INSULATION CO-ORDINATION –
Part 11: Definitions, principles and rules for HVDC system

1 Scope
This part of IEC 60071 applies to high-voltage direct current (HVDC) systems. It specifies the
principles on the procedures for the determination of the specified withstand voltages, creepage
distance and air clearances for the equipment and the installations of these systems.
This document gives the insulation co-ordination principles related to line commutated
converter (LCC) and voltage sourced converters (VSC) HVDC systems. The main principles of
this document also apply to other special converter configurations of LCC, such as the capacitor
commutated converter (CCC) as well as the controlled series compensated converter (CSCC),
etc.
This document applies to insulation co-ordination of equipment connected between the
converter AC bus (including the AC harmonic filters, the converter transformer, the circuit
breakers) and the DC line side. The line and cable terminations in so far as they influence the
insulation co-ordination of converter station equipment are also covered.
This document applies only for HVDC applications in power systems and not for industrial
conversion equipment. Principles and guidance given are for insulation co-ordination purposes
only. The requirements for human safety are not covered by this document.
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 60060-1, High-voltage test techniques – Part 1: General definitions and test requirements
IEC 60071-1:2019, Insulation co-ordination – Part 1: Definitions, principles and rules
IEC 60071-2:2018, Insulation co-ordination – Part 2: Application guidelines
IEC 60099-4:2014, Surge arresters – Part 4: Metal-oxide surge arresters without gaps for a.c.
systems
IEC TS 60815-1:2008, Selection and dimensioning of high-voltage insulators intended for use
in polluted conditions – Part 1: Definitions, information and general principles
IEC TS 60815-2:2008, Selection and dimensioning of high-voltage insulators intended for use
in polluted conditions – Part 2: Ceramic and glass insulators for a.c. systems
IEC TS 60815-3:2008, Selection and dimensioning of high-voltage insulators intended for use
in polluted conditions – Part 3: Polymer insulators for a.c. systems

– 8 – IEC 60071-11:2022 © IEC 2022
3 Terms and definitions
For the purposes of this document, the following terms and definitions 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
NOTE Many of the following definitions refer to insulation co-ordination concepts (IEC 60071-1), or to arrester
parameters (IEC 60099-4).
3.1
insulation co-ordination
selection of the dielectric strength of equipment in relation to the operating voltages and
overvoltages which can appear on the system for which the equipment is intended and taking
into account the service environment and the characteristics of the available preventing and
protective devices
[SOURCE: IEC 60071-1:2019, 3.1, modified – The note to entry has been removed.]
3.2
nominal DC voltage
mean value of the DC voltage required to transmit nominal power at nominal current
3.3
highest DC voltage
highest value of DC voltage for which the equipment and system is designed to operate
continuously, in respect of its insulation as well as other characteristics
3.4
symmetrical monopole
HVDC converter with symmetrical DC voltage outputs on the two pole terminals
Note 1 to entry: A symmetrical monopole is generally applicable only to the VSC HVDC systems.
Note 2 to entry: “Symmetrical monopole” is used even though there are two polarities with DC voltages, because
only one converter is unable to provide the redundancy which is generally provided by “bipole”.
Note 3 to entry: In the symmetrical monopole operation, persistent overvoltage appears at the sound (healthy) pole
when a fault occurs at the opposite pole.
3.5
asymmetrical monopole
for the HVDC converter with asymmetrical DC voltage outputs on the two terminals, one terminal
is generally earthed
3.6
bipole
in general, two asymmetrical monopoles form a bipolar DC circuit
3.7
overvoltage
voltage having a value exceeding the corresponding highest steady state voltage of the system
Note 1 to entry: Table 1 presents (as per IEC 60071-1) the classification of these voltages which are defined in
3.7.1 to 3.7.2.3.
Table 1 – Classes and shapes of overvoltages, standard voltage shapes
and standard withstand voltage tests
Low frequency Transient
Class
Continuous Temporary Slow-front Fast-front Very-fast-front
1/f
Tf
1/f
1/f
Voltage or 2
over-
T
p
voltage
T
shapes Tt Tt 1/f
1/f1
T2
Tt T
T ≤ 100 ns
f
T ≥ 3 600s
t
10 Hz < f <
Range of
20 μs < T ≤ 0,1 μs < T ≤
f = 50 Hz or
p 1
500 Hz 0,3 MHz < f <
voltage or
1/T = f =
60 Hz
t 1 5 000 μs 20 μs
over-
100 MHz
0 Hz
0,02 s ≤ T ≤
voltage
T ≥ 3 600s t
T ≤ 20 ms T ≤ 300 μs
t
2 2
30 kHz < f <
shapes
3 600 s
f < 2 500 Hz
300 kHz
1/f
1/f
Standard
u +Δu
n
un
u -Δu a
voltage n
shapes
sec)
T Tt T
t T 1
p
Tt
T
T
∆U
f = 50 Hz 48 Hz ≤ f ≤
≤3% T = 250 μs T = 1,2 μs
b
p 1
U or 60 Hz 62 Hz
n
T = 2 500 μs T = 50 μs
a
2 2
a T T = 60 s
T t t
t
Short-
Standard
duration
withstand DC voltage Switching Lightning impulse
a a
power
a
voltage impulse test test
test
frequency
test
test
a
To be specified by the relevant apparatus committees.
b
Unless otherwise specified by the relevant Technical Committees, standard voltage shapes should be in
accordance with IEC 60060-1.
3.7.1
temporary overvoltage
overvoltages of relatively long duration (ranging from 0,02 to 3 600 s as per IEC 60071-1)
Note 1 to entry: The overvoltage can be undamped or weakly damped.
3.7.2
transient overvoltage
short-duration overvoltage of a few millisecond or less, oscillatory or non-oscillatory, usually
highly damped
[SOURCE: IEC 60071-1: 2019, 3.17.2, modified – The note to entry has been removed.]
3.7.2.1
slow-front overvoltage
transient overvoltage, usually unidirectional, with time to peak 20 μs < T ≤ 5 000 μs, and tail
p
duration T ≤ 20 ms
– 10 – IEC 60071-11:2022 © IEC 2022
Note 1 to entry: For the purpose of insulation co-ordination, slow-front overvoltages are classified according to their
shape, regardless of their origin. Although considerable deviations from the standard shapes occur on actual
systems, in this standard it is considered sufficient in most cases to describe such overvoltages by their classification
and peak value.
3.7.2.2
fast-front overvoltage
overvoltage at a given location on a system, due to a lightning discharge or other cause, the
shape of which can be regarded, for insulation co-ordination purposes, as similar to that of the
standard impulse (IEC 60060-1) used for lightning impulse tests
Note 1 to entry: Fast-front overvoltage is defined as transient overvoltage, usually unidirectional, with time to peak
0,1 μs < T ≤ 20 μs, and tail duration T ≤ 300 μs in IEC 60071-1:2019, 3.17.2.2.
1 2
Note 2 to entry: For the purpose of insulation co-ordination, fast-front overvoltages are classified according to their
shape, regardless of their origin. Although considerable deviations from the standard shapes occur on actual
systems, in this standard it is considered sufficient in most cases to describe such overvoltages by their classification
and peak value.
3.7.2.3
very-fast-front overvoltage
transient overvoltage, usually unidirectional, with time to peak T < 0,1 μs, and with or without
f
superimposed oscillations at frequency 30 kHz < f < 100 MHz
[SOURCE: IEC 60071-1:2019, 3.17.2.3, modified – The abbreviated term VFFO has been
removed.]
3.7.2.4
steep-front overvoltage
transient overvoltage classified as a kind of fast-front overvoltage with time to peak 3 ns
< T < 1,2 μs
Note 1 to entry: A steep-front impulse voltage for test purposes is defined in IEC 60700-1.
Note 2 to entry: The front time is decided by means of system studies.
3.7.2.5
combined overvoltage
overvoltage consisting of two voltage components simultaneously applied between each of the
two-phase terminals of a phase-to-phase (or longitudinal) insulation and earth
Note 1 to entry: Combined overvoltage can include temporary, slow-front, fast-front or very-fast front overvoltages.
Note 2 to entry: It is classified by the component of higher peak value.
3.8
representative overvoltage
U
rp
overvoltage assumed to produce the same dielectric effect on the insulation as overvoltage of
a given class occurring in service due to various origins
Note 1 to entry: In this document, it is generally assumed that the representative overvoltages are characterized by
their assumed or obtained maximum values.
[SOURCE: IEC 60071-1:2019, 3.19, modified – The notes to entry have been removed and
replaced by a new Note 1.]
3.8.1
representative slow-front overvoltage
RSFO
voltage value between terminals of an equipment having the shape of a standard switching
impulse
3.8.2
representative fast-front overvoltage
RFFO
voltage value between terminals of an equipment having the shape of a standard lightning
impulse
3.8.3
representative steep-front overvoltage
RSTO
voltage value with a standard shape having a time to crest less than that of a standard lightning
impulse, but not less than that of a very-fast-front overvoltage as defined by IEC 60071-1
Note 1 to entry: A steep-front impulse voltage for test purposes is defined in Figure 1 of IEC 60700-1:2015. The
front time is decided by means of system studies.
3.9
co-ordination withstand voltage
U
cw
for each class of voltage, value of the withstand voltage of the insulation configuration, in actual
service conditions, that meets the performance criterion
[SOURCE: IEC 60071-1:2019, 3.25]
3.10
required withstand voltage
U
rw
test voltage that the insulation must withstand in a standard withstand voltage test to ensure
that the insulation will meet the performance criterion when subjected to a given class of
overvoltages in actual service conditions and for the whole service duration
Note 1 to entry: The required withstand voltage has the shape of the co-ordination withstand voltage, and is
specified with reference to all the conditions of the standard withstand voltage test selected to verify it
[SOURCE: IEC 60071-1:2019, 3.28]
3.11
specified withstand voltage
U
w
)
test voltage suitably selected equal to or above the required withstand voltage (U
rw
Note 1 to entry: For AC equipment, values of withstand voltages U are standardized as per IEC 60071-1. For
w
HVDC equipment, the specified withstand voltages are rounded up to convenient practical values.
Note 2 to entry: The standard impulse shapes used for withstand tests on equipment as well as the test procedures
are defined in IEC 60060-1 and IEC 60071-1. For some DC equipment (e.g. the thyristor valves), the standard
impulse shapes may be modified in order to more realistically reflect expected conditions.
3.11.1
switching impulse withstand voltage
SIWV
withstand voltage of insulation with the shape of the standard switching impulse
3.11.2
lightning impulse withstand voltage
LIWV
withstand voltage of insulation with the shape of the standard lightning impulse

– 12 – IEC 60071-11:2022 © IEC 2022
3.11.3
steep-front impulse withstand voltage
STIWV
withstand voltage of insulation with the shape parameters in 3.7.2.4
3.12
continuous operating voltage of an arrester
U
c
permissible r.m.s. value of power frequency voltage that may be applied continuously between
the terminals of the arrester
[SOURCE: IEC 60099-4:2014, 3.10, modified – The words “designated” at the beginning of the
definition and “in accordance with 8.7” at the end have been removed.]
3.13
equivalent continuous operating voltage of an arrester
ECOV
r.m.s. value of the sinusoidal power frequency voltage at a metal-oxide surge arrester stressed
by operating voltage of any wave-shape that generates the same power losses in the metal
oxide materials as the actual operating voltage
3.14
residual voltage of an arrester
U
res
peak value of voltage that appears between the terminals of an arrester during the passage of
a discharge current
[SOURCE: IEC 60099-4:2014, 3.58, modified – The note to entry has been removed.]
3.15
co-ordination currents of an arrester
for a given system under study and for each class of overvoltage, the current through the
arrester for which the representative overvoltage is determined
Note 1 to entry: Standard shapes of co-ordination currents for steep-front, lightning and switching current impulses
are given in IEC 60099-4.
Note 2 to entry: The co-ordination currents are determined by system studies.
3.16
protective levels of an arrester
for each voltage class, residual voltage that appears between the terminals of an arrester during
the passage of a discharge current corresponding to the co-ordination current
Note 1 to entry: For HVDC converter equipment, the following specific definitions 3.16.1 to 3.16.3 apply.
3.16.1
switching impulse protective level
SIPL
residual voltage of a surge arrester subjected to a discharge current corresponding to the
co-ordination switching impulse current
3.16.2
lightning impulse protective level
LIPL
residual voltage of a surge arrester subjected to a discharge current corresponding to the
co-ordination lightning impulse current

3.16.3
steep-front impulse protective level
STIPL
residual voltage of a surge arrester subjected to a discharge current corresponding to the
co-ordination steep-front impulse current
3.17
directly protected equipment
equipment connected in parallel to a surge arrester for which the separation distance can be
neglected and any representative overvoltage be considered equal to the corresponding
protective level
3.18
creepage distance
shortest distance, or the sum of the shortest distances, along the insulating parts of the insulator
between those parts which normally have the operating voltage between them
Note 1 to entry: The surface of cement or of any other non-insulating jointing material is not considered as forming
part of the creepage distance.
Note 2 to entry: If a high resistance coating, e.g. semi-conductive glaze, is applied to parts of the insulating part of
an insulator, such parts are considered to be effective insulating surfaces and the distance over them is included in
the creepage distance.
[SOURCE: IEC TS 60815-1: 2008, 3.1.5]
3.19
unified specific creepage distance
USCD
creepage distance of an insulator divided by the maximum operating voltage across the
insulator. It is generally expressed in mm/kV
[SOURCE: IEC TS 60815-4:2016, 3.1.1, modified – The note to entry has been removed.]
3.20
separation distance
distance between the high voltage terminal of the protected equipment and the connection point
of the arrester high voltage conductor
3.21
performance criterion
basis on which the insulation is selected so as to reduce to an economically and operationally
acceptable level the probability that the resulting voltage stresses imposed on the equipment
will cause damage to equipment insulation or affect continuity of service
Note 1 to entry: The performance criterion is usually expressed in terms of an acceptable failure rate (number of
failures per year, years between failures, risk of failure, etc.) of the insulation configuration.
[SOURCE: IEC 60071-1:2019, 3.23]
4 Symbols and abbreviations
4.1 General
The list provided in 4.2 below covers only the most frequently used symbols and abbreviations,
some of which are illustrated graphically in the single-line diagram of Figure A.1 and Table A.1.
For a more complete list of symbols which has been adopted for HVDC converter stations, and
also for insulation co-ordination, refer to the standards listed in the normative references
(Clause 2) and to the Bibliography.

– 14 – IEC 60071-11:2022 © IEC 2022
4.2 Subscripts
0(zero) at no load (IEC 60633)
d direct current or voltage (IEC 60633)
i ideal (IEC 60633)
max maximum (IEC 60633)
n pertaining to harmonic component of order n (IEC 60633)

4.3 Letter symbols
K
altitude correction factor (IEC 60071-1)
a
K
co-ordination factor (IEC 60071-1)
c
K
safety factor (IEC 60071-1)
s
U
continuous operating voltage of an arrester
c
U
continuous operating voltage of an arrester including harmonics
ch
U
nominal voltage of DC system
n
U
highest voltage of an AC system (IEC 60071-1 and 60071-2)
s
U
highest voltage for the equipment
m
U
50 % disruptive discharge voltage
U
representative overvoltage
rp
U
co-ordination withstand voltage
cw
U
required withstand voltage
rw
U
specified withstand voltage (standard withstand voltage in AC)
w
σ the standard deviation
N the number of the conventional deviations

4.4 Abbreviations
HVDC high voltage direct current
DC (d.c.) direct current
AC (a.c.) alternating current
LCC line commutated converter
VSC voltage sourced converter
CCC capacitor commutated converter
CSCC controlled series compensated converter
CCOV crest value of continuous operating voltage
PCOV peak continuous operating voltage
ECOV equivalent continuous operating voltage
RSFO representative slow-front overvoltage (the maximum voltage stress value)
RFFO representative fast-front overvoltage (the maximum voltage stress value)
RSTO representative steep-front overvoltage (the maximum voltage stress value)
RSIWV required switching impulse withstand voltage
RLIWV required lightning impulse withstand voltage
RSTIWV required steep-front impulse withstand voltage
SIPL switching impulse protective level
LIPL lightning impulse protective level
STIPL steep-front impulse protective level
SIWV switching impulse withstand voltage
LIWV lightning impulse withstand voltage
STIWV steep-front impulse withstand voltage
USCD unified specific creepage distance
RUSCD reference unified specific creepage distance

5 Principles of insulation co-ordination
5.1 General
The primary objectives of insulation co-ordination are:
– to determine the maximum steady state, temporary and transient overvoltage levels to which
the various components of a system may be subjected in practice;
– to select the insulation strength and characteristics of equipment, including the protective
devices, used in order to ensure a safe, economic and reliable installation in the event of
overvoltages.
5.2 Essential differences between AC and DC systems
The insulation co-ordination applied to an HVDC converter station is basically the same in
principle as that of an AC substation. However, essential differences exist which warrant
particular consideration when dealing with HVDC converter stations. For example, there is a
need to consider the following:
a) the requirements of series-connected valve groups involving surge arresters connected
across individual valves and between terminals away from earth potential which involves
the use of different insulation levels for different parts of the HVDC converter station;

– 16 – IEC 60071-11:2022 © IEC 2022
b) the topology of the converter circuits with no direct exposure to the external overvoltage
since these circuits are bounded by inductances of converter transformers and smoothing
reactors;
c) the presence of reactive power sources and harmonic filters on both the AC and DC sides
giving rise to potential overvoltages and higher probability of resonance conditions;
d) applications involving long overhead transmission lines and/or cables without intervening
switching stations, with potential for resonance conditions on the DC side;
e) the presence of converter transformers with the valve side not directly connected to earth
potential, and a DC voltage offset;
f) the characteristics of the converter valves resulting in composite voltage wave shapes
(which include in some cases a combination of direct voltage, fundamental frequency
voltage, harmonic voltages and high frequency components), commutation failures, etc.;
g) control malfunction resulting in possible valve misfires, trigger failure, current extinction;
h) fast control and protection action reducing overvoltages;
i) voltage polarity effects of DC stress which, by attracting greater contaminants to the DC
insulation because of constant polarity, lead to greater creepage and clearance
requirements and to worse pollution and flashover performance compared with AC insulation
under the same environment;
j) interaction between the AC and DC systems, particularly where the AC system is relatively
weak;
k) the various operating modes of the converter such as monopolar, bipolar, parallel or
multiterminal;
l) no standard insulation levels exist in the case of DC systems so far.
5.3 Insulation co-ordination procedure
The general method of investigation for an HVDC converter station contains the following:
a) selection of the DC circuit configuration, for example location of the DC smoothing reactors,
location of the DC side earthing, converter transformer valve winding connection (star or
delta) to the higher DC voltage terminal;
b) selection of arrester arrangement according to the selected DC circuit configuration;
c) evaluation of the characteristics of the AC system at the commutation bus and the DC
system and their interaction to determine different representative overvoltages and
current/energy stresses imposed on surge arresters;
d) optimization of the design by iterative assessment of equipment insulation and arrester
requirements.
5.4 Differences of withstand voltage selection in AC and DC systems
As described in IEC 60071-1, there are four main steps in the insulation co-ordination procedure
which can be identified in Table 2.
Table 2 – Comparison of the insulation co-ordination procedure of AC and DC systems
Procedure AC systems DC systems
step 1: determination of the representative overvoltages (U )
rp
step 2: determination of the co-ordination withstand voltages (U )
cw
step 3: determination of the required withstand voltages (U )
rw
step 4: determination of the standard rated withstand d
...