IEC 60071-12:2022

IEC 60071-12 ®
Edition 1.0 2022-10
INTERNATIONAL
STANDARD
NORME
INTERNATIONALE
Insulation co-ordination –
Part 12: Application guidelines for LCC HVDC converter stations

Coordination de l’isolement –
Partie 12: Lignes directrices en matière d’application pour stations de
conversion à courant continu haute tension (CCHT) équipées de convertisseurs
commutés par le réseau (LCC)
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 60071-12 ®
Edition 1.0 2022-10
INTERNATIONAL
STANDARD
NORME
INTERNATIONALE
Insulation co-ordination –
Part 12: Application guidelines for LCC HVDC converter stations

Coordination de l’isolement –
Partie 12: Lignes directrices en matière d’application pour stations de

conversion à courant continu haute tension (CCHT) équipées de convertisseurs

commutés par le réseau (LCC)
INTERNATIONAL
ELECTROTECHNICAL
COMMISSION
COMMISSION
ELECTROTECHNIQUE
INTERNATIONALE
ICS 29.080.30 ISBN 978-2-8322-5845-3

– 2 – IEC 60071-12:2022 © IEC 2022
CONTENTS
FOREWORD . 5
1 Scope . 7
2 Normative references . 7
3 Terms, definitions, symbols and abbreviated terms . 7
3.1 Terms and definition . 7
3.2 Symbols and abbreviated terms . 8
3.2.1 General . 8
3.2.2 Subscripts . 8
3.2.3 Letter symbols . 8
3.2.4 Abbreviated terms . 9
4 Typical LCC HVDC converter station schemes . 9
5 Voltages and overvoltages in service . 12
5.1 Continuous operating voltages at various locations in the converter station . 12
5.2 Peak continuous operating voltage (PCOV) and crest continuous operating
voltage (CCOV) . 16
5.3 Sources and types of overvoItages . 18
5.4 Temporary overvoltage . 18
5.4.1 General . 18
5.4.2 Temporary overvoltage on the AC side . 18
5.4.3 Temporary overvoltages on the DC side . 19
5.5 Slow-front overvoltages . 19
5.5.1 General . 19
5.5.2 Slow-front overvoltages on the AC side . 19
5.5.3 Slow-front overvoltages on the DC side . 20
5.6 Fast-front, very-fast-front and steep-front overvoltages . 20
6 Arrester characteristics and stresses . 21
6.1 Arrester characteristics . 21
6.2 Arrester specification . 22
6.3 Arrester stresses. 23
6.3.1 General . 23
6.3.2 AC bus arrester (A) . 24
6.3.3 AC filter arrester (FA) . 24
6.3.4 Transformer valve winding arresters (T). 25
6.3.5 Valve arrester (V) . 25
6.3.6 Bridge arrester (B) . 28
6.3.7 Converter unit arrester (C) . 28
6.3.8 Mid-point DC bus arrester (M) . 29
6.3.9 Converter unit DC bus arrester (CB) . 29
6.3.10 DC bus and DC line/cable arrester (DB and DL/DC) . 30
6.3.11 Neutral bus arrester (E, EL, EM in Figure 3, EB, E1, EL, EM in Figure 1) . 30
6.3.12 DC reactor arrester (DR) . 31
6.3.13 DC filter arrester (FD) . 32
6.3.14 Earth electrode station arrester. 32
6.4 Protection strategy . 32
6.4.1 General . 32
6.4.2 Insulation directly protected by a single arrester . 32
6.4.3 Insulation protected by more than one arrester in series . 32

6.4.4 Valve side neutral point of transformers . 33
6.4.5 Insulation between phase conductors of the converter transformer . 33
6.4.6 Summary of protection strategy . 33
6.5 Summary of events and stresses . 36
7 Design procedure of insulation co-ordination . 37
7.1 General . 37
7.2 Arrester requirements . 38
7.3 Representative overvoltages (U ) . 38
rp
7.4 Determination of the co-ordination withstand voltages (U ) . 40
cw
7.5 Determination of the required withstand voltages (U ) . 40
rw
7.6 Determination of the specified withstand voltage (U ) . 40
w
8 Study tools and system modelling . 40
8.1 General . 40
8.2 Study approach and tooIs . 40
8.3 System details . 41
8.3.1 Modelling and system representation . 41
8.3.2 AC network and AC side of the LCC HVDC converter station . 43
8.3.3 DC overhead line/cable and earth electrode line details . 44
8.3.4 DC side of an LCC HVDC converter station details . 44
Annex A (informative) Example of insulation co-ordination for LCC HVDC converter
stations . 45
A.1 General . 45
A.2 Example for LCC HVDC converter station in a pole with one 12-pulse

converter . 45
A.2.1 Arrester protective scheme . 45
A.2.2 Arrester stresses, protection and insulation levels . 45
A.2.3 Transformer valve side withstand voltages. 50
A.2.4 Air-insulated smoothing reactors withstand voltages . 50
A.2.5 Results . 52
A.3 Example for LCC HVDC converter station in a pole with two 12-pulse
converters in series . 54
A.3.1 Arrester protective scheme . 54
A.3.2 Arrester stresses, protection and insulation levels . 55
A.3.3 Transformer valve side withstand voltages. 59
A.3.4 Smoothing reactor withstand voltages . 61
A.3.5 Results . 62
Bibliography . 64

Figure 1 – Possible arrester locations in a pole with two 12-pulse converters in series . 11
Figure 2 – Possible arrester locations for a back-to-back converter station . 12
Figure 3 – LCC HVDC converter station in a pole with one 12-pulse converter . 13
Figure 4 – Continuous operating voltages at various locations (location identification
according to Figure 3) . 15
Figure 5 – Operating voltage of a valve arrester (V), rectifier operation and definition of
PCOV and CCOV . 17
Figure 6 – Operating voltage of a mid-point arrester (M), rectifier operation . 17
Figure 7 – Operating voltage of a converter bus arrester (CB), rectifier operation . 17
Figure 8 – One pole of an LCC HVDC converter station . 43

– 4 – IEC 60071-12:2022 © IEC 2022
Figure A.1 – AC and DC arresters (LCC HVDC converter station in a pole with one 12-
pulse converter) . 52
Figure A.2 – Valve arrester stresses for slow-front overvoltages from AC side . 53
Figure A.3 – Arrester V2 stress for slow-front overvoltage from AC side . 53
Figure A.4 – Valve arrester stresses for earth fault between valve and upper bridge

transformer bushing . 54
Figure A.5 – Arrester V1 stress for earth fault between valve and upper bridge
transformer bushing . 54
Figure A.6 – AC and DC arresters (LCC HVDC converter station in a pole with two 12-
pulse converters in series) . 63

Table 1 – Symbol description . 12
Table 2 – Arrester protection on the DC side: one 12-pulse converter (Figure 3) . 34
Table 3 – Arrester protection on the DC side: two 12-pulse converters in series
(Figure 1) . 35
Table 4 – Events stressing arresters: one 12-pulse converter (Figure 3) . 36
Table 5 – Types of arrester stresses for different events: one 12-pulse converter
(Figure 3) . 37
Table 6 – Arrester requirements . 38
Table 7 – Representative overvoltages and required withstand voltages . 39
Table 8 – Origin of overvoltages and associated frequency ranges . 42

INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
INSULATION CO-ORDINATION –
Part 12: Application guidelines for LCC HVDC converter stations

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 60071-12 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.
On the basis of technical experience gained and the development of HVDC, sufficient
consensus has emerged to establish a series insulation co-ordination standard for HVDC
system. The standard series for HVDC system belongs to IEC 60071 standard series, and 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.

– 6 – IEC 60071-12:2022 © IEC 2022
This International Standard replaces, in conjunction with IEC 60071-11 , IEC 60071-5
published in 2014. IEC 60071-5 provides basic principles and guidance for insulation
coordination of high-voltage direct current (HVDC) converter stations. IEC 60071-11 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.
IEC 60071-12 provides guidelines on the procedures for insulation co-ordination of line
commutated converter (LCC) stations for high-voltage direct current (HVDC) project, whose aim
is to give guidance for the determination of the specified withstand voltages for equipment.
IEC 60071-12 retains the technical content of IEC 60071-5 of the guidelines on the procedures
for insulation coordination of LCC converter stations, and there are no essentially technical
amendments. An example for LCC HVDC converter station in a pole with two 12-pulse
converters in series is provided in annex. Examples of insulation co-ordination for controlled
series capacitor converter (CSCC) and capacitor commutated converters (CCC) in IEC 60071-
5 are no longer dealt with in this document.
The text of this International Standard is based on the following documents:
Draft Report on voting
99/368/FDIS 99/379/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.
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.
___________
Under preparation. Stage at the time of publication: IEC/CFDIS 60071-11:2022.

INSULATION CO-ORDINATION –
Part 12: Application guidelines for LCC HVDC converter stations

1 Scope
This part of IEC 60071 applies guidelines on the procedures for insulation co-ordination of line
commutated converter (LCC) stations for high-voltage direct current (HVDC) project, whose aim
is evaluating the overvoltage stresses on the converter station equipment subjected to
combined DC, AC power frequency, harmonic and impulse voltages, and determining the
specified withstand voltages for equipment.
This document deals only with metal-oxide surge arresters, without gaps, which are used in
modern HVDC converter stations. The criteria for determining the protective levels of series
and/or parallel combinations of surge arresters used to ensure optimal protection are also
presented. Typical arrester protection schemes and stresses of arresters are presented.
Annex A contains examples of insulation co-ordination for LCC HVDC converters which support
the concepts described in the main text, and the basic analytical techniques used.
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 60071-11 , Insulation co-ordination – Part 11：Definitions, principles and rules for HVDC
system
IEC 60099-4, Surge arresters – Part 4: Metal-oxide surge arresters without gaps for a.c.
systems
IEC 60633, High-voltage direct current (HVDC) transmission – Vocabulary
3 Terms, definitions, symbols and abbreviated terms
3.1 Terms and definition
For the purposes of this document, the terms and definitions given in IEC 60071-11 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
___________
Under preparation. Stage at the time of publication: IEC/CFDIS 60071-11:2022.

– 8 – IEC 60071-12:2022 © IEC 2022
3.1.1
crest value of continuous operating voltage
CCOV
highest continuously occurring crest value of the voltage at the equipment on the DC side of
the converter station excluding commutation overshoots
3.1.2
peak value of continuous operating voltage
PCOV
highest continuously occurring crest value of the voltage at the equipment on the DC side of
the converter station including commutation overshoots and commutation notches
3.1.3
valve protective firing
means of protecting the thyristors from excessive forward voltage, rate of change of voltage or
forward voltage applied during the reverse recovery time, by firing the thyristors into conduction
3.2 Symbols and abbreviated terms
3.2.1 General
The list covers only the most frequently used symbols and abbreviated terms, some of which
are illustrated graphically in the single-line diagram of Figure 1 and Figure 2. For a more
complete list of symbols which has been adopted for LCC HVDC converter stations, and also
for insulation co-ordination, refer to the standards listed in the normative references (Clause 2)
and to the Bibliography.
3.2.2 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)

3.2.3 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 crest value of continuous operating voltage
ccov
U continuous operating voltage of an arrester including harmonics
ch
U ideal no-load direct voltage (IEC 60633)
di0
U maximum value of U taking into account AC voltage measuring
di0max di0
tolerances, and transformer tap-changer offset by one step
U highest voltage of an AC system (IEC 60071-1 and IEC 60071-2)
s
U highest voltage for the equipment
m
U no-load phase-to-phase voltage on the valve side of converter transformer,
v0
r.m.s. value excluding harmonics

U representative overvoltage
rp
U co-ordination withstand voltage
cw
U required withstand voltage
rw
U specified withstand voltage (standard withstand voltage in alternating current)
w
α delay angle (IEC 60633); “firing angle” also used in this standard
β advance angle (IEC 60633)
γ extinction angle (IEC 60633)
μ overlap angle (IEC 60633)
3.2.4 Abbreviated terms
LCC line commutated converter
VSC voltage sourced converter
HVDC high voltage direct current
HV high voltage
LV low voltage
CCOV crest value of continuous operating voltage
GIS gas-insulated switchgear
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
p.u. per unit
4 Typical LCC HVDC converter station schemes
Figure 1 shows the single line diagram of typical LCC HVDC converter stations equipped with
two 12-pulse converters in series. It can be noted that Figure 1 shows possible arrester
locations covered in this document. Some of these arresters can be redundant and could be
excluded depending on the specific design.
Figure 2 shows an example for a single line diagram and arrester arrangement of a back-to
back converter station. Other arrangements with different earthing connections are also
common, e.g., earthing at the mid-point between the two six-pulse bridges. The location of the
smoothing reactor, if applicable, can change accordingly.

– 10 – IEC 60071-12:2022 © IEC 2022
The AC and DC filter configurations could be more complex than those shown in these figures.
Table 1 presents the graphical symbols used in this document.
The thyristor valves being voltage sensitive require strict overvoltage protection, which is
provided by valve arresters that are connected directly across the valve terminals.
The valve arresters in combination with other arresters typically provide protection to
transformer valve windings and in general separate phase-phase and phase-earth arresters are
not provided. Transformer valve winding phase-to-earth arresters can be considered at 800 kV
and above to lower the insulation levels especially to the top valve group.
Each voltage level and component are protected by either a single arrester or a combination of
series or parallel connected arresters.
Arrester designations and details on their design and specific roles are presented in Clause 6.

Key
A: AC bus arrester FA: AC filter arrester
FD: DC filter arrester EL: electrode line arrester
E1: DC neutral bus arrester EM: metallic return arrester
EB: converter neutral arrester B: bridge arrester (6-pulse)
V: valve arrester CB: converter unit DC bus arrester
T: transformer valve winding arrester DB: DC bus arrester
DR: smoothing reactor arrester DC: DC cable arrester
DL: DC line arrester CM: arrester between converters unit
CL: LV converter unit arrester MH: mid-point bridge arrester (HV bridge)
CH: HV converter unit arrester ML: mid-point bridge arrester (LV bridge)

Figure 1 – Possible arrester locations in
a pole with two 12-pulse converters in series

– 12 – IEC 60071-12:2022 © IEC 2022

Key
A: AC bus arrester FA: AC filter arrester
V: valve arrester
Figure 2 – Possible arrester locations for a back-to-back converter station
Table 1 – Symbol description
Symbol Description
Single valve (thyristor)
IEC 60617-S00057:2001-07
Arrester
IEC 60617-S00373:2001-07
Reactor
IEC 60617-S00849:2001-07
Capacitor
IEC 60617-S00567:2001-07
Earth
IEC 60617-S00200:2001-07
5 Voltages and overvoltages in service
5.1 Continuous operating voltages at various locations in the converter station
The continuous operating voltages at various locations in an LCC HVDC converter station differ
from the AC system in that they consist of not simply the fundamental frequency voltages. They
could be a combination of direct voltage, fundamental frequency voltage, harmonic voltages,
and high frequency transients, depending upon the location.

Table 3 shows an LCC HVDC converter station in a pole with one 12-pulse converter
configuration. In general phase-earth arresters on the valve side of the converter transformer
(T) are not provided for LCC HVDC schemes up to 600 kV.
Figure 1 shows an LCC HVDC scheme with two 12-pulse converters in series per pole
configuration, which has been used for the early 600 kV scheme and some 800 kV schemes.
Figure 4 shows typical waveforms of continuous operating voltages excluding commutation
overshoots at various locations in the LCC HVDC converter station either to earth (G) or to
another point for the typical configuration of Figure 3. The numbers and alphabetical
designations, in Figure 3, identify node numbers and arrester designations respectively. These
waveforms have been produced with a simulation tool considering typical DC parameters.
Note that Figure 1, Figure 2 and Figure 3 show possible arrester locations, and some of them
can be eliminated because of specific designs.

Key
A: AC bus arrester CB: converter unit DC bus arrester
M: mid-point bridge arrester EM: metallic return arrester
E: DC neutral bus arrester EL: electrode line arrester
V: valve arrester B: bridge arrester (6-pulse)
T: transformer valve winding arrester C: converter unit arrester
DR: smoothing reactor arrester DB: DC bus arrester
DL: DC line arrester DC: DC cable arrester
FA1, FA2: AC filter arresters FD1, FD2: DC filter arresters

Figure 3 – LCC HVDC converter station in a pole with one 12-pulse converter

– 14 – IEC 60071-12:2022 © IEC 2022

Figure 4 – Continuous operating voltages at various locations
(location identification according to Figure 3)
The harmonics generated on the AC side are assumed to be filtered by the connected filters
and thus the voltage at Loc. (1-G) and (4-G) is considered sine wave of fundamental frequency
without any harmonics.
Voltage shape at Loc. (1-2) is also predominantly a fundamental frequency sine wave but
superimposed by harmonics. The content of harmonics strongly depends on the filter
configuration, tuning frequencies as well as operating condition of the converters. Typically, the
content is less than 30 % of the fundamental frequency.
The voltages across the 6-pulse bridges (Loc. (7-8) and (9-7)) are the DC voltages across the
bridges consisting of about 60° arcs of line-line AC voltages (60°- μ, duration) and the average
of line-line voltages (duration, μ).
The voltage at the 6-pulse bridge to earth (Loc. (7-G)) can be identical to Loc. (7-8) if the station
is earthed via the station earth as well as during symmetrical operation of a bipole. However,
in case of unsymmetrical bipolar operation or monopolar operation an additional DC offset will
be superimposed.
The voltage across the 12-pulse converter (Loc. (9-8)) comprises of 30° arcs of line-line AC
voltages with superimposed influence of firing delay and overlap angles.
The voltage across the 12-pulse converter to earth (Loc. (9-G)) can be identical to Loc. (9-8) or
include an additional dc offset due to the same reasons as described for Loc. (7-G) (see above).

– 16 – IEC 60071-12:2022 © IEC 2022
Voltage shapes of Loc. (5b-6a) and (5c-6a) show the voltage between two different phases of
the two six-pulse groups. This wave shape is relevant only in case of three-phase 3-winding
transformers.
The voltage at Loc. (10-G) is the smoothed out voltage due to the influence of the smoothing
reactor and DC filter, if applicable.
The voltages at Loc. (6-8) and (9-5) are the voltages across a valve in rectifier mode indicating
the valve conduction period and commutation in its own row and the other row of thyristors in a
6-pulse bridge.
The voltage across the transformer valve winding phase-phase is shown in Loc. (5), (6) (ph-ph).
The zero voltage shows the commutation process involving the valves connected to the
corresponding two phases, while the notches indicate the commutation involving valves that
are connected to one of the phases.
Neutral bus voltage (Loc. (8-G)) and voltages across the filters are indicative of typical voltages
and they depend on electrode circuit and filter parameters. Loc. (8-G) can also include a DC
offset especially during monopolar metallic return operation.
The voltage at location (n-G) has a DC component equal to 3/4 of pole voltage (Loc. (10-G))
plus the ripple of the lower 6-pulse bridge and half of the ripple of the upper 6-pulse bridge.
5.2 Peak continuous operating voltage (PCOV) and crest continuous operating
voltage (CCOV)
The switching action of the valves produces high frequency turn-on and turn-off commutation
transient voltages which are superimposed on the commutation voltage. The overshoot at turn
off increases the transformer valve-side winding voltage and in particular the off-state (reverse-
blocking) voltage across the valves and associated valve arresters. The amplitude of the
overshoot is determined by:
a) the inherent characteristics of the thyristors (particularly the recovery charge);
b) the distribution of the recovery charge in a series-connected string of thyristors in a valve;
c) the damping resistors and capacitors at individual thyristor levels;
d) the various capacitances and inductances within the valve and commutation circuit;
e) the firing and overlap angles;
f) the valve commutation voltage at the instant of turn-off.
The continuous operating voltage waveform across the (Loc. (6-8) and (9-5)) and valve arrester
(V), during rectifier operation, is shown in Figure 5.
The CCOV (defined in Clause 3) is proportional to the U , and is given by:
di0max
π
U = U = 2 ⋅ U
⋅
ccov di0max v0
Refer to 3.2.3 for the definition of U and U .
di0max v0
Operation with large delay angles α increases the commutation overshoots, and special care
shall be taken that these do not overstress the arresters.

Figure 5 – Operating voltage of a valve arrester (V), rectifier operation
and definition of PCOV and CCOV
The continuous operating voltage waveforms across the mid-point arrester (M) (Loc. (7-G)) and
across the converter bus arrester (CB) (Loc. (9-G)) are shown in Figure 6 and Figure 7,
respectively.
Figure 6 – Operating voltage of a mid-point arrester (M), rectifier operation

Figure 7 – Operating voltage of a converter bus arrester (CB), rectifier operation

– 18 – IEC 60071-12:2022 © IEC 2022
5.3 Sources and types of overvoItages
Overvoltages on the AC side can originate from switching, faults, load rejection or lightning.
The dynamic characteristics of the AC network, its impedance and also its effective damping at
dominant transient oscillation frequencies, and the proper modelling of the converter
transformers, static and synchronous compensators and the filter components, are important in
evaluating the overvoltages. If the lengths of busbars in the AC switchyard are significant, they
shall be taken into account in the evaluation of lightning and fast-front overvoltages (e.g.,
distance effects) and in the location of arresters.
Overvoltages on the DC side can originate from either the AC system or the DC line and/or
cable, or from in-station flashovers. or other fault events.
In assessing the overvoltages, the configuration of the AC and DC systems shall be taken into
account as well as the dynamic performance of the valves and controls, and credible worst case
combinations, as discussed in Clauses 6 and 8.
Impacts on arrester requirements are discussed in Clause 6.
While the origin of overvoltages can result from different phenomena (switching, fault and
lightning) and described above, the overvoltages are categorized according to their shape and
duration as:
• temporary overvoltages (power frequency overvoltage of relatively long duration),
• transient overvoltages (short-duration overvoltage of few milliseconds or less, oscillatory or
non-oscillatory, usually highly damped).
Transient overvoltages can be further classified as:
• slow front overvoltages,
• fast-front overvoltages,
• very-fast-front overvoltages,
• steep-front overvoltages.
5.4 Temporary overvoltage
5.4.1 General
A temporary overvoltage is defined as an oscillatory overvoltage of relatively long duration
which is undamped or only weakly damped. The temporary overvoltages can originate either
from the AC side or the DC side.
5.4.2 Temporary overvoltage on the AC side
These overvoltages are usually generated due to switching operations or faults. The highest
temporary overvoltages usually occur in conjunction with sudden loss of load caused by faults
either on the AC system or the DC system with AC reactive sources still connected. If the
connected reactive elements and the AC system result in resonance conditions the temporary
overvoltages can be more severe both from the overvoltage magnitude and arrester energy
duty point of view.
Together with the highest AC operating voltages (U ), the temporary overvoltage will be decisive
s
for setting the rated voltage of AC bus arresters (A).
Temporary overvoltages together with high firing or extinction angles should also be considered
for valve arresters (V).
IEC 60071-12:202
...