IEC 60071-5:2014

IEC 60071-5 ®
Edition 1.0 2014-10
INTERNATIONAL
STANDARD
NORME
INTERNATIONALE
colour
inside
Insulation co-ordination –
Part 5: Procedures for high-voltage direct current (HVDC) converter stations

Coordination de l’isolement –
Partie 5: Procédures pour les stations de conversion à courant continu haute
tension (CCHT)
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IEC 60071-5 ®
Edition 1.0 2014-10
INTERNATIONAL
STANDARD
NORME
INTERNATIONALE
colour
inside
Insulation co-ordination –
Part 5: Procedures for high-voltage direct current (HVDC) converter stations

Coordination de l’isolement –
Partie 5: Procédures pour les stations de conversion à courant continu haute

tension (CCHT)
INTERNATIONAL
ELECTROTECHNICAL
COMMISSION
COMMISSION
ELECTROTECHNIQUE
PRICE CODE
INTERNATIONALE
CODE PRIX XD
ICS 29.080.30 ISBN 978-2-8322-1887-7

– 2 – IEC 60071-5:2014 © IEC 2014

CONTENTS
FOREWORD . 6
INTRODUCTION . 8
1 General . 9
1.1 Scope . 9
1.2 Additional background . 9
2 Normative references . 10
3 Terms and definitions. 10
4 Symbols and abbreviations . 16
4.1 General . 16
4.2 Subscripts . 16
4.3 Letter symbols . 16
4.4 Abbreviations . 17
5 Typical HVDC converter station schemes . 17
6 Principles of insulation co-ordination . 21
6.1 General . 21
6.2 Essential differences between a.c. and d.c. systems . 21
6.3 Insulation co-ordination procedure . 21
6.4 Comparison of withstand voltage selection in a.c. and d.c. systems . 22
7 Voltages and overvoltages in service . 24
7.1 Continuous operating voltages at various locations in the converter station . 24
7.2 Peak continuous operating voltage (PCOV) and crest continuous operating
voltage (CCOV) . 28
7.3 Sources and types of overvoltages . 30
7.4 Temporary overvoltages . 31
7.4.1 General . 31
7.4.2 Temporary overvoltages on the a.c. side . 31
7.4.3 Temporary overvoltages on the d.c. side . 31
7.5 Slow-front overvoltages . 31
7.5.1 General . 31
7.5.2 Slow-front overvoltages on the a.c. side . 31
7.5.3 Slow-front overvoltages on the d.c. side . 32
7.6 Fast-front, very-fast-front and steep-front overvoltages . 33
8 Arrester characteristics and stresses. 34
8.1 Arrester characteristics . 34
8.2 Arrester specification. 35
8.3 Arrester stresses . 35
8.3.1 General . 35
8.3.2 AC bus arrester (A) . 36
8.3.3 AC filter arrester (FA) . 37
8.3.4 Transformer valve winding arresters (T) . 37
8.3.5 Valve arrester (V) . 37
8.3.6 Bridge arrester (B) . 40
8.3.7 Converter unit arrester (C) . 41
8.3.8 Mid-point d.c. bus arrester (M) . 41
8.3.9 Converter unit d.c. bus arrester (CB) . 42
8.3.10 DC bus and d.c. line/cable arrester (DB and DL/DC) . 42

8.3.11 Neutral bus arrester (E, EL, EM in Figure 3, EB, E1, EL, EM in Figure 1) . 42
8.3.12 DC reactor arrester (DR) . 43
8.3.13 DC filter arrester (FD) . 44
8.3.14 Earth electrode station arrester . 44
8.4 Protection strategy . 44
8.4.1 General . 44
8.4.2 Insulation directly protected by a single arrester . 44
8.4.3 Insulation protected by more than one arrester in series . 45
8.4.4 Valve side neutral point of transformers . 45
8.4.5 Insulation between phase conductors of the converter transformer . 45
8.4.6 Summary of protection strategy . 45
8.5 Summary of events and stresses . 47
9 Design procedure of insulation co-ordination . 49
9.1 General . 49
9.2 Arrester requirements. 49
9.3 Characteristics of insulation. 51
9.4 Representative overvoltages (U ) . 51
rp
9.5 Determination of the co-ordination withstand voltages (U ) . 52
cw
9.6 Determination of the required withstand voltages (U ) . 52
rw
9.7 Determination of the specified withstand voltage (U ) . 54
w
10 Study tools and system modelling . 54
10.1 General . 54
10.2 Study approach and tools . 54
10.3 System details . 55
10.3.1 Modelling and system representation . 55
10.3.2 AC network and a.c. side of the HVDC converter station . 57
10.3.3 DC overhead line/cable and earth electrode line details . 58
10.3.4 DC side of an HVDC converter station details . 58
11 Creepage distances . 59
11.1 General . 59
11.2 Base voltage for creepage distance . 59
11.3 Creepage distance for outdoor insulation under d.c. voltage . 59
11.4 Creepage distance for indoor insulation under d.c. or mixed voltage . 60
11.5 Creepage distance of a.c. insulators . 60
12 Clearances in air . 60
Annex A (informative) Example of insulation co-ordination for conventional HVDC
converters . 62
A.1 General . 62
A.2 Arrester protective scheme . 62
A.3 Arrester stresses, protection and insulation levels. 62
A.3.1 General . 62
A.3.2 Slow-front overvoltages transferred from the a.c. side . 63
A.3.3 Earth fault between valve and upper bridge transformer bushing . 63
A.4 Transformer valve side withstand voltages . 66
A.4.1 Phase-to-phase . 66
A.4.2 Upper bridge transformer phase-to-earth (star). 67
A.4.3 Lower bridge transformer phase-to-earth (delta) . 67
A.5 Air-insulated smoothing reactors withstand voltages . 67
A.5.1 Terminal-to-terminal slow-front overvoltages . 67

– 4 – IEC 60071-5:2014 © IEC 2014

A.5.2 Terminal-to-earth . 68
A.6 Results . 68
Annex B (informative) Example of insulation co-ordination for capacitor commutated
converters (CCC) and controlled series capacitor converters (CSCC) . 72
B.1 General . 72
B.2 Arrester protective scheme . 72
B.3 Arrester stresses, protection and insulation levels. 72
B.3.1 General . 72
B.3.2 Transferred slow-front overvoltages from the a.c. side . 73
B.3.3 Earth fault between valve and upper bridge transformer bushing . 74
B.4 Transformer valve side withstand voltages . 77
B.4.1 Phase-to-phase . 77
B.4.2 Upper bridge transformer phase-to-earth (star). 77
B.4.3 Lower bridge transformer phase-to-earth (delta) . 77
B.5 Air-insulated smoothing reactors withstand voltages . 78
B.5.1 Slow-front terminal-to-terminal overvoltages . 78
B.5.2 Terminal-to-earth . 78
B.6 Results . 79
Annex C (informative) Considerations for insulation co-ordination of some special
converter configurations . 87
C.1 Procedure for insulation co-ordination of back-to-back type of HVDC links . 87
C.2 Procedure for insulation co-ordination of parallel valve groups . 87
C.2.1 General . 87
C.2.2 AC bus arrester (A) . 88
C.2.3 AC filter arrester (FA) . 88
C.2.4 Valve arrester (V) . 88
C.2.5 Bridge arrester (B) and converter unit arrester (C) . 88
C.2.6 Mid-point arrester (M) . 88
C.2.7 Converter unit d.c. bus arrester (CB) . 88
C.2.8 DC bus and d.c. line/cable arrester (DB and DL) . 89
C.2.9 Neutral bus arrester (E) . 89
C.2.10 DC reactor arrester (DR) . 89
C.2.11 DC filter arrester (FD) . 89
C.2.12 New converter stations with parallel valve groups . 89
C.3 Procedure for insulation co-ordination of upgrading existing systems with
series-connected valve groups . 89
C.3.1 General . 89
C.3.2 AC bus arrester (A) . 90
C.3.3 AC filter arrester (FA) . 90
C.3.4 Valve arrester (V) . 90
C.3.5 Bridge arrester (B) and converter unit arrester (C) . 90
C.3.6 Mid-point arrester (M) . 90
C.3.7 Converter unit d.c. bus arrester (CB), d.c. bus and d.c. line/cable
arrester (DB and DL) . 91
C.3.8 Neutral bus arrester (E) . 91
C.3.9 DC reactor arrester (DR) . 91
C.3.10 DC filter arrester (FD) . 91
C.4 Overvoltages in the a.c. network due to closely coupled HVDC links . 91
C.5 Effect of gas-insulated switchgear on insulation co-ordination of HVDC
converter stations . 92

Annex D (informative) Typical arrester characteristics . 93
Bibliography . 94

Figure 1 – Possible arrester locations in a pole with two 12-pulse converters in series . 19
Figure 2 – Possible arrester locations for a back-to-back converter station . 20
Figure 3 – HVDC converter station with one 12-pulse converter bridge per pole . 25
Figure 4 – Continuous operating voltages at various locations (location identification
according to Figure 3) . 27
Figure 5 – Operating voltage of a valve arrester (V), rectifier operation . 29
Figure 6 – Operating voltage of a mid-point arrester (M), rectifier operation . 29
Figure 7 – Operating voltage of a converter bus arrester (CB), rectifier operation . 30
Figure 8 – One pole of an HVDC converter station . 57
Figure A.1 – AC and d.c. arresters . 69
Figure A.2 – Valve arrester stresses for slow-front overvoltages from a.c. side . 69
Figure A.3 – Arrester V2 stress for slow-front overvoltage from a.c. side . 70
Figure A.4 – Valve arrester stresses for earth fault between valve and upper bridge
transformer bushing . 70
Figure A.5 – Arrester V1 stress for earth fault between valve and upper bridge
transformer bushing . 71
Figure B.1 – AC and d.c. arresters for CCC and CSCC converters . 80
Figure B.2 – Valve arrester stresses for slow-front overvoltages from a.c. side . 81
Figure B.3 – Arrester V2 stress for slow-front overvoltage from a.c. side . 82
Figure B.4 – Valve arrester stresses for earth fault between valve and upper bridge
transformer bushing . 84
Figure B.5 – Arrester V1 stress for earth fault between valve and upper bridge
transformer bushing . 85
Figure B.6 – Stresses on capacitor arresters C and C during earth fault between
cc sc
valve and upper bridge transformer bushing . 86
Figure C.1 – Expanded HVDC converter with parallel valve groups . 88
Figure C.2 – Upgraded HVDC converter with series valve group . 90
Figure D.1 – Typical arrester V-I characteristics . 93

Table 1 – Classes and shapes of overvoltages, standard voltage shapes and standard
withstand voltage tests . 11
Table 2 – Symbol description. 20
Table 3 – Comparison of the selection of withstand voltages for a.c. equipment with
that for HVDC converter station equipment . 23
Table 4 – Arrester protection on the d.c. side: Single 12-pulse converter (Figure 3) . 46
Table 5 – Arrester protection on the d.c. side: Two 12-pulse converters (Figure 1) . 46
Table 6 – Events stressing arresters: Single 12-pulse converter (Figure 3). 48
Table 7 – Types of arrester stresses for different events: Single 12-pulse converter
(Figure 3) . 48
Table 8 – Arrester requirements . 50
Table 9 – Representative overvoltages and required withstand voltages. 51
Table 10 – Indicative values of ratios of required impulse withstand voltage to impulse

protective level . 54
Table 11 – Origin of overvoltages and associated frequency ranges. 56

– 6 – IEC 60071-5:2014 © IEC 2014

INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
INSULATION CO-ORDINATION –
Part 5: Procedures for high-voltage
direct current (HVDC) converter stations

FOREWORD
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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.
International Standard IEC 60071-5 has been prepared by IEC technical committee 28:
Insulation co-ordination.
This International Standard cancels and replaces IEC TS 60071-5 published in 2002. On the
basis of technical experience gained since the Technical Specification was published, sufficient
consensus has emerged for transformation of the Technical Specification into an International
Standard.
The technical content is essentially the same as that contained in the Technical Specification
with amendments mainly for user convenience. The structure of the document has been
changed to allow division and subdivision into complete integral parts to facilitate
comprehension and ease of referencing.
In addition to the high level revisions above, the following main technical changes have been
made with respect to the previous edition:

– arresters have been added to several locations to reflect some recent 800 kV HVDC
scheme practice, along with their justifications, expected voltages, overvoltages and
arrester stresses in service;
– significant changes have been made in Clause 8 – all subclauses on the characteristics,
schemes, stresses and specification of arresters have been consolidated into a single entity,
Clause 8;
– the implications of a smoothing reactor and of a neutral blocking filter located on the neutral
bus (as on some recent 800 kV schemes), on coordination of arresters connected to the
neutral end have been added;
– possible use of sacrificial arresters on the neutral bus is introduced to cater for excessive
arrester energy in the rather unlikely event of a particular rare fault;
– all subclauses dealing with study tools and modelling details have been consolidated into
Clause 10;
– creepage distances and clearances have been consolidated into Clauses 11 and 12,
respectively, with more details added.
The text of this standard is based on the following documents:
FDIS Report on voting
28/218/FDIS 28/221/RVD
Full information on the voting for the approval of this standard can be found in the report on
voting indicated in the above table.
This publication has been drafted in accordance with the ISO/IEC Directives, Part 2.
A list of all parts in the IEC 600071 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 publication will remain unchanged until the
stability date indicated on the IEC web site under "http://webstore.iec.ch" in the data related to
the specific publication. At this date, the publication will be
• reconfirmed,
• withdrawn,
• replaced by a revised edition, or
• amended.
IMPORTANT – The 'colour inside' logo on the cover page of this publication indicates
that it contains colours which are considered to be useful for the correct understanding
of its contents. Users should therefore print this document using a colour printer.

– 8 – IEC 60071-5:2014 © IEC 2014

INTRODUCTION
The IEC 60071 series consists of the following parts under the general title Insulation co-
ordination:
Part 1: Definitions, principles and rules
Part 2: Application guide
Part 4: Computational guide to insulation co-ordination and modelling of electrical networks
Part 5: Procedures for high-voltage direct current (HVDC) converter stations

INSULATION CO-ORDINATION –
Part 5: Procedures for high-voltage
direct current (HVDC) converter stations

1 General
1.1 Scope
This part of IEC 60071 provides guidance on the procedures for insulation co-ordination of
high-voltage direct current (HVDC) converter stations, without prescribing standardized
insulation levels.
This standard applies only for HVDC applications in high-voltage a.c. 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 standard.
1.2 Additional background
The use of power electronic thyristor valves in a series and/or parallel arrangement, along with
the unique control and protection strategies employed in the conversion process, has
ramifications requiring particular consideration of overvoltage protection of equipment in
converter stations compared with substations in a.c. systems. This standard outlines the
procedures for evaluating the overvoltage stresses on the converter station equipment
subjected to combined d.c., a.c. power frequency, harmonic and impulse voltages. The criteria
for determining the protective levels of series and/or parallel combinations of surge arresters
used to ensure optimal protection are also presented.
The basic principles and design objectives of insulation co-ordination of converter stations, in
so far as they differ from normal a.c. system practice, are described.
Concerning surge arrester protection, this standard deals only with metal-oxide surge arresters,
without gaps, which are used in modern HVDC converter stations. The basic arrester
characteristics, requirements for these arresters and the process of evaluating the maximum
overvoltages to which they may be exposed in service, are presented. Typical arrester
protection schemes and stresses of arresters are presented, along with methods to be applied
for determining these stresses.
This standard includes insulation co-ordination of equipment connected between the converter
a.c. bus (including the a.c. harmonic filters, the converter transformer, the circuit breakers) and
the d.c. line side of the smoothing reactor. The line and cable terminations in so far as they
influence the insulation co-ordination of converter station equipment are also covered.
Although the main focus of the standard is on conventional HVDC systems where the
commutation voltage bus is at the a.c. filter bus, outlines of insulation co-ordination for the
capacitor commutated converter (CCC) as well as the controlled series compensated converter
(CSCC) and some other special converter configurations are covered in the annexes.
This standard discusses insulation co-ordination related to line commutated converter (LCC)
stations. The insulation coordination of voltage sourced converters (VSC) is not part of this
standard.
– 10 – IEC 60071-5:2014 © IEC 2014

2 Normative references
The following documents, in whole or in part, are normatively referenced in this document and
are indispensable for its application. 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 60071-1:2006, Insulation co-ordination – Part 1: Definitions, principles and rules
IEC 60071-2:1996, Insulation co-ordination – Part 2: Application guide
IEC 60099-4:2004, Surge arresters – Part 4: Metal-oxide surge arresters without gaps for a.c.
systems
IEC 60633, Terminology for high-voltage direct current (HVDC) transmission
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
3 Terms and definitions
For the purposes of this document, the following terms and definitions apply.
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: 2006, 3.1]
3.2
nominal d.c. voltage
mean value of the direct voltage required to transmit nominal power at nominal current
3.3
highest d.c. voltage
highest value of d.c. voltage for which the equipment is designed to operate continuously, in
respect of its insulation as well as other characteristics
3.4
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
3.4.1 to 3.4.2.3.
Table 1 – Classes and shapes of overvoltages, standard voltage shapes
and standard withstand voltage tests

3.4.1
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 may be undamped or weakly damped.
3.4.2
transient overvoltage
short-duration overvoltage of a few millisecond or less, oscillatory or non-oscillatory, usually
highly damped
[SOURCE: IEC 60071-1: 2006, 3.17.3]
3.4.2.1
slow-front overvoltage
transient overvoltage, usually unidirectional, with time to peak 20 µs < T ≤ 5 000 µs, and tail
p
duration T ≤ 20 ms
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.

– 12 – IEC 60071-5:2014 © IEC 2014

[SOURCE: IEC 60071-1:2006, 3.17.3.1]
3.4.2.2
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 60071-1:2006, 3.17.3.2.
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.
3.4.2.3
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 60071-1:2006, 3.17.3.3]
3.4.2.4
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.
3.4.2.5
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.
3.5
representative overvoltages
U
rp
overvoltages assumed to produce the same dielectric effect on the insulation as overvoltages
of a given class occurring in service due to various origins
Note 1 to entry: In this standard it is generally assumed that the representative overvoltages are characterized by
their assumed or obtained maximum values.
[SOURCE: IEC 60071-1:2006, 3.19]
3.5.1
representative slow-front overvoltage
RSFO
voltage value between terminals of an equipment having the shape of a standard switching
impulse
Note 1 to entry: This note applies
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