IEC TS 60071-5:2002

TECHNICAL IEC
SPECIFICATION TS 60071-5
First edition
2002-06
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 CCHT

Reference number
IEC/TS 60071-5:2002(E)
Publication numbering
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TECHNICAL IEC
SPECIFICATION TS 60071-5
First edition
2002-06
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 CCHT

© IEC 2002 ⎯ Copyright - all rights reserved
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 the publisher.
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– 2 – TS 60071-5  IEC:2002(E)
CONTENTS
FOREWORD.5
1 General .7
1.1 Scope.7
1.2 Additional background .7
2 Normative references.8
3 Definitions .8
4 Symbols and abbreviations .12
4.1 Subscripts .12
4.2 Letter symbols .12
4.3 Abbreviations.12
4.4 Typical HVDC converter station schemes and associated graphical symbols .13
5 Principles of insulation co-ordination .15
5.1 Essential differences between a.c. and d.c. systems .15
5.2 Insulation co-ordination procedure .16
6 Voltages and overvoltages in service .18
6.1 Arrangements of arresters .18
6.2 Continuous operating voltages at various locations in the converter station .19
6.3 Peak (PCOV) and crest value (CCOV) of continuous operating voltage
applied to valves and arresters .20
6.4 Sources and types of overvoltages.21
6.5 Overvoltage limiting characteristics of arresters .24
6.6 Valve protection strategy .25
6.7 Methods and tools for overvoltage and surge arrester characteristic studies .25
6.8 Necessary system details .28
7 Design objectives of insulation co-ordination .30
7.1 Arrester requirements .31
7.2 Characteristics of insulation .33
7.3 Representative overvoltages.33
7.4 Determination of the required withstand voltage .36
7.5 Determination of the specified withstand voltage .37
7.6 Creepage distances.37
7.7 Clearances in air .37
8 Creepage distances and clearances in air .37
8.1 Creepage distance for outdoor insulation under d.c. voltage.38
8.2 Creepage distance for indoor insulation under d.c. voltage .38
8.3 Creepage distance of a.c. insulators (external).38
8.4 Clearances in air .39
9 Arrester requirements .39
9.1 Arrester specification .39
9.2 AC bus arrester (A).40
9.3 AC filter arrester (FA) .40
9.4 Valve arrester (V) .40
9.5 Bridge arrester (B).42
9.6 Converter unit arrester (C) .43

TS 60071-5  IEC:2002(E) – 3 –
9.7 Mid-point d.c. bus arrester (M) .43
9.8 Converter unit d.c. bus arrester (CB).43
9.9 DC bus and d.c. line/cable arrester (DB and DL) .44
9.10 Neutral bus arrester (E) .44
9.11 DC reactor arrester (DR).45
9.12 DC filter arrester (FD) .45
9.13 Earth electrode station arrester.45
Annex A (informative) Example of insulation co-ordination for conventional HVDC
converters .46
Annex B (informative) Example of insulation co-ordination for Controlled Series
Capacitor Converters (CSCC) and Capacitor Commutated Converters (CCC).55
Annex C (informative) Considerations for insulation co-ordination of some special
converter configurations.69
Bibliography .75
Figure 1 – Single line diagram of typical converter pole with two 12-pulse
converters in series.13
Figure 2 – Single line diagram of typical capacitor commutated converter (CCC) pole
with two 12-pulse converters in series .14
Figure 3 – Single line diagram of typical controlled series compensated converter
(CSCC) pole with two 12-pulse converters in series .14
Figure 4 – HVDC converter station diagram with 12-pulse converter bridges .18
Figure 5 – Continuous operating voltages at various locations (location identification
according to figure 4) .20
Figure 6 – Operating voltage of a valve arrester (V), rectifier operation .21
Figure 7 – One pole of an HVDC converter station.29
Figure A.1 – AC and DC arresters (400 kV a.c. side for conventional HVDC converters) .52
Figure A.2 – Simplified circuit configuration for stresses of valve arrester at slow-front
overvoltages from a.c. side (conventional HVDC converters) – Illustration of
slow-front overvoltage wave (applied voltage).53
Figure A.3 – Stresses on valve arrester V2 at slow-front overvoltage from a.c. side
(conventional HVDC converter ) .53
Figure A.4 – Circuit configuration for stresses on valve arrester at earth fault on
transformer HV bushing (conventional HVDC converters) .54
Figure A.5 – Stresses on valve arrester V1 during earth fault on HV bushing of
converter transformer (conventional HVDC converter) .54
Figure B.1a – AC and DC arresters (400 kV a.c. side for CCC converters) .62
Figure B.1b – AC and DC arresters (400 kV a.c. side for CSCC converter) .63
Figure B.2a – Simplified circuit configuration for stresses on valve arrester at slow-front
overvoltages from a.c. side (CCC converter) .64
Figure B.2b – Simplified circuit configuration for stresses on valve arrester at slow-front
overvoltages from a.c. side (CSCC converter) .64
Figure B.3a – Stresses on valve arrester V2 at slow-front overvoltage from a.c. side
(CCC converter).65
Figure B.3b – Stresses on valve arrester V2 at slow-front overvoltage from a.c. side
(CSCC converter).65

– 4 – TS 60071-5  IEC:2002(E)
Figure B.4a – Circuit configuration for stresses on valve arrester at earth fault on HV
bushing of converter transformer (CCC converter).66
Figure B.4b – Circuit configuration for stresses on valve arrester at earth fault
on HV bushing of converter transformer (CSCC converter) .66
Figure B.5a – Stresses on valve arrester V1 during earth fault on HV bushing of
converter transformer (CCC converter).67
Figure B.5b – Stresses on valve arrester V1 during earth fault on HV bushing of
converter transformer (CSCC converter) .67
Figure B.6a – Stresses on CCC capacitor arrester Ccc during earth fault on HV bushing
of converter transformer (CCC converter).68
Figure B.6b – Stresses on CSCC capacitor arrester Csc during earth fault on HV
bushing of converter transformer (CSCC converter).68
Figure C.1 – Expanded HVDC converter with parallel valve groups .70
Figure C.2 – Upgraded HVDC converter with series valve group .72
Table 1 – Symbol description .14
Table 2 – Comparison of the selection of withstand voltages for three-phase a.c.
equipment with that for HVDC converter station equipment.17
Table 3 – Events stressing the different arresters .27
Table 4 – Types of stresses on arresters for different events .27
Table 5 – Origin of overvoltages and associated frequency ranges .28
Table 6 – Table for arrester requirements.32
Table 7 – Arrester protection of d.c. side of a HVDC converter station .34
Table 8 – Table gathering representative overvoltage levels and required withstand
voltage levels. .35
Table 9 – Indicative values of ratios of required impulse withstand voltage to impulse
protective level.37

TS 60071-5  IEC:2002(E) – 5 –
INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
INSULATION CO-ORDINATION –
Part 5: Procedures for high-voltage direct current (HVDC)
converter stations
FOREWORD
1) The IEC (International Electrotechnical Commission) is a worldwide organization for standardization comprising
all national electrotechnical committees (IEC National Committees). The object of the 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, the IEC publishes International Standards. 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. The 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 the 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 National Committees.
3) The documents produced have the form of recommendations for international use and are published in the form
of standards, technical specifications, technical reports or guides and they are accepted by the National
Committees in that sense.
4) In order to promote international unification, IEC National Committees undertake to apply IEC International
Standards transparently to the maximum extent possible in their national and regional standards. Any
divergence between the IEC Standard and the corresponding national or regional standard shall be clearly
indicated in the latter.
5) The IEC provides no marking procedure to indicate its approval and cannot be rendered responsible for any
equipment declared to be in conformity with one of its standards.
6) Attention is drawn to the possibility that some of the elements of this technical specification may be the subject
of patent rights. The IEC shall not be held responsible for identifying any or all such patent rights.
The main task of IEC technical committees is to prepare International Standards. In
exceptional circumstances, a technical committee may propose the publication of a technical
specification when
• the required support cannot be obtained for the publication of an International Standard,
despite repeated efforts, or
• the subject is still under technical development or where, for any other reason, there is the
future but no immediate possibility of an agreement on an International Standard.
Technical specifications are subject to review within three years of publication to decide
whether they can be transformed into International Standards.
IEC 60071-5, which is a technical specification, has been prepared by IEC technical committee
28: Insulation co-ordination.
The text of this technical specification is based on the following documents:
Enquiry draft Report on voting
28/139/CDV 28/144A/RVC
Full information on the voting for the approval of this technical specification 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 3.

– 6 – TS 60071-5  IEC:2002(E)
This technical specification is published in English only.
Annexes A, B and C are for information only.
The committee has decided that the contents of this publication will remain unchanged until
2008. At this date, the publication will be
• transformed into an International standard
• reconfirmed;
• withdrawn;
• replaced by a revised edition, or
• amended.
TS 60071-5  IEC:2002(E) – 7 –
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.
The guide 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 application guide.
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 guide 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 is 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 guide 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 guide 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 guide 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.

– 8 – TS 60071-5  IEC:2002(E)
2 Normative references
The following referenced documents are indispensable for the application 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 60060-1:1989, High-voltage test techniques – Part 1: General definitions and test
requirements
IEC 60071-1:1993, Insulation co-ordination – Part 1: Definitions, principles and rules
IEC 60071-2:1996, Insulation co-ordination – Part 2: Application guide
IEC 60099-4:1991, Surge arresters – Part 4: Metal-oxide surge arresters without gaps for a.c.
systems
IEC 60633:1998, Terminology for high-voltage direct current (HVDC) transmission
IEC 60700-1:1998,Thyristor valves for high-voltage direct current (HVDC) power transmission –
Part 1: Electrical testing
IEC 60815:1986, Guide for the selection of insulators in respect of polluted conditions
3 Definitions
For the purposes of this part of IEC 60071, the following terms and definitions apply.
Many of the following definitions refer to actual insulation co-ordination concepts, or to actual
arrester parameters. For more information on these, please refer to IEC 60071-1 or to
IEC 60099-4, respectively.
3.1
d.c. system voltage
highest mean or average operating voltage to earth, excluding harmonics and commutation
overshoots (IEC 123 pollution test of HVDC insulator)
3.2
peak value of continuous operating voltage (PCOV)
highest continuously occurring crest value of the voltage at the equipment on the d.c. side
of the converter station including commutation overshoots and commutation notches (see
figure 6)
3.3
crest value of continuous operating voltage (CCOV)
highest continuously occurring crest value of the voltage at the equipment on the d.c. side of
the converter station excluding commutation overshoots (see figure 6)
3.4
overvoltage
voltage between one phase conductor and earth or between phase conductors having a peak
value exceeding the corresponding peak of the highest voltage of the system on the a.c. side
and the PCOV on the d.c. side of the HVDC converter station

TS 60071-5  IEC:2002(E) – 9 –
3.4.1
temporary overvoltage (TOV)
power frequency overvoltage of relatively long duration (IEC 60071-1)
NOTE The overvoltage may be undamped or weakly damped. In some cases its frequency may be several times
smaller or higher than power frequency.
3.4.2
slow-front overvoltage
transient overvoltage, usually unidirectional, with time to peak 20 μs < Tp < 5 000 μs, and tail
duration T2 < 50 ms (IEC 60071-1)
NOTE 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.
3.4.3
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.
Transient overvoltage, usually unidirectional, with time to peak 0,1 μs < T1 < 20 μs, and tail
duration T2 < 300 μs (IEC 60071-1).
NOTE For the purpose of insulation co-ordination, slow-front and 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.
3.4.4
very fast-front overvoltage
transient overvoltage, usually unidirectional, with time to peak T < 0,1 μs, total duration
f
< 3 ms, and with superimposed oscillations at frequency 30 kHz < f < 100 MHz (IEC 60071-1)
3.4.5
steep-front overvoltage
transient overvoltage classified as a kind of fast-front overvoltage with time to peak 3 ns < T1
< 1,2 μs). A steep-front impulse voltage for test purposes is defined in figure 1 of IEC 60700-1
NOTE The front time is decided by means of system studies.
3.4.6
combined overvoltage (temporary, slow-front, fast-front, very fast-front)
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. It is classified
by the component of higher peak value
3.5
representative overvoltages
overvoltages assumed to produce the same dielectric effect on the insulation as overvoltages
of a given class occurring in service due to various origins (IEC 60071-1)
NOTE In this specification it is generally assumed that the representative overvoltages are characterized by their
assumed or obtained maximum values.
3.5.1
representative slow-front overvoltage (RSLO)
voltage value between terminals of an equipment having the shape of a standard switching
impulse
– 10 – TS 60071-5  IEC:2002(E)
3.5.2
representative fast-front overvoltage (RFAO)
voltage value between terminals of an equipment having the shape of a standard lightning
impulse
3.5.3
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 A steep-front impulse voltage for test purposes is defined in figure 1 of IEC 60700-1. The front time is
decided by means of system studies.
3.6
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 in accordance with IEC 60099-4.
3.7
continuous operating voltage of an arrester including harmonics (U )
ch
r.m.s. value of the combination of power frequency voltage and harmonics that may be applied
continuously between the terminals of the arrester
3.8
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
3.9
residual voltage of an arrester
peak value of voltage that appears between the terminals of an arrester during the passage of
a discharge current (IEC 60099-4)
3.10
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. Standard shapes of
co-ordination currents for steep-front, lightning and switching current impulses are given in
IEC 60099-4
NOTE The co-ordination currents are determined by system studies.
3.11
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
3.12
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
For HVDC converter equipment the following specific definitions 3.12.1 to 3.12.3 apply.

TS 60071-5  IEC:2002(E) – 11 –
3.12.1
switching impulse protective level (SIPL)
residual voltage of a surge arrester subjected to a discharge current corresponding to the
co-ordination switching impulse current
3.12.2
lightning impulse protective level (LIPL)
residual voltage of a surge arrester subjected to a discharge current corresponding to the co-
ordination lightning impulse current
3.12.3
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
3.13
co-ordination withstand voltage
for each class of voltage, value of the withstand voltage of the insulation configuration,
in actual service conditions, that meets the performance criterion (IEC 60071-1)
3.14
required withstand voltage
test voltage that the insulation withstands in a standard withstand test to ensure that the
insulation will meet the co-ordination withstand voltage in actual service
(IEC 60071-1 modified)
3.15
specified withstand voltage
test voltage suitably selected equal or above the required withstand voltage (see 3.14)
NOTE 1 For a.c. equipment, values of specified withstand voltages are standardized as per IEC 60071-1. For
HVDC equipment, there is no standardized values for the specified withstand voltages which are rounded up to
convenient practical values.
NOTE 2 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 d.c. equipment (e.g. the thyristor valves), the standard impulse
shapes may be modified in order to more realistically reflect expected conditions.
3.15.1
specified switching impulse withstand voltage (SSIWV)
withstand voltage of insulation with the shape of the standard switching impulse
3.15.2
specified lightning impulse withstand voltage (SLIWV)
withstand voltage of insulation with the shape of the standard lightning impulse
3.15.3
specified steep-front impulse withstand voltage (SSFIWV)
withstand voltage of insulation with the shape specified in IEC 60700-1
3.16
thyristor valve protective firing (PF)
method of protecting the thyristors from excessive voltage in the forward direction by firing
them at a pre-determined voltage

– 12 – TS 60071-5  IEC:2002(E)
4 Symbols and abbreviations
The list covers only the most frequently used symbols and abbreviations some of which are
illustrated graphically in the single-line diagram of figure 1 and table 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 and to the bibliography.
4.1 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)
4.2 Letter symbols
K atmospheric 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 including harmonics
ch
U ideal no-load direct voltage (IEC 60633)
dio
U maximum value of U taking into account a.c. voltage measuring
dio
dim
tolerances, and transformer tap-changer offset by one step
U highest voltage of an a.c. system (IEC 60071-1 and 60071-2)
s
U no-load phase-to-phase voltage on the valve side of converter
v0
transformer, r.m.s. value excluding harmonics
α delay angle (IEC 60633); “firing angle” also used in this standard
advance angle (IEC 60633)
β
extinction angle (IEC 60633)
γ
overlap angle (IEC 60633)
μ
4.3 Abbreviations
CCC capacitor commutated converter
CSCC controlled series compensated converter
CCOV crest value of continuous operating voltage
ECOV equivalent continuous operating voltage
LIPL lightning impulse protective level
PCOV peak continuous operating voltage
PF protective firing
RFAO representative fast-front overvoltage (the maximum voltage stress value)
RSLO representative slow-front overvoltage (the maximum voltage stress value)
RSTO representative steep-front overvoltage (the maximum voltage stress value)
RLIWV required lightning impulse withstand voltage
RSIWV required switching impulse withstand voltage
RSFIWV required steep-front impulse withstand voltage
SIPL switching impulse protective level

TS 60071-5  IEC:2002(E) – 13 –
STIPL steep-front impulse protective level
SLIWV specified lightning impulse withstand voltage
SSIWV specified switching impulse withstand voltage
SSFIWV specified steep-front impulse withstand voltage
TOV temporary overvoltage
4.4 Typical HVDC converter station schemes and associated graphical symbols
Figures 1, 2 and 3 show the single line diagrams of typical HVDC converter stations equipped
with two 12-pulse converter bridges in series. The main differences between the schemes
consist in the presence, or not, of commutated capacitors (figure 2) or controlled series
capacitors (figure 3) on the a.c. side of the HVDC converter station.
NOTE Figures 1, 2 and 3 show all the possible arresters covered in this standard. However, some of them may be
eliminated because of specific designs.
Table 1 presents the specific graphical symbols associated with figures 1, 2 and 3 and which
are defined for the purpose of this report. Arrester designations and details on their design and
specific roles are presented in clause 9.
DC line / cable
Valve
arrester
[DR]
[V]
DC reactor
Bridge
DC bus DC line/cable
arrester
arrester
arrester arrester
[V]
[B] [CB]
[A]
Converter
[V] [DB] [DL]
d.c. bus
arrester
[V]
AC bus
DC filter
[V]
arrester
arrester
[FD]
[V]
Mid-point
[M] d.c. bus
arrester
[V]
AC filter AC reactor
[A]
arrester
arrester
[V]
[SR]
Neutral bus Electrode line
[FA]
Neutral bus
arrester
[E]
IEC  1610/02
Figure 1 – Single line diagram of typical converter pole
with two 12-pulse converters in series

– 14 – TS 60071-5  IEC:2002(E)
DC line/cable
Valve
arrester
[CC]
[DR]
[V]
DC reactor
Bridge DC bus DC line
arrester
arrester arrester
arrester
[V]
Capacitor
[B] [CB]
arrester
[A] Converter
[V] [DB] [DL]
d.c. bus
arrester
[V]
[CC]
DC filter
AC bus
[V]
arrester arrester
[FD]
[V]
Mid-point
[CC]
d.c. bus
[M]
arrester
[V]
[A]
AC reactor
AC filter
arrester arrester
[V]
[SR]
[CC]
Neutral bus Electrode line
[FA]
Neutral bus
arrester
[E]
IEC  1611/02
Figure 2 – Single line diagram of typical capacitor commutated converter (CCC) pole
with two 12-pulse converters in series
Valve
arrester
DC line/cable
[DR]
[V]
DC reactor
DC bus
Bridge DC line
arrester
arrester
arrester arrester
[V]
[B]
[CB]
Converter
[A]
[V] [DB] [DL]
d.c. bus
arrester
[SC]   [CSC]
[V]
AC bus DC filter
[V]
arrester
arrester
[FD]
[V]
Mid-point
[M] d.c. bus
arrester
[V]
AC filter AC reactor
[A]
arrester
arrester
[V]
[SR]
Neutral bus Electrode line
[FA]
Neutral bus
arrester
[E]
IEC  1612/02
Figure 3 – Single line diagram of typical controlled series
compensated converter (CSCC) pole with two 12-pulse converters in series
Table 1 – Symbol description
Symbol Description
Valve (commutation group)
Valve (one arm)
Arrester
Resistor
Reactor
Capacitor
Transformer with two windings
Earth (ground)
TS 60071-5  IEC:2002(E) – 15 –
5 Principles of insulation co-ordination
The primary objectives of insulation co-ordination are
– to establish 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 those for
protective devices, used in order to ensure a safe, economic and reliable installation in the
event of the above overvoltages.
5.1 Essential differences between a.c. and d.c. systems
In terms of the above objectives, insulation co-ordination applied to an HVDC converter station
is basically the same in principle as that of an a.c. substation. However, essential differences
exist which warrant particular consideration when dealing with HVDC converter station
insulation co-ordination. For example, there is a need to consider the following:
• 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;
• 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 (see also 9.4.3);
• the presence of reactive power sources and harmonic filters on both the a.c. and d.c. sides;
• the presence of converter transformers with two major windings including the valve side
winding floating from earth potential when the valves are not conducting, and a d.c.
component of current flowing when the valves are conducting;
• the characteristics of the converter valves, including their controls;
• the impact of control and protection in reducing overvoltages;
• voltage polarity effects of d.c. stress which, by attracting greater contaminants to the d.c.
insulation because of constant polarity, lead to greater creepage and clearance
requirements and to worse pollution and flashover performance compared with a.c.
insulation under the same environment;
• long overhead transmission lines and cables without intervening switching stations;
• interaction between the a.c. and d.c. systems, particularly where the a.c. system is
relatively weak;
• composite continuous operating voltages which include in some cases direct voltage,
fundamental frequency voltage, harmonic voltages and high frequency components;
• the various operating modes of the converter such as monopolar, bipolar, parallel or multi-
terminal.
– 16 – TS 60071-5  IEC:2002(E)
5.2 Insulation co-ordination procedure
Table 2 is a flow chart showing the comparison between the insulation co-ordination procedure
for a.c. systems (refer to figure 1 of IEC 60071-1) and for HVDC converter stations.
The general method of investigation is basically the same for an a.c. scheme as it is for an
HVDC converter station. This requires:
• an evaluation of characteristics of the system and the HVDC converter station;
• an assessment of the nature of the insulation in each equipment;
• the determination of different representative overvoltages;
• consideration of the type of overvoltage protection adopted and of current/energy stresses
imposed to surge arresters and determinant on their design.
However, characteristics of insulation and voltage distribution are different for a.c. and d.c.
systems.
TS 60071-5  IEC:2002(E) – 17 –
Table 2 – Comparison of the selection of withstand voltages for three-phase a.c.
equipment with that for HVDC converter station equipment
Flow chart for the
determination of rated or
Deviations from IEC 60071-1
standard insulation levels
in the selection of withstand
for three-phase a.c.
voltages for HVDC converter
equipment according to
station equipment
IEC 60071-1
System analysis.
System analysis
Same approach as for a.c.
Representative voltages and overvoltages.
Representative voltages
Same approach as for a.c.
and overvoltages
Selection of the insulation
Selection of the insulation meeting the
performance criterion.
meeting the performance
Same approach as for a.c.
criterion
In general, co-ordination withstand voltages
are determined in the same way as for a.c.
Co-ordination
For HVDC converter equipment requiring a
withstand very close protection with surge arresters
(directly protected equipment), co-ordination
voltages
withstand voltages are deduced from a
process involving the determination of the
co-ordination currents
Application of factors to account
Application of factors to account for the
for the differences between type
differences between test conditions and
test conditions and actual
actual service conditions.
service conditions
Same approach as for a.c.
Required withstand voltages.
Required withstand voltages
Same approach as for a.c.
Selection of standard withstand voltages for
a.c. side equipment only.
The present step is skipped for equipment
Selection of standard
on d.c. side because there are no
withstand voltages
standardized withstand voltage levels for
such equipment
Set of standard withstand voltages is
Rated or standard insulation
applicable only for equipment on the a.c. side.
level: set of standard
For equipment on the d.c. side, specified
withstand voltages insulation levels are rounded up to
convenient practical values
– 18 – TS 60071-5  IEC:2002(E)
6 Voltages and overvoltages in service
6.1 Arrangements of arresters
Since the late 1970s, overvoltage protection of HVDC converter stations has been based
exclusively on metal-oxide surge arresters. This is largely due to their superior protection
characteristics compared with the gapped SiC arresters (earlier technology) and their reliable
performance when connected in series or parallel with other arresters. The actual arrangement
of the arresters depends on the configuration of the HVDC converter station and the type of
transmission circuit. The basic criteria used however is that each voltage level and the
equipment connected to it is adequately protected at a cost commensurate with the desired
reliability and equipment withstand capability.
A typical arrester arrangement between the a.c. side of the converter bridges and the d.c.
transmission circuit is shown in figure 4 for a two terminal bipolar HVDC scheme with one 12-
pulse converter per pole. It should be noted however, that some of the arresters may be
deleted, depending upon the overvoltage withstand capability of the equipment connected at
that point, and upon the overvoltage protection afforded by a combination of other arresters
at the same point. For example, the d.c. bus can be protected by a series combination of
the bridge (B) and mid-point d.c. bus (M) arresters, instead of the converter unit d.c.
bus arrester (CB).
AC bus
FA1
9 DC line
...