SIST-TP CLC/TR 60919-2:2011

SLOVENSKI STANDARD
01-januar-2011
Lastnosti visokonapetostnih enosmernih (HVDC) sistemov z vodovno
komutiranimi pretvorniki - 2. del: Okvare in stikanje (IEC/TR 60919-2:2008)
Performance of high-voltage direct current (HVDC) systems with linecommutated
converters - Part 2: Faults and switching (IEC/TR 60919-2:2008)
Betriebsverhalten netzgeführter Stromrichter in Hochspannungsgleichstrom (HGÜ)-
Systemen - Teil 2: Fehler und Schalten (IEC/TR 60919-2:2008)
Fonctionnement des systèmes à courant continu haute tension (CCHT) munis de
convertisseurs commutés par le réseau - Partie 2: Défauts et manoeuvres (CEI/TR
60919-2:2008)
Ta slovenski standard je istoveten z: CLC/TR 60919-2:2010
ICS:
29.200 8VPHUQLNL3UHWYRUQLNL Rectifiers. Convertors.
6WDELOL]LUDQRHOHNWULþQR Stabilized power supply
QDSDMDQMH
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

TECHNICAL REPORT
CLC/TR 60919-2
RAPPORT TECHNIQUE
October 2010
TECHNISCHER BERICHT
ICS 29.200; 29.240.99
English version
Performance of high-voltage direct current (HVDC) systems with line-
commutated converters -
Part 2: Faults and switching
(IEC/TR 60919-2:2008)
Fonctionnement des systèmes à courant Betriebsverhalten netzgeführter
continu haute tension (CCHT) munis de Stromrichter in
convertisseurs commutés par le réseau - Hochspannungsgleichstrom (HGÜ)-
Partie 2: Défauts et manoeuvres Systemen -
Teil 2: Fehler und Schalten
(CEI/TR 60919-2:2008)
(IEC/TR 60919-2:2008)
This Technical Report was approved by CENELEC on 2010-09-17.

CENELEC members are the national electrotechnical committees of Austria, Belgium, Bulgaria, Croatia, Cyprus,
the Czech Republic, Denmark, Estonia, Finland, France, Germany, Greece, Hungary, Iceland, Ireland, Italy,
Latvia, Lithuania, Luxembourg, Malta, the Netherlands, Norway, Poland, Portugal, Romania, Slovakia, Slovenia,
Spain, Sweden, Switzerland and the United Kingdom.

CENELEC
European Committee for Electrotechnical Standardization
Comité Européen de Normalisation Electrotechnique
Europäisches Komitee für Elektrotechnische Normung

Management Centre: Avenue Marnix 17, B - 1000 Brussels

© 2010 CENELEC - All rights of exploitation in any form and by any means reserved worldwide for CENELEC members.
Ref. No. CLC/TR 60919-2:2010 E

Foreword
This Technical Report consists of the text of the International Technical Report IEC/TR 60919-2:2008
prepared by SC 22F, Power electronics for electrical transmission and distribution systems, of
IEC TC 22, Power electronic systems and equipment.
It was circulated for voting in accordance with the Internal Regulations, Part 2, Subclause 11.4.3.3
(simple majority) and was accepted by CENELEC as CLC/TR 60919-2 on 2010-09-17.
Attention is drawn to the possibility that some of the elements of this document may be the subject of
patent rights. CEN and CENELEC shall not be held responsible for identifying any or all such patent
rights.
Annex ZA has been added by CENELEC.
__________
Endorsement notice
The text of the Technical Report IEC/TR 60919-2:2008 was approved by CENELEC as a Technical
Report without any modification.
In the official version, for Bibliography, the following notes have to be added for the standards
indicated:
IEC 60071-1:2006 NOTE  Harmonized as EN 60071-1:2006 (not modified).
IEC 60071-2:1996 NOTE  Harmonized as EN 60071-2:1997 (not modified).
IEC 60076 series NOTE  Harmonized in EN 60076 series (partially modified).
IEC 60044-5:2004 NOTE  Harmonized as EN 60044-5:2004 (not modified).
IEC 60076-6:2007 NOTE  Harmonized as EN 60076-6:2008 (not modified).
IEC 60505:2004 NOTE  Harmonized as EN 60505:2004 (not modified).
__________
– 3 – CLC/TR 60919-2:2010
Annex ZA
(normative)
Normative references to international publications
with their corresponding European publications
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.
NOTE  Where an International Publication has been modified by common modifications, indicated by (mod), the relevant
EN/HD applies.
Publication Year Title EN/HD Year
IEC 60071-1 - Insulation co-ordination - - -
Part 1: Terms, definitions, principles and rules
IEC 60146-1-1 1991 Semiconductor convertors - General EN 60146-1-1 1993
+ A1 1996 requirements and line commutated convertors - + A1 1997
Part 1-1: Specifications of basic requirements
IEC/TR 60146-1-2 - Semiconductor convertors - General - -
requirements and line commutated convertors -
Part 1-2: Application guide
IEC 60146-1-3 - Semiconductor convertors - General EN 60146-1-3 -
requirements and line commutated convertors -
Part 1-3: Transformers and reactors
IEC 60633 - Terminology for high-voltage direct current EN 60633 -
(HVDC) transmission
IEC 60700-1 - Thyristor valves for high voltage direct current EN 60700-1 -
(HVDC) power transmission -
Part 1: Electrical testing
IEC/TR 60919-1 2005 Performance of high-voltage direct current CLC/TR 6091-1 2005
(HVDC) systems with line-commutated
converters -
Part 1: Steady-state conditions
IEC 60919-3 - Performance of high-voltage direct current - -
(HVDC) systems -
Part 3: Dynamic conditions
IEC/TR 60919-2
Edition 2.0 2008-11
TECHNICAL
REPORT
RAPPORT
TECHNIQUE
Performance of high-voltage direct current (HVDC) systems with line-
commutated converters –
Part 2: Faults and switching
Fonctionnement des systèmes à courant continu haute tension (CCHT) munis
de convertisseurs commutés par le réseau –
Partie 2: Défauts et manœuvres

INTERNATIONAL
ELECTROTECHNICAL
COMMISSION
COMMISSION
ELECTROTECHNIQUE
PRICE CODE
INTERNATIONALE
XB
CODE PRIX
ICS 29.200; 29.240.99 ISBN 2-8318-1006-0
– 2 – TR 60919-2 © IEC:2008
CONTENTS
FOREWORD.6
1 Scope.8
2 Normative references .9
3 Outline of HVDC transient performance specifications .9
3.1 Transient performance specifications .9
3.2 General comment .10
4 Switching transients without faults.10
4.1 General .10
4.2 Energization and de-energization of a.c. side equipment .10
4.3 Load rejection .12
4.4 Start-up and shut-down of converter units .13
4.5 Operation of d.c. breakers and d.c. switches .13
5 AC system faults .15
5.1 General .15
5.2 Fault categories .16
5.3 Specification matters affecting transient performance.16
5.3.1 Effective a.c. system impedance.16
5.3.2 Power transfer during faults.16
5.3.3 Recovery following fault clearing .17
5.3.4 Reactive power consumption during fault and post-fault recovery
periods .18
5.3.5 Load rejection due to a.c. faults.18
5.3.6 Switching of reactive power equipment .19
5.3.7 Effects of harmonic voltages and current during faults .19
5.3.8 Shift in control modes of operation .19
5.3.9 Power modulation on the HVDC system.20
5.3.10 Emergency power reductions.20
5.4 Specification impact on control strategy .20
6 AC filters, reactive power equipment and a.c. bus faults .21
6.1 General .21
6.2 Transient overvoltages in filter banks .21
6.3 Transient overcurrents in filter and capacitor banks.22
6.4 Capacitor unbalance protection .23
6.5 Examples of protection of filters and capacitor banks .23
6.6 Shunt reactor protection .24
6.7 AC bus protection.24
7 Converter unit faults .27
7.1 General .27
7.2 Short circuits .27
7.3 Failure of converter unit to perform its intended function .28
7.3.1 General .28
7.3.2 Rectifier operation .28
7.3.3 Inverter operation .29
7.4 Converter unit protection .29
7.4.1 Converter differential protection.29
7.4.2 Overcurrent protection.29

TR 60919-2 © IEC:2008 – 3 –
7.4.3 AC overvoltage protection .29
7.4.4 Protection against large delay angle operation .29
7.4.5 Commutation failure protection .30
7.4.6 Thyristor valve protections.30
7.4.7 Transformer protection .30
7.4.8 Transformer tap-changer unbalance protection.30
7.4.9 AC connection earth fault protection .30
7.5 Additional protection aspects of series connected converter units .30
7.6 Additional protection aspects of parallel connected converter units .31
8 DC reactor, d.c. filter and other d.c. equipment faults .33
8.1 General .33
8.2 Fault types .34
8.3 Protection zones .34
8.4 Neutral protection.34
8.4.1 General .34
8.4.2 Neutral fault detection .34
8.4.3 Neutral bus fault isolation .35
8.4.4 Bipolar neutral bus faults .35
8.5 DC reactor protection .35
8.6 DC harmonic filter protection .35
8.6.1 General .35
8.6.2 Filter bank fault protection .36
8.6.3 DC filter capacitor unit protection.36
8.7 DC harmonic protection.36
8.8 DC overvoltage protection .36
8.9 DC side switching protection .37
9 DC line faults.38
9.1 Overhead line faults .38
9.2 Cable faults .39
9.3 DC fault characteristics .39
9.4 Functional d.c. fault detection requirements .40
9.5 Protective sequence.40
9.5.1 Overhead line faults .40
9.5.2 Faults in cable systems .40
9.5.3 Faults in an overhead line/cable system .40
9.5.4 Faults in one of a system of parallel-connected cables .40
9.5.5 Fault in a system of parallel overhead lines .41
9.6 Fault protection schemes .41
9.7 Open circuit on the d.c. side .42
9.8 Power line cross protection .42
10 Earth electrode line faults.42
10.1 General .42
10.2 Specific requirements – Earth electrode line.42
10.3 Electrode line supervision .43
11 Metallic return conductor faults.43
11.1 Conductor for the return circuit .43
11.2 Metallic return faults.43
11.3 Fault detection – Metallic return .44

– 4 – TR 60919-2 © IEC:2008
11.4 Metallic return fault protection systems.44
12 Insulation co-ordination – HVDC systems .47
12.1 General .47
12.2 Protection schemes using surge arresters .47
12.3 Switching overvoltages and temporary overvoltages on the a.c. side .48
12.4 Switching overvoltages and temporary overvoltages on the d.c. side .48
12.5 Lightning and steep fronted surges .48
12.6 Protective margins.49
12.7 Arrester duties .50
12.7.1 AC bus arresters (A , A and A ).50
1 2 3
12.7.2 Arrester across filter reactors (FA).50
12.7.3 Valve arresters (V) .51
12.7.4 Mid-point d.c. bus arrester (M).51
12.7.5 Converter unit d.c. bus arresters (CB) and converter unit arresters.51
12.7.6 DC bus and d.c. line arresters (DB and DL) .51
12.7.7 Neutral bus arresters (E and E ) .52
1 2
12.7.8 DC reactor arrester (R).52
12.7.9 DC filter arresters (FD) .52
12.8 Prevention of protective relay action due to arrester currents.52
12.9 Insulation clearances.52
12.10 Creepage distances for the insulation .52
12.10.1 Outdoor insulation .52
12.10.2 Indoor insulation.53
13 Telecommunication requirements .56
13.1 General .56
13.2 Specific requirements - Telecommunication systems.56
13.3 Consequence of telecommunication system outages .57
13.4 Special considerations for power line carrier (PLC) systems.57
14 Auxiliary systems.58
14.1 General .58
14.2 Electrical auxiliary systems .58
14.2.1 General requirements .58
14.2.2 Specific requirements .59
14.3 Mechanical auxiliary systems .59
Bibliography.61

Figure 1 – DC-side switches for an HVDC substation with series-connected converter unit.15
Figure 2 – Example of voltage dependent control characteristics .21
Figure 3 – Example of arrangement of a.c. filters and capacitor and reactor banks for
large bipolar HVDC.25
Figure 4 – Example of current transformer arrangements for a.c. filters and a.c. bus
differential protections .25
Figure 5 – Example of restricted ground fault protection of filter.26
Figure 6 – Example of current transformers arrangement for capacitor bank unbalance
protecttion and overload protection of double tuned filter arm .26
Figure 7 – Examples of a.c. phase short circuits, pole short circuits and faults in a
twelve-pulse converter unit .32
Figure 8 – Protection zones in series-connected converter units .33

TR 60919-2 © IEC:2008 – 5 –
Figure 9 – Protection zones in parallel-connected converter units .33
Figure 10 – Example of d.c. protection zones for series-connected converter units .37
Figure 11 – Example of d.c. protection zones for parallel-connected converter pole.38
Figure 12 – Monopolar metallic return system showing metallic return transfer breaker
(MRTB).45
Figure 13 – Monopolar operation of a bipolar system during converter pole outages.45
Figure 14 – DC current flowing into an a.c. system during a fault on a metallic return
conductor when the HVDC substation mat is used for grounding of the d.c. circuit.45
Figure 15 – Earth current flowing during line faults .46
Figure 16 – Example of metallic return fault detection system by means of auxiliary a.c.
signal.46
Figure 17 – Example of use of MRTB to quench fault to earth on metallic return
conductor.47
Figure 18 – Example of an arrester protection schene for an HVDC substation .53
Figure 19 – Example of a d.c. arrester protection scheme for a back to back HVDC
substation .54
Figure 20 –Example of an arrester protection arrangement for a capacitor commutated
converter HVDC substation.54
Figure 21 – Example of an a.c. arrester protection arrangement for an HVDC
substation .55
Figure 22 – Example of an arrester protection scheme in a HVDC substation with
series-connected converters. .55

– 6 – TR 60919-2 © IEC:2008
INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
PERFORMANCE OF HIGH-VOLTAGE DIRECT CURRENT
(HVDC) SYSTEMS WITH LINE-COMMUTATED CONVERTERS –

Part 2: Faults and switching
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 provides no marking procedure to indicate its approval and cannot be rendered responsible for any
equipment declared to be in conformity with an IEC Publication.
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.
The main task of IEC technical committees is to prepare International Standards. However, a
technical committee may propose the publication of a technical report when it has collected
data of a different kind from that which is normally published as an International Standard, for
example "state of the art".
IEC 60919-2, which is a technical report, has been prepared by subcommittee 22F: Power
electronics for electrical transmission and distribution systems, of IEC technical committee 22:
Power electronic systems and equipment.
This second edition cancels and replaces the first edition, published in 1991, and constitutes
a technical revision.
This edition includes the following main changes with respect to the previous edition:
a) this report concerns only line-commutated converters;
b) significant changes have been made to the control system technology;

TR 60919-2 © IEC:2008 – 7 –
c) some environmental constraints, for example audible noise limits, have been added;
d) the capacitor coupled converters (CCC) and controlled series capacitor converters (CSCC)
have been included.
The text of this technical report is based on the following documents:
Enquiry draft Report on voting
22F/160/DTR 22F/165/RVC
Full information on the voting for the approval of this technical report 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 of the IEC 60919 series, under the general title: Performance of high-voltage
direct current (HVDC) systems with line-commutated converters, can be found on the IEC
website.
The committee has decided that the contents of this publication will remain unchanged until
the maintenance result 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.
– 8 – TR 60919-2 © IEC:2008
PERFORMANCE OF HIGH-VOLTAGE DIRECT CURRENT
(HVDC) SYSTEMS WITH LINE-COMMUTATED CONVERTERS –

Part 2: Faults and switching
1 Scope
This part of IEC 60919 which is a technical report provides guidance on the transient
performance and fault protection requirements of high voltage direct current (HVDC) systems.
It concerns the transient performance related to faults and switching for two-terminal HVDC
systems utilizing 12-pulse converter units comprised of three-phase bridge (double way)
connections but it does not cover multi-terminal HVDC transmission systems. However,
certain aspects of parallel converters and parallel lines, if part of a two-terminal system， are
discussed. The converters are assumed to use thyristor valves as the bridge arms, with
gapless metal oxide arresters for insulation co-ordination and to have power flow capability in
both directions. Diode valves are not considered in this report.
Only line-commutated converters are covered in this report, which includes capacitor
commutated converter circuit configurations. General requirements for semiconductor line-
commutated converters are given in IEC 60146-1-1, IEC 60146-1-2 and IEC 60146-1-3.
Voltage-sourced converters are not considered.
The report is comprised of three parts. IEC 60919-2, which covers transient performance, will
be accompanied by companion documents, IEC 60919-1 for steady-state performance and
IEC 60919-3 for dynamic performance. An effort has been made to avoid duplication in the
three parts. Consequently users of this report are urged to consider all three parts when
preparing a specification for purchase of a two-terminal HVDC system.
Readers are cautioned to be aware of the difference between system performance
specifications and equipment design specifications for individual components of a system.
While equipment specifications and testing requirements are not defined herein, attention is
drawn to those which could affect performance specifications for a system. Note that detailed
seismic performance requirements are excluded from this technical report. In addition,
because of the many possible variations between different HVDC systems, these are not
considered in detail. Consequently this report should not be used directly as a specification
for a specific project, but rather to provide the basis for an appropriate specification tailored to
fit actual system requirements for a particular electric power transmission scheme. This report
does not intend to discriminate the responsibility of users and manufacturers for the work
specified.
Terms and definitions for high-voltage direct current (HVDC) transmission used in this report
are given in IEC 60633.
Since the equipment items are usually separately specified and purchased, the HVDC
transmission line, earth electrode line and earth electrode are included only because of their
influence on the HVDC system performance.
For the purpose of this report, an HVDC substation is assumed to consist of one or more
converter units installed in a single location together with buildings, reactors, filters, reactive
power supply, control, monitoring, protective, measuring and auxiliary equipment. While there
is no discussion of a.c. switching substations in this report, a.c. filters and reactive power
sources are included, although they may be connected to an a.c. bus separate from the HVDC
substation.
TR 60919-2 © IEC:2008 – 9 –
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 60146-1-1, Semiconductor converters – General requirements and line commutated
converters – Part 1-1: Specifications of basic requirements
Amendment 1 (1996)
IEC 60146-1-2, Semiconductor converters – General requirements and line commutated
converters – Part 1-2: Application guide
IEC 60146-1-3, Semiconductor converters – General requirements and line commutated
converters – Part 1-3: Transformers and reactors
IEC 60633, Terminology for high-voltage direct current (HVDC) transmission
IEC 60071-1, Insulation co-ordination – Part 1: Terms, definitions, principles and rules
IEC 60700-1, Thyristor valves for high-voltage direct current (HVDC) power transmission –
Part 1: Electrical testing
IEC/TR 60919-1:2005, Performance of high-voltage direct current (HVDC) systems with line-
commutated converters – Part 1: Steady-state conditions
IEC 60919-3, Performance of high-voltage direct current (HVDC) systems with line-
commutated converters – Part 3: Dynamic conditions
3 Outline of HVDC transient performance specifications
3.1 Transient performance specifications
A complete performance specification related to transient performance of an HVDC system
during faults and switching should also include fault protection requirements.
These concepts are introduced at the appropriate locations in the following transient
performance and related clauses:
– Clause 4 – Switching transients without faults
– Clause 5 – AC system faults
– Clause 6 – AC filter, reactive power equipment and a.c. bus faults
– Clause 7 – Converter unit faults
– Clause 8 – DC reactor, d.c. filter and other d.c. equipment faults
– Clause 9 – DC line faults
– Clause 10 – Earth electrode line faults
– Clause 11 – Metallic return conductor faults
– Clause 12 – Insulation co-ordination - HVDC systems
– Clause 13 – Telecommunication requirements
– Clause 14 – Auxiliary systems

– 10 – TR 60919-2 © IEC:2008
Discussion in the following clauses on the d.c. line, earth electrode line and earth electrode is
limited to the relationships between these and either the transient performance or protection
of HVDC converter stations.
3.2 General comment
In general, control strategies can be used to minimize the effect of disturbances, but when the
safety of equipment depends on their correct performance, this should be identified.
4 Switching transients without faults
4.1 General
This clause deals with the transient behaviour of the HVDC system during and after switching
operations both on the a.c. and the d.c. sides of converter substations, and is not related to
equipment or line faults which are treated in the following clauses of this report.
Switching operations without faults can be classified as follows:
a) energization and de-energization of a.c. side equipment such as converter transformers,
a.c. filters, shunt reactors, capacitor banks, a.c. lines, static var compensators (SVC), and
synchronous compensators;
b) load rejection;
c) starting and removal from service of converter units;
d) operation of d.c. breakers and d.c. switches for paralleling of poles and lines; connection
or disconnection of d.c. lines (poles), earth electrode lines, metallic return paths, d.c.
filters, etc.
4.2 Energization and de-energization of a.c. side equipment
During the operating life of an HVDC transmission system, energization and de-energization
of converter transformers, a.c. filters, shunt reactors, capacitor banks, SVCs, and other
equipment may occur many times. Depending on the characteristics of the a.c. system and
the equipment being switched, resulting current and voltage stresses will be imposed on
equipment being switched and generally impinge as well on part of the overall a.c. system.
The overvoltages and overcurrents which are critical for plant design are usually due to faults
(Clauses 5 to 9), and not to normal switching operations. Nevertheless, they are discussed
here for completeness. They are relevant in consideration of disturbances to a.c. system
voltages.
Filter switching will also result in transient distortion of the bus voltage. This could disturb the
commutation process and in a weak system could lead to commutation failure.
Thus equipment switching should be investigated to:
– determine critical a.c. network and equipment conditions which may contribute to such
abnormal stresses and actions which may be taken to mitigate them;
– design the equipment;
– verify arrester duties.
Transients occur routinely when filters and capacitor banks are switched as necessary to
control harmonic interference and steady-state terminal voltages.
Because of the frequency of occurrence of switching overvoltages it is generally desirable that
the overvoltage protective devices do not absorb appreciable energy during such operations.
For example the amplitudes of overvoltages arising from routine switching operations can be

TR 60919-2 © IEC:2008 – 11 –
minimized by the use of suitable resistors incorporated in the circuit-breakers associated with
filters and capacitor banks or by synchronizing the closing of the circuit-breakers. This can
also reduce the possibility of inverter commutation failures. The HVDC control system can
also be used effectively to damp certain overvoltages.
Restrike-free switching devices should be used for capacitor switching to avoid onerous
overvoltages from restriking which otherwise could occur when disconnecting filters or
capacitor banks.
Transformer energization inrush currents can cause an undesirable interaction in the a.c.
and d.c. systems. When disconnecting a converter transformer from the ac network, the
transformer should be disconnected maintaining the ac filters connected in parallel if possible,
instead of disconnecting the transformer alone. In that way, residual saturation will be
decreased, and inrush currents would be reduced. After some hundreds of milliseconds the
filters could be disconnected from the transformer.
To reduce inrush currents, typical control measures include circuit-breaker pre-insertion
resistors, using the synchronized circuit-breaker, or setting of the transformer on-load tap
changers at their highest tap changer positions. Highest tap changer position refers to the tap
changer position with highest number of winding turns. Synchronization requires switching at
an optimum instant in each phase, i.e. breaker closing 90 degrees after voltage zero crossing.
This implies that the three poles of a circuit-breaker cannot switch simultaneously. For
breakers with one-pole operating mechanisms (and thus a separate synchronizing unit), this is
not a problem. The synchronizing unit is simply programmed to give switching orders suitably
separated in time to the poles. Some breakers with three-pole operating mechanism can also
be used for synchronized switching if the operating mechanism can be arranged to give a
However it should also be noted that saturation of already energized
mechanical time delay.
converter transformers can arise from energization of another transformer in the converter
station or from switching of an SVC.
Also the application of low order harmonic filters can be helpful in reducing the problems with
inrush currents. The effectiveness of such measures depends largely on the system and
pertinent equipment characteristics. In addition, the response of the a.c. system can be
sensitive to the number of converter transformers already energized, especially if they are not
yet loaded as for series connections of multiple converter units.
Energization of capacitor and filter banks changes the system impedance characteristic. In
case of system with relatively small short circuit capacity, adding capacitive component shifts
high impedance peak of frequency-impedance curve to lower frequency side. If the high
impedance peak becomes closer to second harmonic, severe overvoltages could be
presumed during faults. To mitigate such situation, damping resister could be added to
capacitors.
The energization of capacitor and filter banks produces oscillations between these elements
and the rest of the network. Again, depending on the size of the banks and the network
characteristics, switching overvoltages can appear along with overcurrents in the already
energized a.c. system components.
Attention should be paid to the possibility of damage to the capacitors during re-energization
of capacitors because of trapped charges in the capacitors from a preceding opening
operation. Measures may be necessary for discharging them before reclosing if their internal
discharge resistors are not sufficiently effective within the desired switching time.
Alternatively, a longer switching time may be necessary.
Energization of filters excites the frequencies to which they, in combination with the a.c.
network, are tuned. Also switching out of filter and capacitor banks can cause the a.c.
system voltage to oscillate.
– 12 – TR 60919-2 © IEC:2008
SVCs can be provided to stabilize the voltage and control temporary overvoltages. Ener-
gization of SVCs should be such as to produce a light or even no transient in the system
voltage. Most of them have an active control which can be used to accomplish this objective.
Connection or disconnection of shunt reactors and capacitors produces change in a.c.
voltage. Size and operation of this equipment should be specified so as to limit switching-
caused voltage changes to acceptable levels.
Energization and de-energization of a.c. transmission lines connected to HVDC sub-stations
generate voltage trans
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