IEC TR 63179:2026

IEC TR 63179 ®
Edition 1.0 2026-01
TECHNICAL
REPORT
Planning of HVDC systems
ICS 29.200; 29.240.01 ISBN 978-2-8327-0938-2

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.

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 - IEC Products & Services Portal - products.iec.ch
webstore.iec.ch/advsearchform Discover our powerful search engine and read freely all the
The advanced search enables to find IEC publications by a
publications previews, graphical symbols and the glossary.
variety of criteria (reference number, text, technical With a subscription you will always have access to up to date
committee, …). It also gives information on projects, content tailored to your needs.
replaced and withdrawn publications.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 22 500 terminological entries in English
details all new publications released. Available online and and French, with equivalent terms in 25 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.
CONTENTS
FOREWORD . 3
1 Scope . 5
2 Normative references . 5
3 Terms and definitions . 5
4 General . 5
5 Selection between HVDC and AC . 7
5.1 Consideration of overall network planning . 7
5.1.1 Overall network planning . 7
5.1.2 Connection topologies . 7
5.2 Consideration of transmission capacity . 8
5.3 Consideration of operation requirements . 8
5.3.1 System fault and stability . 8
5.3.2 Voltage regulation and reactive power compensation . 8
5.4 Consideration of costs . 9
5.5 Consideration of other aspects . 9
6 HVDC solutions . 10
6.1 Main circuit topologies. 10
6.1.1 Asymmetrical monopolar HVDC transmission system . 10
6.1.2 Symmetrical monopole HVDC transmission system . 11
6.1.3 Bipolar HVDC transmission system . 11
6.2 Key DC rating parameters . 13
6.2.1 Nominal DC power . 13
6.2.2 Nominal DC voltage . 13
6.2.3 Nominal DC current . 14
6.3 Converters . 14
6.3.1 General. 14
6.3.2 LCC . 15
6.3.3 VSC . 15
6.3.4 Comparisons between LCC and VSC . 16
6.3.5 Applicable scenarios of LCC and VSC . 19
6.3.6 Converter connection and combination in a project . 19
6.3.7 Converter/interface transformer . 20
6.4 Other main equipment . 21
6.4.1 General. 21
6.4.2 AC filtering equipment . 21
6.4.3 Dynamic braking system . 22
6.4.4 DC circuit breakers . 22
6.5 Line conductor . 22
6.6 Station sites and transmission line routes . 23
6.6.1 HVDC converter station sites . 23
6.6.2 Overhead line route. 24
6.6.3 Electrode station sites . 24
6.6.4 Submarine cable route. 25
6.6.5 Land cable route . 25
7 Interconnection system scheme and stability analysis. 25
7.1 Scheme for HVDC access to AC system . 25
7.2 Interface requirements between AC network and converter station . 26
7.3 Requirements of HVDC control systems . 27
7.3.1 LCC control . 27
7.3.2 VSC control . 27
7.4 Stability analysis. 28
7.4.1 Static analysis . 28
7.4.2 Transient analysis . 31
7.4.3 Dynamic analysis . 31
8 Economic comparison of the alternatives . 33
8.1 General . 33
8.2 Main factors to be considered . 33
8.3 Indexes to be considered. 33
8.4 Sensitivity analysis . 34
8.5 Economic conclusion for recommended solution . 34
9 Study conclusions and recommended solution . 34
Annex A (informative) Introduction to three of the broadband oscillation analysis
methods . 35
A.1 Electromagnetic transient simulation method . 35
A.2 Eigenvalue modelling method . 35
A.3 impedance modelling method . 36
Bibliography . 37

Figure 1 – Phases during integration of a new HVDC system into the electrical power
network . 6
Figure 2 – Procedure for planning an HVDC system . 7
Figure 3 – Asymmetrical monopolar HVDC transmission system with earth return . 10
Figure 4 – Asymmetrical monopolar HVDC transmission system with dedicated metallic
return . 10
Figure 5 – Symmetrical monopolar HVDC transmission system. with AC side ground
impedance . 11
Figure 6 – Bipolar HVDC transmission system with earth return . 12
Figure 7 – Bipolar HVDC transmission system with dedicated metallic return . 12
Figure 8 – Rigid bipolar HVDC transmission system . 13
Figure 9 – Different MMC submodule topologies . 15
Figure 10 – Bipolar configuration of the Baihetan-Jiangsu UHVDC project with LCC
and VSC in the inverter station . 20

Table 1 – Comparison between LCC and VSC HVDC system on technical items . 16

INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
Planning of HVDC systems
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) IEC draws attention to the possibility that the implementation of this document may involve the use of (a)
patent(s). IEC takes no position concerning the evidence, validity or applicability of any claimed patent rights in
respect thereof. As of the date of publication of this document, IEC had not received notice of (a) patent(s), which
may be required to implement this document. However, implementers are cautioned that this may not represent
the latest information, which may be obtained from the patent database available at https://patents.iec.ch. IEC
shall not be held responsible for identifying any or all such patent rights.
IEC TR 63179 has been prepared by IEC technical committee 115: High Voltage Direct Current
(HVDC) transmission for DC voltages above 100 kV. It is a Technical Report.
The text of this Technical Report is based on the following documents:
Draft Report on voting
115/418/DTR 115/427/RVDTR
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 Technical Report 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/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, or
– revised.
1 Scope
This document provides technical information for planning high-voltage direct current (HVDC)
systems with line-commutated converters (LCC), voltage sourced converters (VSC), or both. It
provides general principles for deciding between HVDC and AC transmission systems, as well
as processes and methods for preliminarily defining the HVDC transmission scheme, including
selection of converter type and key parameters, grid stability analysis, and technical-economic
comparison among various solutions. In addition, this document gives the objectives to be
achieved in the planning phase.
This document is applicable for planning a point-to-point or a back-to-back HVDC system.
This document can also be used for DC grid systems (including multi-terminal HVDC systems)
as a reference.
This document is not exhaustive. It is possible that there are other specific aspects, that are
particularly important for a specific HVDC project.
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 60633, High-voltage direct current (HVDC) transmission - Vocabulary
IEC 62747, Terminology for voltage-sourced converters (VSC) for high-voltage direct current
(HVDC) systems
3 Terms and definitions
For the purposes of this document, the terms and definitions given in IEC 60633 and IEC 62747
apply.
ISO and IEC maintain terminology databases for use in standardization at the following
addresses:
– IEC Electropedia: available at https://www.electropedia.org/
– ISO Online browsing platform: available at https://www.iso.org/obp
4 General
The HVDC system development and integration cycle can be described in terms of six phases,
as shown in Figure 1.
The main task of HVDC system planning is to develop and select an HVDC scheme based on
the conclusions of electrical power network development planning where the network
requirements are defined. HVDC system planning uses as a minimum the total transmission
capacity and range of connection points previously determined by electrical power network
development planning, taking into account current and future conditions of the power system,
environment, and other contributing factors.
There is a certain degree of repetition and iteration between HVDC system planning and system
design (refer to Figure 1). For the purpose of project feasibility study and scheme comparison,
some investigation would be carried out during the system planning phase, and the detailed
studies and final design would be accomplished during the system design phase.
Figure 1 – Phases during integration of a new HVDC system into the electrical power
network
The work contents and procedure for planning of an HVDC system are as follows:
a) compare HVDC and AC solutions at high level according to the specific requirements (see
Clause 5);
b) when HVDC is the only technically feasible solution, or the use of an HVDC scheme has
overwhelming advantages, a number of alternative HVDC solutions could be investigated
(see Clause 6). When both HVDC and AC alternatives are technically feasible and neither
of them has overwhelming advantages, further analysis would be conducted to confirm the
preferred solution;
c) verify the security of supply and stability of each alternative (see Clause 7);
d) compare the economic efficiency of alternative solutions (see Clause 8);
e) present the recommended solution (see Clause 9).
The above steps in the planning of an HVDC system are shown in Figure 2.
Figure 2 – Procedure for planning an HVDC system
5 Selection between HVDC and AC
5.1 Consideration of overall network planning
5.1.1 Overall network planning
When a new line between two areas is planned, it will be suitable to consider all aspects of
transmission planning, including the current and future power demands, line corridor conditions,
operation and maintenance, energy dispatch and overall costs.
5.1.2 Connection topologies
Generally, an AC system is used for the majority of connections, while an HVDC system is
mainly used in the following two cases:
a) HVDC interconnection between two asynchronous AC power networks;
1) both AC power networks are typical transmission systems;
2) one AC power network is a typical transmission system and one is an islanded AC power
network (e.g. an offshore wind farm).
b) Embedded HVDC system. An embedded HVDC system is an HVDC link with at least two
parts being physically connected within a single AC synchronous network.
In addition, a multi-terminal HVDC link could also be considered both in a) and b) (see above).
5.2 Consideration of transmission capacity
Available transmission power between two networks through an AC transmission line is
approximately given by the following expression:
VV
SR
P= sinθ
(1)
SR
X
SR
where
V and V are the voltages at the sending and receiving ends, respectively;
S R
X is the series reactance between the two ends;
SR
θ is the load angle (phase difference between the two voltages).
SR
To ensure that synchronism between the two networks is maintained following major
disturbances, the load angle is kept low during steady state operation. As a result, the power
transfer capability of the AC line could be smaller than its steady state thermal capability. The
power flow in HVDC is not dependent on the load angles of the buses at rectifier and inverter,
however proper care will be taken when planning HVDC in an embedded system so that parallel
AC line flows are properly accounted during steady state conditions, contingency conditions
and dynamic conditions.
For high-voltage AC cable transmission over certain distances, the capacitive current becomes
a major contributor to the thermal loading of the cable, due to its large shunt capacitance. This
therefore limits the active current that the AC transmission circuit can carry. With DC
transmission, no capacitive current problems occur and therefore the useful load is also
generally only limited by the thermal capability of the cable.
5.3 Consideration of operation requirements
5.3.1 System fault and stability
Contrary to AC transmissions, HVDC transmission systems do not significantly increase the
short-circuit currents in either sending or receiving ends of AC power networks.
HVDC transmission systems have the capability to decouple the AC networks. The faults
causing significant voltage variation or power swings in one side of an HVDC link will not be
transferred to the other side when the HVDC capacity is much smaller than the short-circuit
capacity of the AC power network. The only impact on the other side is a temporary reduction
of power.
An HVDC link does not suffer from the power angle stability problems which frequently occur
with long AC transmission lines. Also, an AC transmission line is sensitive to disturbances of
the imbalanced power in AC power networks, and the power flow within connecting AC lines is
not easy to control, whereas the controllability of an HVDC system can be used to support the
stability of the connected AC networks. However, an HVDC link has a limited short time
overloading capability. For a short time during a transient event, an AC line might be able to
transmit more power than a DC link, even beyond its steady state thermal capacity, while the
transient overload allowed by the HVDC converter stations is usually smaller. It is suitable to
check the overload capabilities of VSC HVDC links during planning, especially the short time
overload capability for contingency conditions.
5.3.2 Voltage regulation and reactive power compensation
An AC transmission line imposes a load-dependent reactive power demand which might impact
the active current rating and might require reactive power compensation at the load end and at
different points along the line, to ensure the desired voltage level and adequate active power
transfer capability. While series or shunt compensation can assist active power transmission
through overhead lines, a technical limit is encountered in the case of transmission through
cables. Even at relatively short distances, the reactive power consumes a large part of the
current carrying capacity of the cable.
Except for the reactive power consumption of LCC HVDC converter stations during operation,
this type of compensation is not essential for HVDC systems and therefore do not present the
same technical limitations in long transmission distance, with no request for special
compensation along the line/cable.
5.4 Consideration of costs
The listed items below will generally be evaluated and compared from a monetary point of view:
a) station costs;
b) line costs;
c) costs due to upgrades of the existing AC network;
d) station and line losses during the life of the project;
e) operational costs;
f) maintenance and refurbishment costs;
g) decommissioning costs;
h) land acquisition and rights of way.
NOTE The above list is not exhaustive.
Generally, for bulk power transfer over long distances, an HVDC transmission project has a
lower cost, whereas an AC transmission project has a lower cost at short distance, given the
same power transfer requirement. There exists a "breakeven distance" at which HVDC and AC
transmission projects have the same cost.
It might not be practical to consider AC cables longer than a certain distance, e.g. 40 km,
without some forms of additional compensation for capacitive currents, but HVDC links using
cables over hundreds of kilometres don't require compensation.
Many factors contribute to the costs of AC and DC transmissions, including ratings, locations,
terrain, and losses, therefore the determination of the actual breakeven distance for a particular
transmission system can be carried out on a case-by-case basis.
5.5 Consideration of other aspects
The items listed below can be evaluated and compared from a strategic point of view, in order
to further assess the most appropriate power transmission solution between AC and HVDC
alternatives:
a) political environment;
b) social impacts;
c) environmental considerations;
d) transmission capacity and integration in the future electrical power networks;
e) human resources for maintenance management and maintenance work;
f) regulatory and statutory requirements.
NOTE The above list is not exhaustive.
6 HVDC solutions
6.1 Main circuit topologies
6.1.1 Asymmetrical monopolar HVDC transmission system
6.1.1.1 Asymmetrical monopolar HVDC transmission system with earth return
This is one of the most cost-effective solutions. This topology is however generally only used
as an emergency operating mode, or as the first stage in construction of a (future) bipolar
system as it presents the following disadvantages:
a) a pole outage means that 100 % of power transfer capability is lost;
b) it requires an electrode line and a continuously operable earth electrode at each end of the
transmission which might cause issues such as corrosion, magnetic field effects, etc., and
might have an impact on the environment and on transformers close to the earth electrodes.
c) This topology is shown in Figure 3.

Figure 3 – Asymmetrical monopolar HVDC transmission system with earth return
NOTE Details about earth electrodes are available in IEC TS 62344.
6.1.1.2 Asymmetrical monopolar HVDC transmission systems with dedicated
metallic return
This topology is generally used:
a) as the first stage in construction of a bipolar system when long term flow of earth current is
not acceptable during the interim period; or
b) if the HVDC transmission line length is short, where electrode lines and earth electrodes
are not economical; or
c) if the earth resistivity is high enough to impose an unacceptable economic penalty; or
d) if the environmental impact due to earth/sea return, such as dryness of land, sea
temperature rise, emission of dissolved gases, etc., is not acceptable.
This topology is shown in Figure 4. Please note that the reference earth can be on any side.

Figure 4 – Asymmetrical monopolar HVDC transmission system with dedicated metallic
return
6.1.2 Symmetrical monopole HVDC transmission system
This topology applies to VSC only and is similar to the monopolar HVDC transmission system
with dedicated metallic return (refer to 6.1.1.2), but with the following features:
a) One converter per pole.
b) The two transmission lines are at the same potential with regard to ground, but with opposite
polarities, achieved by a defined impedance to ground either on DC side or on the valve
side of converter transformers.
This configuration has the following advantages:
c) The interface transformer does not experience DC voltage under normal conditions and
therefore can be designed as a conventional AC transmission transformer.
d) For the same power and current, voltage rating of the converter equipment and the DC
transmission lines, with regard to ground, is halved.
Figure 5 shows a simplified illustration of a symmetrical monopole system with ground
impedance at the valve side of converter transformers.

Figure 5 – Symmetrical monopolar HVDC transmission system. with AC side ground
impedance
6.1.3 Bipolar HVDC transmission system
6.1.3.1 Bipolar HVDC transmission system with earth or dedicated metallic return
The advantages of the bipolar topology compared to monopolar are as follows:
a) lower losses for a given transmitted power;
b) a pole outage means only 50 % of the total power transfer capability of the HVDC link is
lost. Owing to this, a bipole HVDC link is compared to a double circuit AC line;
c) lower earth current flow.
The disadvantages of this topology (compared to monopolar) include the following:
d) higher converter station costs;
e) more converter station equipment and therefore more land usage.
It is noted that the bipolar HVDC system with earth return can be designed such that when one
pole converter is out of service, the healthy pole can use the faulty pole's high-voltage line as
a metallic return to avoid environmental impact.
This topology is shown in Figure 6 and Figure 7. Please note that the reference earth can be
on any side in Figure 7.
Figure 6 – Bipolar HVDC transmission system with earth return

Figure 7 – Bipolar HVDC transmission system with dedicated metallic return
6.1.3.2 Rigid bipolar HVDC system
In this topology, there is no neutral connection between both converter stations. Since only two
(pole) conductors exist, no unbalance current between the two poles is possible. In case of
interruption of power transfer of one converter pole, the current of the other pole also has to be
interrupted (at least for a limited time to allow reconfiguration of the DC circuit).
The advantages of the rigid bipolar topology compared to the conventional bipolar topology
described in 6.1.3.1 are as follows:
a) no environmental impact due to earth current;
b) lower cost;
c) a return conductor or an electrode station is not required.
The disadvantages of the rigid bipolar topology compared to the conventional bipolar topology
described in 6.1.3.1 are as follows:
d) lower availability because the outage on one of the two pole conductors will lead to a bipole
(100 %) power loss and any failure of main equipment, excluding pole conductors, will result
in at least a short total power interruption until re-configuration of the DC circuit with
bypassing failed converter;
e) impossibility to have a scheduled outage of an individual pole including the associated pole
conductor without an outage of the complete bipole, though there is possibility to have a
scheduled outage of an individual pole excluding the associated pole conductor but with a
short total power interruption to re-configure the DC circuit.
This topology is shown in Figure 8. Please note that the reference earth can be on any side.
Figure 8 – Rigid bipolar HVDC transmission system
6.2 Key DC rating parameters
6.2.1 Nominal DC power
Nominal DC power refers to the power transmitted continuously by an HVDC link under the
specified environmental conditions without the redundant cooling equipment in service. The
nominal power is generally defined at the rectifier DC terminals, but can also be defined at the
inverter DC terminals, or at either of the two converter AC terminals. The nominal power of an
HVDC link is decided based on many aspects. Below are three of the key considerations:
a) power levels required to be transmitted in the link, considering the minimum and maximum
power levels according to short and long-term electrical power network planning;
b) overall transmission network considerations, including capacity in the connection points;
c) techno-economic factors to reach a cost-optimized power level.
The maximum continuous and short-term overload capacities will be defined carefully, as these
might impact the redundant cooling requirements for the valves and converter transformers,
and the ratings of main circuit equipment, including valves, the overhead line or cable conductor,
etc. The overload capability of the HVDC transmission system can be determined according to
the stability and reliability requirements of the power system and the overload capability of
valves.
For LCC schemes, the minimum power will be defined as this could have an impact on the rating
of converter equipment and on other power circuit equipment such as smoothing reactor, due
to discontinuous current. In the case of a very weak AC network, it is appropriate for the value
of minimum power to be low enough to keep the network stable when the HVDC system starts.
Further, for LCC HVDC systems, power direction will also be defined. It can be fully bidirectional
(symmetrical ratings), unidirectional, or bidirectional with a reduced power rating in the direction
opposite to the "normal" direction. Specifying a scheme for fully bidirectional power
transmission when this is not actually required could add extra cost due to the needs of
increasing the capacity of reactive power compensation devices and converter transformers at
receiving end of HVDC link.
6.2.2 Nominal DC voltage
The nominal DC voltage is the continuous DC voltage at which the nominal power is to be
transmitted. For an HVDC system, the nominal DC voltage is generally defined at the rectifier
terminals but might also be defined at the inverter terminals or at a point along the DC
transmission line depended on a specific project.
For an HVDC transmission system, the nominal voltage is selected through analysis considering
many aspects including total investment and operation cost including cost of capitalized losses.
Below are three of the key considerations:
a) the transmission nominal power;
b) the manufacturing capability of equipment;
c) techno-economic factors to reach a cost-optimum power level.
For LCC project, the preferred nominal voltage ranges under various transmission powers and
distances are listed in IEC TR 63127.
For VSC projects, the preferred nominal voltage ranges under various transmission powers and
distances could be similar to those of LCC shown in IEC TR 63127. However, a different
nominal voltage might be considered for a specific HVDC project, which might be determined
based on its special situation through studying and evaluating the above three considerations.
Extreme environmental conditions which might adversely affect the insulation capability of the
HVDC link will be considered during the planning stage. Depending on the frequency of
occurrence of these conditions, it might be necessary to temporarily impose a lower operating
limit on the HVDC voltage during the operation of an HVDC system. Under these circumstances,
the required voltage, together with the transmitted power requirements, during reduced voltage
operation will be stated.
For a back-to-back system, nominal DC voltages are generally left to the manufacturer to decide
during planning phase according to their design requirements. It is generally the nominal power
divided by the nominal current. The nominal voltage is determined through the maximum
utilization of current capability of converter valves, as converter station space and converter
cost saving are major concerns.
For preferred DC voltages for HVDC grid applications, refer to IEC TS 63471.
6.2.3 Nominal DC current
For an HVDC transmission system, the nominal current is the nominal power divided by the
nominal DC voltage.
Nominal current is a key factor in the design of all main circuit equipment in the HVDC system,
as the current influences the specification of all these individual components, which requires
an assessment of the physical weight and dimensions, cooling, mounting, and cost. This also
applies to the overhead line or cable conductors. For these reasons the selection of both the
nominal DC current and DC voltage might take all factors into account to achieve a fully
cost-optimized solution for a given power rating.
For a back-to-back system, the nominal DC current will be selected based on the current
carrying capability of the converter equipment. A higher nominal DC current is preferred such
that nominal DC voltage can be lower, thus leading to lower converter equipment costs.
However, this will be subject to other factors such as converter loss evaluation, overload
requirements, etc. This is generally decided in the detailed design phase.
6.3 Converters
6.3.1 General
A converter comprises one or more converter bridges, one or more converter/interface
transformers, converter control equipment, essential protective and switching devices and
auxiliaries, if any, used for conversion, according to IEC 60633 and IEC 62747.
Converters in HVDC projects mainly include two types: LCC or VSC, or both. Other types are
not discussed in this document.
The optimum converter type and topology are selected through their respective characteristics
and suitable application scenarios described below. Particularly, factors that can be taken into
account include, but are not limited to the following:
a) the maximum capacity of a single converter in terms of ability to manufacture and transport;
b) reliability and strength/weakness of the AC power network connected to the HVDC link and
fault ride through capability of the HVDC system;
c) reliability of the HVDC system;
d) cost-effectiveness of the project;
e) flexibility of operation, for example under-voltage operation, STATCOM operation, etc.
especially during maintenance and fault conditions;
f) AC/DC harmonic current/voltage impact and required solution.
The converter type and topology might be selected independently for each converter station.
For example, LCC in rectifier station and VSC in inverter station, however there is less
operational experience (2025) for such combinations.
6.3.2 LCC
LCC has been extensively used for several decades in the worldwide HVDC projects. Its
classification and characteristics of each type will not be elaborated further here, refer to
IEC TR 60919-1 for details.
6.3.3 VSC
VSC mainly includes two-level converters, three-level or cascaded two-level (CTL) converters,
and modular multilevel converters (MMC). Among these types of VSC, MMC is the most widely
used one in HVDC power transmission and distribution.
MMC is composed of several submodules in series. There are two main types of sub-modules,
namely half bridge submodule (HBSM) and full bridge submodule (FBSM). Traditional
topologies of HBSM and FBSM are shown in Figure 9.

a) HBSM b) FBSM
Figure 9 – Different MMC submodule topologies
Compared with the FBSM converter, the HBSM converter uses fewer devices, so it has the
lower expense and the lower power loss. HBSM however cannot suppress DC side fault, so
when applied to overhead line or multi-terminal DC projects, DC circuit breakers might be
required. Otherwise, for clearing DC line fault, the AC circuit breakers for all connected
converters are required to isolate the whole relevant HVDC system from the surrounding AC
power grid. Accordingly, the DC projects are unable to realize fault ride through.
As shown in Figure 9 b), the main improvement with FBSM is the ability to handle DC side
faults. However, such improvements means that FBSM requires twice the number of IGBTs
compared to HBSM; in addition, converter losses also increase.
Therefore, hybrid VSC valve based MMC, which includes full bridge submodules and half bridge
submodules in one arm of MMC, has been used in VSC projects to reduce power loss and limit
DC-short circuit current.
Furthermore, considerable research and development efforts are being invested in VSC sub-
module topology. Several new types of submodule topologies have been proposed, the purpose
of which is to have the fault self-clearing capability compared with HBSM and simultaneously
have the cost advantage compared with FBSM. Some of these types might become available
after the issue of this document.
6.3.4 Comparisons between LCC and VSC
6.3.4.1 Comparison on technical items
Comparisons between LCC and VSC operational capability and performance of typical
evaluation items are summarized in Table 1.
Table 1 – Comparison between LCC and VSC HVDC system on technical items
Evaluation items LCC HVDC system VSC HVDC system Remarks
OHTL: 11 000 MW/Bi-pole OHTL: 5 000 MW/Bi-pole
Nominal
configuration configuration
transmission
Cable：2 200 MW Bi-pole Cable：2 000 MW/Bi-pole
capacity (max.)
configuration configuration
OHTL: ±1 100 kV OHTL: ±800 kV
Nominal DC voltage
level (max.)
Cable: ±600 kV Cable: ±525 kV
OHTL: 5 000 A OHTL: 3 000A
Nominal DC Current
Level (max.)
Cable: 3 000 A Cable: 2 100 A
For FBSM MMC, fast restart is
DC fault ride For LCC, fast restart: fewer available. However, for HBSM-
through for point-to- than several hundreds of MMC, fast restart usually considers

point (PTP) HVDC milliseconds, are possible additional equipment, for example
project with OHTL without additional equipment DCCB, for fault clearing and
sequential restart
CF is possible in LCC and
countermeasure is always
Commutation failure
required to avoid or minimize free of CF
(CF)
its occurrence and mitigate
the consequences
Depending on
Very flexible AC fault ride through
AC fault ride Individual control application electrical power
capability, easier to achieve than
through capability is considered network
for LCC
requirements
Only P control with
compensating Q is available. Simultaneously P & Q control is
P & Q contr
...