IEC 62747:2014

IEC 62747 ®
Edition 1.0 2014-07
INTERNATIONAL
STANDARD
NORME
INTERNATIONALE
colour
inside
Terminology for voltage-sourced converters (VSC) for high-voltage direct
current (HVDC) systems
Terminologie relative aux convertisseurs de source de tension (VSC) des
systèmes en courant continu à haute tension (CCHT)
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IEC 62747 ®
Edition 1.0 2014-07
INTERNATIONAL
STANDARD
NORME
INTERNATIONALE
colour
inside
Terminology for voltage-sourced converters (VSC) for high-voltage direct

current (HVDC) systems
Terminologie relative aux convertisseurs de source de tension (VSC) des

systèmes en courant continu à haute tension (CCHT)

INTERNATIONAL
ELECTROTECHNICAL
COMMISSION
COMMISSION
ELECTROTECHNIQUE
PRICE CODE
INTERNATIONALE
CODE PRIX V
ICS 29.200; 29.240 ISBN 978-2-8322-1702-3

– 2 – IEC 62747:2014  IEC 2014
CONTENTS
FOREWORD . 3
1  Scope . 5
2  Normative references. 5
3  Symbols and abbreviations . 5
3.1  List of letter symbols . 5
3.2  List of subscripts . 6
3.3  List of abbreviations . 7
4  Graphical symbols . 8
5  General terms related to converter circuits . 9
6  VSC topologies . 10
7  Converter units and valves . 10
8  Converter operating conditions. 16
9  HVDC systems and substations . 20
10  HVDC substation equipment . 23
11  Modes of control . 26
12  Control systems . 27
Bibliography . . 30

Figure 1 – Converter symbol identifications . 7
Figure 2 – Graphical symbols . 8
Figure 3 – Voltage-sourced converter unit . 11
Figure 4 – Phase unit of the modular multi-level converter (MMC) in basic half-bridge,
two-level arrangement, with submodules . 13
Figure 5 – Phase unit of the cascaded two-level converter (CTL) in half-bridge form . 14
Figure 6 – Phasor diagram showing a.c. system voltage, converter a.c. voltage and
converter a.c. current . . 18
Figure 7 – Example of bipolar VSC transmission with earth return . 21
Figure 8 – VSC transmission with a symmetrical monopole illustrated with capacitive
earthing on the d.c. side . 22
Figure 9 – VSC transmission with an asymmetrical monopole with metallic return . 22
Figure 10 – VSC transmission with an asymmetrical monopole with earth return . 22
Figure 11 – Major components that may be found in a VSC substation . 25
Figure 12 – Hierarchical structure of an HVDC control system . 29

INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
TERMINOLOGY FOR VOLTAGE-SOURCED CONVERTERS (VSC)
FOR HIGH-VOLTAGE DIRECT CURRENT (HVDC) SYSTEMS

FOREWORD
1) The International Electrotechnical Commission (IEC) is a worldwide organization for standardization comprising
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8) Attention is drawn to the Normative references cited in this publication. Use of the referenced publications is
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9) Attention is drawn to the possibility that some of the elements of this IEC Publication may be the subject of
patent rights. IEC shall not be held responsible for identifying any or all such patent rights.
International Standard IEC 62747 has been prepared by subcommittee 22F: Power
electronics for electrical transmission and distribution systems, of IEC technical committee 22:
Power electronic systems and equipment.
The text of this standard is based on the following documents:
CDV Report on voting
22F/301/CDV 22F/317A/RVC
Full information on the voting for the approval of this standard can be found in the report on
voting indicated in the above table.
This publication has been drafted in accordance with the ISO/IEC Directives, Part 2.

– 4 – IEC 62747:2014  IEC 2014
The committee has decided that the contents of this publication will remain unchanged until
the stability date indicated on the IEC web site under "http://webstore.iec.ch" in the data
related to the specific publication. At this date, the publication will be
• reconfirmed,
• withdrawn,
• replaced by a revised edition, or
• amended.
The contents of the corrigendum of February 2015 have been included in this copy.

IMPORTANT – The 'colour inside' logo on the cover page of this publication indicates
that it contains colours which are considered to be useful for the correct
understanding of its contents. Users should therefore print this document using a
colour printer.
TERMINOLOGY FOR VOLTAGE-SOURCED CONVERTERS (VSC)
FOR HIGH-VOLTAGE DIRECT CURRENT (HVDC) SYSTEMS

1 Scope
This International Standard defines terms for the subject of self-commutated voltage-sourced
converters used for transmission of power by high voltage direct current (HVDC).
The standard is written mainly for the case of application of insulated gate bipolar transistors
(IGBTs) in voltage sourced converters (VSC) but may also be used for guidance in the event
that other types of semiconductor devices which can both be turned on and turned off by
control action are used.
Line-commutated and current-sourced converters for high-voltage direct current (HVDC)
power transmission systems are specifically excluded from this standard.
2 Normative references
The following documents, in whole or in part, are normatively referenced in this document and
are indispensable for its application. For dated references, only the edition cited applies. For
undated references, the latest edition of the referenced document (including any
amendments) applies.
IEC 60027 (all parts), Letter symbols to be used in electrical technology
IEC 60617, Graphical symbols for diagrams
3 Symbols and abbreviations
3.1 List of letter symbols
Essential terms and definitions necessary for the understanding of this standard are given
here; other terminology is as per relevant parts of IEC 60747.
The list covers only the most frequently used symbols (see Figure 1). IEC 60027 shall be
used for a more complete list of the symbols which have been adopted for static converters.
See also other standards listed in the normative references and the bibliography.
U direct voltage
d
U converter d.c. voltage
dc
U pole-to-earth direct voltage
dpe
U pole-to-pole direct voltage
dpp
U rated pole-to-pole direct voltage
dppN
U rated pole-to-earth direct voltage
dpeN
U line-to-line voltage on line side of interface transformer, r.m.s. value including
L
harmonics
– 6 – IEC 62747:2014  IEC 2014
U line-to-earth voltage on line side of interface transformer, r.m.s. value including
Le
harmonics
U rated value of U
LN L
U line-to-line voltage on valve side of interface transformer, r.m.s. value including
v
harmonics
U line-to-earth voltage on valve side of interface transformer, r.m.s. value including
ve
harmonics
U line-to-line converter voltage, r.m.s. value including harmonics
c
NOTE U is equal to U minus the voltage drop across the phase and valve reactors. However, U has only a
c v c
clear meaning during balanced conditions (steady state).
U line-to-earth converter voltage , r.m.s. value including harmonics
ce
U voltage between terminals of a valve (any defined value)
valve
I direct current (any defined value)
d
I rated direct current
dN
I current on line side of interface transformer, r.m.s. value including harmonics
L
I rated value of I
LN L
I current on valve side of interface transformer, r.m.s. value including harmonics
ν
I current through a valve
νalve
3.2 List of subscripts
0 (zero) at no load
e earth
p pole
N rated value or at rated load
d direct current or voltage
L line side of interface transformer
c converter
v valve side of interface transformer
valve through or across one valve
max maximum
min minimum
n pertaining to harmonic component of order n

Positive d.c. terminal
I I
valve d
U
valve
U U U
Le ve dpe
U
c
U
ce
U
L
I
L I
v
U
dpp
U
v
transformer transformer
line side valve side
I
d
I
valve
Negative d.c. terminal
IEC
Figure 1 – Converter symbol identifications
3.3 List of abbreviations
The following abbreviations are always in capital letters and without dots.
CTL cascaded two-level converter
ERTB earth return transfer breaker
ESCR effective short-circuit ratio
FWD free-wheeling diode
HF high frequency
HVDC high-voltage direct current
IGBT insulated gate bipolar transistor
MMC modular multilevel converter
MRTB metallic return transfer breaker
MTDC multi-terminal HVDC transmission system
MVU multiple valve (unit)
NBS neutral bus switch
NGBS neutral bus grounding switch
PCC point of common coupling
PCC-DC point of common coupling – d.c. side

– 8 – IEC 62747:2014  IEC 2014
SCR short-circuit ratio
VBE valve base electronics
VCU valve control unit
VSC voltage-sourced converter
NOTE Even though the word “breaker” is used in the abbreviations, it does not necessarily imply the ability to
interrupt fault currents.
4 Graphical symbols
Figure 2 shows the specific graphical symbols which are defined only for the purposes of this
standard. IEC 60617 shall be used for a more complete list of the graphical symbols which
have been adopted for static converters.
IEC
Figure 2 – Graphical symbols
5 General terms related to converter circuits
5.1
conversion
in the context of HVDC, the transfer of energy from a.c. to d.c. or vice versa, or a combination
of these operations
5.2
converter
in the context of HVDC, the device employed to transfer of energy from a.c. to d.c. or vice
versa, it connects between three a.c. terminals and two d.c. terminals
5.3
voltage-sourced converter
VSC
electronic a.c./d.c. converter having an essentially smooth d.c. voltage provided by e.g. a
common d.c. link capacitor or distributed d.c. capacitors within the converter arms
5.4
arm
converter arm
part of a converter connecting the a.c. phase terminal with the d.c. pole terminal
5.5
commutation
transfer of current between any two paths with both paths carrying current simultaneously
during this process
5.6
line commutation
method of commutation whereby the commutating voltage is supplied by the a.c. system
5.7
self-commutation
commutation where the commutating voltage is supplied by components within the converter
or the electronic switch
5.8
commutating voltage
voltage which causes the current to commutate, provided either by the system or by a
switching action of valve/semiconductor devices
5.9
commutation inductance
total inductance included in the commutation circuit, in series with the commutating voltage
Note 1 to entry: The commutation inductance is typically referred as stray inductance or loop inductance.
5.10
coupling inductance
equivalent inductance referred to the converter side of the interface transformer between the
point of common coupling (PCC) and the d.c. terminal of the valve

– 10 – IEC 62747:2014  IEC 2014
6 VSC topologies
6.1
two-level converter
converter in which the voltage between the a.c. terminals of the VSC unit (see 7.6) and VSC
unit midpoint (see 7.28) is switched between two discrete d.c. voltage levels
6.2
three-level converter
converter in which the voltage between the a.c. terminals of the VSC unit (see 7.6) and VSC
unit midpoint (see 7.28) is switched between three discrete d.c. voltage levels
6.3
multi-level converter
converter in which the voltage between the a.c. terminals of the VSC unit (see 7.6) and VSC
unit midpoint (see 7.28) is switched between more than three discrete d.c. voltage levels
6.4
modular multi-level converter
MMC
multi-level converter in which each VSC valve (see 7.8, 7.9) consists of a number of MMC
building blocks (see 7.11) connected in series
Note 1 to entry: See also Figure 4.
6.5
cascaded two-level converter
CTL
modular multi-level converter in which each switch position consists of more than one IGBT-
diode pair connected in series
Note 1 to entry: See Figure 5.
7 Converter units and valves
7.1
turn-off semiconductor device
controllable semiconductor device which may be turned on and off by a control signal, for
example an IGBT
7.2
insulated gate bipolar transistor
IGBT
turn-off semiconductor device with three terminals: a gate terminal (G) and two load terminals
emitter (E) and collector (C)
7.3
free-wheeling diode
FWD
power semiconductor device with diode characteristic
Note 1 to entry: A FWD has two terminals: an anode (A) and a cathode (K).
Note 2 to entry: The current through FWDs is in the opposite direction to the IGBT current.
7.4
IGBT-diode pair
arrangement of IGBT and FWD connected in inverse parallel

Note 1 to entry: An IGBT-diode pair is usually in one common package, however, it can include individual IGBTs
and/or diodes packages connected in parallel.
7.5
converter unit
indivisible operative unit comprising all equipment between the point of common coupling on
the a.c. side (see 9.25) and the point of common coupling – d.c. side (see 9.26), essentially
one or more VSC units, together with one or more interface transformers, converter unit
control equipment, essential protective and switching devices and auxiliaries, if any, used for
conversion
Note 1 to entry: See Figure 3.
7.6
VSC unit
three VSC phase units, together with VSC unit control equipment, essential protective and
switching devices, d.c. storage capacitors, phase reactors and auxiliaries, if any, used for
conversion
Note 1 to entry: See Figure 3.
7.7
VSC phase unit
equipment used to connect the two d.c. terminals to one a.c. terminal
Note 1 to entry: In the simplest implementation, the VSC phase unit consists of two VSC valves, and in some
case, it may include also valve reactors. The VSC phase unit may also include control and protection equipment,
and other components.
7.8
VSC valve
arrangement of IGBT-diode pairs connected in series and arranged to be
switched simultaneously as a single function unit
VSC valveVSC valve VSC valveVSC valve VSC valveVSC valve
D.c.
capacitor
ValveValve ValveValve ValveValve
reactorsreactors reactorsreactors reactorsreactors
PhasePhase PhasePhase PhasePhase
reactorreactor reactorreactor reactorreactor
VSC valveVSC valve VSC valveVSC valve VSC valveVSC valve
Phase PhasePhase PhasePhase
unit unitunit unitunit
D.c. Protective Dynamic
Converter control
capacitor devices braking valve
IEC
Figure 3 – Voltage-sourced converter unit
Note 1 to entry: In some designs of VSC, the phase reactors may fulfill part of the function of the converter-side
high frequency filter. In addition, in some designs of VSC, part or all of the phase reactor may be built into the
three “phase units” of the VSC unit, as “valve reactors”.
Note 2 to entry: In some designs of VSC, the VSC d.c. capacitor may be partly or entirely distributed amongst the
three “phase units” of the VSC unit, where it is referred to as d.c. submodule capacitors.
Note 3 to entry: Valve and/or phase reactors shown above show optional configurations which may not be
included in all schemes.
– 12 – IEC 62747:2014  IEC 2014
Note 4 to entry: Just a typical example of how a VSC unit could look like is shown in Figure 3, differences may
exist at all levels.
7.9
VSC valve
complete controllable voltage source assembly, which is
generally connected between one a.c. terminal and one d.c. terminal
7.10
VSC valve level
the smallest indivisible functional unit of VSC valve
Note 1 to entry: For any VSC valve in which IGBTs are connected in series and operated simultaneously, one
VSC valve level is one IGBT-diode pair including its auxiliaries (see Figure 4). For MMC type without IGBT-diode
pairs connected in series, one valve level is one submodule together with its auxiliaries (see Figure 5).
7.11
MMC building block
self-contained, two-terminal controllable voltage source together with d.c. capacitor(s) and
immediate auxiliaries, forming part of a MMC
7.12
switch position
semiconductor function which behaves as a single, indivisible switch
Note 1 to entry: A switch position may consist of a single IGBT-diode pair or, in the case of the Cascaded Two
Level converter, a series connection of multiple IGBT-diode pairs.
7.13
submodule
MMC building block where each switch position consists of only one IGBT-diode pair
Note 1 to entry: See Figure 4.

P
Valve
MMC building block
= Submodule
= VSC valve level
Switch
position
AC
Valve
N
IEC
Figure 4 – Phase unit of the modular multi-level converter (MMC)
in basic half-bridge, two-level arrangement, with submodules
7.14
cell
MMC building block where each switch position consists of more than one IGBT-diode pair
connected in series
Note 1 to entry: See Figure 5.

– 14 – IEC 62747:2014  IEC 2014
P
Valve
VSC valve level
MMC building block
= Cell
Switch
position
AC
Valve
N
IEC
Figure 5 – Phase unit of the cascaded two-level converter (CTL) in half-bridge form
7.15
diode valve
semiconductor valve containing only diodes as the main semiconductor devices and
associated circuits and components if any, which might be used in some VSC topologies
7.16
diode valve level
part of a diode valve composed of a diode and associated circuits and components, if any
7.17
dynamic braking valve
complete controllable device assembly, which is used to control energy absorption in a
dynamic braking resistor
7.18
dynamic braking valve level
part of a dynamic braking valve comprising a turn-off semiconductor device and an associated
diode, or controllable switches and diodes connected in parallel, or turn-off semiconductor
devices and diodes connected to a half bridge arrangement, together with their immediate
auxiliaries, storage capacitor, if any
7.19
valve
VSC valve, dynamic braking valve or diode valve according to the context
7.20
redundant levels
the maximum number of series connected VSC valve levels or diode valve levels in a valve
that may be short-circuited externally or internally without affecting the safe operation of the
valve as demonstrated by type tests, and which if and when exceeded, would require
shutdown of the valve to replace the failed levels or acceptance of increased risk of failures
Note 1 to entry: In valve designs such as the cascaded two level converter, which contain two or more conduction
paths within each cell and have series-connected VSC valve levels in each path, redundant levels shall be counted
only in one conduction path in each cell.
7.21
d.c. capacitor
capacitor which is used as part of a voltage-sourced converter which experiences mainly d.c.
voltage between its terminals
Note 1 to entry: For valves of the controllable switch type, the d.c. capacitor is usually arranged as a single
device between the d.c. terminals. For valves of the controllable voltage-sourced type the d.c. capacitor is usually
distributed amongst the MMC building blocks.
7.22
valve reactor
reactor (if any) which is connected in series to a VSC valve of the controllable voltage-source
type
Note 1 to entry: One or more valve reactors can be associated to one VSC valve and might be connected at
different positions within the valve. According to the definition, valve reactors are not part of the VSC valve.
However, it is also possible to integrate the valve reactors in the structural design of the VSC valve, e.g. into each
valve level.
7.23
valve module
the largest factory-assembled and tested building block of the valve, consisting of one or
more VSC valve levels, submodules or cells connected electrically in series
7.24
valve structure
structural components of a valve, required in order to mechanically support the valve modules
7.25
valve support
that part of the valve which mechanically supports and electrically insulates the active part of
the valve from earth
Note 1 to entry: A part of a valve which is clearly identifiable in a discrete form to be a valve support may not
exist in all designs of valves.

– 16 – IEC 62747:2014  IEC 2014
7.26
multiple valve unit
MVU
mechanical arrangement of 2 or more valves or 1 or more VSC phase units sharing a common
valve support
Note 1 to entry: A MVU might not exist in all topologies and physical arrangement of converters.
7.27
valve section
electrical assembly defined for test purposes, comprising a number valve levels and other
components, which exhibits pro-rated electrical properties of a complete valve
Note 1 to entry: For valves of controllable voltage source type, the valve section includes d.c. capacitor in
addition to VSC valve levels.
7.28
VSC unit midpoint
point in a VSC unit whose electrical potential is equal to the average of the potentials of the
positive and negative d.c. terminals of the VSC unit
Note 1 to entry: In some applications, the VSC unit midpoint may exist only as a virtual point, not corresponding
to a physical node in the circuit.
8 Converter operating conditions
8.1
rectifier operation
rectification
mode of operation of a converter or an HVDC substation when energy is transferred from the
a.c. side to the d.c. side
Note 1 to entry: Phasor diagram showing a.c. system voltage, converter a.c. voltage and converter a.c. current for
rectifier operation is shown in Figure 6.
8.2
inverter operation
inversion
mode of operation of a converter or an HVDC substation when energy is transferred from the
d.c. side to the a.c. side
Note 1 to entry: Phasor diagram showing a.c. system voltage, converter a.c. voltage and converter a.c. current for
inverter operation is shown in Figure 6.
8.3
capacitive operation
operation in which the converter feeds reactive power into the a.c. system with or without
exchanging active power
8.4
inductive operation
operation in which the converter consums reactive power from the a.c. system with or without
exchanging active power
8.5
STATCOM operation
mode of operation of a converter when only reactive power (capacitive or inductive) is
exchanged with the a.c. system

8.6
operating state
condition in which the HVDC substation is energized and the converters are de-blocked
Note 1 to entry: Unlike line-commutated converter, VSC can operate with zero active/reactive power output.
8.7
no-load operating state
condition in which the HVDC substation is energized but the IGBTs are blocked and all
necessary substation service loads and auxiliary equipment are connected
8.8
idling operating state
condition in which the HVDC substation is energized and the IGBTs are de-blocked but with
no active or reactive power output at the point of common connection to the a.c. network
Note 1 to entry: The “idling operating” and “no-load” conditions are similar but from the no-load state, several
seconds may be needed before power can be transmitted, while from the idling operating state, power transmission
may be commenced almost immediately (less than 3 power frequency cycles).
Note 2 to entry: In the idling operating state, the converter is capable of actively controlling the d.c. voltage, in
contrast to the no-load state, where the behavior of the converter is essentially “passive”.
Note 3 to entry: Losses will generally be slightly lower in the no-load state than in the idling operating state,
therefore this operating mode is preferred where the arrangement of the VSC system permits it.
8.9
blocked state
condition in which all valves of the VSC unit are blocked
8.10
converter charging
transitional condition of the converter when the a.c. system voltage is applied to the converter
via a pre-insertion resistor
Note 1 to entry: Pre-insertion resistor may not be necessary in all applications.
8.11
modulation index
M
ratio of the peak line to ground a.c. converter voltage, to half of the converter d.c. terminal to
terminal voltage
2U
c1
M
U
dc
3
where
U is the r.m.s value of the fundamental frequency component of the line-to-line voltage U ;
c1 c
U is the output voltage of one VSC phase unit at its a.c. terminal;
c
U is the output voltage of one VSC phase unit at its d.c. terminals.
dc
Note 1 to entry: Some sources define modulation index in a different way such that a modulation index of 1 refers
to a square-wave output, which means that the modulation index can never exceed 1. The modulation index
according to that definition is given simply by M·(/4). However, that definition is relevant mainly to two-level
converters using pulse width modulation (PWM).

– 18 – IEC 62747:2014  IEC 2014
V =jX *I
LI L LI
U
CEI
U
LI
X
L
I
LI δ
R
δ
I
ϕ
R
I
LEI
ϕ
I
V
LI
U
U CEI
LEI
IEC
Figure 6 – Phasor diagram showing a.c. system voltage,
converter a.c. voltage and converter a.c. current
8.12
positive conducting state
condition of an IGBT-diode pair in which load current flows through the IGBT from collector to
emitter
8.13
negative conducting state
condition of an IGBT-diode pair in which load current flows through the free-wheeling diode
from anode to cathode
8.14
positive valve current
direction of current flow through the valve from positive d.c. terminal to negative d.c. terminal
or, in case of a diode valve, in the direction that forward biases the diode valve
8.15
negative valve current
direction of current flow through the valve from negative d.c. terminal to positive d.c. terminal
8.16
positive valve terminal
terminal of the valve that is closest to the positive d.c. terminal of the VSC unit
8.17
negative valve terminal
terminal of the valve that is closest to the negative d.c. terminal of the VSC unit
8.18
valve voltage
potential difference between the positive valve terminal and negative valve terminal
8.19
valve blocking state
condition of a valve when all IGBTs are turned off
8.20
IGBT gating
control action carried out to establish a current or interrupt a current in an IGBT

8.21
short-circuit failure mode
condition of an IGBT in which it is no longer capable of withstanding voltage but can safely
conduct current in either direction
8.22
MMC building block operating states
possible states under which MMC building blocks can be operated
8.22.1
bypassed
operating state where the IGBT(s) of one or more switch positions are turned on such that the
valve current does not flow through the cell/submodule d.c. capacitor
8.22.2
active
operating state where the IGBT(s) of one or more switch positions are turned on such that the
valve current flows through the cell/submodule d.c. capacitor
8.22.3
protectively bypassed
emergency operating state where the valve current flows through a protective device other
than the IGBT(s)/diode(s) in order to prevent damage to the MMC building block or its
components
Note 1 to entry: Protective bypassing may be used for either permanent or temporary conditions depending on the
type of fault.
8.22.4
converter blocking
operation to initiate a mode change from operating state to blocked state of a VSC unit
8.23
valve protective blocking
means of protecting the valve or converter from excessive electrical stress by the emergency
turn-off of all IGBTs in one or more valves
8.24
converter deblocking
operation to initiate a mode change from blocked state to operating state of a VSC unit
8.25
short-circuit ratio
SCR
ratio of the a.c network short-circuit level (in MVA) at 1 p.u. voltage at the point of connection
to the HVDC substation a.c. bus, to the rated d.c. power of the HVDC substation (in MW)
8.26
effective short-circuit ratio
ESCR
ratio of the a.c. network short-circuit level (in MVA) at 1 p.u. voltage at the point of connection
to the HVDC substation a.c. bus, reduced by the reactive power of the shunt capacitor banks
and a.c. filters, if any, connected to this point (in MVAr), to the rated d.c. power of the HVDC
substation (in MW)
– 20 – IEC 62747:2014  IEC 2014
9 HVDC systems and substations
9.1
HVDC system
electrical power system which transfers energy in the form of high-voltage direct current
between two or more a.c. buses
9.2
HVDC transmission system
HVDC system which transfers energy between two or more geographic locations
9.3
two-terminal HVDC transmission system
HVDC transmission system consisting of two HVDC transmission substations and the
connected HVDC transmission line(s)
9.4
multiterminal HVDC transmission system
MTDC
HVDC transmission system consisting of more than two separated HVDC substations and the
interconnecting HVDC transmission lines
9.5
symmetrical monopole
single VSC converter with symmetrical d.c. voltage output on the two terminals
Note 1 to entry: The term “symmetrical monopole” is used even though there are two polarities with d.c. voltages,
because with only one converter it is not possible to provide the redundancy which is normally associated with the
term “bipole”.
9.6
asymmetrical monopole
single VSC converter with asymmetrical d.c. voltage output on the two terminals, normally
with one terminal earthed
9.7
bipole
two or more VSC asymmetrical monopoles forming a bipolar d.c. circuit
9.8
parallel converter configuration
two or more converters located in the same substation and connected to the same a.c. and
d.c. transmission system connected in parallel
9.9
series converter configuration
two or more converters located in the same substation and connected to the same a.c. and
d.c. transmission systems, connected in parallel on the a.c. side and in series in the d.c. side
9.10
bi-directional HVDC system
HVDC system for the transfer of energy in either direction
9.11
uni-directional HVDC system
HVDC system for the transfer of energy in only one direction
Note 1 to entry: Most HVDC systems are inherently bi-directional. However, some systems may be optimized to
transmit power in only one preferred direction. Such systems may still be considered as “bi-directional”.

9.12
HVDC back-to-back system
HVDC system which transfers energy between a.c. buses at the same location
9.13
(HVDC) (system) pole
part of an HVDC system consisting of all the equipment in the HVDC substations and the
interconnecting transmission lines, if any, which during normal operation exhibit a common
direct voltage polarity with respect to earth
Note 1 to entry: See Figure 7.
9.14
(HVDC) (system) bipole
part of an HVDC system consisting of two HVDC system poles, which during normal
operation, exhibit opposite direct voltage polarities with respect to earth
9.15
bipolar (HVDC) system
HVDC system with two independently operable poles of opposite polarity with respect to earth
Pole
DC :Line or cable
~ ~
=
=
~ ~
DC :Line or cable
=
=
Pole
IEC
Figure 7 – Example of bipolar VSC transmission with earth return
9.16
earth return
operation mode in which the return current path between neutrals of the HVDC substations is
through the earth
9.17
metallic return
operation mode in which the return current path between neutrals of the HVDC substations is
through a dedicated conductor
Note 1 to entry: The metallic return conductor may be either a dedicated neutral conductor or another high
voltage conductor.
9.18
monopolar (HVDC system)
HVDC system with only one pole

– 22 – IEC 62747:2014  IEC 2014
9.19
symmetrical monopolar HVDC system
HVDC system consisting of a single converter unit or a parallel connection of two or more
converter units at each substation operated such that the two d.c. output terminals are at
symmetrical voltages with respect to earth
Note 1 to entry: See Figure 8.
IEC
Figure 8 – VSC transmission with a symmetrical monopole illustrated with capacitive
earthing on the d.c. side
9.20
asymmetrical monopolar HVDC system
HVDC system consisting of a single converter unit or a parallel connection of two or more
converter units at each substation operated such that one of the two d.c. output terminals of
at least one substation is earthed
Note 1 to entry: See Figures 9 and 10.
~ ~
=
=
IEC
Figure 9 – VSC transmission with an asymmetrical monopole with metallic return
~ ~
=
=
IEC
Figure 10 – VSC transmission with an asymmetrical monopole with earth return
9.21
HVDC substation
HVDC converter station
part of an HVDC system which consists of one or more converter units installed in a single
location together with buildings, reactors, filters, reactive power supply, co
...