SIST EN 62751-2:2014

SLOVENSKI STANDARD
01-december-2014
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Determination of power losses in voltage sourced converter (VSC) valves for high
voltage direct current (HVDC) systems - Part 2: Modular multilevel converters
Bestimmung der Leistungsverluste in Spannungszwischenkreis-Stromrichtern (VSC) für
Hochspannungsgleichstrom(HGÜ)-Systeme -- Teil 2: Modulare Mehrstufen-Stromrichter
Determination des pertes de puissance dans les valves à convertisseur de source de
tension (VSC) des systemes en courant continu a haute tension (CCHT) -- Partie 2:
Convertisseurs multiniveaux modulaires
Ta slovenski standard je istoveten z: EN 62751-2:2014
ICS:
29.200 8VPHUQLNL3UHWYRUQLNL Rectifiers. Convertors.
6WDELOL]LUDQRHOHNWULþQR Stabilized power supply
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29.240.01 2PUHåMD]DSUHQRVLQ Power transmission and
GLVWULEXFLMRHOHNWULþQHHQHUJLMH distribution networks in
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2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

EUROPEAN STANDARD EN 62751-2
NORME EUROPÉENNE
EUROPÄISCHE NORM
October 2014
ICS 29.200; 29.240
English Version
Power losses in voltage sourced converter (VSC) valves for
high-voltage direct current (HVDC) systems - Part 2: Modular
multilevel converters
(IEC 62751-2:2014)
Pertes de puissance dans les valves à convertisseur de Bestimmung der Leistungsverluste in
source de tension (VSC) des systèmes en courant continu Spannungszwischenkreis-Stromrichtern (VSC) für
à haute tension (CCHT) - Partie 2: Convertisseurs Hochspannungsgleichstrom(HGÜ)-Systeme - Teil 2:
multiniveaux modulaires Modulare Mehrstufen-Stromrichter
(CEI 62751-2:2014) (IEC 62751-2:2014)
This European Standard was approved by CENELEC on 2014-10-01. CENELEC members are bound to comply with the CEN/CENELEC
Internal Regulations which stipulate the conditions for giving this European Standard the status of a national standard without any alteration.
Up-to-date lists and bibliographical references concerning such national standards may be obtained on application to the CEN-CENELEC
Management Centre or to any CENELEC member.
This European Standard exists in three official versions (English, French, German). A version in any other language made by translation
under the responsibility of a CENELEC member into its own language and notified to the CEN-CENELEC Management Centre has the
same status as the official versions.
CENELEC members are the national electrotechnical committees of Austria, Belgium, Bulgaria, Croatia, Cyprus, the Czech Republic,
Denmark, Estonia, Finland, Former Yugoslav Republic of Macedonia, France, Germany, Greece, Hungary, Iceland, Ireland, Italy, Latvia,
Lithuania, Luxembourg, Malta, the Netherlands, Norway, Poland, Portugal, Romania, Slovakia, Slovenia, Spain, Sweden, Switzerland,
Turkey and the United Kingdom.

European Committee for Electrotechnical Standardization
Comité Européen de Normalisation Electrotechnique
Europäisches Komitee für Elektrotechnische Normung
CEN-CENELEC Management Centre: Avenue Marnix 17, B-1000 Brussels
© 2014 CENELEC All rights of exploitation in any form and by any means reserved worldwide for CENELEC Members.
Ref. No. EN 62751-2:2014 E
Foreword
The text of document 22F/303/CDV, future edition 1 of IEC 62751-2, prepared by SC 22F "Power
electronics for electrical transmission and distribution systems", of IEC/TC 22 "Power electronic
systems and equipment" was submitted to the IEC-CENELEC parallel vote and approved by
CENELEC as EN 62751-2:2014.
The following dates are fixed:
– latest date by which the document has to be implemented at (dop) 2015-07-01
national level by publication of an identical national
standard or by endorsement
– latest date by which the national standards conflicting with (dow) 2017-10-01
the document have to be withdrawn

Attention is drawn to the possibility that some of the elements of this document may be the subject of
patent rights. CENELEC [and/or CEN] shall not be held responsible for identifying any or all such
patent rights.
Endorsement notice
The text of the International Standard IEC 62751-2:2014 was approved by CENELEC as a European
Standard without any modification.
In the official version, for Bibliography, the following note has to be added for the standard indicated:
IEC 61803:1999 NOTE Harmonised as EN 61803:1999.

- 3 - EN 62751-2:2014
Annex ZA
(normative)
Normative references to international publications
with their corresponding European publications
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.
NOTE 1 When an International Publication has been modified by common modifications, indicated by (mod),
the relevant EN/HD applies.
NOTE 2 Up-to-date information on the latest versions of the European Standards listed in this annex is
available here: www.cenelec.eu.
Publication Year Title EN/HD Year
IEC 60633 -  Terminology for high-voltage direct current EN 60633 -
(HVDC) transmission
IEC 62747 -  Terminology for voltage-sourced EN 62747 -
converters (VSC) for high-voltage direct
current (HVDC) systems
IEC 62751-1 2014 Determination of power losses in voltage EN 62751-1 2014
sourced converter (VSC) valves for high-
voltage direct current (HVDC) systems --
Part 1: General requirements
ISO/IEC Guide 98-3 -  Uncertainty of measurement -- Part-3: - -
Guide to the expression of uncertainty in
measurement (GUM:1995)
IEC 62751-2 ®
Edition 1.0 2014-08
INTERNATIONAL
STANDARD
NORME
INTERNATIONALE
colour
inside
Power losses in voltage sourced converter (VSC) valves for high-voltage direct

current (HVDC) systems –
Part 2: Modular multilevel converters

Pertes de puissance dans les valves à convertisseur de source de tension (VSC)

des systèmes en courant continu à haute tension (CCHT) –

Partie 2: Convertisseurs multiniveaux modulaires

INTERNATIONAL
ELECTROTECHNICAL
COMMISSION
COMMISSION
ELECTROTECHNIQUE
PRICE CODE
INTERNATIONALE
CODE PRIX XA
ICS 29.200; 29.240 ISBN 978-2-8322-1836-5

– 2 – IEC 62751-2:2014 © IEC 2014
CONTENTS
FOREWORD . 5
1 Scope . 7
2 Normative references . 7
3 Terms, definitions, symbols and abbreviated terms . 7
3.1 Terms and definitions . 8
3.2 Symbols and abbreviated terms . 9
3.2.1 Valve and simulation data . 9
3.2.2 Semiconductor device characteristics . 10
3.2.3 Other component characteristics. 10
3.2.4 Operating parameters . 10
3.2.5 Loss parameters . 11
4 General conditions. 11
4.1 General . 11
4.2 Principles for loss determination . 12
4.3 Categories of valve losses . 12
4.4 Loss calculation method . 13
4.5 Input parameters . 13
4.5.1 General . 13
4.5.2 Input data for numerical simulations . 13
4.5.3 Input data coming from numerical simulations . 14
4.5.4 Converter station data . 14
4.5.5 Operating conditions . 15
5 Conduction losses . 15
5.1 General . 15
5.2 IGBT conduction losses . 16
5.3 Diode conduction losses . 17
5.4 Other conduction losses . 18
6 DC voltage-dependent losses . 19
7 Losses in d.c. capacitors of the valve . 19
8 Switching losses . 20
8.1 General . 20
8.2 IGBT switching losses . 20
8.3 Diode switching losses . 21
9 Other losses . 21
9.1 Snubber circuit losses . 21
9.2 Valve electronics power consumption. 22
9.2.1 General . 22
9.2.2 Power supply from off-state voltage across each IGBT . 23
9.2.3 Power supply from the d.c. capacitor . 23
10 Total valve losses per HVDC substation . 24
Annex A (informative) Description of power loss mechanisms in MMC valves . 26
A.1 Introduction to MMC Converter topology . 26
A.2 Valve voltage and current stresses . 29
A.2.1 Simplified analysis with voltage and current in phase . 29
A.2.2 Generalised analysis with voltage and current out of phase . 30

IEC 62751-2:2014 © IEC 2014 – 3 –
A.2.3 Effects of third harmonic injection . 31
A.3 Conduction losses in MMC building blocks . 32
A.3.1 Description of conduction paths . 32
A.3.2 Conduction losses in semiconductors . 38
A.3.3 MMC building block d.c. capacitor losses . 42
A.3.4 Other conduction losses . 42
A.4 Switching losses . 42
A.4.1 Description of state changes . 42
A.4.2 Analysis of state changes during cycle . 44
A.4.3 Worked example of switching losses . 44
A.5 Other losses . 47
A.5.1 Snubber losses . 47
A.5.2 DC voltage-dependent losses . 47
A.5.3 Valve electronics power consumption . 50
A.6 Application to other variants of valve. 52
A.6.1 General . 52
A.6.2 Two-level full-bridge MMC building block . 52
A.6.3 Multi-level MMC building blocks . 53
Bibliography . 55

Figure 1 – Two basic versions of MMC building block designs . 15
Figure 2 – Conduction paths in MMC building blocks . 16
Figure A.1 – Phase unit of the modular multi-level converter (MMC) in basic half-
bridge, two-level arrangement, with submodules . 27
Figure A.2 – Phase unit of the cascaded two-level converter (CTL) in half-bridge form . 28
Figure A.3 – Basic operation of the MMC converters . 29
Figure A.4 – MMC converters showing composition of valve current . 30
Figure A.5 – Phasor diagram showing a.c. system voltage, converter a.c. voltage and
converter a.c. current . 31
rd
Figure A.6 – Effect of 3 harmonic injection on converter voltage and current . 32
Figure A.7 – Two functionally equivalent variants of a “half-bridge”, two-level MMC
building block . 33
Figure A.8 – Conducting states in “half-bridge”, two-level MMC building block . 34
Figure A.9 – Typical patterns of conduction for inverter operation (left) and rectifier
operation (right) . 35
Figure A.10 – Example of converter with only one MMC building block per valve to
illustrate switching behaviour . 36
Figure A.11 – Inverter operation example of switching events . 36
Figure A.12 – Rectifier operation example of switching events . 37
Figure A.13 – Valve current and mean rectified valve current . 39
Figure A.14 – IGBT and diode switching energy as a function of collector current . 43
Figure A.15 – Valve voltage, current and switching behaviour for a hypothetical MMC
valve consisting of 5 submodules . 45
Figure A.16 – Power supply from IGBT terminals . 50
Figure A.17 – Power supply from IGBT terminals in cell . 51
Figure A.18 – Power supply from d.c. capacitor in submodule . 52
Figure A.19 – One “full-bridge”, two-level MMC building block . 52

– 4 – IEC 62751-2:2014 © IEC 2014
Figure A.20 – Four possible variants of three-level MMC building block . 54

Table 1 – Contributions to valve losses in different operating modes . 25
Table A.1 – Hard switching events . 42
Table A.2 – Soft switching events . 44
Table A.3 – Summary of switching events from Figure A.15 . 46

IEC 62751-2:2014 © IEC 2014 – 5 –
INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
POWER LOSSES IN VOLTAGE SOURCED
CONVERTER (VSC) VALVES FOR HIGH-VOLTAGE
DIRECT CURRENT (HVDC) SYSTEMS –

Part 2: Modular multilevel converters

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
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2) The formal decisions or agreements of IEC on technical matters express, as nearly as possible, an international
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5) IEC itself does not provide any attestation of conformity. Independent certification bodies provide conformity
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6) All users should ensure that they have the latest edition of this publication.
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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.
International Standard IEC 62751-2 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/303/CDV 22F/322A/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.

– 6 – IEC 62751-2:2014 © IEC 2014
A list of all parts in the IEC 62751series, published under the general title Power losses in
voltage sourced converter (VSC) valves for high-voltage direct current (HVDC) systems, can
be found on the IEC website.
The committee has decided that the contents of this publication will remain unchanged until
the stability date indicated on the IEC web site under "http://webstore.iec.ch" in the data
related to the specific publication. At this date, the publication will be
• reconfirmed,
• withdrawn,
• replaced by a revised edition, or
• amended.
IMPORTANT – The 'colour inside' logo on the cover page of this publication indicates
that it contains colours which are considered to be useful for the correct
understanding of its contents. Users should therefore print this document using a
colour printer.
IEC 62751-2:2014 © IEC 2014 – 7 –
POWER LOSSES IN VOLTAGE SOURCED
CONVERTER (VSC) VALVES FOR HIGH-VOLTAGE
DIRECT CURRENT (HVDC) SYSTEMS –

Part 2: Modular multilevel converters

1 Scope
This part of IEC 62751 gives the detailed method to be adopted for calculating the power
losses in the valves for an HVDC system based on the “modular multi-level converter”, where
each valve in the converter consists of a number of self-contained, two-terminal controllable
voltage sources connected in series. It is applicable both for the cases where each modular
cell uses only a single turn-off semiconductor device in each switch position, and the case
where each switch position consists of a number of turn-off semiconductor devices in series
(topology also referred to as “cascaded two-level converter”). The main formulae are given for
the two-level “half-bridge” configuration but guidance is also given in Annex A as to how to
extend the results to certain other types of MMC building block configuration.
The standard is written mainly for insulated gate bipolar transistors (IGBTs) but may also be
used for guidance in the event that other types of turn-off semiconductor devices are used.
Power losses in other items of equipment in the HVDC station, apart from the converter
valves, are excluded from the scope of this standard.
This standard does not apply to converter valves for line-commutated converter HVDC
systems.
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 60633, Terminology for high-voltage direct-current (HVDC) transmission
IEC 62747, Terminology for voltage-sourced converters (VSC) for high-voltage direct current
(HVDC) systems
IEC 62751-1:2014, Power losses in voltage sourced converter (VSC) valves for high-voltage
direct current (HVDC) systems – Part 1: General requirements
ISO/IEC Guide 98-3, Uncertainty of measurement – Part 3: Guide to the expression of
uncertainty in measurement (GUM:1995)
3 Terms, definitions, symbols and abbreviated terms
For the purposes of this document, the terms and definitions given in IEC 60633, IEC 62747,
IEC 62751-1, as well as the following apply.

– 8 – IEC 62751-2:2014 © IEC 2014
3.1 Terms and definitions
3.1.1
modular multi-level converter
MMC
multi-level converter in which each VSC valve consists of a number of MMC building blocks
connected in series
Note 1 to entry: This note applies to the French language only.
3.1.2
MMC building block
self-contained, two-terminal controllable voltage source together with d.c. capacitor(s) and
immediate auxiliaries, forming part of a MMC
3.1.3
IGBT-diode pair
arrangement of IGBT and free-wheeling diode connected in inverse parallel
3.1.4
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.
3.1.5
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: This note applies to the French language only.
3.1.6
submodule
MMC building block where each switch position consists of only one IGBT-diode pair
3.1.7
cell
MMC building block where each switch position consists of more than one IGBT-diode pair
connected in series
3.1.8
turn-off semiconductor device
controllable semiconductor device which may be turned on and off by a control signal, for
example an IGBT
3.1.9
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)
Note 1 to entry: This note applies to the French language only.
3.1.10
operating state
condition in which the HVDC substation is energized and the converters are de-blocked

IEC 62751-2:2014 © IEC 2014 – 9 –
Note 1 to entry: Unlike line-commutated converter, VSC can operate with zero active/reactive power output.
3.1.11
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
3.1.12
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.
3.1.13
modulation index of PWM converters
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·(π/4). However, that definition is relevant mainly to two-level
converters using PWM.
3.2 Symbols and abbreviated terms
3.2.1 Valve and simulation data
N number of MMC building blocks per valve
tc
N number of series-connected semiconductor devices per switch position
c
N total number of series resistive elements contributing to conduction losses in the
sr
valve, other than in the IGBTs and diodes
N number of d.c. capacitors in the valve
cv
N number of switching cycles (on or off) experienced by each VSC valve level
s
during the integration time t
i
N total number of parallel resistive elements contributing to d.c. voltage dependent
pr
losses in the valve
N number of snubber circuits per valve
sn
t integration time used in the simulation
i
– 10 – IEC 62751-2:2014 © IEC 2014
3.2.2 Semiconductor device characteristics
V average IGBT threshold voltage for the relevant operating conditions
0T
R average IGBT slope resistance for the relevant operating conditions, valid at the
0T
device terminals
V average diode threshold voltage for the relevant operating conditions
0D
R average diode slope resistance for the relevant operating condition, valid at the
0D
device terminals
E average turn-on energy dissipated in the IGBT for the relevant operating
on
conditions
E average turn-off energy dissipated in the IGBT(s) for the relevant operating
off
conditions
th th
E turn-on energy dissipated in IGBT T1 in the j MMC building block for the k
on,T1_j,k
turn-on event for the relevant operating conditions (voltage, current and junction
temperature)
th th
E turn-on energy dissipated in IGBT T2 in the j MMC building block for the k
on,T2_j,k
turn-on event for the relevant operating conditions (voltage, current and junction
temperature)
th th
E turn-off energy dissipated in IGBT T1 in the j MMC building block for the k
off,T1_j,k
turn-off event for the relevant operating conditions (voltage, current and junction
temperature)
th th
E turn-off energy dissipated in IGBT T2 in the j MMC building block for the k
off,T2_j,k
turn-off event for the relevant operating conditions (voltage, current and junction
temperature)
th
E diode recovery energy dissipated in diode D1 in the j MMC building block for
rec,D1_j,k
th
the k diode turn-off event for the relevant operating conditions (voltage, current
and junction temperature)
th
E diode recovery energy dissipated in diode D2 in the j MMC building block for
rec,D2_j,k
th
the k diode turn-off event for the relevant operating conditions (voltage, current
and junction temperature)
3.2.3 Other component characteristics
th
R total resistance of the k series resistive elements in the valve contributing to
s_k
other conduction losses
th
R resistance of the k parallel resistive component in the valve
dc_k
th
R average equivalent series resistance of the j d.c. capacitor
ESR_j
th th
E energy dissipated in the snubber resistor of the j snubber circuit for the k turn-
sn,on_j,k
on event for the relevant operating conditions (voltage, and current where
relevant to the design of the snubber)
th th
E energy dissipated in the snubber resistor of the j snubber circuit for the k turn-
sn,off_j,k
off event for the relevant operating conditions (voltage, and current where
relevant to the design of the snubber)
3.2.4 Operating parameters
th
I mean current of IGBT T1 in the j MMC building block, averaged over an
T1av_j
integration time t
i
th
I mean current of IGBT T2 in the j MMC building block, averaged over an
T2av_j
integration time t
i
th
I rms current of IGBT T1 in the j MMC building block, averaged over an
T1rms_j
integration time t
i
th
I rms current of IGBT T2 in the j MMC building block, averaged over an
T2rms_j
integration time t
i
IEC 62751-2:2014 © IEC 2014 – 11 –
th
I mean current of diode D1 in the j MMC building block, averaged over an
D1av_j
integration time t
i
th
I mean current of diode D2 in the j MMC building block, averaged over an
D2av_j
integration time t
i
th
I rms current of diode D1 in the j MMC building block, averaged over an
D1rms_j
integration time t
i
th
I rms current of diode D2 in the j MMC building block, averaged over an
D2rms_j
integration time t
i
th
I rms current flowing in the k series resistive element for the relevant operating
rms_k
conditions
th
U rms value (including d.c. component) of the voltage across the k parallel
rms_k
resistive component in the valve
th
I rms current flowing in the j d.c. capacitor of the valve
crms_j
th th
P average power input to the power supply of k IGBT in j MMC building block
GU_j,k
th th
p (t) instantaneous power input to the power supply of k IGBT in j MMC building
GU_j,k
block
th th
u (t) instantaneous voltage input to the power supply of k IGBT in j MMC building
GU_j,k
block
th th
i (t) instantaneous current input to the power supply of k IGBT in j MMC building
GU_j,k
block
th
P average power input to the power supply in j MMC building block
GU_j
th
p (t) instantaneous power input to the power supply in j MMC building block
GU_j
th
u (t) instantaneous voltage input to the power supply in j MMC building block
GU_j
th
i (t) instantaneous current input to the power supply in j MMC building block
GU_j
3.2.5 Loss parameters
P IGBT conduction losses
V1
P diode conduction losses
V2
P other valve conduction losses
V3
P d.c. voltage-dependent losses
V4
P d.c. capacitor losses
V5
P IGBT switching losses
V6
P diode turn-off losses
V7
P snubber losses
V8
P valve electronics power consumption
V9
P total valve losses
Vt
4 General conditions
4.1 General
Modular multi-level converters (MMC) are a family of converters in which each valve forms a
controllable voltage source. The converter a.c. voltage is synthesized by switching large
numbers of relatively small, self-contained, two-terminal controllable voltage sources at
different times, thereby obtaining a high-quality converter waveform with low switching losses
and therefore a high overall efficiency. The MMC building blocks from which the overall
converter is built up may use multiple IGBT-diode pairs connected in series (in which case the
converter is referred to as the “Cascaded two level converter”, CTLC) or only a single IGBT-
diode pair per switch position. A detailed description of these types of converter is beyond the
scope of this standard; however, Annex A includes a general description of the operation of
the MMC (see also IEC TR 62543).

– 12 – IEC 62751-2:2014 © IEC 2014
4.2 Principles for loss determination
Theoretically, the losses of a converter station can be determined either by direct
measurements of the input and output powers or by means of component characteristics,
using suitable mathematical models of the individual components of a converter. The
selection of the principle under which the losses are to be determined shall take into
consideration the uncertainties.
The overall uncertainty of the value of losses is an important parameter for a converter and
for a converter station since it is used to compare investment cost to capitalised cost over the
life-time of the converter station. To ensure that estimates are undisputed, adherence to the
provisions of this standard and the provisions of ISO/IEC Guide 98-3 is indispensable. All
measurements shall furthermore be traceable to national and/or international standards of
measurement.
As mentioned above, a determination of the losses of a converter station could in principle be
performed by making a direct measurement a.c. and d.c. side power. The difference between
these two power values is however small, and a good accuracy will be very difficult to reach.
In practice this measurement would require the use of state-of-the-art measurement
equipment that rivals the best equipment available at national metrology institutes, equipment
that is not intended for on-site use. While not impossible, this method is unlikely to be used.
In rare cases, where there are two converters in a substation, there may be an opportunity to
connect the two converters in a temporary back-to-back configuration and circulate d.c. power
between them, with their total loss being supplied by the a.c. grid. This loss can be measured,
using standard energy meters and voltage transformers, but with special current transformers
that have a rated current that is on the order of 5 % to 10 % of the normal operating current of
the converters in order to reach sufficient accuracy at the power levels to be measured. In
order to enable back-to-back measurements, additional equipment and/or control and
protection enhancements could be needed, which will increase the investment cost of the
converter station.
For most cases, however, the losses have to be estimated from component characteristics,
using suitable mathematical models of the converters, as discussed in this standard. It is
however important that all such estimates have a base in actual measurements having
sufficiently low uncertainty. Care should also be taken to show the propagation of
uncertainties from measurements and how they interact with the model. Estimates of the
uncertainty contributions from imperfections in the models themselves should also be
considered.
4.3 Categories of valve losses
The various components of valve losses are subdivided into terms referred to as P to P :
V1 V9
P IGBT conduction losses
V1
P diode conduction losses
V2
P other valve conduction losses
V3
P d.c. voltage-dependent losses
V4
P d.c. capacitor losses
V5
P IGBT switching losses
V6
P diode turn-off losses
V7
P snubber losses
V8
P valve electronics power consumption
V9
For the MMC topology, because the switching frequency is usually low (less than 200 Hz) the
largest contributors to the valve losses are usually the IGBT and diode conduction losses P
V1
and P . With half-bridge converters, P is dominant in inverter mode and P is dominant in
V2 V1 V2
IEC 62751-2:2014 © IEC 2014 – 13 –
rectifier mode, while with full-bridge converters there is no major difference between rectifier
and inverter modes. IGBT switching losses P and diode turn-off losses P are also a
V6 V7
significant (although not dominant) contribution. The other components of valve losses are
generally minor.
4.4 Loss calculation method
The proposed method for determining valve losses is based on analytical formulae for the
operating conditions. However, some of the necessary input parameters are difficult to obtain
by purely analytical means. Numerical solutions using real-time or non-real-time simulations
shall be applied, to derive such input parameters, for example valve currents and switching
energies. For that, the input parameters described in 4.5 are required.
Important requirement for such simulations is an accurate modelling of the system under
investigation. Multi-level converters offer a high degree of freedom in terms of control
strategies. Therefore the resulting valve currents strongly depend on the realization and the
algorithms of the control itself.
In alignment with the statement presented in 4.2, uncertainties of numerical simulations shall
be clearly stated and justified by the manufacturer.
4.5 Input parameters
4.5.1 General
This subclause describes the input parameters necessary for the calculation of power losses
in the valves of an MMC to take place. These input parameters refer to the data needed for
the performance of numerical simulations as well as the converter and component data
needed for calculation of losses. At the same time, converter and component data is divided
into two categories: converter station data such as the number of MMC building blocks per
valve and the on-state characteristics of the IGBT and free-wheeling diode, and operating
parameters such as the converter a.c. and d.c. currents, converter voltage (amplitude and
rd
harmonic injection where applicable), a.c. system frequency and
waveshape, including 3
mean MMC building block switching frequency.
4.5.2 Input data for numerical simulations
For numerical simulations, the following requirements shall be considered.
– The simulation model shall include a control block which represents the real control
behaviour and realistic behaviour regarding measurements, dead and transfer times,
interlocking times, etc.
– The calculations shall be performed for a period of time with stable conditions in terms of
active and reactive power transfer on the a.c. side and active power on the d.c. side.
– After the simulation has settled to steady-state conditions, a minimum integration time t of
i
1 s shall be used for operating state losses, but a longer time may be required for standby
and no-load operating states, depending on the switching strategies used.
– The simulation models shall represent real conditions of the converter station in terms of
number of MMC building blocks, main components, parasitic elements, original control
algorithms, voltage and current sensors. For the calculation of valve losses, all redundant
VSC levels shall be assumed to be in operation.
– A simplification of the simulation model with a reduced number of MMC building blocks is
possible if it can be demonstrated that the resulting valve currents are not influenced by
the simplification.
– The simulation shall also consider the junction temperature dependent semiconductor
properties, such as on-state voltages, switching and recovery losses. These properties are
based on the characterisation testing as described in IEC 62751-1:2014, 4.4.2. The
steady-state junction temperatures of the semiconductors are calculated iteratively for the
relevant operating point to derive the semiconductor losses. Further outputs of the

– 14 – IEC 62751-2:2014 © IEC 2014
simulation are converter valve currents and MMC building block capacitor currents, which
are the basis for the calculation of corresponding losses.
4.5.3 Input data coming from numerical simulations
From the numerical simulations the currents through the devices of the valve that are needed
as input for calculation of losses are determined. The
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