IEC 61378-1:2011

Converter transformers - Part 1: Transformers for industrial applications

IEC 61378-1:2011 deals with the specification, design and testing of power transformers and reactors which are intended for integration within semiconductor converter plants. The scope of this International Standard is limited to application of power converters of any power rating. Typical applications are:
- thyristor rectifiers for electrolysis;
- diode rectifiers for electrolysis;
- thyristor rectifiers for large drives;
- thyristor rectifiers for scrap melting furnaces, and
- diode rectifiers feeding inverters for variable speed drives. The standard also covers the regulating unit utilized in such application as step down regulating transformers or autotransformers. This edition includes the following significant technical changes with respect to the previous edition:
- addition of winding connections with phase displacement (30°);
- addition of transformers with more than one active part in the same tank;
- change of reference power definition;
- addition of regulating transformer feeding converter transformer;
- addition of considerations about current sharing and hot spot temperature in high current windings for various winding arrangements;
- addition of transductors used for d.c. voltage regulation together with diode rectifiers. The contents of the corrigendum of January 2012 have been included in this copy.

Transformateurs de conversion - Partie 1: Transformateurs pour applications industrielles

L'IEC 61378-1:2011 traite de la spécification, la conception et l'essai des transformateurs de puissance et des réacteurs qui sont destinés à l'intégration au sein des convertisseurs à semi-conducteurs, il n'est pas applicable aux transformateurs conçus pour la distribution industrielle ou publique de pouvoir a.c. en général. Cette édition inclut les changements techniques significatifs suivants par rapport à la précédente édition:
- Addition de couplages d'enroulements (zigzag, triangle prolongé, etc.) avec déphasage ( - Addition de transformateurs ayant plus d'une partie active dans la même cuve.
- Changement de la définition de la puissance de référence (elle est désormais basée sur la composante fondamentale du courant).
- Addition de considérations pour des lignes directrices relatives à la sélection de changeurs de prise en charge (OLTC, On-Load Tap-Changers).
- Addition du transformateur de régulation alimentant un transformateur de conversion.
- Addition de considérations relatives à la répartition des courants et aux points chauds dans les enroulements à fort courant pour diverses dispositions d'enroulements.
- Addition de transducteurs magnétiques pour la régulation de tension continue avec des redresseurs à diodes.
- Amélioration des anciennes annexes avec plusieurs exemples de calcul.
- Additions de nouvelles annexes pour les montages de mesure spéciaux.

General Information

• Status: Published
• Publication Date: 25-Jul-2011
• ICS: 29.180 - Transformers. Reactors
• Technical Committee: TC 14 - Power transformers
• Drafting Committee: MT 7 - TC 14/MT 7
• Current Stage: PPUB - Publication issued
• Start Date: 26-Jul-2011
• Completion Date: 15-Aug-2011

Relations

• Corrected By: IEC 61378-1:2011/COR1:2012 - Corrigendum 1 - Converter transformers - Part 1: Transformers for industrial applications
• Effective Date: 05-Sep-2023
• Revises: IEC 61378-1:1997 - Convertor transformers - Part 1: Transformers for industrial applications
• Effective Date: 05-Sep-2023

Standards Content (Sample)

IEC 61378-1 ®
Edition 2.0 2011-07
INTERNATIONAL
STANDARD
colour
inside
Converter transformers –
Part 1: Transformers for industrial applications

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IEC 61378-1 ®
Edition 2.0 2011-07
INTERNATIONAL
STANDARD
colour
inside
Converter transformers –
Part 1: Transformers for industrial applications

INTERNATIONAL
ELECTROTECHNICAL
COMMISSION
PRICE CODE
XC
ICS 29.180 ISBN 978-2-88912-602-6

– 2 – 61378-1 © IEC:2011(E)
CONTENTS
FOREWORD . 6
1 Scope . 8
2 Normative references. 9
3 Terms, definitions and acronyms . 9
3.1 Terms and definitions . 9
3.2 Acronyms . 10
4 Classification . 11
4.1 General . 11
4.2 Normal service conditions . 11
4.3 Provision for unusual service conditions . 12
5 Ratings . 12
5.1 General . 12
5.2 Rated power at rated frequency and load capability . 12
5.3 Rated and service voltages . 13
5.3.1 Transformer energized from an a.c. power system . 13
5.3.2 Transformer energized from a converter/inverter with or without
variable frequency . 13
5.4 Rated current . 13
5.5 Phase displacement and terminal identification for three-phase transformer . 13
5.6 Rating plate . 14
5.7 Units with tertiary windings loaded with filter and compensation . 14
5.8 On load tap-changers . 15
6 Load loss and voltage drop in transformers and reactors . 15
6.1 General . 15
6.2 Determination of transformer load loss under distorted current loading . 15
6.3 Current sharing, losses and hot spot in high current windings . 19
6.4 Effect of geometrical winding arrangement and magnetic coupling between
windings on their eddy current losses due to harmonics in transformers with
three or more windings wound on the same core limb . 20
6.5 Losses in interphase transformers, current-balancing reactors, series-
smoothing reactors and transductors . 26
6.5.1 General . 26
6.5.2 Interphase transformers . 26
6.5.3 Current-balancing reactors . 26
6.5.4 Series-smoothing reactors . 26
6.5.5 Transductors . 26
6.6 Voltage drops in transformers and reactors . 27
6.6.1 General . 27
6.6.2 Transductors . 28
7 Tests for converter transformers . 29
7.1 General . 29
7.2 Measurement of commutating reactance and determination of the inductive
voltage drop . 30
7.2.1 Commutating reactance . 30
7.2.2 Inductive voltage regulation . 30
7.3 Measurement of voltage ratio and phase displacement . 31
7.4 Dielectric tests . 31

61378-1 © IEC:2011(E) – 3 –
7.4.1 General . 31
7.4.2 Dielectric test between interleaved valve windings . 31
7.5 Load loss test . 32
7.5.1 General . 32
7.5.2 Load loss measurement in rectifier transformers with transductors in
the same tank . 32
7.5.3 Test bus bars configuration for short circuit of high current valve
windings . 32
7.6 Temperature rise tests . 32
7.6.1 General . 32
7.6.2 Total loss injection . 33
7.6.3 Rated load loss injection . 33
7.6.4 Test of temperature rise on dry-type transformers . 35
8 On load noise level with transductors and/or IPT . 35
Annex A (informative) Determination of transformer service load loss at rated non-
sinusoidal converter current from measurements with rated transformer current
of fundamental frequency . 38
Annex B (informative) Short-circuit test currents and load losses in transformers for
single-way converters (total loss injection) . 56
Annex C (informative) Current sharing measurement in high current valve windings . 57
Annex D (informative) Examples of duty cycles . 66
Annex E (informative) Guidelines for design review . 67
Annex F (informative) Determination of loss in transformer tank due to magnetic field.
3D simulation and guidelines for tank losses evaluation and tank hotspot
calculation . 70
Annex G (informative) Short-circuit measurements of rectifier transformers equipped
with built in transductors . 71
Annex H (informative) Determination of the transformer voltage ratio and phase
displacement by the turn ratio measurements . 73
Annex I (informative) Phase displacement connections and terminal indications of
converter transformers . 78
Annex J (normative) Correlation between IEC 61378-1 and IEC 60146-1-1 ratings . 83
Bibliography . 90

Figure 1 – B6U or DB 6 pulse double bridge connection . 10
Figure 2 – DSS 6 pulse connection . 11
Figure 3 – Leakage fields for a three-winding transformer with closely coupled valve
windings. 22
Figure 4 – Leakage fields for a three-winding transformer with decoupled valve
windings. 23
Figure 5 – Leakage fields for a three winding transformer with loosely coupled double
concentric valve windings . 24
Figure 6 – Leakage fields for a three winding transformer with loosely coupled double-
tier valve windings . 25
Figure 7 – Typical transductor regulating curve (with max voltage drop at zero control
current) and tolerance band . 28
Figure A.1 – Cross-section of a winding strand . 40
Figure A.2 – Terminal identification for winding connection Y y0y6 . 43
Figure A.3 – Terminal identification for winding connection D d0y1 . 46

– 4 – 61378-1 © IEC:2011(E)
Figure A.4 – Valve current DB connection rectangular shape positive shape . 47
Figure A.5 – Valve current DB connection rectangular shape positive and negative
shape. 48
Figure A.6 – Valve current DSS connection rectangular shape . 52
Figure C.1 – Example of valve high current winding and measurement equipment
disposition . 58
Figure C.2 – Transformer windings arrangement . 59
Figure C.3 – Measurement circuit for the in-phase measurement . 60
Figure C.4 – Measurement circuit for the in-opposition measurement. 61
Figure C.5 – Measurements and comparison with the simulations made by finite
element method software for the in-phase current distribution . 63
Figure C.6 – Measurements and comparison with the simulations made by finite
element method software for the in-opposition current distribution . 65
Figure H.1 – Yd1 connection . 74
Figure H.2 – Yd11 connection . 74
Figure H.3 – Pd0+7,5 connection . 75
Figure H.4 – Oscilloscope connection . 76
Figure H.5 – Oscilloscope with phase B + 7,5° lag referring to phase A . 76
Figure H.6 – Oscilloscope with phase B – 7,5° lead referring to phase A . 77
Figure I.1 – Counterclockwise phase displacement . 78
Figure I.2 – Yd11 connection . 78
Figure I.3 – Yd1 connection . 78
Figure I.4 – Example I.1 phase displacement . 79
Figure I.5 – Example I.2 phase displacement . 79
Figure J.1 – DB connection ideal rectangular current blocks . 83
Figure J.2 – DSS Connection rectangular current blocks . 84

Table 1 – Connections and calculation factors . 36
Table A.1 – Specified harmonic currents and phase displacement in the valve windings . 41
Table A.2 – Resistance measurements at 20 °C winding temperature . 42
Table A.3 – Specified harmonic currents and phase displacement in the line and valve
windings. 45
Table A.4 – Measurements from test report . 46
Table A.5 – Resulting current harmonics . 48
Table A.6 – Resulting current harmonics . 49
Table A.7 – Resulting current harmonics . 50
Table A.8 – Detailed transformer load losses at rated tap position, with tertiary unloaded . 51
Table A.9 – Resulting current harmonics . 52
Table A.10 – Specified harmonic currents and phase displacement in the line and valve
windings. 53
Table A.11 – Resulting current harmonics . 54
Table A.12 – Detailed transformer load losses at rated tap position, with tertiary
unloaded . 55
Table C.1 – Measurements and comparison with the simulations made by finite element
method software for the in-phase current distribution . 62

61378-1 © IEC:2011(E) – 5 –
Table C.2 – Measurements and comparison with the simulations made by finite element
method software for the in-opposition current distribution . 64
Table D.1 – Examples of duty cycles for different applications . 66
Table H.1 – Single phase ratio measurements . 73
th
Table J.1 – Harmonics content up to 25 in DB 6 pulse connection (ideal rectangular
current waveshape) . 84
th
Table J.2 – Harmonics content up to 25 in DSS 6 pulse connection (ideal rectangular
current waveshape) . 85
Table J.3 – Calculation factor comparison example . 86
Table J.4 – Calculation factor comparison general factors . 87

– 6 – 61378-1 © IEC:2011(E)
INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
CONVERTER TRANSFORMERS –
Part 1: Transformers for industrial applications

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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3) IEC Publications have the form of recommendations for international use and are accepted by IEC National
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4) In order to promote international uniformity, IEC National Committees undertake to apply IEC Publications
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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) 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 61378-1 has been prepared by IEC technical committee 14: Power
transformers.
This second edition cancels and replaces the first edition published in 1997. It constitutes a
technical revision.
This edition includes the following significant technical changes with respect to the previous
edition:
• addition of winding connections (zig-zag, extended delta, etc.) with phase displacement
(<30º);
• addition of transformers with more than one active part in the same tank;
• change of reference power definition (it is now based on fundamental component of the
current);
• addition of considerations for guidelines for OLTC selection;
• addition of regulating transformer feeding converter transformer;

61378-1 © IEC:2011(E) – 7 –
 addition of considerations about current sharing and hot spot temperature in high current
windings for various winding arrangements;
 addition of transductors used for d.c. voltage regulation together with diode rectifiers;
 improved old annexes with several calculation examples;
 addition of new annexes for special measurements setups.
The text of this standard is based on the following documents:
FDIS Report on voting
14/686/FDIS 14/695/RVD
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.
A list of all parts of the IEC 61378 series can be found, under the general title Converter
transformers, 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.
A bilingual version of this standard may be issued at a later date.
The contents of the corrigendum of January 2012 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.
– 8 – 61378-1 © IEC:2011(E)
CONVERTER TRANSFORMERS –
Part 1: Transformers for industrial applications

1 Scope
This Part of IEC 61378 deals with the specification, design and testing of power transformers
and reactors which are intended for integration within semiconductor converter plants; it is not
applicable to transformers designed for industrial or public distribution of a.c. power in general.
The scope of this International Standard is limited to application of power converters of any
power rating. Typical applications are: thyristor rectifiers for electrolysis; diode rectifiers for
electrolysis; thyristor rectifiers for large drives; thyristor rectifiers for scrap melting furnaces,
and diode rectifiers feeding inverters for variable speed drives. The standard also covers the
regulating unit utilized in such application as step down regulating transformers or
autotransformers. The valve winding highest voltage for equipment is limited to 36 kV.
This standard is not applicable to transformers for HVDC power transmission. These are high-
voltage transformers, and they are subjected to d.c. voltage tests.
The standards for the complete converter plant (IEC 60146 series, or other publications
dedicated to particular fields of application) may contain requirements of guarantees and tests
(such as insulation and power loss) for the whole plant, including the converter transformer and
possibly auxiliary transformers and reactor equipment. This does not relieve the application of
the requirements of this standard concerning the guarantees and tests applicable to the
converter transformer itself as a separate component before being assembled with the
remainder of the converter plant.
The guarantees, service and type tests defined in this standard apply equally to transformers
supplied as part of an overall converter package, or to those transformers ordered separately
but for use with converter equipment. Any supplementary guarantee or special verification has
to be specifically agreed in the transformer contract.
The converter transformers covered by this standard may be of the oil-immersed or dry-type
design. Unless specific exceptions are stated in this standard, the transformers comply with
IEC 60076 series for oil-immersed transformers, and with IEC 60076-11 for dry-type
transformers.
NOTE For some converter applications, it is possible to use common distribution transformers of standard design.
The use of such standard transformers in the special converter applications may require a certain derating. This
matter is not specifically covered in this standard, which deals with the requirements to be placed on specially
designed units. It is possible to estimate this derating from the formulae given in 5.1, and also from Clause 9 of
IEC 60076-8:1997.
This standard deals with transformers with one or more active parts installed in the same tank
like regulating (auto)transformer and one or two rectifier transformers. It also covers
transformers with transductors and/or one or more interphase transformers.
For any combination not listed above an agreement between the purchaser and manufacturer
is necessary regarding the determination and the measurement of the total losses.
This standard deals with transformers star Y and delta D and any other phase shifting
connections (like zig-zag, extended delta, polygon etc.). Phase shifting windings can be placed
on either the regulating or rectifier transformer.

61378-1 © IEC:2011(E) – 9 –
2 Normative references
The following referenced documents are indispensable for the application of this document. For
dated references, only the edition cited applies. For undated references, the latest edition of
the referenced document (including any amendments) applies.
IEC 60050-421:1990, International Electrotechnical Vocabulary (IEV) – Chapter 421: Power
transformers and reactors
IEC 60076 (all parts), Power transformers
IEC 60076-1:2011, Power transformers – Part 1: General
IEC 60076-2:2011, Power transformers – Part 2: Temperature rise for liquid-immersed transformers
IEC 60076-3:2000, Power transformers – Part 3: Insulation levels, dielectric tests and external
clearances in air
IEC 60076-6:2007, Power transformers – Part 6: Reactors
IEC 60076-8:1997, Power transformers – Part 8: Application guide
IEC 60076-11:2004, Power transformers – Part 11: Dry-type transformers
IEC 60146 (all parts), Semiconductor converters – General requirements and line commutated
converters
IEC 60146-1-1:2009, Semiconductor converters – General requirements and line commutated
converters – Part 1-1: Specifications of basic requirements
IEC/TR 60146-1-2:2011, Semiconductor converters – General requirements and line
commutated converters – Part 1-2: Application guide
IEC/TR 60616:1978, Terminal and tapping markings for power transformers
3 Terms, definitions and acronyms
3.1 Terms and definitions
For the purposes of this document, the terms and definitions given in IEC 60050-421,
IEC 60076-1 and IEC 60146-1-1, as well as the following apply.
3.1.1
polygon connection
P
the winding connection in which each phase winding consists of two parts in which phase
displaced voltages are induced. One part of each phase is connected in series to the other part
of a different phase and then closed in a delta (see Annex I)
3.1.2
extended delta connection
E
the winding connection in which each phase winding consists of two parts in which phase
displaced voltages are induced. One part of each phase is delta connected and it is then
connected to its appropriate line terminal through the other part (see Annex I)

– 10 – 61378-1 © IEC:2011(E)
3.1.3
phase shifting angle
Γ
the angle with sign, expressed in degrees and decimal of degrees, which needs to be added to
the nearest clock number to obtain the phase displacement
3.1.4
transductor
device consisting of one or more ferromagnetic cores with windings, by means of which an a.c.
or d.c. current or voltage can be varied by an independent voltage or current, utilizing
saturation phenomena in the magnetic circuit
NOTE The French term transducteur magnétique (English: transductor) should not be confused with the more
general French term transducteur (English: transducer). The use of the term transducteur in the sense of
transducteur magnétique is permissible when no ambiguity is possible.
[IEC 60050-431:1980, 431-01-01]
3.1.5
interphase transformer
an electromagnetic device enabling the operation in parallel of two or more phase displaced
commutating groups through inductive coupling between the windings placed on the same core
[IEC 60050-551:1998, 551-14-16]
3.1.6
line side
transformer winding connected to the a.c. network
3.1.7
valve side
transformer winding connected to the converter
3.2 Acronyms
B6U 6-pulse double bridge connection (see Figure 1 below)
DB double bridge connection (see Figure 1 below)
NOTE The transformer windings can be star or delta connected.
+
1V 2v
2u
2v
2u
1U
2w
1W 2w
-
IEC  1720/11
Figure 1 – B6U or DB 6 pulse double bridge connection
DSS double star with interphase transformer (see Figure 2 below)

61378-1 © IEC:2011(E) – 11 –
2u
2v
2v
w
2u
1V
2w
2n
1U 3u
3n
w
1W
3u 3v
+
3w
3v
2n
3n
-
IEC  1721/11
Figure 2 – DSS 6 pulse connection
IPT see definition 3.1.5
SR see definition 3.1.4
FFT fast fourier transformation
4 Classification
4.1 General
Classification of converters and converter applications are given in 4.1 of IEC 60146-1-1:2009
and in 4.1 of IEC/TR 60146-1-2:2011. From the aspect of transformer design, it is important to
distinguish between
– applications with essentially sinusoidal voltage across the transformer, and
– applications with non-sinusoidal voltage where the transformer primary is energized from a
converter circuit for a.c. power control or variable frequency conversion.
It is also important to distinguish between
– applications characterized by a continuous load, such as electrolysis, d.c. arc furnace etc.,
and
– applications with short-time cyclic or irregular load variation, such as reversible mill-motor
drives, etc.
Information about the converter application should be supplied in the transformer specification.
This is detailed further in following subclauses of this standard.
4.2 Normal service conditions
Normal service conditions for the transformer are in accordance with IEC 60076-1, IEC 60076-
2, IEC 60076-11 and IEC 60146-1-1.
Any deviation of the a.c. voltage from the rated voltage value or tapping voltage value,
sinusoidal wave shape or three-phase symmetry should be within the limits of immunity class
B, according to 5.4 of IEC 60146-1-1:2009. If the converter transformer is supplied with non-
sinusoidal voltage, inverter or frequency converter application, it is necessary that information
on the range of variation of service voltage shape and frequency variation shall be submitted in

– 12 – 61378-1 © IEC:2011(E)
the specification. It is also important that information is given regarding the d.c. component of
the applied voltage cycle.
4.3 Provision for unusual service conditions
In addition to the unusual service conditions to be specified for power transformers, in case of
transformers with more than two windings, each loading combination of the windings is to be
clearly specified. Each loading combination shall include the respective current harmonic
components.
Examples of this type of unusual service conditions are no or reduced load on tertiary
compensation winding or on one valve winding.
5 Ratings
5.1 General
IEC 60076-1 applies, with the following additions and explanations.
Transformers for converter application are loaded with non-sinusoidal current, and sometimes
work with non-sinusoidal voltage. Even the frequency may vary considerably in certain
applications.
The rating of the transformers on which the tests will be conducted and to which the
corresponding guarantees are related is expressed in sinusoidal quantities of fundamental
frequency in steady state.
The following subclauses provide guidance as to how to determine the transformer rating when
the details of the converter and other information about the loading are available.
5.2 Rated power at rated frequency and load capability
The rated power of the converter transformer line side winding is based on the fundamental
frequency components of voltage and current, hence the rated three-phase power is:
S = 3× U × I (1)
R 1
where
U is the r.m.s. value of the fundamental component of the line-to-line voltage;
I is the r.m.s. value of the fundamental component of the rated line side current. This
fundamental component is calculated from an ideal rectangular waveshape current (see
Table 1).
The rated power S and line current I shall be used for guaranteed load losses and short
R
circuit impedance.
The rated power of the valve windings S is equal to the rated power of the line winding
V
multiplied by a factor which is a function of number of valve windings and type of rectifier
(single or double way). This factor is defined in Table 1.
The thermal design and cooling system of the transformers shall be determined after allowance
is made for the increased losses due to harmonics (including d.c. components) by means of an
equivalent thermal current to be used in temperature rise test (see Clause 6).
In case of cyclic loading, the load variation pattern shall be included by the purchaser in the
transformer specification.
61378-1 © IEC:2011(E) – 13 –
5.3 Rated and service voltages
5.3.1 Transformer energized from an a.c. power system
For a converter transformer connected to an a.c. power system, the rated voltage shall be as
specified in 5.4 of IEC 60076-1:2011 and in IEC 60076-8.
5.3.2 Transformer energized from a converter/inverter with or without variable
frequency
For a converter application with a considerably distorted transformer voltage, the rated voltage
shall be the r.m.s value of the sinusoidal fundamental component derived from the Fourier
series analysis of the maximum continuous service voltage.
For applications with such a distorted transformer voltage, or with variable frequency,
information shall be given in the specification concerning the applied voltage under various
service conditions.
NOTE For the above applications, the amplitude of flux density in the magnetic circuit is the determining
parameter, and not the amplitude of a non-sinusoidal voltage. The value of flux is determined by the voltage-time
integral over a half-cycle. This value will be the maximum value in continuous service. If short-time higher values of
the voltage-time integral exist, they should also be included in the specification, to permit checking against possible
overfluxing.
5.4 Rated current
The rated current of the transformer is the r.m.s. value of the fundamental component of
current corresponding to rated power according to 5.2.
5.5 Phase displacement and terminal identification for three-phase transformer
The definition of phase displacement is described in 3.10.6 of IEC 60076-1:2011.
Whenever the ‘clock number’ notation outlined in the Clause 7 of IEC 60076-1:2011 is not
sufficient to identify the phase displacement; the nearest clock number shall be used followed
by the value with sign of the angle Γ which has to be added to obtain the exact phase
displacement. The indication of the sign of the Γ has to follow the definition of the leading and
lagging displacement included in 3.10.6 of IEC 60076-1:2011 (see Annex I).
The terminal identification of a converter transformer shall also include the information
regarding the sequence of the commutating valve. Therefore the terminals are expressed by a
code of three symbols as described below.
First symbol: Number, that refers the different winding systems (with 1 for line
side winding).
Second symbol:   Letter, that refers the sequence of the phases according to the IEC
60616.
Third symbol (optional):  “+” or “–“ that refers to which polarity of the rectifier the terminal is
connected to.
Examples of different type of connections, phase displacement and terminal indications are
included in the Annex I.
If the phase displacement changes with tap position, the one on the nominal tap shall be
indicated and the range of variation shall be agreed at the tender stage.

– 14 – 61378-1 © IEC:2011(E)
5.6 Rating plate
In addition to the information normally provided for power transformers, the following data shall
be included in the rating plate:
– connection and phasor diagram with indication of angular displacement (as per 5.5);
– eddy loss enhancement factor;
– r.m.s. value of the load current (which includes fundamental and harmonics);
NOTE In case of a power transformer loaded with pure sinusoidal current the eddy loss enhancement factor
would be equal to 1 and the r.m.s. load current is equal to the rated current.
– type of rectifier to which the transformer will be connected (single/double way, diodes,
thyristors);
– cooling characteristics, if water cooled, water flow rate, inlet pressure and pressure drop in
the cooling equipment;
– in case of regulating (auto)transformer and rectifier transformer(s) in the same tank, the
primary voltage of the rectifier transformer.
In case transductors are present, then the following information shall be supplied:
• number of turns of bias, control and test (if present) windings;
• rated current of bias, control and test (if present) windings;
• d.c. voltage drop;
• connection diagram of the bias, control and test (if present) windings with terminal
identification.
5.7 Units with tertiary windings loaded with filter and compensation
A tertiary winding on the transformer may be requested for power factor compensation and
harmonic filtering purposes.
In addition to specifying the rated power and voltage of the tertiary winding, the purchaser shall
specify the combinations of transformer loading conditions. This concerns:
• primary supply voltage, including variation limit;
• secondary side: voltage, power factor and current (fundamental and harmonics);
• tertiary side: voltage, power factor and current (fundamental and harmonics).
The rated current of the tertiary winding is defined as the rms value in ampere of the
fundamental component of the current at rated tertiary voltage (I = fundamental component
rated
of I = U / Z, where Z is the impedance of the compensation / filtering bank).
rated
Current harmonics caused by line voltage harmonics flow to the filters. A resonance might
occur by the transformer and the network reactance and the compensation capacitor. In this
case, large harmonic current flows to the capacitor. The purchaser shall consider the network
condition and specify harmonics accordingly.
The following conditions shall be taken into account, if specified by the purchaser, when
designing the transformer:
• when overfluxing occurs, the power supplied by the tertiary winding varies with the square
value of the voltage;
• because the power factor compensation has the effect of decreasing the supplied power,
the purchaser shall specify whether the primary winding shall be designed for the power
when the compensation capacitor bank is disconnected;

61378-1 © IEC:2011(E) – 15 –
• depending on the winding arrangement overfluxing may occur at reduced or no load on the
secondary winding while the tertiary winding remains connected to the compensation
capacitor.
NOTE These information are a result of an interactive process and may change during project development.
5.8 On load tap-changers
The breaking capacity of an on-load tap-changer depends on the maximum slope of the current
after it crosses zero value. For converter industrial applications, this value differs from the one
found in applications for power transformers (as described in IEC 60076-1) and is essential for
the proper selection of the on-load tap-changer. This value, expressed in A/s, shall be provided
to the transformer manufacturer by the purchaser.
6 Load loss and voltage drop in transformers and reactors
6.1 General
The measurement of load loss shall be carried out with the rated current. The comparison with
the guaranteed load losses defined before manufacturing shall be based on this measurement.
The actual load loss in service includes additional loss due to distorted current. This value shall
be calculated in accordance with the procedure of 6.2. It is not guaranteed, but shall be
provided by the transformer manufacturer for the purchaser.
...

IEC 61378-1 ®
Edition 2.0 2011-07
INTERNATIONAL
STANDARD
NORME
INTERNATIONALE
colour
inside
Converter transformers –
Part 1: Transformers for industrial applications

Transformateurs de conversion -
Partie 1: Transformateurs pour applications industrielles

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IEC 61378-1 ®
Edition 2.0 2011-07
INTERNATIONAL
STANDARD
NORME
INTERNATIONALE
colour
inside
Converter transformers –
Part 1: Transformers for industrial applications

Transformateurs de conversion -

Partie 1: Transformateurs pour applications industrielles

INTERNATIONAL
ELECTROTECHNICAL
COMMISSION
COMMISSION
ELECTROTECHNIQUE
PRICE CODE
INTERNATIONALE
CODE PRIX XC
ICS 29.180 ISBN 978-2-8322-1698-9

– 2 – IEC 61378-1:2011 © IEC 2011
CONTENTS
FOREWORD . 6
1 Scope . 8
2 Normative references. 9
3 Terms, definitions and acronyms . 9
3.1 Terms and definitions . 9
3.2 Acronyms . 10
4 Classification . 11
4.1 General . 11
4.2 Normal service conditions . 11
4.3 Provision for unusual service conditions . 12
5 Ratings . 12
5.1 General . 12
5.2 Rated power at rated frequency and load capability . 12
5.3 Rated and service voltages . 13
5.3.1 Transformer energized from an a.c. power system . 13
5.3.2 Transformer energized from a converter/inverter with or without
variable frequency . 13
5.4 Rated current . 13
5.5 Phase displacement and terminal identification for three-phase transformer . 13
5.6 Rating plate . 14
5.7 Units with tertiary windings loaded with filter and compensation . 14
5.8 On load tap-changers . 15
6 Load loss and voltage drop in transformers and reactors . 15
6.1 General . 15
6.2 Determination of transformer load loss under distorted current loading . 15
6.3 Current sharing, losses and hot spot in high current windings . 19
6.4 Effect of geometrical winding arrangement and magnetic coupling between
windings on their eddy current losses due to harmonics in transformers with
three or more windings wound on the same core limb . 20
6.5 Losses in interphase transformers, current-balancing reactors, series-
smoothing reactors and transductors . 26
6.5.1 General . 26
6.5.2 Interphase transformers . 26
6.5.3 Current-balancing reactors . 26
6.5.4 Series-smoothing reactors . 26
6.5.5 Transductors . 26
6.6 Voltage drops in transformers and reactors . 27
6.6.1 General . 27
6.6.2 Transductors . 28
7 Tests for converter transformers . 29
7.1 General . 29
7.2 Measurement of commutating reactance and determination of the inductive
voltage drop . 30
7.2.1 Commutating reactance . 30
7.2.2 Inductive voltage regulation . 30
7.3 Measurement of voltage ratio and phase displacement . 31
7.4 Dielectric tests . 31

7.4.1 General . 31
7.4.2 Dielectric test between interleaved valve windings . 31
7.5 Load loss test . 32
7.5.1 General . 32
7.5.2 Load loss measurement in rectifier transformers with transductors in
the same tank . 32
7.5.3 Test bus bars configuration for short circuit of high current valve
windings . 32
7.6 Temperature rise tests . 32
7.6.1 General . 32
7.6.2 Total loss injection . 33
7.6.3 Rated load loss injection . 33
7.6.4 Test of temperature rise on dry-type transformers . 35
8 On load noise level with transductors and/or IPT . 35
Annex A (informative) Determination of transformer service load loss at rated non-
sinusoidal converter current from measurements with rated transformer current
of fundamental frequency . 38
Annex B (informative) Short-circuit test currents and load losses in transformers for
single-way converters (total loss injection) . 56
Annex C (informative) Current sharing measurement in high current valve windings . 57
Annex D (informative) Examples of duty cycles . 66
Annex E (informative) Guidelines for design review . 67
Annex F (informative) Determination of loss in transformer tank due to magnetic field.
3D simulation and guidelines for tank losses evaluation and tank hotspot calculation . 70
Annex G (informative) Short-circuit measurements of rectifier transformers equipped
with built in transductors . 71
Annex H (informative) Determination of the transformer voltage ratio and phase
displacement by the turn ratio measurements . 73
Annex I (informative) Phase displacement connections and terminal indications of
converter transformers . 78
Annex J (normative) Correlation between IEC 61378-1 and IEC 60146-1-1 ratings . 83
Bibliography . 90

Figure 1 – B6U or DB 6 pulse double bridge connection . 10
Figure 2 – DSS 6 pulse connection . 11
Figure 3 – Leakage fields for a three-winding transformer with closely coupled valve
windings. 22
Figure 4 – Leakage fields for a three-winding transformer with decoupled valve
windings. 23
Figure 5 – Leakage fields for a three winding transformer with loosely coupled double
concentric valve windings . 24
Figure 6 – Leakage fields for a three winding transformer with loosely coupled double-
tier valve windings . 25
Figure 7 – Typical transductor regulating curve (with max voltage drop at zero control
current) and tolerance band . 28
Figure A.1 – Cross-section of a winding strand . 40
Figure A.2 – Terminal identification for winding connection Y y0y6 . 43
Figure A.3 – Terminal identification for winding connection D d0y1 . 46
Figure A.4 – Valve current DB connection rectangular shape positive shape . 47

– 4 – IEC 61378-1:2011 © IEC 2011
Figure A.5 – Valve current DB connection rectangular shape positive and negative
shape. 48
Figure A.6 – Valve current DSS connection rectangular shape . 52
Figure C.1 – Example of valve high current winding and measurement equipment
disposition . 58
Figure C.2 – Transformer windings arrangement . 59
Figure C.3 – Measurement circuit for the in-phase measurement . 60
Figure C.4 – Measurement circuit for the in-opposition measurement. 61
Figure C.5 – Measurements and comparison with the simulations made by finite
element method software for the in-phase current distribution . 63
Figure C.6 – Measurements and comparison with the simulations made by finite
element method software for the in-opposition current distribution . 65
Figure H.1 – Yd1 connection . 74
Figure H.2 – Yd11 connection . 74
Figure H.3 – Pd0+7,5 connection . 75
Figure H.4 – Oscilloscope connection . 76
Figure H.5 – Oscilloscope with phase B + 7,5 ° lag referring to phase A . 76
Figure H.6 – Oscilloscope with phase B – 7,5 ° lead referring to phase A . 77
Figure I.1 – Counterclockwise phase displacement . 78
Figure I.2 – Yd11 connection . 78
Figure I.3 – Yd1 connection . 78
Figure I.4 – Example I.1 phase displacement . 79
Figure I.5 – Example I.2 phase displacement . 79
Figure J.1 – DB connection ideal rectangular current blocks . 83
Figure J.2 – DSS Connection rectangular current blocks . 84

Table 1 – Connections and calculation factors . 36
Table A.1 – Specified harmonic currents and phase displacement in the valve windings . 41
Table A.2 – Resistance measurements at 20 °C winding temperature . 42
Table A.3 – Specified harmonic currents and phase displacement in the line and valve
windings. 45
Table A.4 – Measurements from test report . 46
Table A.5 – Resulting current harmonics . 48
Table A.6 – Resulting current harmonics . 49
Table A.7 – Resulting current harmonics . 50
Table A.8 – Detailed transformer load losses at rated tap position, with tertiary unloaded . 51
Table A.9 – Resulting current harmonics . 52
Table A.10 – Specified harmonic currents and phase displacement in the line and valve
windings. 53
Table A.11 – Resulting current harmonics . 54
Table A.12 – Detailed transformer load losses at rated tap position, with tertiary
unloaded . 55
Table C.1 – Measurements and comparison with the simulations made by finite element
method software for the in-phase current distribution . 62
Table C.2 – Measurements and comparison with the simulations made by finite element
method software for the in-opposition current distribution . 64

Table D.1 – Examples of duty cycles for different applications . 66
Table H.1 – Single phase ratio measurements . 73
th
Table J.1 – Harmonics content up to 25 in DB 6 pulse connection (ideal rectangular
current waveshape) . 84
th
Table J.2 – Harmonics content up to 25 in DSS 6 pulse connection (ideal rectangular
current waveshape) . 85
Table J.3 – Calculation factor comparison example . 86
Table J.4 – Calculation factor comparison general factors . 87

– 6 – IEC 61378-1:2011 © IEC 2011
INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
CONVERTER TRANSFORMERS –
Part 1: Transformers for industrial applications

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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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
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4) In order to promote international uniformity, IEC National Committees undertake to apply IEC Publications
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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
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other damage of any nature whatsoever, whether direct or indirect, or for costs (including legal fees) and
expenses arising out of the publication, use of, or reliance upon, this IEC Publication or any other IEC
Publications.
8) Attention is drawn to the Normative references cited in this publication. Use of the referenced publications is
indispensable for the correct application of this publication.
9) Attention is drawn to the possibility that some of the elements of this IEC Publication may be the subject of
patent rights. IEC shall not be held responsible for identifying any or all such patent rights.
International Standard IEC 61378-1 has been prepared by IEC technical committee 14: Power
transformers.
This bilingual version (2014-07) corresponds to the English version, published in 2011-07.
This second edition cancels and replaces the first edition published in 1997. It constitutes a
technical revision.
This edition includes the following significant technical changes with respect to the previous
edition:
• addition of winding connections (zig-zag, extended delta, etc.) with phase displacement
(< 30 º);
• addition of transformers with more than one active part in the same tank;
• change of reference power definition (it is now based on fundamental component of the
current);
• addition of considerations for guidelines for OLTC selection;
• addition of regulating transformer feeding converter transformer;
• addition of considerations about current sharing and hot spot temperature in high current
windings for various winding arrangements;
• addition of transductors used for d.c. voltage regulation together with diode rectifiers;
• improved old annexes with several calculation examples;
• addition of new annexes for special measurements setups.
The text of this standard is based on the following documents:
FDIS Report on voting
14/686/FDIS 14/695/RVD
Full information on the voting for the approval of this standard can be found in the report on
voting indicated in the above table.
The French version of this standard has not been voted upon.
This publication has been drafted in accordance with the ISO/IEC Directives, Part 2.
A list of all parts of the IEC 61378 series can be found, under the general title Converter
transformers, 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.
The contents of the corrigendum of January 2012 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.

– 8 – IEC 61378-1:2011 © IEC 2011
CONVERTER TRANSFORMERS –
Part 1: Transformers for industrial applications

1 Scope
This Part of IEC 61378 deals with the specification, design and testing of power transformers
and reactors which are intended for integration within semiconductor converter plants; it is not
applicable to transformers designed for industrial or public distribution of a.c. power in general.
The scope of this International Standard is limited to application of power converters of any
power rating. Typical applications are: thyristor rectifiers for electrolysis; diode rectifiers for
electrolysis; thyristor rectifiers for large drives; thyristor rectifiers for scrap melting furnaces,
and diode rectifiers feeding inverters for variable speed drives. The standard also covers the
regulating unit utilized in such application as step down regulating transformers or
autotransformers. The valve winding highest voltage for equipment is limited to 36 kV.
This standard is not applicable to transformers for HVDC power transmission. These are high-
voltage transformers, and they are subjected to d.c. voltage tests.
The standards for the complete converter plant (IEC 60146 series, or other publications
dedicated to particular fields of application) may contain requirements of guarantees and tests
(such as insulation and power loss) for the whole plant, including the converter transformer and
possibly auxiliary transformers and reactor equipment. This does not relieve the application of
the requirements of this standard concerning the guarantees and tests applicable to the
converter transformer itself as a separate component before being assembled with the
remainder of the converter plant.
The guarantees, service and type tests defined in this standard apply equally to transformers
supplied as part of an overall converter package, or to those transformers ordered separately
but for use with converter equipment. Any supplementary guarantee or special verification has
to be specifically agreed in the transformer contract.
The converter transformers covered by this standard may be of the oil-immersed or dry-type
design. Unless specific exceptions are stated in this standard, the transformers comply with
IEC 60076 series for oil-immersed transformers, and with IEC 60076-11 for dry-type
transformers.
NOTE For some converter applications, it is possible to use common distribution transformers of standard design.
The use of such standard transformers in the special converter applications may require a certain derating. This
matter is not specifically covered in this standard, which deals with the requirements to be placed on specially
designed units. It is possible to estimate this derating from the formulae given in 5.1, and also from Clause 9 of
IEC 60076-8:1997.
This standard deals with transformers with one or more active parts installed in the same tank
like regulating (auto)transformer and one or two rectifier transformers. It also covers
transformers with transductors and/or one or more interphase transformers.
For any combination not listed above an agreement between the purchaser and manufacturer
is necessary regarding the determination and the measurement of the total losses.
This standard deals with transformers star Y and delta D and any other phase shifting
connections (like zig-zag, extended delta, polygon etc.). Phase shifting windings can be placed
on either the regulating or rectifier transformer.

2 Normative references
The following referenced documents are indispensable for the application of this document. For
dated references, only the edition cited applies. For undated references, the latest edition of
the referenced document (including any amendments) applies.
IEC 60050-421:1990, International Electrotechnical Vocabulary (IEV) – Chapter 421: Power
transformers and reactors
IEC 60076 (all parts), Power transformers
IEC 60076-1:2011, Power transformers – Part 1: General
IEC 60076-2:2011, Power transformers – Part 2: Temperature rise for liquid-immersed transformers
IEC 60076-3:2000, Power transformers – Part 3: Insulation levels, dielectric tests and external
clearances in air
IEC 60076-6:2007, Power transformers – Part 6: Reactors
IEC 60076-8:1997, Power transformers – Part 8: Application guide
IEC 60076-11:2004, Power transformers – Part 11: Dry-type transformers
IEC 60146 (all parts), Semiconductor converters – General requirements and line commutated
converters
IEC 60146-1-1:2009, Semiconductor converters – General requirements and line commutated
converters – Part 1-1: Specifications of basic requirements
IEC/TR 60146-1-2:2011, Semiconductor converters – General requirements and line
commutated converters – Part 1-2: Application guide
IEC/TR 60616:1978, Terminal and tapping markings for power transformers
3 Terms, definitions and acronyms
3.1 Terms and definitions
For the purposes of this document, the terms and definitions given in IEC 60050-421,
IEC 60076-1 and IEC 60146-1-1, as well as the following apply.
3.1.1
polygon connection
P
the winding connection in which each phase winding consists of two parts in which phase
displaced voltages are induced. One part of each phase is connected in series to the other part
of a different phase and then closed in a delta (see Annex I)
3.1.2
extended delta connection
E
the winding connection in which each phase winding consists of two parts in which phase
displaced voltages are induced. One part of each phase is delta connected and it is then
connected to its appropriate line terminal through the other part (see Annex I)

– 10 – IEC 61378-1:2011 © IEC 2011
3.1.3
phase shifting angle
Γ
the angle with sign, expressed in degrees and decimal of degrees, which needs to be added to
the nearest clock number to obtain the phase displacement
3.1.4
transductor
device consisting of one or more ferromagnetic cores with windings, by means of which an a.c.
or d.c. current or voltage can be varied by an independent voltage or current, utilizing
saturation phenomena in the magnetic circuit
NOTE The French term transducteur magnétique (English: transductor) should not be confused with the more
general French term transducteur (English: transducer). The use of the term transducteur in the sense of
transducteur magnétique is permissible when no ambiguity is possible.
[IEC 60050-431:1980, 431-01-01]
3.1.5
interphase transformer
an electromagnetic device enabling the operation in parallel of two or more phase displaced
commutating groups through inductive coupling between the windings placed on the same core
[IEC 60050-551:1998, 551-14-16]
3.1.6
line side
transformer winding connected to the a.c. network
3.1.7
valve side
transformer winding connected to the converter
3.2 Acronyms
B6U 6-pulse double bridge connection (see Figure 1 below)
DB double bridge connection (see Figure 1 below)
NOTE The transformer windings can be star or delta connected.
+
1V 2v
2u
2v
2u
1U
2w
1W 2w
-
IEC  1720/11
Figure 1 – B6U or DB 6 pulse double bridge connection
DSS double star with interphase transformer (see Figure 2 below)

2u
2v
2v
w
2u
1V
2w
2n
1U 3u
3n
w
1W
3u 3v
+
3w
3v
2n
3n
-
IEC  1721/11
Figure 2 – DSS 6 pulse connection
IPT see definition 3.1.5
SR see definition 3.1.4
FFT fast fourier transformation
4 Classification
4.1 General
Classification of converters and converter applications are given in 4.1 of IEC 60146-1-1:2009
and in 4.1 of IEC/TR 60146-1-2:2011. From the aspect of transformer design, it is important to
distinguish between
– applications with essentially sinusoidal voltage across the transformer, and
– applications with non-sinusoidal voltage where the transformer primary is energized from a
converter circuit for a.c. power control or variable frequency conversion.
It is also important to distinguish between
– applications characterized by a continuous load, such as electrolysis, d.c. arc furnace etc.,
and
– applications with short-time cyclic or irregular load variation, such as reversible mill-motor
drives, etc.
Information about the converter application should be supplied in the transformer specification.
This is detailed further in following subclauses of this standard.
4.2 Normal service conditions
Normal service conditions for the transformer are in accordance with IEC 60076-1, IEC 60076-
2, IEC 60076-11 and IEC 60146-1-1.
Any deviation of the a.c. voltage from the rated voltage value or tapping voltage value,
sinusoidal wave shape or three-phase symmetry should be within the limits of immunity class
B, according to 5.4 of IEC 60146-1-1:2009. If the converter transformer is supplied with non-
sinusoidal voltage, inverter or frequency converter application, it is necessary that information
on the range of variation of service voltage shape and frequency variation shall be submitted in

– 12 – IEC 61378-1:2011 © IEC 2011
the specification. It is also important that information is given regarding the d.c. component of
the applied voltage cycle.
4.3 Provision for unusual service conditions
In addition to the unusual service conditions to be specified for power transformers, in case of
transformers with more than two windings, each loading combination of the windings is to be
clearly specified. Each loading combination shall include the respective current harmonic
components.
Examples of this type of unusual service conditions are no or reduced load on tertiary
compensation winding or on one valve winding.
5 Ratings
5.1 General
IEC 60076-1 applies, with the following additions and explanations.
Transformers for converter application are loaded with non-sinusoidal current, and sometimes
work with non-sinusoidal voltage. Even the frequency may vary considerably in certain
applications.
The rating of the transformers on which the tests will be conducted and to which the
corresponding guarantees are related is expressed in sinusoidal quantities of fundamental
frequency in steady state.
The following subclauses provide guidance as to how to determine the transformer rating when
the details of the converter and other information about the loading are available.
5.2 Rated power at rated frequency and load capability
The rated power of the converter transformer line side winding is based on the fundamental
frequency components of voltage and current, hence the rated three-phase power is:
S = 3 × U × I (1)
R 1
where
U is the r.m.s. value of the fundamental component of the line-to-line voltage;
I is the r.m.s. value of the fundamental component of the rated line side current. This
fundamental component is calculated from an ideal rectangular waveshape current (see
Table 1).
The rated power S and line current I shall be used for guaranteed load losses and short
R
circuit impedance.
The rated power of the valve windings S is equal to the rated power of the line winding
V
multiplied by a factor which is a function of number of valve windings and type of rectifier
(single or double way). This factor is defined in Table 1.
The thermal design and cooling system of the transformers shall be determined after allowance
is made for the increased losses due to harmonics (including d.c. components) by means of an
equivalent thermal current to be used in temperature rise test (see Clause 6).
In case of cyclic loading, the load variation pattern shall be included by the purchaser in the
transformer specification.
5.3 Rated and service voltages
5.3.1 Transformer energized from an a.c. power system
For a converter transformer connected to an a.c. power system, the rated voltage shall be as
specified in 5.4 of IEC 60076-1:2011 and in IEC 60076-8.
5.3.2 Transformer energized from a converter/inverter with or without variable
frequency
For a converter application with a considerably distorted transformer voltage, the rated voltage
shall be the r.m.s value of the sinusoidal fundamental component derived from the Fourier
series analysis of the maximum continuous service voltage.
For applications with such a distorted transformer voltage, or with variable frequency,
information shall be given in the specification concerning the applied voltage under various
service conditions.
NOTE For the above applications, the amplitude of flux density in the magnetic circuit is the determining
parameter, and not the amplitude of a non-sinusoidal voltage. The value of flux is determined by the voltage-time
integral over a half-cycle. This value will be the maximum value in continuous service. If short-time higher values of
the voltage-time integral exist, they should also be included in the specification, to permit checking against possible
overfluxing.
5.4 Rated current
The rated current of the transformer is the r.m.s. value of the fundamental component of
current corresponding to rated power according to 5.2.
5.5 Phase displacement and terminal identification for three-phase transformer
The definition of phase displacement is described in 3.10.6 of IEC 60076-1:2011.
Whenever the ‘clock number’ notation outlined in the Clause 7 of IEC 60076-1:2011 is not
sufficient to identify the phase displacement; the nearest clock number shall be used followed
by the value with sign of the angle Γ which has to be added to obtain the exact phase
displacement. The indication of the sign of the Γ has to follow the definition of the leading and
lagging displacement included in 3.10.6 of IEC 60076-1:2011 (see Annex I).
The terminal identification of a converter transformer shall also include the information
regarding the sequence of the commutating valve. Therefore the terminals are expressed by a
code of three symbols as described below.
First symbol: Number, that refers the different winding systems (with 1 for line
side winding).
Second symbol:  Letter, that refers the sequence of the phases according to the
IEC 60616.
Third symbol (optional):  “+” or “–“ that refers to which polarity of the rectifier the terminal is
connected to.
Examples of different type of connections, phase displacement and terminal indications are
included in the Annex I.
If the phase displacement changes with tap position, the one on the nominal tap shall be
indicated and the range of variation shall be agreed at the tender stage.

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5.6 Rating plate
In addition to the information normally provided for power transformers, the following data shall
be included in the rating plate:
– connection and phasor diagram with indication of angular displacement (as per 5.5);
– eddy loss enhancement factor;
– r.m.s. value of the load current (wh
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