SIST EN 60076-16:2012

SLOVENSKI STANDARD
01-januar-2012
0RþQRVWQLWUDQVIRUPDWRUMLGHO7UDQVIRUPDWRUML]DXSRUDERSULYHWUQLK
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Power transformers - Part 16: Transformers for wind turbines application
Leistungstransformatoren - Teil 16: Transformatoren für Windenergieanlagen-
Anwendungen
Transformateurs de puissance - Partie 16: Transformateurs pour applications éoliennes
Ta slovenski standard je istoveten z: EN 60076-16:2011
ICS:
29.180 Transformatorji. Dušilke Transformers. Reactors
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

EUROPEAN STANDARD
EN 60076-16
NORME EUROPÉENNE
October 2011
EUROPÄISCHE NORM
ICS 29.180
English version
Power transformers -
Part 16: Transformers for wind turbines applications
(IEC 60076-16:2011)
Transformateurs de puissance -  Leistungstransformatoren -
Partie 16: Transformateurs pour Teil 16: Transformatoren für
applications éoliennes Windenergieanlagen-Anwendungen
(CEI 60076-16:2011) (IEC 60076-16:2011)

This European Standard was approved by CENELEC on 2011-09-29. 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, France, Germany, Greece, Hungary, Iceland, Ireland, Italy,
Latvia, Lithuania, Luxembourg, Malta, the Netherlands, Norway, Poland, Portugal, Romania, Slovakia, Slovenia,
Spain, Sweden, Switzerland and the United Kingdom.

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

Management Centre: Avenue Marnix 17, B - 1000 Brussels

© 2011 CENELEC - All rights of exploitation in any form and by any means reserved worldwide for CENELEC members.
Ref. No. EN 60076-16:2011 E
Foreword
The text of document 14/690/FDIS, future edition 1 of IEC 60076-16, prepared by IEC/TC 14 "Power
transformers", was submitted to the IEC-CENELEC parallel vote and approved by CENELEC as
The following dates are fixed:
(dop) 2012-06-29
• latest date by which the document has
to be implemented at national level by
publication of an identical national
standard or by endorsement
(dow) 2014-09-29
• latest date by which the national
standards conflicting with 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 60076-16:2011 was approved by CENELEC as a European
Standard without any modification.
In the official version, for Bibliography, the following notes have to be added for the standards indicated:
IEC 60071-1:2006 NOTE  Harmonized as EN 60071-1:2006 (not modified).
IEC 60071-2:1996 NOTE  Harmonized as EN 60071-2:1997 (not modified).
IEC 60137:2008 NOTE  Harmonized as EN 60137:2008 (not modified).
IEC 60270:2000 NOTE  Harmonized as EN 60270:2001 (not modified).
IEC 62271-100:2008 NOTE  Harmonized as EN 62271-100:2009 (not modified).
IEC 62271-202:2006 NOTE  Harmonized as EN 62271-202:2007 (not modified).

- 3 - EN 60076-16:2011
Annex ZA
(normative)
Normative references to international publications
with their corresponding European publications

The following referenced documents are indispensable for the application of this document. For dated
references, only the edition cited applies. For undated references, the latest edition of the referenced
document (including any amendments) applies.

NOTE  When an international publication has been modified by common modifications, indicated by (mod), the relevant EN/HD
applies.
Publication Year Title EN/HD Year

IEC 60076-1 2011 Power transformers - EN 60076-1 2011
Part 1: General
IEC 60076-2 2011 Power transformers - EN 60076-2 2011
Part 2: Temperature rise for liquid-immersed
transformers
IEC 60076-3 2000 Power transformers - EN 60076-3 2001
+ corr. December 2000 Part 3: Insulation levels, dielectric tests and
external clearances in air
IEC 60076-5 2006 Power transformers - EN 60076-5 2006
Part 5: Ability to withstand short circuit

IEC 60076-7 2005 Power transformers - - -
Part 7: Loading guide for oil-immersed power
transformers
IEC 60076-8 1997 Power transformers - - -
Part 8: Application guide
IEC 60076-11 2004 Power transformers - EN 60076-11 2004
Part 11: Dry-type transformers

IEC 60076-12 2008 Power transformers - - -
Part 12: Loading guide for dry-type power
transformers
IEC 60076-13 2006 Power transformers - EN 60076-13 2006
Part 13: Self-protected liquid-filled
transformers
IEC 61100 - Classification of insulating liquids according EN 61100 -
to fire point and net calorific value

IEC 61378-1 2011 Convertor transformers - EN 61378-1 2011
Part 1: Transformers for industrial applications

IEC 61378-3 2006 Convertor transformers - - -
Part 3: Application guide
IEC 61400-1 2005 Wind turbines - EN 61400-1 2005
Part 1: Design requirements
ISO 12944 Series Paints and varnishes - Corrosion protection of - -
steel structures by protective paint systems

IEC 60076-16 ®
Edition 1.0 2011-08
INTERNATIONAL
STANDARD
NORME
INTERNATIONALE
colour
inside
Power transformers –
Part 16: Transformers for wind turbine applications

Transformateurs de puissance –
Partie 16: Transformateurs pour applications éoliennes

INTERNATIONAL
ELECTROTECHNICAL
COMMISSION
COMMISSION
ELECTROTECHNIQUE
PRICE CODE
INTERNATIONALE
CODE PRIX W
ICS 29.180 ISBN 978-2-88912-670-5

– 2 – 60076-16 © IEC:2011
CONTENTS
FOREWORD . 4
INTRODUCTION . 6
1 Scope . 7
2 Normative references . 7
3 Terms and definitions . 8
4 Service conditions . 8
4.1 Normal service conditions . 8
4.2 Altitude . 8
4.3 Temperature of cooling air . 8
4.4 Content of harmonic currents in the transformer . 9
4.5 Wave-shape of supply voltage . 9
4.6 Transient over and under voltages . 9
4.7 Humidity and salinity . 10
4.8 Special electrical and environmental conditions around the transformer . 10
4.9 Level of vibration . 11
4.10 Provision for unusual service conditions for transformers for wind turbine
applications . 11
4.11 Transportation and storage conditions . 11
4.12 Corrosion protection . 11
5 Electrical characteristics . 11
5.1 Rated power . 11
5.2 Highest voltage for equipment . 11
5.3 Tappings . 12
5.4 Connection group . 12
5.5 Dimensioning of neutral terminal . 12
5.6 Short circuit impedance . 12
5.7 Insulation levels for high voltage and low voltage windings . 12
5.8 Temperature rise guaranteed at rated conditions . 12
5.9 Overload capability . 13
5.10 Inrush current . 13
5.11 Ability to withstand short circuit . 13
5.12 Operation with forced cooling . 13
6 Rating plate . 13
7 Tests . 13
7.1 List and classification of tests (routine, type and special tests) . 13
7.2 Routine tests . 13
7.3 Type tests . 14
7.4 Special tests . 14
7.4.1 General . 14
7.4.2 Chopped wave test . 14
7.4.3 Electrical resonance frequency test . 14
7.4.4 Climatic tests . 14
7.4.5 Environmental test E3 . 14
7.4.6 Fire behavior test . 15
Annex A (informative) Calculation method and tables . 16
Bibliography . 36

60076-16 © IEC:2011 – 3 –
Figure A.1 – Heat dissipation in a natural ventilated room . 17
Figure A.2 – Schematic diagram of power frequency current injection apparatus . 30
Figure A.3 – Switched transformer winding voltage responses with capacitor injection . 31
Figure A.4 – HV Injection test figure . 32
Figure A.5 – Example of measurement device . 33

Table 1 – Insulation levels . 10
Table A.1 – Impact of harmonics content on liquid-immersed transformer losses . 23
Table A.2 – Impact of harmonics content on dry type transformers losses. 26
Table A.3 – Example of voltage harmonic order . 29

– 4 – 60076-16 © IEC:2011
INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
POWER TRANSFORMERS –
Part 16: Transformers for wind turbine 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
this end and in addition to other activities, IEC publishes International Standards, Technical Specifications,
Technical Reports, Publicly Available Specifications (PAS) and Guides (hereafter referred to as “IEC
Publication(s)”). Their preparation is entrusted to technical committees; any IEC National Committee interested
in the subject dealt with may participate in this preparatory work. International, governmental and non-
governmental organizations liaising with the IEC also participate in this preparation. IEC collaborates closely
with the International Organization for Standardization (ISO) in accordance with conditions determined by
agreement between the two organizations.
2) The formal decisions or agreements of IEC on technical matters express, as nearly as possible, an international
consensus of opinion on the relevant subjects since each technical committee has representation from all
interested IEC National Committees.
3) IEC Publications have the form of recommendations for international use and are accepted by IEC National
Committees in that sense. While all reasonable efforts are made to ensure that the technical content of IEC
Publications is accurate, IEC cannot be held responsible for the way in which they are used or for any
misinterpretation by any end user.
4) In order to promote international uniformity, IEC National Committees undertake to apply IEC Publications
transparently to the maximum extent possible in their national and regional publications. Any divergence
between any IEC Publication and the corresponding national or regional publication shall be clearly indicated in
the latter.
5) IEC itself does not provide any attestation of conformity. Independent certification bodies provide conformity
assessment services and, in some areas, access to IEC marks of conformity. IEC is not responsible for any
services carried out by independent certification bodies.
6) All users should ensure that they have the latest edition of this publication.
7) No liability shall attach to IEC or its directors, employees, servants or agents including individual experts and
members of its technical committees and IEC National Committees for any personal injury, property damage or
other damage of any nature whatsoever, whether direct or indirect, or for costs (including legal fees) and
expenses arising out of the publication, use of, or reliance upon, this IEC Publication or any other IEC
Publications.
8) Attention is drawn to the Normative references cited in this publication. Use of the referenced publications is
indispensable for the correct application of this publication.
9) 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 60076-16 has been prepared by IEC technical committee 14:
Power transformers.
The text of this standard is based on the following documents:
FDIS Report on voting
14/690/FDIS 14/698/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.

60076-16 © IEC:2011 – 5 –
A list of all parts of the IEC 60076 series can be found, under the general title Power
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.
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.
– 6 – 60076-16 © IEC:2011
INTRODUCTION
This part of IEC 60076 is intended to specify the additional requirements for the transformers
for installation in wind turbine applications.
Wind turbines use generator step-up transformers to connect the turbines to a network. These
transformers can be installed in the nacelle or in the tower or outside close to the wind
turbine.
This standard covers transformers for wind turbine applications or wind farms where the
constraints on transformers exceed the requirement of the present IEC 60076 series. The
constraints are not often known or recognized by the transformer manufacturers, wind turbine
manufacturers and operators and as a result the level of reliability of these transformers can
be lower than those used for conventional applications.
The transformers for wind turbine applications are not included in the present list of
IEC 60076 standard series.
The purpose of this standard is help to obtain the same level of reliability as transformers for
more common applications.
This standard deals particularly with the effects of repeated high frequency transient over-
voltages, electrical, environmental, thermal, loading, installation and maintenance conditions
that are specific for wind turbines or wind farms.
On site measurements, investigations and observations in wind turbines have detected risks
for some different kind of installations:
– repeated high frequency transient over or under voltages in the range of kHz;
– over and under frequency due to turbine control;
– values of over voltage;
– over voltage or under voltage coming from LV side;
– high level of transient over voltages due to switching;
– presence of partial discharge around the transformer;
– harmonic contents current and voltage;
– overloading under ambient conditions;
– fast transient overload;
– clearances not in compliance with the minimum prescribed;
– installation conditions and connections;
– restricted conditions of cooling;
– water droplets;
– humidity levels that exceed the maximum permissible values;
– salt and dust pollution and extreme climatic conditions;
– high levels of vibration;
– mechanical stresses.
Therefore it is necessary to take into account in the design of the transformer the constraints
of this application, or to define some protective devices to protect the transformer. Additional
or improved routine, type or special tests for these transformers have to be specified to be in
compliance with the constraints on the network.

60076-16 © IEC:2011 – 7 –
POWER TRANSFORMERS –
Part 16: Transformers for wind turbine applications

1 Scope
This part of IEC 60076 applies to dry-type and liquid-immersed transformers for rated power
100 kVA up to 10 000 kVA for wind turbine applications having a winding with highest voltage
for equipment up to and including 36 kV and at least one winding operating at a voltage
greater than 1,1 kV.
Transformers covered by this standard comply with the relevant requirements prescribed in
the IEC 60076 standards.
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 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 60076-5:2006, Power transformers – Part 5: Ability to withstand short circuit
IEC 60076-7:2005, Power transformers – Part 7: Loading guide for oil-immersed power
transformers
IEC 60076-8:1997, Power transformers – Application guide
IEC 60076-11:2004, Power transformers – Part 11: Dry-type transformers
IEC 60076-12:2008, Power transformers – Part 12: Loading guide for dry-type power
transformers
IEC 60076-13:2006, Power transformers – Part 13: Self-protected liquid-filled transformers
IEC 61100, Classification of insulating liquids according to fire-point and net calorific value
IEC 61378-1:2011, Converter transformers – Part 1: Transformers for industrial applications
IEC 61378-3:2006, Converter transformers – Part 3: Application guide
IEC 61400-1:2005, Wind turbines – Part 1: Design requirements

– 8 – 60076-16 © IEC:2011
ISO 12944 (all parts), Paints and varnishes – Corrosion protection of steel structures by
protective paint systems
3 Terms and definitions
For the purposes of this document, the following terms and definitions apply.
3.1
wind turbine transformer
generator step up transformer connecting the wind turbine to the power collection network of
the wind farm
3.2
tower
part of the supporting structure of wind turbine on top of which the nacelle with generator and
other equipments are located
3.3
nacelle
housing that contains the drive-train and other elements on top of a horizontal-axis wind
turbine tower
4 Service conditions
4.1 Normal service conditions
Unless otherwise stated in this standard, the service conditions in IEC 60076-11 and
IEC 60076-1 apply.
4.2 Altitude
IEC 60076 series applies.
4.3 Temperature of cooling air
The installation of transformers inside an enclosure without active cooling systems increases
the transformer temperature.
The purchaser shall specify the maximum cooling air temperatures if they are different from
those stated in IEC 60076-2.
The transformer shall be designed according to real ambient temperatures and installation
real conditions as described by the purchaser at enquiry stage.
Clause A.1 provides considerations for transformers installed in a naturally ventilated area
like at the rear of the nacelle or in a separate enclosure installed outside the tower and
equipped with air inlet and outlet.
In case of transformer installed in the tower or in an enclosure where natural ventilation is not
provided the formula in A.1 is not applicable. For transformers operating under these
conditions, the effects of air inlet and outlet, cooling conditions, efficiency of air cooling and
ventilation shall be considered.
The purchaser shall prescribe the air ambient temperature and air flow inside the tower at the
enquiry stage. If no temperature or air flow is specified, an internal ambient temperature
inside the tower of 10 K higher than external temperature shall be assumed and not limited air
circulation around the transformers.

60076-16 © IEC:2011 – 9 –
The effect of external direct solar radiation is not taken into account at the design stage. This
can increase the temperature of transformers parts and therefore information should be given
by purchaser at enquiry time.
4.4 Content of harmonic currents in the transformer
At the enquiry stage the purchaser shall specify the magnitude and frequency of all harmonic
currents supplied to the transformer. The manufacturer shall take the losses caused by these
harmonic currents into account in the transformer design to prevent that the winding and
liquid temperature rises exceed the permissible limits.
A method to calculate the impact of the harmonic currents on the design of the transformer is
given in A.2.
The transformer shall be designed to take into account the increased rating required due to
the harmonic currents. The temperature rise test shall be carried out with the equivalent rated
power due to the harmonics defined in A.2. The result of the test shall be in compliance with
temperature limits guaranteed for the transformer and related to the transformer insulation
thermal class.
4.5 Wave-shape of supply voltage
U
Within the prescribed value of a transformer shall be capable of continuous service at full
m
load without damage under conditions of ‘overfluxing’ where the ratio of voltage over
frequency exceeds the corresponding ratio at rated voltage and rated frequency according to
IEC 60076-1.
The wind turbine manufacturer shall state at enquiry stage the maximum ratio between the
voltage and the frequency. The transformer manufacturer shall take into account this value in
the design of the transformer.
The purchaser shall specify in the inquiry the magnitude and frequency of any harmonic
voltages present in the supply. A method to calculate the impact of the voltage harmonics on
the design of the transformer is given in A.3.
4.6 Transient over and under voltages
The risk of failures of a wind turbine transformer is higher due to the fact of repeated transient
over and under voltages on each side on transformer.
Several solutions are available to increase the reliability of the transformer against these fast
transient interactions:
– to evaluate the insulation level of the transformer and if necessary apply one or more of
the following solutions. This can be done by modeling or measuring the system by high
frequency resonance analysis. The resonance frequency test is a special test. The test
method shall be agreed between manufacturer and purchaser. One method is described in
A.4;
– to install standard protection technique such as surge arresters (HV, LV), or RC circuit or
surge capacitor.
The choice of the lists 2 or 3 in Table 1 shall be the responsibility of the system engineer
based on specific insulation co-ordination (IEC 60071-1 and -2) and risk assessment.
The list 3 covers transformers with increased ability to withstand repeated transient over
voltages and increases the reliability of the transformer.

– 10 – 60076-16 © IEC:2011
Table 1 – Insulation levels
Highest voltage Rated short Rated lightning impulse
for equipment duration withstand voltage (peak
separated source value) in kV
U
(rms) kV
m
AC withstand
List 2 List 3
voltage (RMS) kV
≤ 1,1 3 - 20
3,6 10 40 50
7,2 20 60 75
12 28 75 95
17,5 38 95 125
24 50 125 150
36 70 170 200
High frequency steep surges can be generated by switching operation on LV or HV side.
These surges are transferred by cables to the terminals of the transformer. Transformers have
different values of resonance frequency. See A.4.
If the high frequency steep surges generated by switching operation on LV and HV side
coincide with the internal frequency of the winding, the result of these surges can resonate
with the winding internal frequencies and cause higher electric stresses than the dielectric
withstand strength of the windings
NOTE For U ≤ 1,1 kV a.c. withstand voltage should have higher value as 10 kV.
m
4.7 Humidity and salinity
An abnormal level of humidity and salinity can lead to failures of dry type transformers and
problems on open type bushings of liquid-immersed transformers or dry type transformers in
enclosures.
The standard pollution levels for open type bushing for liquid-immersed transformers are
defined in IEC 60815 series. There are also simulated rain tests defined in IEC 60137.
According to IEC 60076-11, the relative humidity in the test chamber shall be maintained
above 93 % for environmental class E2 transformers. Salinity shall be such as the
conductivity of the water in E2 test shall be in the range of 0,5 to1,5 S/m.
If a dry type transformer shall operate under more severe conditions than corresponding to
class E2 without a protective enclosure against humidity and salinity, the capability of the
transformer design shall be demonstrated by the test according to class E3 described in 7.4.5
in this standard.
IEC 61400-1 states that relative humidity up to 95 % shall be taken into account as a normal
environmental condition.
Higher values of humidity and salinity shall be given at enquiry stage.
4.8 Special electrical and environmental conditions around the transformer
IEC 60076-3 recommends general minimum clearances between transformer live parts and
conductive parts of the wind turbine.

60076-16 © IEC:2011 – 11 –
Any part of the wind turbine made of insulation material becomes conductive when moistened
with rain water, salt water or other conductive liquids. Partial discharges in the surroundings
of the transformer can decrease the dielectric strength of the air.
Therefore the clearances between these wind turbine parts and the live parts of the
transformer shall not be less than the clearances recommended in IEC 60076-3.
The transformer manufacturer shall indicate the required minimum clearances on the outline
drawing of the transformer and it is the responsibility of the purchaser to follow up that these
requirements will be met.
4.9 Level of vibration
Vibrations of the structure where the transformer is to be installed shall be taken into account
when designing the transformer and special consideration shall be given in the stress
transferred to connection terminals.
The purchaser shall specify vibration spectrum at the enquiry stage. The procedure of
vibration test if any should be agreed at enquiry stage between purchaser and manufacturer.
4.10 Provision for unusual service conditions for transformers for wind turbine
applications
Provision for unusual service conditions are indicated in IEC 60076-1 for liquid-immersed
transformers and IEC 60076-11 for dry type transformers.
4.11 Transportation and storage conditions
Transportation and storage conditions are indicated in IEC 60076-1 for liquid-immersed
transformers and IEC 60076-11 for dry type transformers.
Storage conditions shall be included in maintenance and operation manuals and shall be
taken into account by the purchaser.
4.12 Corrosion protection
Depending on the kind of the installation, the purchaser should choose a protection class
defined in ISO 12944 or otherwise agreed between purchaser and manufacturer.
5 Electrical characteristics
5.1 Rated power
The rated power shall be in accordance with 5.1 of IEC 60076-1.
The rated power S of the transformer is based on the fundamental frequency of the voltage
r
U and of the current I . The rated power of a three phase transformer is therefore:
1 1
S = 3 × U × I
r 1 1
The temperature rise and the cooling requirements of the transformer shall be determined
after allowance is made for any increased losses due to harmonics.
5.2 Highest voltage for equipment
The highest voltage for equipment shall be chosen in accordance with Clause 5 of
IEC 60076-3:2000.
– 12 – 60076-16 © IEC:2011
The wind turbine designer shall inform the transformer manufacturer of peak voltages,
frequencies and durations of any transient and repeated over voltages (see also Table 1 of
this standard).
Information about insulation coordination is described in IEC 60071-1 and IEC 60071-2.
5.3 Tappings
The requirements in Clause 5 of IEC 60076-1:2011 apply.
The preferred tapping range if any is either:
• +5 % to –5 % in steps of 2,5 %,
or
• +5 % to –5 % in steps of 5 %.
Tapping selection shall be made by means of off-circuit bolted links or an off-circuit tap
changer.
5.4 Connection group
Unless otherwise specified by the purchaser, transformer connections shall be Dyn with clock
hour figure 5 or 11 in accordance with Clause 7 of IEC 60076-1:2011.
5.5 Dimensioning of neutral terminal
The neutral terminal shall be capable of carrying full phase rated current unless otherwise
specified by the purchaser.
5.6 Short circuit impedance
For general purpose the impedance voltage shall be in accordance with IEC 60076-5.
For auxiliary windings when the combined impedance voltage of the tertiary winding and the
system result in short circuit current levels for which the transformer cannot feasibly or
economically be designed to withstand, the manufacturer and the purchaser shall mutually
agree on the maximum allowed over current. In this case, provision should be made by the
purchaser to limit the over current to the maximum value determined by the manufacturer and
stated on the rating plate.
5.7 Insulation levels for high voltage and low voltage windings
The selected insulation level for the high voltage and low voltage windings shall be in
accordance with Table 1 of this standard.
5.8 Temperature rise guaranteed at rated conditions
The design of the transformer shall be in accordance with the operating conditions (harmonic
contents, ambient temperature) stated by the purchaser at the enquiry stage.
The guaranteed temperature rise shall take into account the additional losses due to
harmonics if specified, which increase eddy losses and stray losses in the windings and
structural/frame parts.
If no harmonics are specified at the design stage but the actual real load current in service
contains harmonics, the load on the transformer may need to be reduced to prevent the
transformer temperature rises exceed the guaranteed limits.

60076-16 © IEC:2011 – 13 –
Examples of calculations of the impact of harmonic currents are given in A.2.
5.9 Overload capability
The loading guides for liquid-immersed transformers in IEC 60076-7 and for dry type
transformers in IEC 60076-12 shall apply.
5.10 Inrush current
Due to frequent energizing of the transformers during wind farm operation, transformers are
frequently exposed to mechanical and thermal effects of inrush currents.
Frequency of energisation (number of energisation per year) shall be given at enquiry stage.
Unless otherwise specified, switching is done on the HV (grid) side. The method of switching
and synchronization shall be described in case of generator side energisation.
System inrush current limitations (maximum value, duration) shall be given at enquiry stage
by the purchaser.
5.11 Ability to withstand short circuit
Transformers shall fulfill the requirements in IEC 60076-5. If the purchaser requires a test to
demonstrate this fulfillment, this test shall be stated in the contract.
5.12 Operation with forced cooling
When additional cooling by means of fans or pumps is provided, the nominal power rating with
and without forced cooling shall be subject to agreement between purchaser and
manufacturer.
The rating plate shall indicate both the power rating without forced cooling and the maximum
power rating with forced cooling.
NOTE In case of forced cooling, the back-to-back method to carry out the temperature rise test for the
transformer is preferred and is subject to agreement between manufacturer and purchaser at enquiry stage.
Temperatures measured by the back-to-back tests correspond more closely to those obtained in practice during
normal operation.
6 Rating plate
See IEC 60076-1 and IEC 60076-11.
7 Tests
7.1 List and classification of tests (routine, type and special tests)
See IEC 60076-1 and IEC 60076-11.
7.2 Routine tests
Tests described in IEC 60076-1 for liquid-immersed transformers and IEC 60076-11 for dry
type transformers apply.
NOTE Impulse test for all transformers type and partial discharge tests for liquid-immersed transformers can be
justified on each unit by agreement between purchaser and manufacturer at enquiry stage. See IEC 60076-13 for
this kind of test cycle for partial discharge test on liquid-immersed transformers.

– 14 – 60076-16 © IEC:2011
7.3 Type tests
Tests described in IEC 60076-1 for liquid-immersed transformers and IEC 60076-11 for dry
type transformers shall apply.
Partial discharge for liquid-immersed transformers less 72,5 kV are not defined in
IEC 60076-3 and consequently test condition of IEC 60076-13 shall apply.
NOTE Chopped wave test can be a part of type testing by agreement between purchaser and manufacturer at
enquiry stage.
7.4 Special tests
7.4.1 General
Special tests shall be defined at enquiry stage by the purchaser.
7.4.2 Chopped wave test
The extension of the lightning impulse test to include impulses chopped on the tail as a
special test is recommended after agreement at enquiry stage.
The peak value of the chopped impulse shall be 110 % of the specified full wave impulse
(BIL).
Clause 14 of IEC 60076-3:2000 shall apply.
7.4.3 Electrical resonance frequency test
The method is described in A.4.
7.4.4 Climatic tests
IEC 60076-11 shall apply for dry type transformers.
7.4.5 Environmental test E3
The transformer shall be placed in a test chamber in which temperature and humidity are kept
under control.
The volume of the chamber shall be at least five times that of the rectangular box
circumscribing the transformer. The clearances from any part of the transformer to walls,
ceiling and spraying nozzles shall be not less than the smallest phase-to-phase clearance
between live parts of the transformer (see IEC 60076-3) and not less than 150 mm according
to 26.3.1 of IEC 60076-11:2004.
The temperature of the air in the test chamber shall be such as to ensure condensation on the
transformer.
The humidity in the chamber shall be maintained above 95 %. This may be achieved by
periodically or continuously atomizing a suitable amount of water.
The conductivity of the water shall be in the range of 3,6 S/m to 4 S/m.
The position of the mechanical atomizers shall be chosen in such a way that the transformer
is not directly sprayed.
The transformer shall be kept in air having a relative humidity above 95 % for not less than
6 h, without being energized.
60076-16 © IEC:2011 – 15 –
Within 5 min thereafter, the transformer shall be submitted to a test with induced voltage as
follows:
a) transformers with windings intended for connection to a system which are solidly earthed
or earthed through a low impedance shall be energised at a voltage of 1,1 times the rated
voltage for a period of 15 min;
b) transformers with windings intended for connection to systems which are isolated or
earthed through considerable impedance shall be submitted to a test with induced voltage
for 3 successive periods of 5 min. During the test, each high voltage terminal in turn shall
be connected to earth and a voltage of 1,1 times the rated voltage shall be applied
between the other terminals and earth. The three-phase test can be replaced by single-
phase tests with the two non-earthed phase terminals being interconnected.
Preferably the dielectric test should be performed in test chamber.
During the voltage application, no flash over shall occur, and visual inspection shall not show
any serious tracking.
If no information in respect of test condition a) or b) is available, test b) should be performed.
7.4.6 Fire behavior test
IEC 60076-11 shall apply for dry type transformers.
Liquids for immersed transformers are described in IEC 61100.

– 16 – 60076-16 © IEC:2011
Annex A
(informative)
Calculation method and tables
A.1 Cooling of transformer in a naturally ventilated room
A.1.1 Assumptions
The room is cooled by naturally air circulation therefore:
Q is the heat dissipation by forced air circulation (kW)
AF
Q = 0 (A.1)
AF
Q Q are losses dissipated respectively through ceiling and the walls (kW)
c, w
Q = Q = 0 (A.2)
c w
In case of harmonics in load current special considerations shall be taken into account
according A.2 or the transformer shall to be derated.
The heat dissipation through ceiling and the walls is generally low. This quantity is depending
on the heat transfer coefficients of the materials of the walls and ceiling, the surface area of
ceiling and the walls and difference between indoor and outdoor temperatures.
See following Figure A.1:
60076-16 © IEC:2011 – 17 –
A
Q
c
θ
Heat losses = no load losses + 1,1 x load losses
H
Q
w
Transformer
A
θ
IEC  2059/11
Key
A
Air inlet effective cross section (m )
A
Air outlet effective cross section (m )
θ , θ Air temperatures of inlet and outlet (°C)
1 2
H Difference in height between mid outlet surface and mid height of transformer (m)
Q Q Losses dissipated respectively through ceiling and the walls (kW)
c, w
Figure A.1 – Heat dissipation in a natural ventilated room
A.1.2 Data for the calculation of ventilation
Transformer produces losses that are dissipated in the room. This subclause gives the
calculation of these losses.
Δθ is the air temperature rise (K):
a
Δθ = θ −θ
approximate value = 15 K (A.3)
a 2 1
is the transformer no load losses (kW);
NLL
are the transformer nominal load losses at reference temperature (kW);
LL
are the transformer heat losses in the room (kW);
HL
Heat losses = No load losses + 1,1 × Load losses:

– 18 – 60076-16 © IEC:2011
HL = NLL +1,1 × LL (kW) (A.4)
NOTE Value 15 K indicated above is common empirical value from the experience of the manufacturers.
A.1.3 Output
Losses produces by the transformers should be dissipated outside the room. This annex
allows to give the surface of the air inlet.
A
is the air inlet effective cross section (m );
A
is the air outlet effective cross section (m ):
A A
/ > 1,1 (minimum 10 % more ) (A.5)
2 1
Q
is the heat dissipation in the transformer’s room (kW):
tot
Q = Q + Q + Q
(kW) (A.6)
tot nac wc AF
Q
is the dissipation by natural air circulati
...