SIST EN 60076-3:2014

SLOVENSKI STANDARD
01-januar-2014
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SIST EN 60076-3:2002
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Power transformers - Part 3: Insulation levels, dielectric tests and external clearances in
air
Leistungstransformatoren - Teil 3: Isolationspegel, Spannungsprüfungen und äußere
Abstände in Luft
Transformateurs de puissance - Partie 3: Niveaux d'isolement, essais diélectriques et
distances d'isolement dans l'air
Ta slovenski standard je istoveten z: EN 60076-3:2013
ICS:
29.180 Transformatorji. Dušilke Transformers. Reactors
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

EUROPEAN STANDARD
EN 60076-3
NORME EUROPÉENNE
November 2013
EUROPÄISCHE NORM
ICS 29.180 Supersedes EN 60076-3:2001

English version
Power transformers -
Part 3: Insulation levels, dielectric tests and external clearances in air
(IEC 60076-3:2013)
Transformateurs de puissance -  Leistungstransformatoren -
Partie 3: Niveaux d'isolement, essais Teil 3: Isolationspegel,
diélectriques et distances d'isolement Spannungsprüfungen und äußere
dans l'air Abstände in Luft
(CEI 60076-3:2013) (IEC 60076-3:2013)

This European Standard was approved by CENELEC on 2013-09-04. CENELEC members are bound to comply
with the CEN/CENELEC Internal Regulations which stipulate the conditions for giving this European Standard
the status of a national standard without any alteration.

Up-to-date lists and bibliographical references concerning such national standards may be obtained on
application to the CEN-CENELEC Management Centre or to any CENELEC member.

This European Standard exists in three official versions (English, French, German). A version in any other
language made by translation under the responsibility of a CENELEC member into its own language and notified
to the CEN-CENELEC Management Centre has the same status as the official versions.

CENELEC members are the national electrotechnical committees of Austria, Belgium, Bulgaria, Croatia, Cyprus,
the Czech Republic, Denmark, Estonia, Finland, Former Yugoslav Republic of Macedonia, France, Germany,
Greece, Hungary, Iceland, Ireland, Italy, Latvia, Lithuania, Luxembourg, Malta, the Netherlands, Norway, Poland,
Portugal, Romania, Slovakia, Slovenia, Spain, Sweden, Switzerland, Turkey and the United Kingdom.

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

CEN-CENELEC Management Centre: Avenue Marnix 17, B - 1000 Brussels

© 2013 CENELEC - All rights of exploitation in any form and by any means reserved worldwide for CENELEC members.
Ref. No. EN 60076-3:2013 E
Foreword
The text of document 14/745/FDIS, future edition 3 of IEC 60076-3, 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) 2014-06-04
• latest date by which the document has
to be implemented at national level by
publication of an identical national
standard or by endorsement
(dow) 2016-09-04
• latest date by which the national
standards conflicting with the
document have to be withdrawn
This document supersedes EN 60076-3:2001.
EN 60073-3:2001:
– Three categories of transformer are clearly identified together with the relevant test requirements,
these are summarised in Table 1.
– Switching impulse levels are defined for all Um>72,5kV.
– The procedure for Induced voltage tests with PD has been revised to ensure adequate phase to
phase test voltages.
– The AC withstand test has been redefined (LTAC instead of ACSD).
– Induced voltage tests are now based on Ur rather than Um.
– New requirements for impulse waveshape (k factor) have been introduced.
– Tables of test levels have been merged and aligned with IEC 60071-1:2010.
– Additional test levels have been introduced for Um > 800kV.
– A new Annex E has been introduced, which sets out the principles used in assigning the tests, test
levels and clearances in air.
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 60076-3:2013 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 60071-2 NOTE  Harmonized as EN 60071-2.
IEC 60076-4 NOTE  Harmonized as EN 60076-4.
IEC 60214-1 NOTE  Harmonized as EN 60214-1.
IEC 61083-1 NOTE  Harmonized as EN 61083-1.
IEC 61083-2 NOTE  Harmonized as EN 61083-2.
IEC 62271-1 NOTE  Harmonized as EN 62271-1.

- 3 - EN 60076-3:2013
Annex ZA
(normative)
Normative references to i nternational publications
with their corresponding European publications

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

NOTE  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 60050-421 International electrotechnical vocabulary - -
(IEV) -
Chapter 421: Power transformers and
reactors
IEC 60060-1 High-voltage test techniques – EN 60060-1
Part 1: General definitions and test
requirements
IEC 60060-2 High-voltage test techniques - EN 60060-2
Part 2: Measuring systems
IEC 60071-1 Insulation co-ordination - EN 60071-1
Part 1: Definitions, principles and rules

IEC 60076-1 Power transformers - EN 60076-1
Part 1: General
IEC 60137 Insulated bushings for alternating voltages EN 60137
above 1 000 V
IEC 60270  High-voltage test techniques - Partial EN 60270
discharge measurements
IEC 60076-3 ®
Edition 3.0 2013-07
INTERNATIONAL
STANDARD
NORME
INTERNATIONALE
Power transformers –
Part 3: Insulation levels, dielectric tests and external clearances in air

Transformateurs de puissance –

Partie 3: Niveaux d'isolement, essais diélectriques et distances d'isolement

dans l'air
INTERNATIONAL
ELECTROTECHNICAL
COMMISSION
COMMISSION
ELECTROTECHNIQUE
PRICE CODE
INTERNATIONALE
CODE PRIX XA
ICS 29.180 ISBN 978-2-83220-830-4

– 2 – 60076-3 © IEC:2013
CONTENTS
FOREWORD . 4
INTRODUCTION . 6
1 Scope . 7
2 Normative references . 7
3 Terms and definitions . 7
4 General . 8
5 Highest voltage for equipment and rated insulation level . 10
6 Transformers with re-connectable windings . 11
7 Dielectric tests . 12
7.1 Overview . 12
7.2 Test requirements . 13
7.2.1 General . 13
7.2.2 Test voltage levels . 14
7.2.3 Test sequence . 17
7.3 Test requirements for specific transformers . 17
7.3.1 Tests for transformers with U ≤ 72,5 kV . 17
m
7.3.2 Tests on transformers with 72,5 kV < U ≤ 170 kV . 18
m
7.3.3 Tests on Transformers with U > 170 kV . 19
m
7.4 Assigning U and test voltages to the neutral terminal of a winding . 20
m
7.4.1 Transformers with U ≤ 72,5 kV . 20
m
7.4.2 Transformers with U > 72,5 kV . 20
m
8 Dielectric tests on transformers that have been in service . 20
9 Insulation of auxiliary wiring (AuxW) . 21
10 Applied voltage test (AV) . 21
11 Induced voltage tests (IVW and IVPD) . 22
11.1 General . 22
11.2 Induced voltage withstand test (IVW) . 22
11.3 Induced voltage test with partial discharge measurement (IVPD) . 23
11.3.1 General . 23
11.3.2 Test duration and frequency . 23
11.3.3 Test sequence . 23
11.3.4 Partial discharge (PD) measurement . 24
11.3.5 Test acceptance criteria . 25
12 Line terminal AC withstand test (LTAC) . 25
13 Lightning impulse tests (LI, LIC, LIN, LIMT) . 26
13.1 Requirements for all lightning impulse tests . 26
13.1.1 General . 26
13.1.2 Tap positions . 26
13.1.3 Records of tests . 26
13.1.4 Test connections. 27
13.2 Full wave lightning impulse test (LI) . 28
13.2.1 Wave shape, determination of test voltage value and tolerances . 28
13.2.2 Tests on transformers without non-linear elements . 29
13.2.3 Tests on transformers with non-linear elements . 30
13.3 Chopped wave lightning impulse test (LIC) . 31

60076-3 © IEC:2013 – 3 –
13.3.1 Wave shape. 31
13.3.2 Tests on transformers without non-linear elements . 31
13.3.3 Tests on transformers with non-linear elements . 32
13.4 Lightning impulse test on a neutral terminal (LIN) . 33
13.4.1 General . 33
13.4.2 Waveshape. 33
13.4.3 Test sequence . 34
13.4.4 Test criteria . 34
14 Switching impulse test (SI) . 34
14.1 General . 34
14.2 Test connections . 34
14.3 Waveshape . 35
14.4 Test sequence . 35
14.5 Test criteria . 35
15 Action following test failure . 36
16 External clearances in air . 36
16.1 General . 36
16.2 Clearance requirements . 37
Annex A (informative) Application guide for partial discharge measurements on

transformers . 40
Annex B (informative) Overvoltage transferred from the high-voltage winding to a low-
voltage winding . 45
Annex C (informative) Information on transformer insulation and dielectric tests to be
supplied with an enquiry and with an order . 47
Annex D (informative) Neutral insulation voltage level calculation . 50
Annex E (informative) Basis for dielectric tests, insulation levels and clearances . 53
Bibliography . 56

Figure 1 – Time sequence for the application of test voltage for induced voltage test with
partial discharge measurement (IVPD) . 24
Figure A.1 – Calibration circuit for partial discharge measurement using the test tap of
condenser type bushing. 41
Figure A.2 – Circuit for partial discharge measurement using a high-voltage coupling
capacitor . 42
Figure B.1 – Equivalent circuit for capacitive transfer of overvoltage . 46

Table 1 – Requirements and tests for different categories of windings. 14
Table 2 – Test voltage levels (1 of 2) . 15
Table 3 – Test voltage levels used in special cases . 16
Table 4 – Minimum clearances in air (1 of 2) . 38

– 4 – 60076-3 © IEC:2013
INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
POWER TRANSFORMERS –
Part 3: Insulation levels, dielectric tests
and external clearances in air

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 60076-3 has been prepared by IEC technical committee 14: Power
transformers.
This third edition of IEC 60076-3 cancels and replaces the second edition published in 2000,
and constitutes a technical revision. The main changes from the previous edition are as follows:
– Three categories of transformer are clearly identified together with the relevant test
requirements, these are summarised in Table 1.
– Switching impulse levels are defined for all U > 72,5kV.
m
– The procedure for Induced voltage tests with PD has been revised to ensure adequate
phase to phase test voltages.
– The AC withstand test has been redefined (LTAC instead of ACSD).
– Induced voltage tests are now based on U rather than U .
r m
– New requirements for impulse waveshape (k factor) have been introduced.

60076-3 © IEC:2013 – 5 –
– Tables of test levels have been merged and aligned with IEC 60071-1:2010.
– Additional test levels have been introduced for U > 800kV.
m
– A new Annex E has been introduced, which sets out the principles used in assigning the
tests, test levels and clearances in air.
The text of this standard is based on the following documents:
FDIS Report on voting
14/745/FDIS 14/749/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 the parts in the IEC 60076 series, under the general title Power transformers, can
be found on the IEC website.
The committee has decided that the contents of this publication will remain unchanged until the
stability date indicated on the IEC web site under "http://webstore.iec.ch" in the data related to
the specific publication. At this date, the publication will be
• reconfirmed,
• withdrawn,
• replaced by a revised edition, or
• amended.
– 6 – 60076-3 © IEC:2013
INTRODUCTION
This part of IEC 60076 specifies the insulation requirements and the corresponding insulation
tests with reference to specific windings and their terminals. It also recommends external
clearances in air (Clause 16).
The insulation levels and dielectric tests which are specified in this standard apply to the
internal insulation only. Whilst it is reasonable that the rated withstand voltage values which are
specified for the internal insulation of the transformer should also be taken as a reference for
its external insulation, this may not be true in all cases. A failure of the non-self-restoring
internal insulation is catastrophic and normally leads to the transformer being out of service for
a long period, while an external flashover may involve only a short interruption of service
without causing lasting damage. Therefore, it may be that, for increased safety, higher test
voltages are specified by the purchaser for the internal insulation of the transformer than for
the external insulation of other components in the system. When such a distinction is made,
the external clearances should be adjusted to fully cover the internal insulation test
requirements.
Annex E sets out some of the principles used in assigning the tests, test levels and clearances
in air to the transformer according to the highest voltage for equipment U .
m
60076-3 © IEC:2013 – 7 –
POWER TRANSFORMERS –
Part 3: Insulation levels, dielectric tests
and external clearances in air

1 Scope
This International Standard applies to power transformers as defined by and in the scope of
IEC 60076-1. It gives details of the applicable dielectric tests and minimum dielectric test levels.
Recommended minimum external clearances in air between live parts and between live parts
and earth are given for use when these clearances are not specified by the purchaser.
For categories of power transformers and reactors which have their own IEC standards, this
standard is applicable only to the extent in which it is specifically called up by cross reference
in the other standards.
2 Normative references
The following documents, in whole or in part, are normatively referenced in this document and
are indispensable for its application. For dated references, only the edition cited applies. For
undated references, the latest edition of the referenced document (including any amendments)
applies.
IEC 60050-421, International Electrotechnical Vocabulary (IEV) – Chapter 421: Power
transformers and reactors
IEC 60060-1, High-voltage test techniques – Part 1: General definitions and test requirements
IEC 60060-2, High-voltage test techniques – Part 2: Measuring systems
IEC 60071-1, Insulation co-ordination – Part 1: Definitions, principles and rules
IEC 60076-1, Power transformers – Part 1: General
IEC 60137, Insulated bushings for alternating voltages above 1 000 V
IEC 60270, High-voltage test techniques – Partial discharge measurements
3 Terms and definitions
For the purposes of this document, the terms and definitions given in IEC 60076-1, IEC 60050-
421 and the following apply.
3.1
highest voltage for equipment applicable to a transformer winding
U
m
highest r.m.s. phase-to-phase voltage in a three-phase system for which a transformer winding
is designed in respect of its insulation

– 8 – 60076-3 © IEC:2013
3.2
rated voltage of a winding
U
r
voltage assigned to be applied, or developed at no-load, between the terminals of an untapped
winding, or a tapped winding connected on the principal tapping, for a three-phase winding it is
the voltage between line terminals
Note 1 to entry: The rated voltages of all windings appear simultaneously at no-load when the voltage applied to
one of them has its rated value.
Note 2 to entry: For single-phase transformers intended to be connected in star to form a three-phase bank or to be
connected between the line and the neutral of a three phase system, the rated voltage is indicated as the phase-to-
phase voltage, divided by 3 , for example 400 / 3 kV.
Note 3 to entry: For single phase transformers intended to be connected between phases of a network, the rated
voltage is indicated as the phase-to-phase voltage.
Note 4 to entry: For the series winding of a three-phase series transformer, which is designed as an open winding,
the rated voltage is indicated as if the windings were connected in star.
[SOURCE: IEC 60076-1:2011, 3.4.3]
3.3
rated insulation level
set of rated withstand voltages which characterise the dielectric strength of the insulation
3.4
rated withstand voltage
value of the assigned test voltage applied in one of the standard dielectric tests that proves
that the insulation complies with the assigned test voltage
3.5
uniform insulation of a transformer winding
insulation of a transformer winding that has all its ends connected to terminals with the same
rated insulation level
3.6
non-uniform insulation of a transformer winding
insulation of a transformer winding when it has a neutral terminal end for direct or indirect
connection to earth, and is designed with a lower insulation level than that assigned to the line
terminal
Note 1 to entry: Non-uniform insulation may also be termed graded insulation.
4 General
The insulation requirements for power transformers and the corresponding insulation tests are
given with reference to specific windings and their terminals.
For liquid-immersed or gas-filled transformers, the requirements apply to the internal insulation
only. Any additional requirements or tests regarding external insulation which are deemed
necessary shall be subject to agreement between manufacturer and purchaser. If the
purchaser does not specify any particular requirements for external clearances then the
provisions of Clause 16 shall apply. If the purchaser intends to make the connections to the
transformer in a way which may reduce the clearances provided by the transformer alone, this
should be indicated in the enquiry.
Bushings shall be subject to separate type and routine tests according to IEC 60137 (including
appropriate bushing test levels for the particular transformer test level), which verify their
phase-to-earth insulation, external as well as internal.

60076-3 © IEC:2013 – 9 –
When a transformer is specified for operation at an altitude higher than 1 000 m, clearances
shall be designed accordingly. It may then be necessary to select bushings designed for higher
insulation levels than otherwise required for operation at lower altitudes, see Clause 16 of this
standard and IEC 60137.
The manufacturer may shield the bushing terminals if necessary during the dielectric tests but
any shielding of the earthed parts closest to the terminals shall form part of the transformer
structure in-service except for shielding required only during partial discharge measurement.
Bushings and tap-changers are specified, designed and tested in accordance with the relevant
IEC standards. The dielectric tests on the complete transformer constitute a check on the
correct application and installation of these components. In the case of tap-changers which
according to IEC 60214-1 are not subjected to dielectric routine tests at the tap-changer
manufacturer’s works then the tests performed according to this standard also serve as the
only dielectric tests routinely performed on this component.
The temperature of the insulation system shall not be less than 10 °C during the tests, but
temperatures higher than those given in IEC 60076-1 may be used.
The transformer shall be completely assembled as in service in respect of all elements that
might influence the dielectric strength of the transformer. It is normally assumed that the
insulating liquid or gas is not circulated during the tests and coolers do not need to be
assembled. Any equipment designed to collect or detect free gas produced by faults in the
insulation shall be installed and monitored during the tests. If free gas is detected during any
test, the nature and cause of the gas shall be investigated and any further actions shall be
agreed between purchaser and manufacturer.
NOTE 1 External overvoltage protection devices such as surge arresters do not need to be assembled and
bushing spark gaps can be removed or their spacing increased to avoid operation during the tests.
NOTE 2 It is common practice for larger transformers for oil samples to be taken for dissolved gas analysis before
and after dielectric tests.
Liquid immersed transformers shall be tested with the same type (mineral, ester, silicone, etc.)
and specification (with respect to the properties that might affect the test performance) of liquid
that it will contain in service.
NOTE 3 Some purchasers can require that the insulating liquid be circulated on OD cooled transformers during an
IVPD test to detect the possibility of static electrification, but this is a very specific requirement and is not covered
by this standard.
Transformers for cable box connection or direct connection to metal-enclosed SF installations
should be designed so that temporary connections can be made for dielectric tests, using
temporary bushings, if necessary. By agreement between manufacturer and purchaser, the
service liquid to SF bushings may be replaced by appropriate liquid to air bushings for test, in
this case the design of the end of the bushing inside the transformer including the positions of
the live parts and the clearances of the substitute bushings inside the transformer shall be the
same (within the normal variation of dimensions of the bushing associated with manufacturing
tolerances) as those of the in-service bushings.
When the manufacturer intends to use non-linear elements (for example surge arresters or
spark gaps), built into the transformer or tap-changer or externally fitted, for the limitation of
overvoltage transients, this shall be brought to the purchaser's attention by the manufacturer at
the tender and order stage and shall be indicated on the transformer rating plate circuit
diagram.
If any terminals of the transformer are to be left open when the transformer is energised in
service then consideration needs to be given to the possibility of a transferred voltage
occurring on the open terminals, see Annex B. During the lightning impulse tests all non-tested
line and neutral terminals are normally connected to earth, see Clause 13.

– 10 – 60076-3 © IEC:2013
5 Highest voltage for equipment and rated insulation level
A value of highest voltage for equipment U (see Clause 3) is assigned to both the line and
m
neutral end of each winding, see IEC 60076-1.
The rules for dielectric testing depend on the value of U . When rules about tests for different
m
windings in a transformer are in conflict, the rule for the winding with the highest U value shall
m
apply for the whole transformer.
Series windings (for example found in autotransformers and phase shifting transformers)
where the rated voltage of the winding is less than the rated voltage of the system, shall be
assigned a value of U corresponding to the rated voltage of the highest voltage system to
m
which the winding is connected.
Standardized values of U are listed in Table 2. Unless otherwise specified, the value to be
m
used for a transformer winding is the one equal to, or nearest above, the value of the rated
voltage of the winding.
NOTE 1 Single-phase transformers intended for connection in star to form a three-phase bank are designated by
phase-to-phase rated voltage divided by , for example kV. The phase-to-phase value determines the
3 400 / 3
choice of U in this case, consequently, U = 420 kV (see also IEC 60076-1). The same principle applies to single-
m m
phase transformers intended for use in a single phase system in that the maximum phase to earth voltage is
multiplied by 3 to obtain the equivalent U in order to define the test voltages.
m
NOTE 2 For transformer windings intended to be used for example in railway supply applications where two
opposite phase to earth voltages are supplied, U relates to the phase to phase voltage unless otherwise specified.
m
NOTE 3 It might happen that certain tapping voltages are chosen slightly higher than a standardized value of U ,
m
but the system to which the winding will be connected has a system highest voltage which stays within the standard
value. The insulation requirements are to be coordinated with actual conditions, and therefore this standard value
can be accepted as U for the transformer, and not the nearest higher value.
m
NOTE 4 In certain applications with very special conditions the specification of other combinations of withstand
voltages can be justified. In such cases, general guidance should be obtained from IEC 60071-1.
NOTE 5 In certain applications, delta-connected windings are earthed through one of the external terminals. In
those applications, a higher withstand voltage with respect to the highest voltage for equipment U can be required
m
for this winding and would need to be agreed between manufacturer and purchaser.
The highest voltage for equipment U and the rated insulation level (the set of assigned rated
m
withstand voltages) determine the dielectric characteristics of a transformer. These
characteristics are verified by a set of dielectric tests, see Clause 7.
The value of U and the rated insulation level which are assigned to each winding of a
m
transformer are part of the information to be supplied with an enquiry and with an order. If there
is a winding with non-uniform insulation, the assigned U and the rated insulation level of the
m
neutral terminal may also be specified by the purchaser, see 7.4.
The rated insulation level shall be characterised as follows:
U / SI / LI / LIC / AC with the associated values (see examples below) for the line terminals of
m
each winding
If the winding does not have an assigned SI or LIC withstand level then the abbreviation is
omitted from the rating so for terminals without an assigned switching impulse withstand level
or chopped wave lightning impulse withstand level and for neutral terminals the abbreviation
would be:
U / LI / AC together with the associated values
m
60076-3 © IEC:2013 – 11 –
If the neutral terminal of a winding has the same rated insulation level as the line terminal then
the rated insulation level of the neutral does not need to be shown separately.
The abbreviations here and in the examples below have the following meaning:
SI is the rated switching impulse withstand voltage level for the line terminals of the winding
with the highest U ;
m
LI is the rated lightning impulse withstand voltage level for the terminal of each individual
winding;
LIC is the rated lightning impulse withstand voltage level for the line terminals of each
individual winding if a chopped wave lightning impulse test was performed;
AC is the highest rated AC withstand voltage level to earth designed for the terminals of each
winding.
NOTE 6 The AC is the value for which the transformer is designed, this is generally the highest AC voltage
required to be achieved on test.
HV high voltage;
LV low voltage;
MV medium voltage (intermediate voltage IEC 60076-1);
N neutral.
The rated withstand voltages for all windings shall appear on the rating plate.
The principles of the standard abbreviated notation are shown in some examples below.
EXAMPLE 1
Transformer with a nominal rated voltage of 66 / 11 kV U (HV) = 72,5 kV and U (LV) = 12 kV, both uniformly
m m
insulated, Y connected, the rating plate would read:
HV U 72,5 / LI 325 / AC 140 kV
m
LV U 12 / LI 75 / AC 28 kV
m
EXAMPLE 2
U (HV) line = 245 kV, Y connected (220 kV rated voltage);
m
U (HV) neutral = 52 kV;
m
U (MV) line = 72,5 kV, uniform insulation, Y connected (LIC not specified);
m
U (LV) line = 24 kV, D connected LIC not required.

m
The rating plate would read:
HV U 245 / SI 750 / LI 950 / LIC 1045 / AC 395 kV
m
HVN U 52 / LI 250 / AC 95 kV
m
MV U 72,5 / LI 325 / AC 140 kV
m
LV U 24 / LI 125 / AC 50 kV
m
6 Transformers with re-connectable windings
Unless otherwise specified, windings which are specified to be capable of being connected in
more than one configuration for service shall be tested in each configuration.

– 12 – 60076-3 © IEC:2013
7 Dielectric tests
7.1 Overview
The dielectric capability of the transformer insulation is verified by dielectric tests. The
following is a general explanation of the different tests.
– Full wave lightning impulse test for the line terminals (LI), see 13.2
The test is intended to verify the capability of the transformer to withstand fast rise time
transients in service typically associated with lightning strikes. The test verifies the
withstand strength of the transformer under test, when the impulse is applied to its line
terminals. The test contains high frequency voltage components and produces non-uniform
stresses in the winding under test different to those for an alternating voltage test.
– Chopped wave lightning impulse test for the line terminals (LIC), see 13.3
As well as covering the intention of the LI test, this test is intended to verify the capability of
the transformer to withstand some high frequency phenomena that may occur in service.
For this test the lightning impulse test includes both full wave impulses and impulses
chopped on the tail to produce a very high rate of change of voltage. The chopped wave
test voltage impulse has a higher peak value and contains higher frequency components
than the full wave impulse.
NOTE 1 According to this standard the LIC test is specified for each winding separately. For example, if a
routine LIC test is required by this standard on the highest voltage winding this would not lead automatically to
LIC tests being required on other winding(s) with U ≤ 170 kV unless LIC tests are specified specifically for
m
these windings by the purchaser.
– Lightning impulse test for the neutral terminal (LIN), see 13.4
The test is intended to verify the impulse withstand voltage of the neutral terminal and it’s
connected winding(s) to earth and other windings, and along the winding(s) under test.
– Switching impulse test for the line terminal (SI), see Clause 14
The test is intended to verify the capability of the transformer to withstand slow rise time
transient voltages typically associated with switching operations in service. The test verifies
the switching impulse withstand strength of the line terminals and the connected winding(s)
to earth and other windings. The test also verifies the withstand strength between phases
and along the winding(s) under test. This is a single-phase test. The voltage is inductively
distributed through all windings of the transformer, line terminals are open circuit for the
test and the line terminals of the tested phase experience a voltage during the test
approximately determined by the transformer turns ratio.
The voltage distribution in the tested phase is similar to that experienced during an induced
voltage withstand test.
– Applied voltage test (AV), see Clause 10
The test is intended to verify the alternating voltage withstand strength of the line and
neutral terminals and their connected windings to earth and other windings. The voltage is
applied to all the terminals of a winding, including the neutral, simultaneously so there is no
turn-to-turn voltage.
– Line terminal AC withstand voltage test (LTAC), see Clause 12
The test is intended to verify the alternating voltage (AC) withstand strength of each line
terminal to earth. During the test, voltage appears at one or more of the line terminals. The
test allows the line terminals of a transformer with non-uniform insulation to be tested at the
applied voltage test level applicable to the line terminals.
– Induced voltage withstand test (IVW), see 11.2
The test is intended to verify the alternating voltage withstand strength of each line terminal
and its connected winding(s) to earth and other windings, along the winding(s) under test
and the withstand strength between phases. The test is performed with the transformer

60076-3 © IEC:2013 – 13 –
connected as for service. During the test, symmetrical voltages appear at all the line
terminals and between turns, with no voltage at the neutral. The test is performed with a
three phase voltage on three phase transformers.
– Induced voltage test with PD measurement (IVPD), see 11.3
This test is intended to verify that the transformer will be free of harmful partial discharges
under normal operating conditions. The test voltage is applied in the same way as the
voltage that the transformer will experience in service. During the test, symmetrical
voltages appear at all the line terminals and between turns, with no vo
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