SIST EN 60076-3:2014

Power transformers - Part 3: Insulation levels, dielectric tests and external clearances in air

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.

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

La CEI 60076-3:2013 spécifie les exigences d'isolement et d'essais diélectriques correspondants en faisant référence aux enroulements spécifiques et à leurs bornes. La présente Norme internationale s'applique aux transformateurs de puissance définis dans le domaine d'application de la CEI 60076-1. Elle recommande également les distances d'isolement dans l'air. Elle fournit les détails des essais diélectriques et des niveaux d'essai diélectriques minimum applicables. Des distances d'isolement dans l'air minimales recommandées entre les parties sous tension et entre ces dernières et la terre sont précisées pour être utilisées lorsque ces distances dans l'air ne sont pas spécifiées par l'acheteur. Pour les catégories de transformateurs de puissance et de bobines d'inductance qui disposent d'une norme CEI qui leur est propre, la présente norme est applicable uniquement dans la mesure où il y est fait explicitement référence dans les autres normes. Cette troisième édition de la CEI 60076-3 annule et remplace la deuxième édition parue en 2000, dont elle constitue une révision technique. Les modifications principales par rapport à l'édition antérieure sont les suivantes:   - Trois catégories de transformateurs sont clairement identifiées ainsi que les exigences d'essais correspondantes, elles sont résumées dans le Tableau 1.   - Les niveaux de tenue au choc de manoeuvre sont définis pour toutes les valeurs de U_m > 72,5 kV.  - La procédure d'essais de tension induite avec PD a été révisée pour garantir des tensions d'essais entre phase adéquates.   - L'essai de tenue en tension alternative a été redéfini (LTAC à la place de ACSD).  - Les niveaux d'essais de tenue en tension induite sont à présent fondés sur U_r plutôt que sur U_m.  - De nouvelles exigences relatives à la forme de l'onde de choc (facteur k) ont été introduites.  - Les tableaux des niveaux d'essai ont été fusionnés et leurs valeurs alignés sur celles de la CEI 60071-1:2010.  - Des niveaux d'essai supplémentaires ont été introduits pour U_m > 800 kV.   - Une nouvelle Annexe E a été introduite, qui fixe certains des principes utilisés pour le choix des essais, des niveaux d'essai et des distances d'isolement dans l'air.

Močnostni transformatorji - 3. del: Izolacijski nivoji, dielektrični preskusi in zunanje zračne razdalje

Ta mednarodni standard se uporablja za močnostne transformatorje, ki so opredeljeni v standardu IEC 60076-1 in spadajo na njegovo področje uporabe. Vsebuje podrobnosti glede ustreznih dielektričnih preskusov in minimalnih nivojev dielektričnih preskusov. Priporočljive minimalne zunanje zračne razdalje med deli pod napetostjo ter med deli pod napetostjo in ozemljitvijo so navedene za uporabo, kadar dobavitelj ne navede teh razdalj. Pri kategorijah močnostnih transformatorjev in reaktorjev, ki imajo svoje standarde IEC, se ta standard uporablja samo v obsegu, v katerem se specifično sklicuje nanj s primerjavo v drugih standardih.

General Information

• Status: Published
• Publication Date: 05-Dec-2013
• ICS: 29.180 - Transformers. Reactors
• Technical Committee: ETR - Power transformers
• Current Stage: 6060 - National Implementation/Publication (Adopted Project)
• Start Date: 03-Dec-2013
• Due Date: 07-Feb-2014
• Completion Date: 06-Dec-2013
• Ref Project: EN 60076-3:2013 - Power transformers - Part 3: Insulation levels, dielectric tests and external clearances in air

Relations

• Replaces: SIST EN 60076-3:2002 - Power transformers -- Part 3: Insulation levels, dielectric tests and external clearances in air
• Effective Date: 01-Jan-2014
• Replaces: SIST EN 60076-3:2002 - Power transformers -- Part 3: Insulation levels, dielectric tests and external clearances in air
• Effective Date: 28-Jan-2023
• Amended By: SIST EN 60076-3:2014/A1:2018 - Power transformers - Part 3: Insulation levels, dielectric tests and external clearances in air
• Effective Date: 01-Oct-2018
• Replaces: SIST EN 60076-3:2002 - Power transformers -- Part 3: Insulation levels, dielectric tests and external clearances in air
• Effective Date: 07-Jun-2022

Standards Content (Sample)

SLOVENSKI STANDARD
SIST EN 60076-3:2014
01-januar-2014
1DGRPHãþD
SIST EN 60076-3:2002
0RþQRVWQLWUDQVIRUPDWRUMLGHO,]RODFLMVNLQLYRMLGLHOHNWULþQLSUHVNXVLLQ
]XQDQMH]UDþQHUD]GDOMH
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
SIST EN 60076-3:2014 en
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

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SIST EN 60076-3:2014

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SIST EN 60076-3:2014

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

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SIST EN 60076-3:2014
EN 60076-3:2013 - 2 -
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
EN 60076-3:2013.
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 60076-3:2013 includes the following significant technical changes with respect to
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.

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SIST EN 60076-3:2014
 - 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

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SIST EN 60076-3:2014

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SIST EN 60076-3:2014



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



Warning! Make sure that you obtained this publication from an authorized distributor.

Attention! Veuillez vous assurer que vous avez obtenu cette publication via un distributeur agréé.

® Registered trademark of the International Electrotechnical Commission
Marque déposée de la Commission Electrotechnique Internationale

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SIST EN 60076-3:2014
– 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

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SIST EN 60076-3:2014
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

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SIST EN 60076-3:2014
– 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.

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SIST EN 60076-3:2014
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.

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SIST EN 60076-3:2014
– 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

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SIST EN 60076-3:2014
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

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SIST EN 60076-3:2014
– 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.

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SIST EN 60076-3:2014
60076-3 © IEC:2013 – 9 –
When a tr
...