EN 60076-3:2001

SLOVENSKI SIST EN 60076-3:2002
prva izdaja
STANDARD
oktober 2002
Power transformers - Part 3: Insulation levels, dielectric tests and external
clearances in air (IEC 60076-3:2000 + Corrigendum 2000)
ICS 29.180 Referenčna številka
©  Standard je založil in izdal Slovenski inštitut za standardizacijo. Razmnoževanje ali kopiranje celote ali delov tega dokumenta ni dovoljeno

EUROPEAN STANDARD EN 60076-3
NORME EUROPÉENNE
EUROPÄISCHE NORM May 2001
ICS 29.180 Supersedes HD 398.3 S1:1986 + A1:1995
English version
Power transformers
Part 3: Insulation levels, dielectric tests and external clearances in air
(IEC 60076-3:2000 + corrigendum 2000)
Transformateurs de puissance Leistungstransformatoren
Partie 3: Niveaux d'isolement, essais Teil 3: Isolationspegel, Spannungs-
diélectriques et distances d'isolement prüfungen und äußere Abstände in Luft
dans l'air (IEC 60076-3:2000 + corrigendum 2000)
(CEI 60076-3:2000 + corrigendum 2000)
This European Standard was approved by CENELEC on 2001-01-01. CENELEC members are bound to
comply with the CEN/CENELEC Internal Regulations which stipulate the conditions for giving this European
Standard the status of a national standard without any alteration.
Up-to-date lists and bibliographical references concerning such national standards may be obtained on
application to the Central Secretariat 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 Central Secretariat has the same status as the official versions.
CENELEC members are the national electrotechnical committees of Austria, Belgium, Czech Republic,
Denmark, Finland, France, Germany, Greece, Iceland, Ireland, Italy, Luxembourg, Netherlands, Norway,
Portugal, Spain, Sweden, Switzerland and United Kingdom.
CENELEC
European Committee for Electrotechnical Standardization
Comité Européen de Normalisation Electrotechnique
Europäisches Komitee für Elektrotechnische Normung
Central Secretariat: rue de Stassart 35, B - 1050 Brussels
© 2001 CENELEC - All rights of exploitation in any form and by any means reserved worldwide for CENELEC members.
Ref. No. EN 60076-3:2001 E
Foreword
The text of document 14/347/FDIS, future edition 2 of IEC 60076-3, prepared by IEC TC 14, Power
transformers, was submitted to the IEC-CENELEC parallel vote and was approved by CENELEC as
EN 60076-3 on 2001-01-01.
This European Standard supersedes HD 398.3 S1:1986 + A1:1995.
The following dates were fixed:
– latest date by which the EN has to be implemented
at national level by publication of an identical
national standard or by endorsement (dop) 2001-10-01
– latest date by which the national standards conflicting
with the EN have to be withdrawn (dow) 2004-01-01
Annexes designated "normative" are part of the body of the standard. Annexes designated
"informative" are given for information only. In this standard, annexes D, ZA and ZB are normative and
annexes A, B and C are informative.
Annexes ZA and ZB have been added by CENELEC.
__________
Endorsement notice
The text of the International Standard IEC 60076-3:2000 + corrigendum December 2000 was approved
by CENELEC as a European Standard with the following editorial modification.
In subclause 12.3, delete note 3.
__________
- 3 - EN 60076-3:2001
Annex ZA
(normative)
Normative references to international publications
with their corresponding European publications
This European Standard incorporates by dated or undated reference, provisions from other
publications. These normative references are cited at the appropriate places in the text and the
publications are listed hereafter. For dated references, subsequent amendments to or revisions of any
of these publications apply to this European Standard only when incorporated in it by amendment or
revision. For undated references the latest edition of the publication referred to applies (including
amendments).
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
)
1)
IEC 60060-1 High-voltage test techniques HD 588.1 S1 1991

Part 1: General definitions and test
requirements
2)
1)
IEC 60060-2 Part 2: Measuring systems EN 60060-2 1994

+ A11 1998
IEC 60071-1 1993 Insulation co-ordination EN 60071-1 1995
Part 1: Definitions, principles and rules
)
IEC 60071-2 1976 Part 2: Application guide HD 540.2 S1 1991
2)
1)
IEC 60076-1 Power transformers EN 60076-1 1997

(mod) Part 1: General
IEC 60137 1995 Insulated bushings for alternating EN 60137 1996
voltages above 1 kV
2)
1)
IEC 60270 High-voltage test techniques - Partial EN 60270 2001

discharge measurements
1)
IEC 60722 Guide to the lightning impulse and--

switching impulse testing of power
transformers and reactors
1) 2)
IEC 60790  Oscilloscopes and peak voltmeters for HD 479 S1 1986
impulse tests
)
Undated reference
)
Valid edition at date of issue
)
HD 540.2 S1 is superseded by EN 60071-2:1997, which is based on IEC 60071-2:1996.

Publication Year Title EN/HD Year
2)
1)
IEC 61083-1 Digital recorders for measurements in EN 61083-1 1993

(mod) high-voltage impulse tests
Part 1: Requirements for digital
recorders
2)
1)
IEC 61083-2 Part 2: Evaluation of software used for EN 61083-2 1997

the determination of the parameters of
impulse waveforms
CISPR 16-1 1993 Specification for radio disturbance and--
immunity measuring apparatus and
methods
Part 1: Radio disturbance and immunity
measuring apparatus
____________
Annex ZB
(normative)
Special national conditions
Special national condition: National characteristic or practice that cannot be changed even over a long
period, e.g. climatic conditions, electrical earthing conditions. If it affects harmonization, it forms part of
the European Standard or Harmonization Document.
For the countries in which the relevant special national apply these provisions are normative, for other
countries they are informative.
Clause Special national condition
12.3 France
Partial discharge measurements during short-duration a.c. test on non-uniform insulated
high-voltage windings are not acceptable.
____________
INTERNATIONAL IEC
STANDARD 60076-3
Second edition
2000-03
Power transformers –
Part 3:
Insulation levels, dielectric tests and
external clearances in air
© IEC 2000 Copyright - all rights reserved
No part of this publication may be reproduced or utilized in any form or by any means, electronic or mechanical,
including photocopying and microfilm, without permission in writing from the publisher.
International Electrotechnical Commission, 3, rue de Varembé, PO Box 131, CH-1211 Geneva 20, Switzerland
Telephone: +41 22 919 02 11 Telefax: +41 22 919 03 00 E-mail: inmail@iec.ch  Web: www.iec.ch
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Commission Electrotechnique Internationale
International Electrotechnical Commission
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For price, see current catalogue

60076-3 © IEC:2000 – 3 –
CONTENTS
Page
FOREWORD . 7
INTRODUCTION . 11
Clause
1 Scope . 13
2 Normative references . 13
3 Definitions. 15
4 General. 15
5 Highest voltage for equipment and insulation level. 17
6 Rules for some particular transformers . 21
7 Insulation requirements and dielectric tests – Basic rules. 23
7.1 General. 23
7.2 Insulation requirements . 25
7.3 Dielectric tests . 29
7.4 Insulation requirements and tests for the neutral terminal of a winding. 33
8 Tests on a transformer having a tapped winding . 35
9 Repeated dielectric tests . 37
10 Insulation of auxiliary wiring. 37
11 Separate source AC withstand voltage test . 37
12 Induced AC voltage tests (ACSD, ACLD) . 39
12.1 General. 39
12.2 Short-duration induced AC withstand voltage test (ACSD) for transformers
with uniformly insulated high-voltage windings . 39
12.3 Short-duration AC withstand voltage test (ACSD) for transformers with
non-uniformly insulated high-voltage windings . 45
12.4 Long-duration induced AC voltage test (ACLD) with non-uniformly and/or
uniformly insulated high-voltage windings, according to table 1 . 49
13 Lightning impulse (LI) test . 55
13.1 General. 55
13.2 Test sequence . 57
13.3 Test connections. 57
13.4 Records of test. 61
13.5 Test criteria. 61
14 Test with lightning impulse chopped on the tail (LIC). 61
14.1 General. 61
14.2 Chopping gap and characteristics of the chopping . 63
14.3 Test sequence and test criteria. 63

60076-3 © IEC:2000 – 5 –
Clause Page
15 Switching impulse test (SI) . 63
15.1 General. 63
15.2 Test sequence and records . 65
15.3 Test connections. 65
15.4 Test criteria. 67
16 External clearances in air . 67
16.1 General. 67
16.2 Bushing clearance requirements as determined by transformer insulation
withstand voltages. 69
Annex A (Informative) Application guide for partial discharge measurements during
induced a.c. withstand voltage test on transformers according to 12.2, 12.3 and 12.4 . 85
Annex B (informative) Overvoltage transferred from the high-voltage winding to
a low-voltage winding. . 99
Annex C (informative) Information on transformer insulation and dielectric tests
to be supplied with an enquiry and with an order . 103
Annex D (normative) ACSD . 105

60076-3 © IEC:2000 – 7 –
INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
POWER TRANSFORMERS –
Part 3: Insulation levels, dielectric tests and
external clearances in air
FOREWORD
1) The IEC (International Electrotechnical Commission) is a worldwide organization for standardization comprising
all national electrotechnical committees (IEC National Committees). The object of the 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, the IEC publishes International Standards. 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. The 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 the 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 National Committees.
3) The documents produced have the form of recommendations for international use and are published in the form
of standards, technical specifications, technical reports or guides and they are accepted by the National
Committees in that sense.
4) In order to promote international unification, IEC National Committees undertake to apply IEC International
Standards transparently to the maximum extent possible in their national and regional standards. Any
divergence between the IEC Standard and the corresponding national or regional standard shall be clearly
indicated in the latter.
5) The IEC provides no marking procedure to indicate its approval and cannot be rendered responsible for any
equipment declared to be in conformity with one of its standards.
6) Attention is drawn to the possibility that some of the elements of this International Standard may be the subject
of patent rights. The 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 second edition cancels and replaces the first edition published in 1980, amendment 1
(1981) and IEC 60076-3-1 (1987).
The text of this standard is based on the following documents:
FDIS Report on voting
14/347/FDIS 14/355/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 3.

60076-3 © IEC:2000 – 9 –
Annexes A, B and C are for information only.
Annex D forms an integral part of this standard.
The committee has decided that the contents of this publication will remain unchanged
until 2008. At this date, the publication will be
 reconfirmed;
 withdrawn;
 replaced by a revised edition, or
 amended.
The contents of the corrigendum of December 2000 have been included in this copy.

60076-3 © IEC:2000 – 11 –
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 clearances in
air between live parts of bushings on oil-immersed power transformers and to objects at earth
potential (clause 16). Guidance can be obtained from IEC 60071.
The insulation levels and dielectric tests which are specified in clauses 4, 5, 6 and 7 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 must be adjusted to fully cover the internal
insulation test requirements.
60076-3 © IEC:2000 – 13 –
POWER TRANSFORMERS –
Part 3: Insulation levels, dielectric tests and
external clearances in air
1 Scope
This International Standard applies to single-phase and three-phase oil-immersed power
transformers (including auto-transformers), with the exception of certain small and special
transformers, as defined in the scope of IEC 60076-1. It identifies transformer windings to their
highest voltage for equipment U associated with their corresponding rated insulation levels
m
and details the relevant applicable dielectric tests and minimum external clearances in air
between live parts of bushings and to objects at earth potential.
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 normative documents contain provisions which, through reference in this text,
constitute provisions of this part of IEC 60076. For dated references, subsequent amendments
to, or revisions of, any of these publications do not apply. However, parties to agreements
based on this part of IEC 60076 are encouraged to investigate the possibility of applying the
most recent editions of the normative documents indicated below. For undated references, the
latest edition of the normative document referred to applies. Members of IEC and ISO maintain
registers of currently valid International Standards.
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 60071-1:1993, Insulation coordination – Part 1: Definitions, principles and rules
IEC 60071-2:1976, Insulation coordination – Part 2: Application guide
IEC 60076-1, Power transformers – Part 1: General
IEC 60137:1995, Bushings for alternating voltages above 1 000 V
IEC 60270, Partial discharge measurements
IEC 60722, Guide to the lightning impulse and switching impulse testing of power transformers
and reactors
60076-3 © IEC:2000 – 15 –
IEC 60790, Oscilloscopes and peak voltmeters for impulse tests
IEC 61083-1, Digital recorders for measurements in high-voltage impulse tests – Part 1:
Requirements for digital recorders
IEC 61083-2, Digital recorders for measurements in high-voltage impulse tests – Part 2:
Evaluation of software used for the determination of the parameters of impulse waveforms
CISPR 16-1:1993, Specification for radio disturbance and immunity measuring apparatus and
methods – Part 1: Radio disturbance and immunity measuring apparatus
3 Definitions
For the purpose of this part of IEC 60076, the following definitions apply. Other terms used
have the meanings ascribed to them in IEC 60076-1 or in IEC 60050(421).
3.1
highest voltage for equipment U applicable to a transformer winding
m
the 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
3.2
rated insulation level
a set of standard withstand voltages which characterize the dielectric strength of the insulation
3.3
standard insulation level
a rated insulation level, the standard withstand voltages of which are associated to U as
m
recommended in tables 2 and 3 of IEC 60071-1
3.4
uniform insulation of a transformer winding
the insulation of a transformer winding when all its ends connected to terminals have the same
rated insulation level
3.5
non-uniform insulation of a transformer winding
the 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 assigned for the line
terminal
4 General
The insulation requirements for power transformers and the corresponding insulation tests are
given with reference to specific windings and their terminals.
For oil-immersed 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 supplier and purchaser, including type tests on a
suitable model of the configuration.

60076-3 © IEC:2000 – 17 –
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.
When an oil-immersed 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 those specified for the internal insulation of the
transformer windings, see clause 16 of this standard and 4.2 of IEC 60137.
Bushings are subject to separate type and routine tests according to IEC 60137, which verify
their phase-to-earth insulation, external as well as internal.
It is presupposed that bushings and tap-changers are specified, designed and tested in
accordance with relevant IEC standards. The insulation tests on the complete transformer,
however, constitute a check on the correct application and installation of these components.
The insulation test shall generally be made at the supplier's works with the transformer
approximately at ambient temperature, but at least at 10 °C.
The transformer shall be completely assembled as in service including supervisory equipment.
It is not necessary, however, to fit elements which do not influence the dielectric strength of the
internal insulation, for example, the external cooling equipment.
If a transformer fails to meet its test requirements and the fault is in a bushing, it is permissible
to replace this bushing temporarily with another bushing and continue the test on the
transformer to completion without delay. A particular case arises for tests with partial discharge
measurements, where certain types of commonly used high-voltage bushings create difficulties
because of their relatively high level of partial discharge in the dielectric. When such bushings
are specified by the purchaser, it is permitted to exchange them for bushings of a partial
discharge free type during the testing of the transformer, see annex A.
Transformers for cable box connection or direct connection to metal-enclosed SF installations
should be designed so that temporary connections can be made for insulation tests, using
temporary bushings, if necessary. By agreement, oil/SF bushings may for that reason be
replaced by appropriate oil/air bushings.
When the supplier intends to use non-linear elements or surge arresters, built into the
transformer or externally fitted, for the limitation of transferred overvoltage transients, this shall
be brought to the purchaser's attention at the tender and order stage and it is recommended
that it be indicated on the transformer rating plate circuit diagram.
5 Highest voltage for equipment and insulation level
To each winding of a transformer, both for the line and neutral side, is assigned a value of
highest voltage for equipment U , see 3.1.
m
The rules for coordination of transformer insulation with respect to transient overvoltages are
formulated differently depending on the value of U .
m
60076-3 © IEC:2000 – 19 –
When rules about related tests for different windings in a transformer are in conflict, the rule for
the winding with the highest U value shall apply for the whole transformer.
m
Rules for a number of special classes of transformers are given in clause 6.
Standardized values of U are listed in tables 2 to 4. The value to be used for a transformer
m
winding is the one equal to, or nearest above, the rated value of the winding.
NOTE 1 Single-phase transformers intended for connection in star to form a three-phase bank are designated by
phase-to-earth rated voltage, for example 400/ 3 kV. The phase-to-phase value determines the choice of U in this
m
case, consequently, U = 420 kV.
m
NOTE 2 It may happen that certain tapping voltages are chosen slightly higher than a standardized value of U ,
m
but that 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 should be accepted as U for the transformer, and not the nearest higher value.
m
NOTE 3 In certain applications with very special conditions the specification of other combinations of withstand
voltages may be justified. In such cases, general guidance should be obtained from IEC 60071-1.
NOTE 4 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 may be required
m
for this winding and should be agreed between supplier and purchaser.
The highest voltage for equipment U and its assigned withstand voltages, that is, their
m
insulation level, determine the dielectric characteristics of a transformer. They are verified by a
set of dielectric tests depending on U , see clause 7.
m
The value of U and the insulation level which are assigned to each winding of a transformer
m
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 insulation level of the neutral
m
terminal shall also be specified by the purchaser, see 7.4.3.
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.
The classifications on the insulation design shall independently of the test procedure be
derived from the values in table 2, 3 and 4 or from IEC 60071-1. Since in most cases the long-
duration induced AC tests are quality control tests in respect to service conditions and not
design proving tests, the insulation level shall be characterized as follows:
U is the highest voltage for equipment
m
SI/LI/AC,
where applicable –/LI/AC.
The abbreviations here and in the examples below have the following meaning:
SI is the switching impulse withstand voltage for the line terminals of the winding with the
highest U ;
m
LI is the lightning impulse withstand voltage for the line and neutral terminals of each
individual winding;
AC is the short duration induced and separate source AC withstand voltage for the line and
neutral terminals of each individual winding;
h.v. high voltage;
l.v. low voltage;
m.v. medium voltage.
60076-3 © IEC:2000 – 21 –
Example 1:
U (h.v.) = 72,5 kV and U (l.v.) = 12 kV, both uniformly insulated, Y connected
m m
Insulation levels: h.v. line terminal and neutral LI/AC 325/140 kV
l.v. line terminal and neutral LI/AC 60/28 kV
Example 2:
U (h.v.) line = 245 kV, Y connected;
m
U (h.v.) neutral = 52 kV;
m
U (m.v.) line = 72,5 kV, uniform insulation, Y connected;
m
U (l.v.) line = 24 kV, D connected.
m
Insulation levels: h.v. line terminal SI/LI 650/850 kV
h.v. neutral LI/AC 250/95 kV
m.v. line terminal and neutral LI/AC 325/140 kV
l.v. line terminal LI/AC 125/50 kV
Example 3:
Auto-transformer with U = 420 kV and 145 kV with an assigned U = 17,5 kV for the neutral
m m
for direct earth connection, Y connected. U (l.v.) line terminal = 24 kV, D connected.
m
Insulation levels: h.v. line terminal SI/LI 1 050/1 300 kV
m.v. line terminal LI/AC  550/230 kV
h.v./m.v.-neutral LI/AC  –/38 kV
l.v. line terminal LI/AC  125/50 kV
or if additionally a short-duration induced test is required:
Insulation levels: h.v. line terminal SI/LI/AC 1 050/1 300/570 kV
m.v. line terminal LI/AC  550/230 kV
h.v./m.v. neutral LI/AC  –/38 kV
l.v. line terminal LI/AC  125/50 kV
6 Rules for some particular transformers
In transformers where uniformly insulated windings having different U values are connected
m
together within the transformer (usually auto-transformers), the separate source AC withstand
test voltages shall be determined by the insulation of the common neutral and its assigned U
m.
In transformers which have one or more non-uniformly insulated windings, the test voltages for
the induced withstand voltage test, and for the switching impulse test if used, are determined
by the winding with the highest U value, and the windings with lower U values may not
m m
receive their appropriate test voltages. This discrepancy should normally be accepted. If the
ratio between the windings is variable by tappings, this should be used to bring the test voltage
for the winding with lower U voltage as close as possible to the appropriate value.
m
60076-3 © IEC:2000 – 23 –
During switching impulse tests, the voltages developed across different windings are
approximately proportional to the ratio of numbers of turns. Rated switching impulse withstand
voltages shall only be assigned to the winding with the highest U . Test stresses in other
m
windings are also proportional to the ratio of numbers of turns and are adjusted by selecting
appropriate tappings to come as close as possible to the assigned value in table 4. The
switching impulse test stresses in other windings shall be limited to approximately 80 % of the
assigned lightning impulse withstand voltages at these terminals.
Series windings in booster regulating transformers, phase shifting transformers, etc. where the
rated voltage of the winding is only a small fraction of the voltage of the system, shall have a
value of U corresponding to the system voltage. It is often impracticable to test such trans-
m
formers in formal compliance with this standard, and it should be agreed between the supplier
and the purchaser as to which tests have to be omitted or modified.
For single-phase transformers intended to be connected between phases, as in the case of
railway traction system supplies, higher test values than indicated in this standard may be
necessary.
Special considerations with respect to test connections and number of tests to be performed on
multiple re-connectable transformers shall be agreed at the time of placing the order.
7 Insulation requirements and dielectric tests – Basic rules
Transformer windings are identified by their highest voltage for equipment U associated to
m
their corresponding insulation levels. This clause details the relevant insulation requirements
and applicable dielectric tests. For categories of power transformers and reactors which have
their own IEC standards, the requirements are applicable only to the extent in which they are
specifically called up by cross reference in the other standards.
7.1 General
The basic rules for insulation requirements and dielectric tests are summarized in table 1.
Levels of standard withstand voltages, identified by the highest voltage for equipment U of a
m
winding are given in tables 2, 3 and 4. The choice between the different levels of standard
withstand voltages in these tables depends on the severity of overvoltage conditions to be
expected in the system and on the importance of the particular installation. Guidance may be
obtained from IEC 60071-1.
NOTE 1 Distribution transformers for suburban or rural installation are, in some countries, severely exposed to
overvoltages. In such cases, higher test voltages, lightning impulse tests and other tests on individual units may be
agreed between supplier and purchaser. They should be clearly stated in the enquiry document.
NOTE 2 Other combinations of U may exist in some countries.
m
Information about the selected transformer insulation requirements and dielectric tests shall be
supplied with an enquiry and with an order, see annex C.
The insulation requirements are specified in 7.2. The verification of the withstand voltages by
dielectric tests is given in 7.3. The insulation requirements and tests for the neutral terminal of
a winding are given in 7.4.
60076-3 © IEC:2000 – 25 –
The extension of the lightning impulse test to include impulses chopped on the tail as a special
test is recommended in cases where the transformer is directly connected to GIS by means of
oil/SF bushings or when the transformer is protected by rod gaps. The peak value of the
chopped impulse shall be 10 % higher than for the full impulse.
For transformers with a high-voltage winding having U > 72,5 kV, lightning impulse tests are
m
routine tests for all windings of the transformer.
Table 1 – Requirements and tests for different categories of windings
Tests
Category of Highest voltage for Lightning Switching Long Short Separate
winding equipment U impulse impulse duration AC duration source
m
(LI) (SI) (ACLD) AC (ACSD) AC
kV (see clause (see (see 12.4) (see 12.2 (see
13 and 14) clause 15) or 12.3) clause 11)
Uniform Type Not Not Routine Routine
U ≤ 72,5
m
insulation (note 1) applicable applicable
(note 1)
72,5 < U ≤ 170 Routine Not Special Routine Routine
m
applicable
Uniform and
non-uniform
170 < U < 300 Routine Routine Routine Special Routine
m
insulation
(note 2) (note 2)
Routine Routine Routine Special Routine
U ≥ 300
m
NOTE 1 In some countries, for transformers with U ≤ 72,5 kV, LI tests are required as routine tests, and ACLD
m
tests are required as routine or type tests.
NOTE 2 If the ACSD test is specified, the SI test is not required. This should be clearly stated in the enquiry
document.
7.2 Insulation requirements
The standard dielectric requirements are:
– if applicable in table 1, a standard switching impulse withstand voltage (SI) for the line
terminals according to table 4;
– a standard lightning impulse withstand voltage (LI) for the line terminals according to
table 2, 3 or 4;
– if specified, a standard impulse withstand voltage (LI) for the neutral terminal; for uniform
insulation, the peak value of the impulse voltage being the same as for the line terminals;
for non-uniform insulation, the peak value of the impulse voltage as specified in 7.4.3;
– a standard separate source AC withstand voltage according to table 2, 3 or 4;
– if applicable in table 1, a standard short-duration AC induced withstand voltage (ACSD) for
the line terminals according to table 2, 3 or 4 and 12.2 or 12.3;
– if applicable in table 1, a long-duration induced AC voltage (ACLD) with partial discharge
measurement according to 12.4.

60076-3 © IEC:2000 – 27 –
Table 2 – Rated withstand voltages for transformer windings with
highest voltage for equipment U ≤ 170 kV –
m
Series I based on European practice
Highest voltage for Rated lightning Rated short duration
equipment U impulse withstand induced or separate source
m
voltage AC withstand voltage
kV r.m.s.
kV r.m.s.
kV peak
3,6 10
7,2 20
12 28
17,5 38
24 50
36 70
52 250 95
60 280 115
72,5 325 140
380 150
450 185
550 230
650 275
750 325
NOTE Dotted lines may require additional phase-to-phase withstand tests to prove
that the required phase-to-phase withstand voltages are met.
Low-voltage windings with U ≤ 1,1 kV shall be tested with 3 kV separate source AC withstand
m
voltage.
60076-3 © IEC:2000 – 29 –
7.3 Dielectric tests
The standard dielectric requirements are verified by dielectric tests. They shall, where
applicable and not otherwise agreed upon, be performed in the sequence as given below.
– Switching impulse test (SI) for the line terminal, see clause 15
The test is intended to verify the switching impulse withstand strength of the line terminals
and its connected winding(s) to earth and other windings, the withstand strength between
phases and along the winding(s) under test.
The test is an essential requirement for transformers subjected to a long-duration induced
AC withstand voltage (ACLD) test.
– Lightning impulse test (LI) for the line terminals, see clause 13
The test is intended to verify the impulse withstand strength of the transformer under test,
when the impulse is applied to its line terminals. If the lightning impulse test includes
impulses chopped on the tail (LIC), the impulse test is modified according to clause 14.
– Lightning impulse test (LI) for the neutral terminal, see 13.3.2
The test is intended to verify the impulse withstand voltage of the neutral terminal and its
connected winding(s) to earth and other windings, and along the winding(s) under test.
This test is required if a standard impulse withstand voltage for the neutral is specified.
– Separate source AC withstand voltage test (applied potential test), see clause 11
The test is intended to verify the AC withstand strength of the line and neutral terminals and
their connected windings to earth and other windings.
– Short-duration induced AC withstand voltage test (ACSD), see 12.2 and 12.3
The test is intended to verify the AC withstand strength of each line terminal and its
connected winding(s) to earth and other windings, the withstand strength between phases
and along the winding(s) under test.
The test shall be performed in accordance with 12.2 for uniform insulation and 12.3 for non-
uniform insulation.
For U > 72,5 kV, the test is normally performed with partial discharge measurements to
m
verify partial discharge free operation of the transformer under operating conditions. By
agreement between supplier and purchaser, the partial discharge measurements may also
be performed for U ≤ 72,5 kV.
m
– Long-duration induced AC voltage test (ACLD), see 12.4
This test is not a design proving test, but a quality control test, and is intended to cover
temporary overvoltages and continuous service stress. It verifies partial discharge-free
operation of the transformer under operating conditions.

60076-3 © IEC:2000 – 31 –
Table 3 – Rated withstand voltages for transformer windings
with highest voltage for equipment U ≤ 169 kV –
m
Series II based on North American practice
Highest voltage Rated lightning impulse Rated short-duration induced or
for withstand voltage separate source AC
equipment U withstand voltage
m
kV peak kV r.m.s.
kV r.m.s.
Distribution (note 1) Class II Distribution and Class II
and class I transformers class I transformers
transformers (note 3) transformers
(note 2)
15 95 110 34 34
125 – 40 –
26,4 150 150 50 50
36,5 200 200 70 70
48,3 250 250 95 95
72,5 350 350 140 140
121 350 140
450 185
145 550 230
650 275
750 325
NOTE 1 Distribution transformers transfer electrical energy from a primary distribution circuit to a secondary
distribution circuit.
NOTE 2 Class I power transformers include high-voltage windings of U ≤ 72,5 kV.
m
NOTE 3 Class II power transformers include high-voltage windings of U 121 kV.
≥
m
60076-3 © IEC:2000 – 33 –
Table 4 – Rated withstand voltages for transformer windings
with U > 170 kV
m
Highest voltage for Rated switching impulse Rated lightning impulse Rated short-duration
equipment U withstand voltage withstand voltage induced or separate
m
phase-to-earth source AC withstand
voltage
kV peak
kV r.m.s kV peak kV r.m.s.
550 325
650 360
245 850
750 395
300 950
850 460
362 1050
950 510
850 460
950 510
420 1300
1050 570
550 1425
1175 630
1300 680
1300 note 3
800 1425 note 3
1550 note 3
NOTE 1 Dotted lines are not in line with IEC 60071-1 but are current practice in some countries.
NOTE 2 For uniformly insulated transformers with extremely low values of rated AC insulation levels, special
measures may have to be taken to perform the short-duration AC induced test, see 12.2.
NOTE 3 Not applicable, unless otherwise agreed.
NOTE 4 For voltages given in the last column, higher test voltages may be required to prove that the required
phase-to-phase withstand voltages are met. This is valid for the lower insulation levels assigned to the
different U in the table.
m
7.4 Insulation requirements and tests for the neutral terminal of a winding
7.4.1 General
The necessary insulation level depends on whether or not the neutral terminal is intended to be
directly earthed, left open or earthed via an impedance. When the neutral terminal is not
directly earthed, an overvoltage protective device should be installed between the neutral
terminal and earth in order to limit transient voltages.
NOTE The recommendations below deal with the determination of the necessary minimum withstand voltage for
the neutral terminal. An increase of the value may sometimes easily be arranged and can improve the
interchangeability of the transformer in the system. For non-uniform insulation it may be necessary to design the
winding with higher neutral insulation level because of the test connection to be used for the AC withstand test of
the transformer, see 12.3.
60076-3 © IEC:2000 – 35 –
7.4.2 Directly earthed neutral terminal
The neutral terminal shall be permanently connected to earth, directly or through a current
transformer, but without any intentionally added impedance in the connection.
In this case, the separate source AC withstand voltage shall be at least either 38 kV (European
practice) or 34 kV (North American practice).
No impulse test on the neutral terminal is recommended. During impulse tests on a line
terminal, the neutral shall be connected directly to earth.
7.4.3 Neutral terminal not directly earthed
The neutral terminal is not to be permanently in direct connection to earth. It may be connected
to earth through a considerable impedance (for example arc-suppression coil earthing).
Separate phase-winding neutral terminals may be connected to a regulating transformer.
It is the responsibility of the purchaser to select the overvoltage protective device, to determine
its impulse protection level, and to specify the corresponding impulse withstand voltage for the
neutr
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