SIST EN 60076-2:2011

SLOVENSKI STANDARD
01-junij-2011
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SIST EN 60076-2:1997
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Power transformers - Part 2: Temperature rise for liquid-immersed transformers (IEC
60076-2:2011)
Leistungstransformatoren - Teil 2: Übertemperaturen für flüssigkeitsgefüllte
Transformatoren (IEC 60076-2:2011)
Transformateurs de puissance - Partie 2: Echauffement des transformateurs immergés
dans le liquide (CEI 60076-2:2011)
Ta slovenski standard je istoveten z: EN 60076-2:2011
ICS:
29.180 Transformatorji. Dušilke Transformers. Reactors
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

EUROPEAN STANDARD
EN 60076-2
NORME EUROPÉENNE
April 2011
EUROPÄISCHE NORM
ICS 29.180 Supersedes EN 60076-2:1997

English version
Power transformers -
Part 2: Temperature rise for liquid-immersed transformers
(IEC 60076-2:2011)
Transformateurs de puissance -  Leistungstransformatoren -
Partie 2: Echauffement des Teil 2: Übertemperaturen für
transformateurs immergés dans le liquide flüssigkeitsgefüllte Transformatoren
(CEI 60076-2:2011) (IEC 60076-2:2011)

This European Standard was approved by CENELEC on 2011-03-30. 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, Bulgaria, Croatia, Cyprus,
the Czech Republic, Denmark, Estonia, Finland, France, Germany, Greece, Hungary, Iceland, Ireland, Italy,
Latvia, Lithuania, Luxembourg, Malta, the Netherlands, Norway, Poland, Portugal, Romania, Slovakia, Slovenia,
Spain, Sweden, Switzerland and the United Kingdom.

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

Management Centre: Avenue Marnix 17, B - 1000 Brussels

© 2011 CENELEC - All rights of exploitation in any form and by any means reserved worldwide for CENELEC members.
Ref. No. EN 60076-2:2011 E
Foreword
The text of document 14/669/FDIS, future edition 3 of IEC 60076-2, prepared by IEC TC 14, Power
transformers, was submitted to the IEC-CENELEC parallel vote and was approved by CENELEC as
EN 60076-2 on 2011-03-30.
This European Standard supersedes EN 60076-2:1997.
— the standard is applicable only to liquid immersed transformers;
— the winding hot-spot temperature rise limit was introduced among the prescriptions;
— the modalities for the temperature rise test were improved in relation to the new thermal
requirements;
— five informative annexes were added in order to facilitate the standard application.
Attention is drawn to the possibility that some of the elements of this document may be the subject of
patent rights. CEN and CENELEC shall not be held responsible for identifying any or all such patent
rights.
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) 2011-12-30
– latest date by which the national standards conflicting
with the EN have to be withdrawn (dow) 2014-03-30
Annex ZA has been added by CENELEC.
__________
Endorsement notice
The text of the International Standard IEC 60076-2:2011 was approved by CENELEC as a European
Standard without any modification.
In the official version, for Bibliography, the following notes have to be added for the standards indicated:
[2] IEC 60296:2003 NOTE  Harmonized as EN 60296:2004 (not modified).
[3] IEC 60567:2005 NOTE  Harmonized as EN 60567:2005 (not modified).
[4] IEC 60599:1999 NOTE  Harmonized as EN 60599:1999 (not modified).
[5] IEC 60836:2005 NOTE  Harmonized as EN 60836:2005 (not modified).
[6] IEC 61099:2010 NOTE  Harmonized as EN 61099:2010 (not modified).
__________
- 3 - EN 60076-2:2011
Annex ZA
(normative)
Normative references to international publications
with their corresponding European publications

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

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

IEC 60076-1 (mod) - Power transformers - EN 60076-1 -
Part 1: General
IEC 60076-8 1997 Power transformers - - -
Part 8: Application guide
IEC 60085 2007 Electrical insulation - EN 60085 2008
Thermal evaluation and designation

IEC 61181 2007 Mineral oil-filled electrical equipment - EN 61181 2007
Application of dissolved gas analysis (DGA)
to factory tests on electrical equipment

IEC Guide 115 2007 Application of uncertainty of measurement - -
to conformity assessment activities in the
electrotechnical sector
IEC 60076-2 ®
Edition 3.0 2011-02
INTERNATIONAL
STANDARD
NORME
INTERNATIONALE
colour
inside
Power transformers –
Part 2: Temperature rise for liquid-immersed transformers

Transformateurs de puissance –
Partie 2: Echauffement des transformateurs immergés dans le liquide

INTERNATIONAL
ELECTROTECHNICAL
COMMISSION
COMMISSION
ELECTROTECHNIQUE
PRICE CODE
INTERNATIONALE
CODE PRIX X
ICS 29.180 ISBN 978-2-88912-346-9

– 2 – 60076-2  IEC:2011
CONTENTS
FOREWORD . 4
1 Scope . 6
2 Normative references . 6
3 Terms and definitions . 6
4 Cooling methods . 8
4.1 Identification symbols . 8
4.2 Transformers with alternative cooling methods . 9
5 Normal cooling conditions. 9
5.1 Air-cooled transformers . 9
5.2 Water-cooled transformers . 10
6 Temperature rise limits . 10
6.1 General . 10
6.2 Temperature rise limits at rated power . 10
6.3 Modified requirements for special cooling conditions . 12
6.3.1 General . 12
6.3.2 Air-cooled transformers . 12
6.3.3 Water-cooled transformers . 13
6.4 Temperature rise during a specified load cycle . 13
7 Temperature rise tests . 13
7.1 General . 13
7.2 Temperature of the cooling media . 13
7.2.1 Ambient temperature . 13
7.2.2 Water temperature . 14
7.3 Test methods for temperature rise determination . 14
7.3.1 General . 14
7.3.2 Test by short-circuit method for two winding transformers. 14
7.3.3 Test modification for particular transformers . 15
7.4 Determination of liquid temperatures . 16
7.4.1 Top-liquid temperature . 16
7.4.2 Bottom and average liquid temperatures . 17
7.5 Determination of top, average and bottom liquid temperature rises . 18
7.6 Determination of average winding temperature . 18
7.7 Determination of winding resistance at the instant of shutdown . 19
7.8 Determination of average winding temperature rise at the instant of
shutdown . 19
7.9 Determination of the average winding to liquid temperature gradient . 19
7.10 Determination of the hot-spot winding temperature rise . 20
7.10.1 General . 20
7.10.2 Determination by calculation . 20
7.10.3 Direct measurement during the temperature rise test . 20
7.11 Uncertainties affecting the results of the temperature rise test. 21
7.12 Dissolved gas-in-oil analysis . 21
7.13 Corrections. 21
Annex A (informative) Hot-spot winding temperature rise determination for OFAF and
OFWF cooled transformers based on the top-liquid temperature in tank . 23
Annex B (informative) Methods to estimate the hot-spot winding temperature rises. 25

60076-2  IEC:2011 – 3 –
Annex C (informative) Techniques used in temperature rise testing of liquid-immersed
transformers . 30
Annex D (informative) Dissolved gases analysis for the detection of local overheating . 39
Annex E (informative) Application of optical fibre sensors for winding hot-spot
measurements . 43
Bibliography . 47

Figure B.1 – Temperature rise distribution model for ON cooling methods . 26
Figure B.2 – Value of factor Q as a function of rated power and strand height (W) . 27
Figure B.3 – Typical liquid flow paths in a disk winding with diverting washers . 28
Figure C.1 – Recommended circuit for transformers with a low resistance winding
using two separate direct current sources, one for each winding . 32
Figure C.2 – Alternative recommended circuit using only one direct current source for
both windings. 32
Figure C.3 – Average winding temperature variation after shutdown . 33
Figure C.4 – Extrapolation of the cooling down curve, using the fitting curve
−t/T
w
θ (t ) = A − kt+ Be . 38
w 0
Figure E.1 – Optical fibre sensor application for a disk winding of core type transformer . 45
Figure E.2 – Optical fibre sensor application for a transposed cable of core type
transformer . 45
Figure E.3 – Modality of optical fibre sensor application in the winding spacer of core
type transformer . 46
Figure E.4 – Optical fibre sensor application for high voltage winding of shell type
transformer . 46

Table 1 – Temperature rise limits . 11
Table 2 – Recommended values of temperature rise corrections in case of special
service conditions . 12
Table 3 – Exponents for the corrections of temperature rise test results . 22
Table A.1 – Hot-spot winding temperature rises for some specific transformers
determined from conventional heat run test data combined with calculated hot-spot
winding temperature rise, and from direct fibre-optic measurements . 24
Table C.1 – Example of cooling down curve calculation spreadsheet . 37
Table D.1 – Minimum detectable value S of gases in oil . 40
D
Table D.2 – Admissible limits for gas rate increases . 41
Table E.1 – Minimum recommended number of sensors for three-phase transformers . 43
Table E.2 – Minimum recommended number of sensors for single-phase transformers . 43

– 4 – 60076-2  IEC:2011
INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
POWER TRANSFORMERS –
Part 2: Temperature rise for liquid-immersed transformers

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 60076-2 has been prepared by IEC technical committee 14: Power
transformers.
This third edition cancels and replaces the second edition published in 1993. It is a technical
revision.
This edition includes the following significant technical changes with respect to the previous
edition:
a) the standard is applicable only to liquid immersed transformers;
b) the winding hot-spot temperature rise limit was introduced among the prescriptions;
c) the modalities for the temperature rise test were improved in relation to the new thermal
requirements;
d) five informative annexes were added in order to facilitate the standard application.

60076-2  IEC:2011 – 5 –
The text of this standard is based on the following documents:
FDIS Report on voting
14/669/FDIS 14/676/RVD
Full information on the voting for the approval of this standard can be found in the report on
voting indicated in the above table.
This publication has been drafted in accordance with the ISO/IEC Directives, Part 2.
A list of all parts of the IEC 60076 series can be found, under the general title Power
transformers, on the IEC website.
The committee has decided that the contents of this publication will remain unchanged until
the stability date indicated on the IEC web site under "http://webstore.iec.ch" in the data
related to the specific publication. At this date, the publication will be
• reconfirmed,
• withdrawn,
• replaced by a revised edition, or
• amended.
IMPORTANT – The 'colour inside' logo on the cover page of this publication indicates
that it contains colours which are considered to be useful for the correct
understanding of its contents. Users should therefore print this document using a
colour printer.
– 6 – 60076-2  IEC:2011
POWER TRANSFORMERS –
Part 2: Temperature rise for liquid-immersed transformers

1 Scope
This part of IEC 60076 applies to liquid-immersed transformers, identifies power
transformers according to their cooling methods, defines temperature rise limits and gives
the methods for temperature rise tests.
2 Normative references
The following referenced documents are indispensable for the application of this document.
For dated references, only the edition cited applies. For undated references, the latest edition
of the referenced document (including any amendments) applies.
IEC 60076-1, Power transformers – Part 1: General
IEC 60076-8:1997, Power transformers – Part 8: Application guide
IEC 60085:2007, Electrical insulation – Thermal evaluation and designation
IEC 61181:2007, Mineral oil-filled electrical equipment – Application of dissolved gas analysis
(DGA) to factory tests on electrical equipment
IEC Guide 115:2007, Application of uncertainty of measurement to conformity assessment
activities in the electrotechnical sector
3 Terms and definitions
For the purposes of this document, the terms and definitions given in IEC 60076-1 and the
following apply.
3.1
external cooling medium
the medium external to the transformer cooling system (air or water) into which the heat
produced by the transformer losses is transferred
3.2
internal cooling medium
the liquid in contact with the windings and other transformer parts by means of which the heat
produced by the losses is transferred to the external cooling medium
NOTE The liquid can be mineral oil or other natural and synthetic liquid.
3.3
temperature rise
the difference between the temperature of the part under consideration (for example, the
average winding temperature) and the temperature of the external cooling medium

60076-2  IEC:2011 – 7 –
3.4
top-liquid temperature
θ
o
the temperature of the insulating liquid at the top of the tank, representative of top-liquid in
the cooling flow stream
3.5
top-liquid temperature rise
∆θ
o
the temperature difference between the top-liquid temperature and the external cooling
medium temperature
3.6
bottom-liquid temperature
θ
b
the temperature of the insulating liquid as measured at the height of the bottom of the
windings or to the liquid flowing from the liquid cooling equipment
3.7
bottom-liquid temperature rise
∆θ
b
the difference between the bottom-liquid temperature and the external cooling medium
temperature
3.8
average liquid temperature
θ
om
the average temperature of the top-liquid and bottom liquid temperatures
3.9
average liquid temperature rise
∆θ
om
the difference between the average liquid temperature and the external cooling medium
temperature
3.10
average winding temperature
θ
w
the winding temperature determined at the end of temperature rise test from the measurement
of winding d.c. resistance
3.11
average winding temperature rise
∆θ
w
the difference between the average winding temperature and the external cooling medium
temperature
3.12
average winding gradient
g
the difference between the average winding temperature and the average insulating liquid
temperature
– 8 – 60076-2  IEC:2011
3.13
hot-spot winding temperature
θ
h
the hottest temperature of winding conductors in contact with solid insulation or insulating
liquid
3.14
hot-spot winding temperature rise
∆θ
h
the difference between hot-spot winding temperature and the external cooling medium
temperature
3.15
hot-spot factor
H
a dimensionless factor to estimate the local increase of the winding gradient due to the
increase of additional loss and variation in the liquid flow stream
NOTE H factor is obtained by the product of the Q and S factors (see 3.16 and 3.17).
3.16
Q factor
a dimensionless factor to estimate the increase of the average winding gradient due to the
local increase of the additional loss
3.17
S factor
a dimensionless factor to estimate the local increase of the average winding gradient due to
the variation in the liquid flow stream
3.18
thermally upgraded paper
cellulose-based paper which has been chemically modified to reduce the rate at which the
paper decomposes
A paper is considered as thermally upgraded if it meets the life criteria of the 50 % retention in
tensile strength after 65 000 h in a sealed tube at 110 °C or any other time/temperature
combination given by the equation:
 
15 000 15 000
 − 
 
θ +273 110+273
 h 
Time (h) = 65 000 e (1)
NOTE 1 Ageing effects are reduced either by partial elimination of water forming agents or by inhibiting the
formation of water through the use of stabilizing agents.
NOTE 2 See IEC 60076-7, for an alternative test method based on the nitrogen content.
4 Cooling methods
4.1 Identification symbols
Transformers shall be identified according to the cooling method employed. For liquid-immersed
transformers, this identification is expressed by a four-letter code as described below.
First letter: Internal cooling medium:
• O: mineral oil or synthetic insulating liquid with fire point ≤ 300 °C;

60076-2  IEC:2011 – 9 –
• K: insulating liquid with fire point > 300 °C;
• L: insulating liquid with no measurable fire point.
Second letter: Circulation mechanism for internal cooling medium:
• N: natural thermosiphon flow through cooling equipment and in windings;
• F: forced circulation through cooling equipment, thermosiphon flow in windings;
• D: forced circulation through cooling equipment, directed from the cooling equipment into
at least the main windings.
Third letter: External cooling medium:
• A: air;
• W: water.
Fourth letter: Circulation mechanism for external cooling medium:
• N: natural convection;
• F: forced circulation (fans, pumps).
NOTE 1 In this standard, the use of insulating liquids K and L is considered only for safety and environmental
reasons.
NOTE 2 In a transformer designated as having forced directed insulating liquid circulation (second code letter D),
the rate of liquid flow through the main windings is determined by the pumps and is not, in principle, determined by
the loading. A minor fraction of the flow of liquid through the cooling equipment may be directed as a controlled
bypass to provide cooling for core and other parts outside the main windings. Regulating windings and/or other
windings having relatively low power may also have non-directed circulation of bypass liquid.
In a transformer with forced, non-directed cooling (second code letter F), the rates of flow of liquid through all the
windings are variable with the loading, and not directly related to the pumped flow through the cooling equipment.
4.2 Transformers with alternative cooling methods
A transformer may be specified with alternative cooling methods. In this case, the
specification and the rating plate shall then carry information about the power values at
which the transformer fulfils the temperature rise limits when these alternatives apply, see
IEC 60076-1.
The power value for the alternative cooling methods with the highest cooling capacity is the
rated power of the transformer (or of an individual winding of a multi-winding transformer,
see IEC 60076-1). The alternatives cooling methods are conventionally listed in rising order
of cooling capacity.
Examples:
• ONAN/ONAF. The transformer has a set of fans which may be put into service as desired
at high loading. The insulating liquid circulation is by thermosiphon effect only, in both
cases.
• ONAN/OFAF. The transformer has cooling equipment with pumps and fans but is also
specified with a reduced rated power under natural cooling (for example, in case of failure
or reduction of auxiliary power).
5 Normal cooling conditions
5.1 Air-cooled transformers
Normal ambient temperature limits for power transformers are given in IEC 60076-1.

– 10 – 60076-2  IEC:2011
With regard to normal temperature rise requirements, the temperatures at the intended
installation site should not exceed:
+ 40 °C at any time;
+ 30 °C monthly average, of the hottest month;
+ 20 °C yearly average.
NOTE The average temperatures are to be derived from meteorological data as follows (see IEC 60076-1).
Monthly average temperature:
– half the sum of the average of the daily maxima and the average of the daily minima during a particular
month, over many years;
Yearly average temperature:
– one-twelfth of the sum of the monthly average temperatures.
5.2 Water-cooled transformers
Normal cooling condition for water cooled transformers is a temperature of cooling water at
the inlet not exceeding 25 °C at any time or a 20 °C yearly average.
If the operating water temperature is higher than this, then a lower temperature rise should be
specified (see IEC 60076-1).
6 Temperature rise limits
6.1 General
Temperature rise requirements are specified according to different options:
• a set of requirements which refer to continuous rated power (see 6.2).
• an additional set of explicitly specified requirements, that relate to a prescribed loading
cycle (see 6.4).
NOTE The additional set of requirements is applicable mainly to large system transformers for which emergency
loading conditions deserve particular attention, and should not be regularly used for small and medium-size
standardized transformers.
It is assumed throughout this part that the service temperatures of different parts of a
transformer can each be described as the sum of the external cooling medium temperature
(ambient air or cooling water) and the temperature rise of the transformer part.
Normal temperature rise limits apply unless other service conditions are specified. In such
cases, the limits of temperature rise shall be modified as indicated in 6.3.
No plus tolerance is permitted on temperature rise limits.
6.2 Temperature rise limits at rated power
For transformers up to 2 500 kVA (833 kVA single-phase) with a tapping range not exceeding
± 5 %, the temperature rise limits shall apply to the principal tapping corresponding to the
rated voltage (see IEC 60076-1).
For transformer rated power larger than 2 500 kVA or if the tapping range exceeds ± 5 %, the
temperature rise limits shall apply to every tapping at the appropriate tapping power, tapping
voltage and tapping current.
NOTE 1 The load losses are different for different tappings and sometimes also the no-load loss when variable
flux voltage variation is specified.

60076-2  IEC:2011 – 11 –
NOTE 2 In a separate winding transformer, the tapping with the highest load loss is normally the tapping with the
maximum current.
NOTE 3 In an auto-transformer with tapping, the tapping with the highest load loss depends on how the tappings
are arranged.
For a multi-winding transformer, when the rated power of one winding is equal to the sum of
the rated powers of the other windings, the temperature rise requirements refer to rated power
in all windings simultaneously. If this is not the case, one or more particular loading
combinations have to be selected and specified for the temperature rise limits.
In the case of a transformer with two or more separate winding sections one above the other,
the winding temperature limit shall apply to the average of the measurements of the stacked
sections, if they are of equal size and rating.
The temperature rise limits given in Table 1 are valid for transformers with solid insulation
designated as class 105 °C according to IEC 60085, and immersed in mineral oil or synthetic
liquid with a fire point not above 300 °C (first code letter: O).
The limits refer to steady state conditions under continuous rated power, and 20 °C average
yearly temperature of the external cooling medium.
If not otherwise agreed between manufacturer and purchaser, the temperature rise limits
given in Table 1 are valid for both Kraft and upgraded paper (see also IEC 60076-7).
Table 1 – Temperature rise limits
Temperature rise limits
Requirements for
K
Top insulating liquid 60
Average winding (by winding resistance variation):

– ON. and OF. cooling systems 65
– OD. cooling system
Hot-spot winding 78
No numerical limits are specified for the temperature rise of magnetic core, bare electrical
connections, electrical or magnetic shields and structural parts in the tank. However, a self-
evident requirement is that they shall not reach a temperature which will cause damages to
adjacent parts or undue ageing of the insulating liquid. If considered necessary, a
temperature rise limit for the magnetic core surface may be agreed between manufacturer and
purchaser.
NOTE 4 For some designs, the hot-spot winding temperature rise limit may imply lower top-liquid and/or average
winding temperature rises than those indicated in the table.
NOTE 5 The rules for determining the hot-spot winding temperature rise are given in 7.10.
NOTE 6 For large power transformers immersed in mineral oil, in-oil dissolved gas analysis (DGA) performed
during the temperature rise test can be a tool for detecting undesirable overheating (see Annex D).
NOTE 7 For large power transformers, the temperature rise of tank and cover surfaces can be checked by means
of a thermographic infrared camera.
On windings of very low resistance with numerous bolted connections (e.g., low voltage
winding of furnace transformers), the determination of the average winding temperature rise
by resistance variation may be difficult and subjected to a large uncertainty. As an alternative
and by agreement between manufacturer and purchaser, the winding temperature rise
requirements may be limited to the hot-spot winding temperature rise which shall be
determined by direct measurement in this case.

– 12 – 60076-2  IEC:2011
Temperature rise limits for transformers having higher temperature resistant insulation
systems and immersed in a less flammable liquid (code letter K or L) are subject to
agreement.
6.3 Modified requirements for special cooling conditions
6.3.1 General
If the service conditions at the intended installation site do not fall within the limits of normal
cooling conditions given in Clause 5, then the limits of temperature rise for the transformer
shall be modified in accordance with the rules indicated below.
6.3.2 Air-cooled transformers
If the temperature of the external cooling medium at site exceeds one or more of the normal
values given in 5.1, all the temperature rise limits indicated in Table 1 shall be corrected by
the same amount as the excess. The obtained values shall be rounded to the nearest whole
number of degrees kelvin.
Recommended ambient temperature reference values and relevant temperature rise limit
corrections are given in Table 2.
Table 2 – Recommended values of temperature rise corrections in case
of special service conditions
Ambient temperatures
Correction of
temperature rise
°C
a
K
Yearly average Monthly average Maximum
20 30 40 0
25 35 45 –5
30 40 50 –10
35 45 55 –15
a
Referred to the values given in Table 1.

NOTE 1 No rules are given for ambient temperatures lower than the normal ones. The temperature rise limits
given in Table 1 are applied unless otherwise specified by the purchaser.
NOTE 2 The values given in the Table 2 may be interpolated.
If the installation site is more than 1 000 m above sea-level but the factory is not, then the
allowable temperature rises during the test in the factory shall be reduced as follows:
• for a naturally cooled transformer (.AN), the limit of top-liquid, average and hot-spot
winding temperature rises shall be reduced by 1 K for every interval of 400 m by which the
installation's altitude exceeds 1 000 m;
• for a forced-cooled transformer (…. AF), the reduction shall be 1 K for every 250 m
exceeding 1 000 m.
A corresponding reverse correction may be applied in cases where altitude of the factory is
above 1 000 m and the altitude of the installation site is below 1 000 m.
Any altitude correction shall be rounded to the nearest whole number of degrees kelvin.
When the specified temperature rise limits of a transformer have been reduced, either
because of high cooling medium temperature or because of high-altitude installation, this shall
be indicated on the rating plate (see IEC 60076-1).

60076-2  IEC:2011 – 13 –
NOTE 3 When standardized transformers are to be used at high altitudes, a reduced value of power may be
calculated, which from the point of view of cooling and temperature rise corresponds to service with rated power
under normal ambient conditions.
6.3.3 Water-cooled transformers
If the maximum and/or the yearly cooling water temperature at site exceeds the values
indicated in 5.2, all the prescribed temperature rise limits shall be reduced by the same
amount as the excess. The values shall be rounded to the nearest whole number of degrees.
NOTE The rule given above does not apply for water temperatures lower than the normal one. In that case, an
agreement between manufacturer and purchaser is necessary.
The influence of differing ambient temperature or altitude on the air cooling of the tank shall
be disregarded.
6.4 Temperature rise during a specified load cycle
By agreement between manufacturer and purchaser, temperature rise limits can be
guaranteed and/or a special test regarding load cycle operation specified (see IEC 60076-7).
7 Temperature rise tests
7.1 General
The following subclauses describe the procedures for the determination of temperature and
temperature rise values during factory testing and also the methods for substituting service
loading conditions by equivalent test procedures.
During the temperature rise test, the transformer shall be equipped with its protective devices
(for example, Buchholz relay). Any indication from these devices during the test shall be
noted and the case investigated.
In the case of a transformer with more than one value of rated power (for example, when two
or more cooling methods are provided), a temperature rise test shall be in principle performed
for each rated power, but by agreement between manufacturer and purchaser the number of
tests can be reduced.
7.2 Temperature of the cooling media
7.2.1 Ambient temperature
For the temperature rise test, the cooling air temperature should be in the range between
10 °C and the maximum ambient temperature for which the transformer is designed.
The interpretation of the test results shall be subject to agreement if the external cooling
medium temperature during the test is outside the limits indicated.
At least four sensors shall be provided and the average of their readings shall be used to
determine the ambient temperature for the evaluation of the test results.
NOTE For tests on large power transformers, the number of sensors should be increased up to six in order to
reduce the uncertainty that can affect the average of the readings.
Readings should be taken at regular intervals (e.g., every ten minutes), or automatic
continuous recording may be used.
Around an ONAN transformer, the ambient sensors shall be placed at a level about halfway
up the cooling surfaces.
– 14 – 60076-2  IEC:2011
The sensors shall be distributed around the tank, about 2 m away from the perimeter of tank
and cooling surfaces, and protected from direct heat radiation.
For a forced-air-cooled transformer, the sensors shall be placed in the air at about 0,5 m from
the intake of the coolers.
In the case of separate cooling equipment placed at a distance of at least 3 m from the
transformer tank, the ambient temperature shall be measured around the cooling equipment
applying the same rules given above.
Attention shall be paid to possible recirculation of hot air. The transformer should be placed
so as to minimize obstructions to the air flow and to provide stable ambient conditions.
Precautions should be taken to minimize variations of cooling-air temperature, particularly
during the last part of the test period when steady state conditions are approached. Rapid
variation of readings due to turbulence should be prevented by appropriate means such as
heat sinks for the temperature sensors of thermal time constant similar to the transformer
thermal time constant.
7.2.2 Water temperature
For the temperature rise test, the cooling water temperature should be in the range between
5 °C and the maximum water temperature for which the transformer is designed.
The interpretation of the test results shall be subject to agreement if the water temperature is
outside the limits indicated above.
The temperature shall be measured at the intake of the cooling equipment. Readings of
temperature and rate of water flow should be taken at regular intervals (e.g., every ten
minutes), or automatic continuous recording may be used.
Precautions shall be taken to minimize the variations of water cooling flow and temperature
during the test period.
7.3 Test methods for temperature rise determination
7.3.1 General
The standard method for the determination of the steady-state temperature rises on the test
floor is the equivalent test in short-circuit connection according to 7.3.2 below.
In special cases, if agreed, the test can be performed applying rated voltage and current by
connection to a suitable load. This is mainly applicable to transformers with low rated power.
A back-to-back method may also be agreed. In this method, two transformers, one of which is
the transformer under test, are connected in parallel and excited at the rated voltage of the
transformer under test. By means of different voltage ratios or an injected voltage, rated
current is made to flow in the windings of the transformer under test.
7.3.2 Test by short-circuit method for two winding transformers
During this test the transformer is not subjected to rated voltage and rated current
simultaneously, but to the calculated total losses, previously obtained by two separate
determinations of losses, namely load loss at reference temperature and no-load loss (see
IEC 60076-1).
The purpose of the test is to establish:

60076-2  IEC:2011 – 15 –
• the top-liquid and average liquid temperature rises in a steady-state condition with
dissipation of total losses;
• the average winding temperature rise at rated current for the average liquid temperature
rise as determined above;
• the hot-spot winding temperature rise at rated current and for the top-liquid temperature
rise as mentioned above.
This
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