Power transformers - Part 2: Temperature rise for liquid-immersed transformers

IEC 60076-2:2011 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. This new edition includes the following significant technical changes with respect to the previous edition:
- the winding hot-spot temperature rise limit was introduced among the prescriptions;
- the procedures for the temperature rise test were improved in relation to the new thermal requirements;
- five informative annexes were added in order to facilitate the implementation of this standard.

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

La CEI 60076-2:2011 s'applique aux transformateurs immergés dans un liquide, identifie les transformateurs de puissance selon leurs méthodes de refroidissement, définit les limites d'échauffement et présente les méthodes d'essais d'échauffement. Cette édition inclut les modifications techniques majeures suivantes par rapport à l'édition précédente:
- les modalités de réalisation des essais d'échauffement ont été améliorées de façon à tenir compte des nouvelles exigences de performances thermiques;
- cinq nouvelles annexes informatives ont été introduites pour faciliter l'application de cette norme.

General Information

Status
Published
Publication Date
22-Feb-2011
Technical Committee
Current Stage
PPUB - Publication issued
Start Date
23-Feb-2011
Completion Date
23-Feb-2011
Ref Project

Buy Standard

Standard
IEC 60076-2:2011 - Power transformers - Part 2: Temperature rise for liquid-immersed transformers
English and French language
95 pages
sale 15% off
Preview
sale 15% off
Preview

Standards Content (sample)

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
IEC 60076-2:2011
---------------------- Page: 1 ----------------------
THIS PUBLICATION IS COPYRIGHT PROTECTED
Copyright © 2011 IEC, Geneva, Switzerland

All rights reserved. Unless otherwise specified, 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 either IEC or

IEC's member National Committee in the country of the requester.

If you have any questions about IEC copyright or have an enquiry about obtaining additional rights to this publication,

please contact the address below or your local IEC member National Committee for further information.

Droits de reproduction réservés. Sauf indication contraire, aucune partie de cette publication ne peut être reproduite

ni utilisée sous quelque forme que ce soit et par aucun procédé, électronique ou mécanique, y compris la photocopie

et les microfilms, sans l'accord écrit de la CEI ou du Comité national de la CEI du pays du demandeur.

Si vous avez des questions sur le copyright de la CEI ou si vous désirez obtenir des droits supplémentaires sur cette

publication, utilisez les coordonnées ci-après ou contactez le Comité national de la CEI de votre pays de résidence.

IEC Central Office
3, rue de Varembé
CH-1211 Geneva 20
Switzerland
Email: inmail@iec.ch
Web: www.iec.ch
About the IEC

The International Electrotechnical Commission (IEC) is the leading global organization that prepares and publishes

International Standards for all electrical, electronic and related technologies.
About IEC publications

The technical content of IEC publications is kept under constant review by the IEC. Please make sure that you have the

latest edition, a corrigenda or an amendment might have been published.
 Catalogue of IEC publications: www.iec.ch/searchpub

The IEC on-line Catalogue enables you to search by a variety of criteria (reference number, text, technical committee,…).

It also gives information on projects, withdrawn and replaced publications.
 IEC Just Published: www.iec.ch/online_news/justpub

Stay up to date on all new IEC publications. Just Published details twice a month all new publications released. Available

on-line and also by email.
 Electropedia: www.electropedia.org

The world's leading online dictionary of electronic and electrical terms containing more than 20 000 terms and definitions

in English and French, with equivalent terms in additional languages. Also known as the International Electrotechnical

Vocabulary online.
 Customer Service Centre: www.iec.ch/webstore/custserv

If you wish to give us your feedback on this publication or need further assistance, please visit the Customer Service

Centre FAQ or contact us:
Email: csc@iec.ch
Tel.: +41 22 919 02 11
Fax: +41 22 919 03 00
A propos de la CEI

La Commission Electrotechnique Internationale (CEI) est la première organisation mondiale qui élabore et publie des

normes internationales pour tout ce qui a trait à l'électricité, à l'électronique et aux technologies apparentées.

A propos des publications CEI

Le contenu technique des publications de la CEI est constamment revu. Veuillez vous assurer que vous possédez

l’édition la plus récente, un corrigendum ou amendement peut avoir été publié.
 Catalogue des publications de la CEI: www.iec.ch/searchpub/cur_fut-f.htm

Le Catalogue en-ligne de la CEI vous permet d’effectuer des recherches en utilisant différents critères (numéro de référence,

texte, comité d’études,…). Il donne aussi des informations sur les projets et les publications retirées ou remplacées.

 Just Published CEI: www.iec.ch/online_news/justpub

Restez informé sur les nouvelles publications de la CEI. Just Published détaille deux fois par mois les nouvelles

publications parues. Disponible en-ligne et aussi par email.
 Electropedia: www.electropedia.org

Le premier dictionnaire en ligne au monde de termes électroniques et électriques. Il contient plus de 20 000 termes et

définitions en anglais et en français, ainsi que les termes équivalents dans les langues additionnelles. Egalement appelé

Vocabulaire Electrotechnique International en ligne.
 Service Clients: www.iec.ch/webstore/custserv/custserv_entry-f.htm

Si vous désirez nous donner des commentaires sur cette publication ou si vous avez des questions, visitez le FAQ du

Service clients ou contactez-nous:
Email: csc@iec.ch
Tél.: +41 22 919 02 11
Fax: +41 22 919 03 00
---------------------- Page: 2 ----------------------
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
® Registered trademark of the International Electrotechnical Commission
Marque déposée de la Commission Electrotechnique Internationale
---------------------- Page: 3 ----------------------
– 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

---------------------- Page: 4 ----------------------
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

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

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

---------------------- Page: 5 ----------------------
– 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.

---------------------- Page: 6 ----------------------
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.
---------------------- Page: 7 ----------------------
– 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
---------------------- Page: 8 ----------------------
60076-2  IEC:2011 – 7 –
3.4
top-liquid temperature

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

the temperature difference between the top-liquid temperature and the external cooling

medium temperature
3.6
bottom-liquid temperature

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

the difference between the bottom-liquid temperature and the external cooling medium

temperature
3.8
average liquid temperature
the average temperature of the top-liquid and bottom liquid temperatures
3.9
average liquid temperature rise

the difference between the average liquid temperature and the external cooling medium

temperature
3.10
average winding temperature

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

the difference between the average winding temperature and the external cooling medium

temperature
3.12
average winding gradient

the difference between the average winding temperature and the average insulating liquid

temperature
---------------------- Page: 9 ----------------------
– 8 – 60076-2  IEC:2011
3.13
hot-spot winding temperature

the hottest temperature of winding conductors in contact with solid insulation or insulating

liquid
3.14
hot-spot winding temperature rise

the difference between hot-spot winding temperature and the external cooling medium

temperature
3.15
hot-spot factor

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;
---------------------- Page: 10 ----------------------
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.

---------------------- Page: 11 ----------------------
– 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 unle
...

Questions, Comments and Discussion

Ask us and Technical Secretary will try to provide an answer. You can facilitate discussion about the standard in here.