IEC 60076-7:2018

Power transformers - Part 7: Loading guide for mineral-oil-immersed power transformers

IEC 60076-7:2018 is now available as IEC 60076-7:2018 RLV which contains the International Standard and its Redline version, showing all changes of the technical content compared to the previous edition.
IEC 60076-7:2018 is applicable to mineral-oil-immersed transformers. It describes the effect of operation under various ambient temperatures and load conditions on transformer life. This document provides guidance for the specification and loading of power transformers from the point of view of operating temperatures and thermal ageing. It provides recommendations for loading above the nameplate rating and guidance for the planner to choose appropriate rated quantities and loading conditions for new installations. This edition includes the following significant technical changes with respect to the previous edition:
a) title has been updated from "oil-immersed power transformers" to "mineral-oil-immersed transformers";
b) insulation life is updated by considering latest research findings;
c) temperature limits have been reviewed and maximum core temperature is recommended;
d) number of fibre optic sensors is recommended for temperature rise test;
e) Q, S and H factors are considered;
f) thermal models are revised and rewritten in generally applicable mathematical form;
g) geomagnetic induced currents are briefly discussed and corresponding temperature limits are suggested;
h) extensive literature review has been performed and a number of references added to bibliography.

General Information

• Status: Published
• Publication Date: 11-Jan-2018
• ICS: 01 - GENERALITIES. TERMINOLOGY. STANDARDIZATION. DOCUMENTATION
• ICS: 29.180 - Transformers. Reactors
• Technical Committee: TC 14 - Power transformers
• Drafting Committee: MT 60076-7 - TC 14/MT 60076-7
• Current Stage: PPUB - Publication issued
• Start Date: 29-Dec-2017
• Completion Date: 12-Jan-2018

Relations

• Revises: IEC 60076-7:2005 - Power transformers - Part 7: Loading guide for oil-immersed power transformers
• Effective Date: 05-Sep-2023

Standards Content (Sample)

IEC 60076-7 ®
Edition 2.0 2018-01
INTERNATIONAL
STANDARD
colour
inside
Power transformers –
Part 7: Loading guide for mineral-oil-immersed power transformers

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.

IEC Central Office Tel.: [phone]
3, rue de Varembé [e-mail]
CH-1211 Geneva 20 www.iec.ch
Switzerland
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.

IEC Catalogue - webstore.iec.ch/catalogue Electropedia - www.electropedia.org
The stand-alone application for consulting the entire The world's leading online dictionary of electronic and
bibliographical information on IEC International Standards, electrical terms containing 21 000 terms and definitions in
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The advanced search enables to find IEC publications by a 67 000 electrotechnical terminology entries in English and
variety of criteria (reference number, text, technical French extracted from the Terms and Definitions clause of
committee,…). It also gives information on projects, replaced IEC publications issued since 2002. Some entries have been
and withdrawn publications. collected from earlier publications of IEC TC 37, 77, 86 and

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IEC 60076-7 ®
Edition 2.0 2018-01
INTERNATIONAL
STANDARD
colour
inside
Power transformers –
Part 7: Loading guide for mineral-oil-immersed power transformers

INTERNATIONAL
ELECTROTECHNICAL
COMMISSION
ICS 29.180 ISBN 978-2-8322-5082-2

– 2 – IEC 60076-7:2018 © IEC 2018
CONTENTS
FOREWORD . 6
INTRODUCTION . 8
1 Scope . 9
2 Normative references . 9
3 Terms and definitions . 9
4 Symbols and abbreviations . 11
5 Effect of loading beyond nameplate rating . 13
5.1 General . 13
5.2 General consequences . 13
5.3 Effects and hazards of short-time emergency loading . 14
5.4 Effects of long-time emergency loading . 15
5.5 Transformer size . 15
6 Relative ageing rate and transformer insulation life . 15
6.1 General . 15
6.2 Insulation life . 16
6.3 Relative ageing rate . 20
6.4 Loss-of-life calculation . 21
7 Limitations . 21
7.1 Temperature limitations . 21
7.2 Current limitations . 22
7.3 Specific limitations for small transformers . 23
7.3.1 Current and temperature limitations . 23
7.3.2 Accessory and other considerations . 23
7.3.3 Indoor transformers . 23
7.3.4 Outdoor ambient conditions . 23
7.4 Specific limitations for medium power transformers . 23
7.4.1 Current and temperature limitations . 23
7.4.2 Accessory, associated equipment and other considerations . 23
7.4.3 Short-circuit withstand requirements . 24
7.4.4 Voltage limitations . 24
7.5 Specific limitations for large power transformers . 24
7.5.1 General . 24
7.5.2 Current and temperature limitations . 24
7.5.3 Accessory, equipment and other considerations . 24
7.5.4 Short-circuit withstand requirements . 25
7.5.5 Voltage limitations . 25
8 Determination of temperatures . 25
8.1 Hot-spot temperature rise in steady state . 25
8.1.1 General . 25
8.1.2 Calculation of hot-spot temperature rise from normal heat-run test data . 25
8.1.3 Direct measurement of hot-spot temperature rise . 26
8.1.4 Hot-spot factor . 29
8.2 Top-oil and hot-spot temperatures at varying ambient temperature and load
conditions . 31
8.2.1 General . 31

8.2.2 Exponential equations solution . 33
8.2.3 Difference equations solution . 37
8.3 Ambient temperature. 39
8.3.1 Outdoor air-cooled transformers . 39
8.3.2 Correction of ambient temperature for transformer enclosure . 39
8.3.3 Water-cooled transformers . 40
9 Influence of tap-changers . 40
9.1 General . 40
9.2 Load loss . 41
9.3 Ratio of losses . 41
9.4 Load factor . 41
Annex A (informative) Insulation life expectancy and relative ageing rate considering
oxygen and water effect . 42
A.1 Insulation life expectancy . 42
A.2 Relative ageing rate considering oxygen and water effect . 44
Annex B (informative) Core temperature . 47
B.1 General . 47
B.2 Core hot-spot locations . 47
Annex C (informative) Specification of loading beyond rated power . 48
Annex D (informative) Description of Q, S and H factors . 50
Annex E (informative) Calculation of winding and oil time constant . 53
Annex F (informative) Thermal model parameters . 55
F.1 General . 55
F.2 Thermal constant estimation: experimental approach . 55
F.3 Dynamic thermal modelling: further development . 57
Annex G (informative) Oil and winding exponents . 58
G.1 General . 58
G.2 Historical background . 58
G.3 Theoretical approach . 60
G.4 Extended temperature rise test approach . 62
Annex H (informative) Practical example of the exponential equations method . 64
H.1 General . 64
H.2 Time period 0 min to 190 min . 65
H.3 Time period 190 min to 365 min . 65
H.4 Time period 365 min to 500 min . 66
H.5 Time period 500 min to 705 min . 66
H.6 Time period 705 min to 730 min . 67
H.7 Time period 730 min to 745 min . 67
H.8 Comparison with measured values . 68
Annex I (informative) Application of the difference equation solution method . 70
I.1 General . 70
I.2 Example. 70
I.3 Use of measured top-oil temperature . 75
Annex J (informative) Flowchart, based on the example in Annex H . 76
Annex K (informative) Example of calculating and presenting overload data . 78
Annex L (informative) Geomagnetic induced currents . 82
L.1 Background. 82

– 4 – IEC 60076-7:2018 © IEC 2018
L.2 GIC capability of power transformers [54], [55]. 82
Annex M (informative) Alternative oils . 84
Bibliography .
...

IEC 60076-7 ®
Edition 2.0 2018-01
REDLINE VERSION
INTERNATIONAL
STANDARD
colour
inside
Power transformers –
Part 7: Loading guide for mineral-oil-immersed power transformers

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.

IEC Central Office Tel.: [phone]
3, rue de Varembé [e-mail]
CH-1211 Geneva 20 www.iec.ch
Switzerland
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.

IEC Catalogue - webstore.iec.ch/catalogue Electropedia - www.electropedia.org
The stand-alone application for consulting the entire The world's leading online dictionary of electronic and
bibliographical information on IEC International Standards, electrical terms containing 21 000 terms and definitions in
Technical Specifications, Technical Reports and other English and French, with equivalent terms in 16 additional
documents. Available for PC, Mac OS, Android Tablets and languages. Also known as the International Electrotechnical
iPad. Vocabulary (IEV) online.

IEC publications search - webstore.iec.ch/advsearchform IEC Glossary - std.iec.ch/glossary
The advanced search enables to find IEC publications by a 67 000 electrotechnical terminology entries in English and
variety of criteria (reference number, text, technical French extracted from the Terms and Definitions clause of
committee,…). It also gives information on projects, replaced IEC publications issued since 2002. Some entries have been
and withdrawn publications. collected from earlier publications of IEC TC 37, 77, 86 and

CISPR.
IEC Just Published - webstore.iec.ch/justpublished
Stay up to date on all new IEC publications. Just Published IEC Customer Service Centre - webstore.iec.ch/csc
details all new publications released. Available online and If you wish to give us your feedback on this publication or
also once a month by email. need further assistance, please contact the Customer Service
Centre: [e-mail].
IEC 60076-7 ®
Edition 2.0 2018-01
REDLINE VERSION
INTERNATIONAL
STANDARD
colour
inside
Power transformers –
Part 7: Loading guide for mineral-oil-immersed power transformers

INTERNATIONAL
ELECTROTECHNICAL
COMMISSION
ICS 29.180 ISBN 978-2-8322-5279-6

– 2 – IEC 60076-7:2018 RLV © IEC 2018
CONTENTS
FOREWORD . 6
INTRODUCTION . 8
1 Scope . 10
2 Normative references . 10
3 Terms and definitions . 10
4 Symbols and abbreviations . 13
5 Effect of loading beyond nameplate rating . 14
5.1 Introduction General . 14
5.2 General consequences . 14
5.3 Effects and hazards of short-time emergency loading . 15
5.4 Effects of long-time emergency loading . 16
5.5 Transformer size . 16
Non-thermally and thermally upgraded insulation paper .
6 Relative ageing rate and transformer insulation life . 18
6.1 General . 18
6.2 Insulation life . 19
6.3 Relative ageing rate . 23
6.4 Loss-of-life calculation . 24
7 Limitations . 25
7.1 Current and Temperature limitations . 25
7.2 Current limitations . 26
7.3 Specific limitations for distribution small transformers . 27
7.3.1 Current and temperature limitations . 27
7.3.2 Accessory and other considerations . 27
7.3.3 Indoor transformers . 27
7.3.4 Outdoor ambient conditions . 27
7.4 Specific limitations for medium power transformers . 27
7.4.1 Current and temperature limitations . 27
7.4.2 Accessory, associated equipment and other considerations . 27
7.4.3 Short-circuit withstand requirements . 28
7.4.4 Voltage limitations . 28
7.5 Specific limitations for large power transformers . 28
7.5.1 General . 28
7.5.2 Current and temperature limitations . 28
7.5.3 Accessory, equipment and other considerations . 28
7.5.4 Short-circuit withstand requirements . 29
7.5.5 Voltage limitations . 29
8 Determination of temperatures . 29
8.1 Hot-spot temperature rise in steady state . 29
8.1.1 General . 29
8.1.2 Calculation of hot-spot temperature rise from normal heat-run test data . 29
8.1.3 Direct measurement of hot-spot temperature rise . 30
8.1.4 Hot-spot factor . 35
8.2 Top-oil and hot-spot temperatures at varying ambient temperature and load
conditions . 37

8.2.1 General . 37
8.2.2 Exponential equations solution . 39
8.2.3 Differential Difference equations solution . 44
8.3 Ambient temperature. 46
8.3.1 Outdoor air-cooled transformers . 46
8.3.2 Correction of ambient temperature for transformer enclosure . 47
8.3.3 Water-cooled transformers . 48
9 Influence of tap-changers . 48
9.1 General . 48
9.2 Short-circuit losses Load loss . 48
9.3 Ratio of losses . 48
9.4 Load factor . 49
Annex A (informative) Insulation life expectancy and relative ageing rate considering
oxygen and water effect . 50
A.1 Insulation life expectancy . 50
A.2 Relative ageing rate considering oxygen and water effect . 52
Annex B (informative) Core temperature . 55
B.1 General . 55
B.2 Core hot-spot locations . 55
Annex C (informative) Specification of loading beyond rated power . 56
Annex D (informative) Description of Q, S and H factors . 58
Annex E (informative) Calculation of winding and oil time constant . 61
Annex F (informative) Thermal model parameters . 63
F.1 General . 63
F.2 Thermal constant estimation: experimental approach . 63
F.3 Dynamic thermal modelling: further development . 65
Annex G (informative) Oil and winding exponents . 66
G.1 General . 66
G.2 Historical background . 66
G.3 Theoretical approach . 68
G.4 Extended temperature rise test approach . 70
Annex H (informative) Practical example of the exponential equations method . 72
H.1 Introduction General . 72
H.2 Time period 0 min to 190 min . 73
H.3 Time period 190 min to 365 min . 74
H.4 Time period 365 min to 500 min . 74
H.5 Time period 500 min to 710 705 min . 75
H.6 Time period 710 705 min to 735 730 min . 76
H.7 Time period 735 730 min to 750 745 min . 77
H.8 Comparison with measured values . 77
Annex I (informative) Illustration Application of the differential difference equation
solution method . 79
I.1 General . 82
I.2 Example. 82
I.3 Use of measured top-oil temperature . 87
Annex J (informative) Flowchart, based on the example in Annex H . 88
Annex K (informative) Example of calculating and presenting overload data . 90

– 4 – IEC 60076-7:2018 RLV © IEC 2018
Annex L (informative) Geomagnetic induced currents . 94
L.1 Background. 94
L.2 GIC capability of power transformers [54], [55]. 94
Annex M (in
...