IEC 60076-7:2018

FOREWORD
INTRODUCTION
1 Scope
2 Normative references
3 Terms and definitions
4 Symbols and abbreviations
5 Effect of loading beyond nameplate rating
6 Relative ageing rate and transformer insulation life
7 Limitations
8 Determination of temperatures
9 Influence of tap-changers
Annex A (informative) - Insulation life expectancy and relative
        ageing rate considering oxygen and water effect
Annex B (informative) - Core temperature
Annex C (informative) - Specification of loading beyond rated
        power
Annex D (informative) - Description of Q, S and H factors
Annex E (informative) - Calculation of winding and oil time
        constant
Annex F (informative) - Thermal model parameters
Annex G (informative) - Oil and winding exponents
Annex H (informative) - Practical example of the exponential
        equations method
Annex I (informative) - Application of the difference equation
        solution method
Annex J (informative) - Flowchart, based on the example in
        Annex H
Annex K (informative) - Example of calculating and presenting
        overload data
Annex L (informative) - Geomagnetic induced currents
Annex M (informative) - Alternative oils
Bibliography

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.