Edition 2.02018-01
INTERNATIONAL STANDARD
PowertransformersPart 7: Loading guide for mineral-oil-immersedpower transformers
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INTERNATIONAL STANDARD
INTERNATIONALELECTROTECHNICALCOMMISSION
ISBN 978-2-8322-5082-2
CONTENTS
FOREWORD.INTRODUCTION. .81 Scope. .92 Normative references3 Terms and definitions ..94 5 Effect of loading beyond nameplate rating Symbols and abbreviations 13 .115.1 General. .135.2 General consequences ..135.3 5.4 Effects and hazards of short-time emergency loading Effects of long-time emergency loading. ..14 155.5 Transformer size.. ..156 Relative ageing rate and transformer insulation life ..1561 6.2 General Insulation life ..16 156.3 Relative ageing rate.... 206.4 Loss-of-life calculation 217.1 Limitations. Temperature limitations. .217.2 Current limitations. 227.3 Specific limitations for small transformers Current and temperature limitations. ..23 237.3.1 7.3.2 Accessory and other considerations .237.3.3 Indoor transformers ..7.4 7.3.4 Specific limitations for medium power transformers. Outdoor ambient conditions .23 237.4.1 Current and temperature limitations. 237.4.2 Accessory associated equipment and other considerations. 237.4.3 7.4.4 Short-circuit withstand requirements. Voltage limitations 24 247.5 Specific limitations for large power transformers 247.5.1 General ...247.5.2 7.5.3 Accessory. equipment and other considerations Current and temperature limitations. .24 .247.5.4 Short-circuit withstand requirements . 25Determination of temperatures. 7.5.5 Voltage limitations. ..25 258 8.1 Hot-spot temperature rise in steady state. 258.1.1 General ..258.1.2 8.1.3 Direct measurement of hot-spot temperature rise Calculation of hot-spot temperature rise from normal heat-run test data ..25 .268.1.4 Hot-spot factor....8.2 Top-oil and hot-spot temperatures at varying ambient temperature and load .318.2.1 conditions General .31
8.2.3 8.2.2 Exponential equations solution Difference equations solution. 37 338.3 Ambient temperature.. ...398.3.1 Outdoor air-cooled transformers 398.3.2 Correction of ambient temperature for transformer enclosure. Water-cooled transformers ..399 Influence of tap-changers ..409.1 General. 409.2 9.3 Load loss. Ratio of losses. ..419.4 Load factor .1Annex A (informative) Insulation life expectancy and relative ageing rate considering oxygen and water effect. ..42A.1 Insulation life expectancy. 42A.2 44Annex B (informative) Core temperature .47B.1 B.2 Core hot-spot locations. General. .47 .47Annex C (informative) Specification of loading beyond rated power .. ...8Annex D (informative) Description of Q. S and H factors .0Annex F (informative) Thermal model parameters. Annex E (informative) Calculation of winding and oil time constant ..53 .55F.1 General. 55F.2 Thermal constant estimation: experimental approach. ..55Annex G (informative) Oil and winding exponents F.3 Dynamic thermal modelling: further development: ...58 57G.1 General. .58G.2 Historical backgroundG.3 G.4 Theoretical approach Extended temperature rise test approach. .60 62Annex H (informative) Practical example of the exponential equations method .64H.1 General. 64H.2 H.3 Time period 0 min to 190 min... Time period 190 min to 365 min.. ..65H.4 Time period 365 min to 500 min. ...6H.5 Time period 500 min to 705 min.. Time period 705 min to 730 min. .66H.6 H.7 Time period 730 min to 745 min.... 67H.8 Comparison with measured values. ..68Annex I (informative) Application of the difference equation solution method .701.1 1.2 Example.. General. .70 .701.3 Use of measured top-oil temperature .75Annex J (informative) Flowchart based on the example in Annex H. ..76Annex K (informative) Example of calculating and presenting overload data Annex L (informative) Geomagnetic induced currents ...82 .78L.1 Background. 82
