CONTENTS
FOREWORD 6
INTRODUCTION 8
1Scope 9
2Normative references 9
3Terms and definitions 9
4Symbols and abbreviations 11
5Effect of loading beyond nameplate rating 13
5.1General 13
5.2General consequences 13
5.3Effects and hazards of short-time emergency loading 14
5.4Effects of long-time emergency loading 15
5.5Transformer size 15
6Relative ageing rate and transformer insulation life 15
6.1General 15
6.2Insulation life 16
6.3Relative ageing rate 20
6.4Loss-of-life calculation 21
7Limitations 21
7.1Temperature limitations 21
7.2Current limitations 22
7.3Specific limitations for small transformers 23
7.3.1Current and temperature limitations 23
7.3.2Accessory and other considerations 23
7.3.3Indoor transformers 23
7.3.4Outdoor ambient conditions 23
7.4Specific limitations for medium power transformers 23
7.4.1Current and temperature limitations 23
7.4.2Accessory, associated equipment and other considerations 23
7.4.3Short-circuit withstand requirements 24
7.4.4Voltage limitations 24
7.5Specific limitations for large power transformers 24
7.5.1General 24
7.5.2Current and temperature limitations 24
7.5.3Accessory, equipment and other considerations 24
7.5.4Short-circuit withstand requirements 25
7.5.5Voltage limitations 25
8Determination of temperatures 25
8.1Hot-spot temperature rise in steady state 25
8.1.1General 25
8.1.2Calculation of hot-spot temperature rise from normal heat-run test data 25
8.1.3Direct measurement of hot-spot temperature rise 26
8.1.4Hot-spot factor 29
8.2Top-oil and hot-spot temperatures at varying ambient temperature and load
conditions 31
General 31]
8.2.2Exponential equations solution 33
8.2.3Difference equations solution 37
8.3Ambient temperature 39
8.3.1Outdoor air-cooled transformers 39
8.3.2Correction of ambient temperature for transformer enclosure 39
8.3.3Water-cooled transformers 40
9Influence of tap-changers 40
9.1General 40
9.2Load loss 41
9.3Ratio of losses 41
9.4Load factor 41
Annex A (informative) Insulation life expectancy and relative ageing rate considering
oxygen and water effect 42
A.1Insulation life expectancy 42
A.2Relative ageing rate considering oxygen and water effect 44
Annex B (informative) Core temperature 47
B.1General 47
B.2Core 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.1General 55
F.2Thermal constant estimation: experimental approach 55
F.3Dynamic thermal modelling: further development 57
Annex G (informative) Oil and winding exponents 58
G.1General 58
G.2Historical background 58
G.3Theoretical approach 60
G.4Extended temperature rise test approach 62
Annex H (informative) Practical example of the exponential equations method 64
H.1General 64
H.2Time period 0 min to 190 min 65
H.3Time period 190 min to 365 min 65
H.4Time period 365 min to 500 min 66
H.5Time period 500 min to 705 min 66
H.6Time period 705 min to 730 min 67
H.7Time period 730 min to 745 min 67
H.8Comparison with measured values 68
Annex I (informative) Application of the difference equation solution method 70
I.1General 70
I.2Example 70
I.3Use 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
Background 82
L.2GIC capability of power transformers [54], [55] 82
Annex M (informative) Alternative oils 84
Bibliography 85
Figure 1 – Structural formula of cellulose 16
Figure 2 – Correlation between tensile strength and DP value 17
Figure 3 – Accelerated ageing in mineral oil at 140 °C, oxygen and moisture contents
maintained at 6 000 ppm and 0,5 %, respectively 18
Figure 4 – Expected life for non-thermally upgraded paper and its dependence upon
moisture, oxygen and temperature 19
Figure 5 – Expected life for thermally upgraded paper and its dependence upon
moisture, oxygen and temperature 20
Figure 6 – Thermal diagram 26
Figure 7 – Temperature rises above top-oil temperature (in tank) 65,8 °C of the zig-zag cooled HV-winding of a 400 MVA ONAF cooled 3-phase transformer, load current 1,0 p.u., tap position (-) 27
Figure 8 – Coil edges, where the sensors should be located in the edge with the higher calculated temperature rise 28
Figure 9 – Temperature rises above top-oil temperature at the end of an 8 h thermal no-load test at 110 % supply voltage 29
Figure 10 – Zigzag-cooled winding where the distance between all sections is the same and the flow-directing washer is installed in the space between sections 30
Figure 11 – Top view section of a rectangular winding with “collapsed cooling duct
arrangement” under the yokes 31
Figure 12 – Block diagram representation of the differential equations 32
Figure 13 – Temperature responses to step changes in the load current 34
Figure 14 – The function θh(t)/θhr generated by the values given in Table 4 37
Figure 15 – Principle of losses as a function of the tap position 41
Figure A.1 – Arrhenius plot for an ageing process 43
Figure F.1 – Hot-spot and top-oil overall model 57
Figure G.1 – Extended temperature rise test 62
Figure G.2 – Transformer exponent estimation plots 63
Figure H.1 – Hot-spot temperature response to step changes in the load current 68
Figure H.2 – Top-oil temperature response to step changes in the load current 68
Figure I.1 – Plotted input data for the example 72
Figure I.2 – Plotted output data for the example 75
Figure K.1 – OF large power transformers: permissible duties for normal loss of life 81
Figure L.1 – GIC flow into a power transformer 82
Table 1 – Relative ageing rates due to hot-spot temperature 21
Table 2 – Maximum permissible temperature limits applicable to loading beyond
nameplate rating 22
Table 3 – Recommended current limits applicable to loading beyond nameplate rating 23
Table 4 – Recommended thermal characteristics for exponential equations 36
Table 5 – Correction for increase in ambient temperature due to enclosure 40
Table A.1 – Activation energy (EA) and environment factor (A) for oxidation, hydrolysis 43
Table A.2 – Expected life of paper under various conditions 44
Table A.3 – Relative ageing rates due to hot-spot temperature, oxygen and moisture
for non-upgraded paper insulation 45
Table A.4 – Relative ageing rates due to hot-spot temperature, oxygen and moisture
for upgraded paper insulation 46
Table H.1 – Load steps of the 250 MVA transformer 64
Table H.2 – Temperatures at the end of each load step 69
Table I.1 – Input data for example 71
Table I.2 – Output data for the example 74
Table K.1 – Example characteristics related to the loadability of transformers 78
Table K.2 – An example table with the permissible duties and corresponding daily loss of life (in “normal” days), and maximum hot-spot temperature rise during the load cycle 80
POWER TRANSFORMERS –
Part 7: Loading guide for mineral-oil-immersed power transformers
1 Scope
This part of IEC 60076 is applicable to mineral-oil-immersed transformers. It describes the effect of operation under various ambient temperatures and load conditions on transformer life.
NOTE For furnace transformers, the manufacturer is consulted in view of the peculiar loading profile.
2 Normative references
The following documents are referred to in the text in such a way that some or all of their content constitutes requirements 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 60076-2, Power transformers – Part 2: Temperature rise for liquid-immersed transformers
IEC 60076-14, Power transformers – Part 14: Liquid-immersed power transformers using high-temperature insulation materials
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