AS 2374.7-1997

1 - AS 2374.7-1997 POWER TRANSFORMERS - LOADING GUIDE FOR OIL-IMMERSED POWER TRANSFORMERS
4 - PREFACE
6 - CONTENTS
7 - Section 1: General
7 - 1.1 Scope
7 - 1.2 Object
8 - 1.3 Definitions
8 - 1.3.1 Distribution transformer
8 - 1.3.2 Medium power transformer
8 - 1.3.3 Large power transformer
9 - 1.4 General limitations and effects of loading beyond nameplate rating
9 - 1.4.1 Effect of loading beyond nameplate rating
11 - 1.4.2 Transformer size
12 - 1.4.3 Current and temperature limitations
13 - 1.5 Specific limitations for distribution transformers
13 - 1.5.1 Rating limitation
13 - 1.5.2 Current and temperature limitations
13 - 1.5.3 Accessory and other considerations
13 - 1.5.4 Indoor transformers
13 - 1.5.5 Outdoor ambient conditions
13 - 1.6 Specific limitations for medium power transformers
13 - 1.6.1 Rating limitations
13 - 1.6.2 Current and temperature limitations
14 - 1.6.3 Accessory, associated equipment and other considerations
14 - 1.6.4 Short-circuit withstand requirements
14 - 1.6.5 Voltage limitations
14 - 1.7 Specific limitations for large power transformers
14 - 1.7.1 General
15 - 1.7.2 Current and temperature limitations
15 - 1.7.3 Accessory, equipment and other considerations
15 - 1.7.4 Short-circuit withstand requirements
15 - 1.7.5 Voltage limitations
16 - Section 2: Determination of temperatures
16 - 2.1 Symbols
16 - 2.1.1 Basic symbols
16 - 2.1.2 Prefixes
16 - 2.1.3 Exponents
17 - 2.1.4 Suffixes (general)
17 - 2.1.5 Specific suffixes relating to oil temperatures
17 - 2.2 Direct measurement of hot-spot temperature
17 - 2.3 Assumed thermal characteristics
17 - 2.3.1 Simplifications made
20 - 2.4 Steady-state temperature equations
20 - 2.4.1 ON cooling
20 - 2.4.2 OF cooling
21 - 2.4.3 OD cooling
21 - 2.4.4 Equation corrections
21 - 2.5 Transient temperature equations
22 - 2.6 Thermal ageing of transformer insulation
22 - 2.6.1 Law of thermal ageing
23 - 2.6.2 Relative thermal-ageing rate
23 - 2.6.3 Loss-of-life calculation
24 - 2.7 Ambient temperature
24 - 2.7.1 General
24 - 2.7.2 Weighted ambient temperature
25 - 2.7.3 Ambient temperature for hot-spot calculation
25 - 2.7.4 Continuously varying ambient temperature
27 - 2.8 Computer program
27 - 2.8.1 Logic diagram
28 - 2.8.2 Example of calculation
35 - Section 3: Loading tables
35 - 3.1 Limitations applicable to loading tables
35 - 3.2 Method of representing an actual load cycle by an equivalent two-step rectangular load cycle
35 - 3.2.1 Use of guide
36 - 3.2.2 Load cycle with one peak
36 - 3.2.3 Load cycle with two peaks of equal amplitude and different duration
37 - 3.2.4 Load cycles with peaks in close succession
37 - 3.3 Normal continuous loading
37 - 3.4 Normal cyclic loading
47 - 3.5 Emergency cyclic loading
72 - Annex A - Equivalent rating for auto-transformers
74 - Annex B - Alternative method for determining the winding oil average temperature rise from temperature-rise test measurement
78 - Annex C - Information to be supplied with enquiry and order
79 - Annex D - Calculation of parameters for sinusoidal variation of ambient temperature
85 - Annex E - Example of simplified application of the guide
87 - APPENDIX ZZ - VARIATIONS FROM IEC 354 FOR AUSTRALIAN CONDITIONS
88 - APPENDIX ZA - DETERMINATION OF THE THERMAL TIME-CONSTANT OF A TRANSFORMER UNDER STEADY LOAD CONDITIONS
89 - APPENDIX ZB - INDIRECT MEASUREMENT OF WINDING HOT-SPOT TEMPERATURE

Provides guidance on determining the acceptable relationship between transformer rating and proposed load cycle when considering the effect of operating temperatures on life expectancy due to insulation deterioration and thermal ageing. Includes recommendations for loading above the nameplate rating and guidance for choosing appropriate rated quantities and loading conditions for new installations. It applies to the same range of transformers complying with AS 2374.1-1997. This Standard is technically equivalent to and reproduced from IEC 354:1991 and includes Australian informative appendices on determination of the thermal time-constant and indirect measurement of winding hot-spot temperature.

This guide is applicable to oil-immersed transformers complying with IEC 76. It indicates how, within limits, transformers may be loaded above rated conditions. For furnace transformers, the manufacturer should be consulted in view of the peculiar loading profile.

First published as AS CC10-1965.
Revised and redesignated AS 1078.1-1972.
Revised and redesignated AS 1078-1984.
Revised and redesignated AS 2374.7-1997.