BS 7735:1994

Committees responsible
Foreword
Section 1. General
1.1 Scope
1.2 Object
1.3 Definitions
1.4 General limitations and effect of loading beyond
      nameplate rating
1.5 Specific limitations for distribution transformers
1.6 Specific limitations for medium power transformers
1.7 Specific limitations for large power transformers
Section 2. Determination of temperatures
2.1 Symbols
2.2 Direct measurement of hot-spot temperature
2.3 Assumed thermal characteristics
2.4 Steady-state temperature equations
2.5 Transient temperature equations
2.6 Thermal ageing of transformer insulation
2.7 Ambient temperature
2.8 Computer program
Section 3. Loading tables
3.1 Limitations applicable to loading tables
3.2 Method of representing an actual load cycle by an
      equivalent two-step rectangular load cycle
3.3 Normal continuous loading
3.4 Normal cyclic loading
3.5 Emergency cyclic loading
Annexes
A. Equivalent rating for auto-transformers
B. Alternative method for determining the winding oil
     average temperature rise from temperature rise test
     measurements
C. Information to be supplied with enquiry and order
D. Calculation of parameters for sinusoidal variation
     of ambient temperature
E. Example of simplified application of the guide
Tables
1. Current and temperature limits applicable to loading
     beyond nameplate rating
2. Thermal characteristics used for the calculation of
     loading table sin section 3
3. Correction for increase in ambient temperature due
     to enclosure
4. Data relating to calculation over a single day with
     a constant weighted ambient temperature
5. Data relating to calculation over a full year with
     double sinusoidal variation of ambient temperature
     and three different load profiles
6. Acceptable load factor for continuous duty K24 at
     different ambient temperatures (ON, OF and OD
     cooling)
7. ONAN distribution transformers: t = 0,5 h Permissible
     duties and corresponding daily loss of life
8. ONAN distribution transformers: t = 1 h Permissible
     duties and corresponding daily loss of life
9. ONAN distribution transformers: t = 2 h Permissible
     duties and corresponding daily loss of life
10. ONAN distribution transformers: t = 4 h Permissible
     duties and corresponding daily loss of life
11. ONAN distribution transformers: t = 8 h Permissible
     duties and corresponding daily loss of life
12. ONAN distribution transformers: t = 24 h Permissible
     duties and corresponding daily loss of life
13. ON medium and large power transformers: t = 0,5 h
     Permissible duties and corresponding daily loss of
     life
14. ON medium and large power transformers: t = 1 h
     Permissible duties and corresponding daily loss of
     life
15. ON medium and large power transformers: t = 2 h
     Permissible duties and corresponding daily loss of
     life
16. ON medium and large power transformers: t = 4 h
     Permissible duties and corresponding daily loss of
     life
17. ON medium and large power transformers: t = 8 h
     Permissible duties and corresponding daily loss of
     life
18. ON medium and large power transformers: t = 24 h
     Permissible duties and corresponding daily loss of
     life
19. OF medium and large power transformers: t = 0,5 h
     Permissible duties and corresponding daily loss of
     life
20. OF medium and large power transformers: t = 1 h
     Permissible duties and corresponding daily loss of
     life
21. OF medium and large power transformers: t = 2 h
     Permissible duties and corresponding daily loss of
     life
22. OF medium and large power transformers: t = 4 h
     Permissible duties and corresponding daily loss of
     life
23. OF medium and large power transformers: t = 9 h
     Permissible duties and corresponding daily loss of
     life
24. OF medium and large power transformers: t = 24 h
     Permissible duties and corresponding daily loss of
     life
25. OD medium and large power transformers: t = 0,5 h
     Permissible duties and corresponding daily loss of
     life
26. OD medium and large power transformers: t = 1 h
     Permissible duties and corresponding daily loss of
     life
27. OD medium and large power transformers: t = 2 h
     Permissible duties and corresponding daily loss of
     life
28. OD medium and large power transformers: t = 4 h
     Permissible duties and corresponding daily loss of
     life
29. OD medium and large power transformers: t = 8 h
     Permissible duties and corresponding daily loss of
     life
30. OD medium and large power transformers: t = 24 h
     Permissible duties and corresponding daily loss of
     life
Figures
1. Thermal diagram
2. Correction between weighted and average ambient
     temperature as a function of the temperature range
3. Logic diagram of a computer program for calculation
     of an acceptable loading factor
4. Equivalent two-step load cycle
5. Load cycle with one peak
6. Load cycle with two peaks of equal amplitude and
     different duration
7. Load cycle with peaks in close succession
8. Illustration of example 2
9. ONAN distribution transformers - Permissible duties
     for normal loss of life
10. ON medium and large power transformers - Permissible
     duties for normal loss of life
11. OF medium and large power transformers - Permissible
     duties for normal loss of life
12. OD medium and large power transformers - Permissible
     duties for normal loss of life
A.1 Auto-transformers: limitations in rated power Sr and
     short-circuit impedance zr
B.1 Determination of average winding rise, gradient and
     winding time constant from resistance cooling curve
B.2 Determination of effective oil time constant, tau o,
     from oil heating curve
D.1 Definition of the parameters for sinusoidal
     variation of ambient temperature
D.2 Logic diagram of a computer program for the
     calculation of parameters for sinusoidal variation
     of ambient temperature
E.1 Example of simplified instructions for loading
     beyond nameplate rating

Applies to transformers covered by BS 171.