CONTENTS
FOREWORD 5
1 Scope 9
2 Normative references 9
3 Requirements with regard to ability to withstand short circuit 9
3.1 General 9
3.2 Overcurrent conditions 11
4 Demonstration of ability to withstand short circuit 17
4.1 Thermal ability to withstand short circuit 17
4.2 Ability to withstand the dynamic effects of short circuit 23
Annex A (informative) Theoretical evaluation of the ability to withstand the dynamic
effects of short circuit 41
Annex B (informative) Definition of similar transformer 71
Figure 1 – Star/delta connected transformer 29
Figure 2 – Star/star auto-transformer 31
Table 1 – Recognized minimum values of short-circuit impedance for transformers
with two separate windings 13
Table 2 – Short-circuit apparent power of the system 13
Table 3 – Maximum permissible values of the average temperature of each winding
after short circuit 21
Table 4 – Values for factor k ´ 25
Table A.1 – Comparison of forces and stresses in core-type transformers 61
Table A.2 – Comparison of forces and stresses in shell-type transformers 65
Table A.3 — Values for factor K3 69
Table A.4 — Values for factor K4 69
POWER TRANSFORMERS –
Part 5: Ability to withstand short circuit
1 Scope
This part of IEC 60076 identifies the requirements for power transformers to sustain without damage the effects of overcurrents originated by external short circuits. It describes the calculation procedures used to demonstrate the thermal ability of a power transformer to withstand such overcurrents and both the special test and the theoretical evaluation method used to demonstrate the ability to withstand the relevant dynamic effects. The requirements apply to transformers as defined in the scope of IEC 60076-1.
2 Normative references
The following referenced documents are indispensable for the application 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 60076-1:1993, Power transformers – Part 1: General
Amendment 1 (1999)2
IEC 60076-3:2000, Power Transformers – Part 3: Insulation levels, dielectric tests and external clearances in air
IEC 60076-8:1997, Power transformers – Part 8: Application guide
IEC 60076-11:2004, Power transformers – Part 11: Dry-type transformers
Definition of similar transformer
A transformer is considered similar to another transformer taken as a reference if it has the following characteristics in common with the latter:
–same type of operation, for example generator step-up unit, distribution, interconnection transformer;
–same conceptual design, for example dry type, oil-immersed type, core type with concentric windings, sandwich type, shell type, circular coils, non-circular coils;
–same arrangement and geometrical sequence of the main windings;
–same type of winding conductors, for example aluminium, aluminium alloy, annealed or work-hardened copper, metal foil, wire, flat conductor, continuously transposed conductors and epoxy bonding, if used;
–same type of main windings, for example helical-, disc-, layer-type, pancake coils;
–absorbed power at short circuit (rated power/per unit short-circuit impedance) between 30 % and 130 % of that relating to the reference transformer;
–axial forces and winding stresses occurring at short circuit not exceeding 120 % of those relating to the reference transformer;
–same manufacturing processes;
–same clamping and winding support arrangement.