Transformers together with all equipment and accessories shall be designed and constructed to withstand without damage the thermal and dynamic effects of external short- circuits.
In accordance with IEC 60076-5 standard and ERDF specification, the short-circuit withstand test is required for each power of transformer and the way it should be conducted is the following:
- Prior to the short-circuit tests, the transformer shall be subjected to the routine tests which are specified in IEC 60076-1.
- 9 short-circuits of 0,5 s duration each (3 on each column).
- All the routine tests, including dielectric tests should be repeated. A lightning impulse test shall be performed at this stage.Inspection of the active part at the end of all the tests : no arcing or overheating marks, no deformation, no loosening and no displacement of any kind (connections, wedges, papers, press-boards, etc.), no particle or deposit in the dielectric liquid [4]
In order to consider the transformer as having passed the short-circuit test, the following conditions should be fulfilled:
The results of the short-circuit tests and the measurements and checks performed during tests do not reveal any condition of faults.
The dielectric tests and other routine tests when applicable, have been successfully repeated and the lightning impulse test, if specified, successfully performed.
The out-of-tank inspection does not reveal any defects such as displacements, shift of laminations, deformation of windings, connections or supporting structures, so significant that they might endanger the safe operation of the transformer.
No traces of internal electrical discharge are found. The short-circuit impedance values, in ohms, evaluated for each phase at the end of the tests, do not differ from the original values by more than 4 % [5].
Calculations