The dynamic behaviour of a transformer winding under axial short-circuit forces

The behaviour of transformer windings under axial short-circuit forces has not been fully understood up to the present time. The paper attempts to remedy this deficiency by describing the results of a comprehensive short-circuit test programme and by producing a theoretical approach to the problem. The authors show that a large power transformer of 33.3 MVA single-phase, when subjected to short-circuits, has a mechanical equivalence for which the relevant motion equations are derived. These control the behaviour of the winding and, when suitably programmed for computer solution, produce calculated values of axial forces and coil displacements. The behaviour is fully borne out by the test results, which also show the influence of the dynamic phenomenon, including the occurrence of a state of resonance for specific conditions of the insulation, under variable-frequency excitation. It becomes apparent that amplification of forces, both within the winding and on the winding supports, can take place and are calculable. These forces can be controlled by judicious use of the controlling parameters, which may be so chosen as to avoid these ill effects, provided that the heat treatment of the windings has been rigorously carried out and that the stability of the insulation is maintained.