Analysis of Short-Circuit Performance of Split-Winding Transformer Using Coupled Field-Circuit Approach

The split-winding arrangement requires special short-circuit design considerations. During short-circuit conditions, there is a considerable distortion of the leakage field, which in turn produces high axial short-circuit forces. This paper deals with the computation and analysis of electromagnetic forces in windings of split-winding transformers. A nonlinear-transient field-circuit coupled finite element model is used to simulate the split-winding transformer. A 70-MVA, three-phase, 220/6.9/6.9-kV split-winding transformer is modeled under preset and postset short-circuit test conditions. Under the preset condition, the transformer is analyzed with one as well as both the LV windings short circuited to compare the axial forces produced in the windings for these two cases. The results show that there is a considerable rise in the axial forces when one winding is short circuited as compared with the case when both windings are short circuited. The effect of initial magnetization of core on axial short-circuit forces is calculated and discussed. It is also shown that even though postset method eliminates inrush related problems there is not much respite in short-circuit forces