A new hybrid model of transformer windings under very fast transient overvoltages

This paper is concerned with the phenomena of very fast transient overvoltages (VFTO) invading the transformer windings. Some literatures have pointed out that the insulation breakdown usually locates near the entrance of the windings. So it is very important to study the voltage distribution along the coils near the entrance of the windings under VFTO. A new hybrid model is presented to simulate this transient process in this paper. In this model, the transformer winding is divided into three parts, which are simulated by the multi-conductor transmission line, single transmission line and equivalent impedance, respectively. The coupling between each part is taken into consideration and is approximated. The voltage distribution and inter-turn voltage of the coils near the entrance of the winding has been calculated. For validation, the calculated results are compared with the measurements on a transformer winding model and the results are satisfactory.