Analysis of internal winding stresses in EHV generator step-up transformer failures

This paper describes studies carried out to investigate the failure of one phase of a 990 MVA generator step-up transformer bank at one of Ontario Hydro´s major generating stations. A high accuracy frequency dependent transformer model was used to determine the behaviour of the transformer at the terminals under transient conditions. With this information, and with measured transfer functions between the terminals and various points inside the windings, it was possible to create internal voltage distribution maps. This information was then used to evaluate the insulation stresses within the winding, and to ascertain that the use of additional capacitance at the HV terminals to reduce the magnitude and frequency of incoming transients would be adequate to reduce the disk-to-disk stresses near the line end of the HV winding. The values of the HV terminal capacitances were chosen so that no undue stresses in other portions of the winding would take place