A study of transformer impedance variation over the range of tap positions applied to fault analysis

It is known that transformer impedances change when the tap positions on the primary or secondary are changed. In this work models were designed that gave the maximum impedance variation for most transformers studied. These models were used to study fault current deviations from the predicted values when the actual transformer impedance was uncertain. The effects of the change in turns ratio was also taken into account, and the base voltages were chosen to accommodate this possible error. Faults were placed on the secondary of a model radial power system, and the resulting primary and secondary fault currents were compared to a base case set. Transformer impedance was then varied + 15% and {minus} 10% from the nominal value selected to simulate the range of tap positions with the appropriate turns ratio selected, and the fault current deviations were compiled. Both unloaded and loaded transformers were tested, and the results of each of the applied faults is shown.