Discrete Fourier Transform (DFT) algorithm, ultra-saturation phenomenon, unloaded transformer energizing with additional line/load from the supplying side, transformer differential protection

One of mal-operations of the transformer differential protection during the unload transformer energizing with additional line/load from the supplying side is ultra-saturation phenomenon. In this paper، first a new model according to Discrete Fourier Transform (DFT) algorithm for investigating the ultra-saturation phenomenon during the unload transformer energizing with additional line/load from the supplying side is presented and its effect on the differential protection of the transformer is considered. In this model، the nonlinear characteristic of the transformer core and the effect of current transformer are taken into account. It is assumed that the additional line/load from the supplying side of the power transformer is a resistive-inductive load. Also، the effect of the residual flux، inception angle and additional line/load from the supplying side on the ultra-saturation phenomenon is investigated. Then، the mal-operation of differential protection by using DFT algorithm is described.