Impedance-based algorithm for the discrimination between inrush and short-circuit currents in single-phase transformers

This paper presents an impedance-based technique for the discrimination between inrush current and short-circuit current in single-phase transformers. The method is based on the simulation of the behavior of the impedance relay. A Hilbert transform is adapted for the numerical calculation of the impedance at the input terminals of the transformer. First, the inrush current is computed numerically by approximating the magnetizing curve. The nonlinearity as well as the saturated portions are taken into account. Second, the transformer input impedance is calculated using the Hilbert transform in a time domain. The input impedance in the short-circuit case is calculated similarly. A comparison between the two input impedances is necessary for protective relay to discriminate between transformer switching transients and short circuits. The method is applied to a 1 kVA, 220/110 V single-phase transformer. The obtained results can be used to design an impedance relay to block the transformer differential protection during the inrush current. The proposed method is simple and computationally efficient.