A new algorithm based on Clarke's Transform and Discrete Wavelet Transform for the differential protection of three-phase power transformers considering the ultra-saturation phenomenon

•An algorithm for differential protection considering ultra-saturation is presented.•To model ultra saturation, a new model is presented for current transformer.•A model to investigate the ultra-saturation in three-phase transformer is presented.•This work can be used as hints for substation personnel to know ultra-saturation.•False trip of differential protection due to ultra-saturation is controlled.

This paper presents, at first, a novel model for investigating the ultra-saturation phenomenon during energization of a loaded three-phase power transformer. Then a new approach is presented to control the unusual false trip of a three-phase power transformer differential protection due to ultra-saturation phenomenon based on Clarke's Transform and Discrete Wavelet Transform (DWT). In this method, the transient phenomena of a power transformer including the magnetic inrush current, the ultra-saturation phenomenon, the external faults, and the internal faults of the power transformer are simulated. To distinguish between these phenomena, appropriate criteria based on Clarke's Transform and DWT are presented using the standard deviation of coefficients and the energy coefficients. The results of this study may be used as notifications by the personnel of substation and relay manufacturers.