Power transformer differential relay using wavelet transform energies

To avoid needless trip by magnetizing inrush current, the second harmonic component is commonly used for blocking the relay in power transformers. Recently, the frequency environment of power systems has been made more complicated and the quantity of second harmonic in inrush currents has been decreased because of the improvement of core steel. This will cause the traditional techniques for transformer differential protection more likely to fail. This paper presents a new scheme that enhances the protection abilities of transformer differential relays. Discrete wavelet transform is used to extract useful information from the low frequency components of the differential currents. The spectral energies of these low frequency components are calculated and the change in these energies is used as the differentiating criterion. The proposed scheme is tested using PSCAD as well as an internal fault simulation program and gave reliable and accurate results.