A transformer differential relay with a core saturation detection algorithm

The differential current for transformer protection has points of inflection at the start and end of each saturation period of the core during magnetic inrush and over-excitation. These points can be detected using the third-difference function of the differential current. This paper proposes a current differential relay for transformer protection that operates in conjunction with a core saturation detection algorithm. If the magnitude of the third-difference function exceeds the threshold, the blocking signal is issued and maintained for one cycle. A method is designed to discriminate between transformer saturation and current transformer (CT) saturation. The performance of the proposed blocking scheme was compared with a conventional harmonic blocking scheme. Test results indicate that the proposed scheme can discriminate between magnetic inrush and internal faults and reduce the delay of the operating time of a relay for an internal fault when a CT saturates.