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Numerical differential protection algorithm for power transformers

The differential protective system establishes the main protection against short circuits occurring on transformer windings. This protective system should operate rapidly when internal faults occur. However, it should not operate under nonfault conditions such as a flowing inrush current. In digital differential relays, differential currents or other required signals are sampled and processed according to protective algorithms. Most of the existing fault detection algorithms in the digital differential protection of power transformers are based on processing the harmonic content of the differential currents. These methods utilize the fact that the ratio of the second harmonic to the fundamental component of differential currents under inrush current conditions is greater in comparison to that under fault conditions. In this paper, transformer terminal voltage and current signals are sensed and utilized to enhance the reliability of differential protection. The proposed algorithm is tested by simulation and compared to a conventional harmonic detection-based algorithm.