Microprocessor-based protection scheme for power transformers

This paper presents a microprocessor based relay scheme for differential protection of large power transformers. This scheme provides protection against internal faults on transformers, and restrains the tripping operation during the inrush condition by detecting the second harmonic component in the current. The functions of various blocks of the relay scheme are explained. A brief review of different relay algorithms for computing fundamental and second harmonic components of the differential current is presented. A block pulse functions based algorithm is explained. Details of simulating the inrush and internal fault currents using a transformer model are described. The relay scheme was tested off-line using the simulated currents, and the test results are included in this paper.