An analysis for digital protection of transformers

Abstract

This thesis presents an analysis for digital differential protection of transformers. For digital differential protection of transformers, it is of utmost importance to test the algorithms on simulated data of both fault and magnetizing inrush currents. For this purpose, a detailed analysis of inrush current for both single and three-phased transformers are carried out. Computer software package programs are developed to simulate the inrush and fault currents. Measured waveforms of the inrushes are compared with the simulated waveforms and the harmonic contents of both the measured and simulated waveforms are analysed up to 4 cycles. There exists a reasonably good agreement between simulated and measured waves. -- Three algorithms are analysed for the digital differential protection purpose. Discrete Fourier technique has been used as the reference algorithm. Rectangular transform and Walsh function are analysed in detail for their suitability in microprocessor applications for both 16 and 8 samples/cycle basis. Their performances are compared with Discrete Fourier on 16 samples/cycle basis. Rectangular transform and Walsh function are tested on simulated inrush and fault current data under various switching conditions. It is found that the Rectangular transform converges more rapidly than the other transforms during a fault in the transformer. Finally, on-line implementation of this new digital relay is suggested in near future.