A STAND-ALONE DIGITAL PROTECTIVE RELAY FOR POWER TRANSFORMERS

This paper deals with the complete design of a stand-alone prototype digital protective relay for three-phase power transformers. The major emphasis of the paper will be on the detailed description of the hardware and software of the prototype relay. The protection functions implemented include: a percentage differential protection with a second-harmonic restraint for magnetizing inrush and a fifth-harmonic restraint for overexcitation conditions, and a separate protection for high impedance primary and secondary ground faults. The present relay design is tested with the Fourier algorithm and any other relay algorithm can be used by replacing only one subroutine. The relay hardware consists of a data acquisition board and a digital processing board which is based on the TMS320E15 processor. Sample real-time test cases are included in the paper. The results show that the relay never misoperated and correctly identified all the faults that are applied.