PC based relaying algorithms for short-circuit and earth fault protection of power transformer using digital filters

The paper reports the development of protective relaying algorithms for the Internal short-circuit and restricted earth fault protection of power transformer using digital filters. Efficient algorithms based on the recursive constraint Butterworth filter transfer function model of the IIR band pass filter have been developed. which extract the various frequency components such as 50 Hz. 100 Hz and 250 Hz for realizing the protective functions such as variable bios differential relaying logic and restricted earth fault protection logic for the transformer. The developed relaying algorithms have been tested for simulated input signals representing various normal and abnormal power transformer conditions and the scheme as a whole operated efficiently by Issuing trip signals during internal fault conditions and restraining from tripping during external fault and normal conditions.