This paper aims to assess the performance of digital filtering algorithms for numeric relays in terms of: ability to reject the DC offset component, transient response, accuracy, sampling frequency and noise immunity. The filters under investigation are the full cycle Fourier transform filters, cosine and two modified DFT methods. The current waveforms used during the tests were generated by mathematical equations and by computational simulation on a test system. The results show that the modified Fourier filters have the best performance, in most cases. These algorithms require a greater computational effort due to the number of mathematical operations involved. However, this fact has not caused considerable increase in simulation time.
Digital filtering; protective relaying; decaying DC offset
