Removal of DC-offset in current waveforms using digital mimic filtering

This paper presents a theoretical investigation of the capability of popular digital filters used in relaying applications to suppress DC offset in current waveforms over a broad range of time constants as encountered on real power systems. First, three mathematical performance indices are introduced to be able to compare filters on the same basis. Then the concept of the FIR type of digital mimic filter is described. It is shown that, for filters exhibiting a null response at frequency zero on their frequency response, the addition of a digital mimic circuit, tuned to the middle of the range of time constants to be suppressed, will almost completely eliminate the effect of an exponentially decaying component. Moreover, this is possible without sacrificing the original response speed, although it increases susceptibility to noisy components