MULTISTAGE ADAPTIVE FILTERS FOR IN-PHASE PROCESSING OF LINE-FREQUENCYSIGNALS

We describe two adaptive multistage digital filters for 50/60-Hz line- frequency signal processing in zero-crossing detectors and synchronous power systems, These filters combine a median filter with adaptive pr edictors, either finite-impulse response (FIR)- or infinite-impulse re sponse (IIR)-based, thus making it possible to extract the sinusoidal signals from noise and strong disturbances without phase shifting the primary frequency signal, The median filter is used as a prefilter bec ause it can remove deep commutation notches from the waveform, Adaptat ion allows the filters to track the exact instantaneous line frequency and avoids the selectivity problem encountered with a fixed filter.