Evaluation of digital filters for rejecting discrete spectral interference in on-site PD measurements

A critical study of the performance of several digital filters for rejecting discrete spectral interference (DSI) in partial-discharge (PD) testing is reported. The filters are evaluated with particular reference to distortion introduced on successive PD pulses. A digital filtering method based on a cascaded second-order IIR lattice notch filter is proposed. The study shows that, for similar performance, methods based on linear prediction require longer filter orders when compared to cascaded IIR notch filters. The nonadaptive methods are found to be stable against impulsive disturbances, while adaptive methods are unstable. The method proposed gives the best performance, low distortion to a sequence of PD pulses, good stability, and fast filtering time. The performance at higher sampling rates, even with PD pulses of long widths, is also found to be superior. The method is implemented using fixed-point arithmetic leading to faster processing. Hence, the method is suitable for on-site PD measurements