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Digital relays traditionally employ sampling rates of less than 100 samples/cycle. In order to avoid aliasing due to fault transients, these relays employ an analog antialiasing filter before critical-sampling (Nyquist rate) the input waveforms coming from instrument transformers. In many applications of electrical engineering, oversampling (greater than the Nyquist rate) has long been used to simplify the requirements of an antialiasing filter with a sharp cutoff; in some cases, the filter can even be eliminated. This paper investigates this option for a digital relay. The performance of a traditional digital relay is compared with a method that uses oversampling without using an antialiasing filter. By processing a comprehensive array of fault waveforms from Electromagnetic Transients Program simulations, a suitable oversampling rate is suggested. A comparison of phasor estimates using the traditional relay and the proposed method is made for different operating and fault conditions. The results suggest that oversampling can eliminate the antialiasing filter traditionally employed in digital relays.
Date of Publication: Oct. 2009