Performance evaluation of digital filters used for distance relaying

To attenuate the postfault waveforms of the voltage and current from high frequency harmonics, a low pass digital filter is used. Accordingly, these low pass digital filters become an integral part of the relay operation. A discussion is presented of the design and operation of these digital filters. Effects of the filter order and the filter time delay on the estimation of the fault location for a three phase symmetrical short circuit are also discussed. Numerical results for a 4th, 8th and 10th order digital filter, when least error squares (LS) and least absolute value (LAV) parameter estimation techniques are used to estimate fault location, are reported and form the basis of the authors conclusions