Calculation of fundamental power frequency for digital relaying algorithms

This paper describes two different measuring methodology for calculating fundamental power system frequency (50 - 60 Hz) with high accuracy for numeric protection algorithms. Especially in distance protection, to measure the fundamental power frequency is a vital fact to locate the fault correctly. The problem is complex because the power frequency changes from 49.0 Hz to 51.0 Hz over a wide range for 50 Hz systems. Assuming the frequency of only 50 Hz or 60 Hz may cause under or over reach problem in distance relaying algorithms. Therefore the aim of this paper is to present a high accuracy, with a wide range frequency estimation algorithm for either sinusoid or non-sinusoid signals. The experimental results reported prove that the first method is more accurate than the second one. In this paper, two different calculation procedures are introduced. The first one is curve fitting approach and needs only six samples of the current or voltage signal. The second one is based on auto correlation function of sampled data. The proposed methods are robust for a wide range of sampling frequency, F_s and give more accurate results than the traditional ones.