A new current estimation algorithm to filter decaying DC component

The decaying DC signals with exponential function occur in fault currents of power system, so direct use of discrete Fourier transform (DFT) algorithm for fundamental current measurement results in big errors. This paper presents a new algorithm to eliminate the decaying DC component from fault currents. This algorithm is based on the principle that a purely sinusoidal signal is integrated into zero over a full cycle of its fundamental frequency and an exponential decaying signal has nonzero integrating value. The sampled data of one full cycle plus one past sampled period are used to estimate exactly the magnitude and time constant of an exponential decaying signal. After each sample data is subtracted by the decaying DC component offset, fault currents can be estimated accurately by full DFT algorithm. A comprehensive simulation test is carried out on Matlab simulation platform, which shows that the proposed algorithm has high measurement accuracy, is easy to be implemented by DSP and can be applied to digital relay protections based on DSP.