An Accurate Filtering Technique to Mitigate Transient Decaying DC Offset

Phasor estimation, as performed by the discrete Fourier transform (DFT), is adversely affected by the decaying dc offset which occurs following a fault. This letter presents an improved algorithm based on the DFT, hereafter named IDFT, which effectively mitigates the decaying dc component. The IDFT uses the even and odd samples of the estimated phasor. The method is based on the fact that, in the phasor domain, the even and odd samples in a purely sinusoidal signal produce the same results for the amplitude estimation. However, in an exponential signal, especially at the beginning of the fault due to a higher slope, they are different. The IDFT technique is compared with the conventional full-cycle DFT algorithm. The results show that IDFT is more accurate and has faster convergence than the desired value compared to the conventional DFT algorithm.

I. Introduction

THE DECAYING dc component in fault currents affects the accuracy and convergence of digital filter algorithms [1]. In digital relaying, discrete Fourier transform (DFT) is the most preferable method to extract the fundamental phasor quantities from the waveforms. DFT has immunity from harmonic components and fast response for the fundamental component calculation [2]. However, it is not immune from the dc component which can cause undesirable oscillations in the DFT results.