Enhancing Immunity of Full-Cycle Discrete Fourier Transform Against Decaying DC Components: a Comparative Analysis

Decaying DC terms of the fault current signal indisputably affect the accuracy of the discrete Fourier transform (DFT) based fundamental phasor estimators of digital relays. This effects will lead to delay in the best case and mal operation in the worst case of the protective relays and it may threat the reliability and security of the power system. This paper aims to conduct a comparative analysis between some decaying DC removal filters for enhancing the immunity if full-cycle DFT against decaying DC components. This paper mainly discusses the effects of DC offsets in the fault current and CT transient responses, harmonics and off-nominal frequency on the performance of DFT. Applying different test signals and also simulated fault current, the effectiveness of the different DC removal filters in combination with DFT is investigated. Analyzing simulation results, it is observed that the DFT phasor estimator should be provided with an algorithm that removes DC offsets due to fault current and CT transient responses under harmonic, noisy and off-nominal frequency conditions.

I. Introduction

Transient fault current signals may contain exponentially decaying DC terms which are the reflections of behavior of power system and also CT transient response[1], [2]. These DC terms produce inappropriate fluctuation in the phasor estimator and as a result the decision making of the protective relays may be badly affected [3]. DFT is one of the most famous phasor estimator that has very excellent performance in harmonic extraction. However, DFT is very sensitive to DC terms in the fault current signals and auxiliary filter must be combined with DFT to enhance the immunity of the phasor estimation against DC terms [3], [4].