A New DFT-Based Current Phasor Estimation for Numerical Protective Relaying

A new algorithm has been developed for phasor estimation of the current signal within numerical protective relays which effectively filters out harmonics, noise, and decaying dc offset. Analytical analysis is presented to show the effect of decaying dc on discrete Fourier transform (DFT)-based phasor estimation. Based on the presented theory, a new method is proposed to first estimate the decaying dc characteristic of the current signal and second to compensate the unwanted effect of decaying dc on current phasor in the phasor domain. The proposed technique adds few basic mathematical operations to conventional DFT by employing the proposed two interim variables to characterize the decaying dc component. The proposed algorithm has been comprehensively evaluated in Matlab and compared with the conventional methods used in modern commercial relays for a wide variety of signals including different levels of decaying dc magnitude and time constants, offnominal frequency operation, signal distortion due to current transformer saturation, and transmission-line series compensation in the presence of harmonics and noise. Standard performance indices including rise time, settling time, and overshoot are considered for evaluation and comparison study.