Abstract

Distance relays serve as critical protection devices for transmission lines in power systems by calculating line impedance up to fault locations. Accurate impedance measurement is essential, as relay malfunction can lead to partial or complete grid blackouts. These relays utilize voltage and current phasors from Phasor Measurement Units (PMUs) for impedance computation. Conventional methods employing Discrete Fourier Transform (DFT) techniques - including Full-Cycle DFT (FCDFT) and Half-Cycle DFT (HCDFT), often combined with mimic filters - exhibit two significant limitations: sensitivity to source impedance and substantial impedance oscillations that frequently cause relay malfunction. This paper presents a modified Least-Squares (LES) algorithm and compares its performance against FCDFT and HCDFT-based phasor estimation with mimic filters. Simulation results demonstrate that the proposed method significantly reduces impedance oscillations in relay measurements, eliminating malfunction during fault diagnosis. Furthermore, the algorithm maintains consistent accuracy regardless of source impedance variations, performing reliably both with and without source impedance present. All simulations were implemented in MATLAB/Simulink to validate these findings.

Key words: Distance relay, Phasor Measurement Units (PMUs), Least-Squares (LES) algorithm, mimic filter, Full-Cycle Discrete Fourier Transform (FCDFT), Half-Cycle Discrete Fourier Transform (HCDFT).