An Intelligent Distance Relay Sensing Methodology Immune to Power Swing and Load Encroachment

A distance relay (DR) is a complex sensor that operates based on apparent impedance computed using inputs from the current and voltage sensors. Power swing and load encroachment are two stressed conditions that a DR finds challenging to differentiate from a fault condition. Conventional DRs use additional techniques to detect such conditions and block the DR. However, a symmetrical fault goes undetected during such a block period and can lead to a blackout in the worst case scenario. Also, a highly reactive load encroachment cannot be detected by the conventional relay and, hence, may maloperate. This article critically analyzes the apparent impedance behavior during a symmetrical fault, power swing, and load encroachment. Using the insights from the analysis, an intelligent DR sensing methodology that does not use additional techniques to sense the power swing and load encroachment conditions is proposed. The results from numerous case studies of faults, power swing, faults during power swing, load encroachment, and faults during load encroachment in the WSCC nine bus system in MATLAB/Simulink prove the efficacy of the proposed methodology. The DR employing the proposed sensing methodology can refrain from issuing false trip signals during power swing and load encroachment conditions. It can detect symmetrical and unsymmetrical faults within one cycle in any given condition. The proposed methodology is superior to other methods in detecting symmetrical and unsymmetrical faults during any given condition within one cycle.