Adaptive underreach protection relay using zero-mode band time-delay dynamics

Traditional three-stage power system protection faces inherent conflicts among its four features. When protection settings avoid mal-operation during severe external faults without boundary elements, internal fault sensitivity is compromised. This limitation causes potential over-reach or under-reach mis-operation in conventional under-reach schemes. In this paper, a novel underreach local measurement-based protection scheme is proposed using zero-mode band time-delay dynamics. First, abrupt voltage changes detected in real-time by a Kalman filter initiate the proposed protection. Second, wavelet packet decomposition processes the zero-mode reverse traveling wave signal across multiple frequency bands. Two specific bands are extracted, Hilbert-transformed, and their first wave-head arrival time difference is calculated as the adaptive characteristic criterion. This value dynamically adjusts to fault locations. The theoretical travel time difference of arrival (TTDoA) serves as the dynamic action threshold. Simulations confirm accurate identification of abrupt changes and activation criteria for single-phase-to-ground faults at varying distances. The scheme reliably protects over 80% of the line length, precisely distinguishes internal from external faults, and operates correctly under high-resistance grounding faults.