A practical first-zone distance relaying algorithm for long parallel transmission lines

This paper presents a novel non-iterative first-zone distance relaying algorithm for single-line-to-ground fault detection in long parallel transmission lines. The conventional distance protection scheme is based on the series impedance line model, in which the shunt capacitance and propagation effects are neglected. Such a simplification, despite being adequate for short lines, can lead to significant errors in the apparent impedance estimation for long-distance faults. The proposed algorithm is derived from a distributed parameter line model applicable to long parallel lines, in which the shunt capacitance and mutual impedance and admittance are all fully considered. Sliding faults simulations were carried out in MATLAB using symmetrical components and graph theory assuming two long parallel transmission lines. It is shown that the proposed algorithm leads to a very good agreement between the apparent and positive-sequence impedances to the faults for line lengths up to approximately 800 km, without using iterative methods. This indicates that the proposed algorithm prevents the distance relay from underreaching for long-distance faults in parallel lines, without additional computational cost.