Mitigating Protection System Maloperations of Mutually Coupled Transmission and Distribution Lines Using Negative Sequence Polarizing Method

Expansion of power network has led to the construction of multiple transmission and distribution lines within constrained spaces inevitably resulting in them being mutually coupled. This introduces a new set of challenges to protection engineers to accurately determine the reach of conventional protection schemes like differential, distance and directional earth fault with carrier aided schemes, making accurate protection grading a complex task. Protection settings without meticulous investigation have led to maloperation of the relays which degrades the overall power system reliability. Zero-sequence impedance-based tripping logics have shown operational issues under such mutual coupling of power lines, whereas secondary tripping logic, such as negative sequence protection has warranted an accurate fault recognition. In this paper, the usability of negative sequence protection of such instances is extensively analysed to mitigate the nuisance tripping of such mutually coupled transmission lines. A realworld case study of relay maloperation is analysed using detailed PSCAD modelling of the network under different fault conditions with protection scheme applied. The results of the case study have revealed that the proposed scheme successfully eliminates the instances of incorrect tripping, ensuring precise fault identification across various fault scenarios, while maintaining compatibility with the existing protection coordination of mutually coupled lines.