An improved dynamic model for overcurrent relays in protection of electrical networks: Addressing two-level fault currents

Abstract

The paper aims to enhance the dynamic model of directional overcurrent relays (DOCRs), ensuring that it satisfies selectivity constraints not only for two-level faults but also for faults with resistance. To this end, a new scaling factor is introduced and integrated with the coefficient of fault current in the relay model which virtually increases the fault currents seen by the corresponding DOCR. In the proposed approach, this scaling factor is considered as an optimization variable and individual setting for the DOCR to create a proper distinction between fault currents and load currents. By doing so, this new variable facilitates the optimal relay coordination within a new framework that results in meeting selectivity constraints and reducing the relays operation times. However, the integration of this scaling factor alongside the coefficient of fault current in the model of relays may leads to unintentional tripping during normal conditions. Therefore, through an innovative modification in the logic of the DOCR, this challenge is addressed. Since the developed logic necessitates the use of numerical relays, the associated costs of replacing traditional relays with numerical ones should be taken into account. Therefore, the proposed coordination scheme is subsequently extended through a techno-economic model and the gray relational analysis (GRA) algorithm is employed to navigate the complex decision-making process from the Pareto front. The results demonstrate that replacing 25 % of the relays with numeric ones can satisfy selectivity requirements even in the presence of fault resistance, while the overall operation times of primary and backup relays can be reduced dramatically by up to 65 %. The proposed is tested not only on the IEEE 14-bus distribution network but also on the 30-bus network, validating its applicability across larger systems.