Coordination of Mixed Overcurrent and voltage-Restrained Overcurrent Relays To Avoid Miscoordination Problems Considering IEC Time-Current Curve Limitations

Nowadays, understanding the fault current characteristics and determining optimal overcurrent relays coordination are one of the major power system operation concerns, due to the high penetration of Distributed Generations (DGs) in the power system. In this paper, the impact of DG on fault currents behavior on the power system protection scheme and Over Current Relays (OCRs) coordination has been investigated. Then, a novel optimal protection scheme is developed to tackle this issue based a new and novel optimum coordination equation considering the limitation of conventional IEC tripping characteristics in digital OCRs. The proposed novel coordination scheme aims to increase the sensitivity, selectivity and reliability of the power protection system by considering the protection of DG in the coordination equation. In addition, unlike the conventional OCRs coordination approaches in the literature review which depends on solving only the phase OCR coordination problem, this paper will present a novel protection scheme and equation by merging the OCR coordination function with the Voltage Restrained OCR (VROCR) of DG protection scheme. This new approach by including the constraints of VROCR in the problem of OCR coordination (VROCRs-OCRs) aims to increase selectivity and speed in isolating faults through adjusting the settings based on the voltage measured at the DG bus-bar. The proposed coordination schemes are evaluated and tested using three-phase faults in different locations within two benchmark IEC microgrid scenarios (With and without DGs). The results of the new protection scheme showed that the proposed scheme have successfully reduced the total operating time of relays and increased the sensitivity and selectivity of power system.