Overcurrent protection coordination with flexible partitioning of active distribution systems into multiple microgrids

•An inverse time admittance relay (ITAR) is introduced.•Impact of microgrid partitioning on the performance of the protection coordination.•Grid-connected, islanded, and partitioned microgrids are considered in protection coordination.•Objective function is formulated based on minimizing overall tripping time of relays.•PG&E 69-bus system is utilized for testing different overcurrent characteristic. Recently, sectionalizing active distribution systems (ADS) into partitioned microgrids has been proposed to enhance the reliability and supply-security but can have adverse effects on the performance of the protection scheme. The previous protection schemes based on overcurrent relays have been proposed for active distribution systems (ADS) with no partitioning capability. This paper analyzes the impact of microgrid partitioning on the performance of the microgrid protection schemes relying on the standard inverse time overcurrent relays (ITOCRs) and introduces a protection scheme that relies on inverse time admittance relay (ITAR) for ADS with partitioning capability. The introduced protection scheme is tested on a modified PG&E 69-bus equipped with distributed generation and partitioned into several microgrids. The protection coordination problem is formulated as a constrained nonlinear programming (NLP) problem considering grid-connected, islanded, as well as partitioned operation modes. Simulation results demonstrate that the ITAR characteristic shows promising performance in comparison to the standard ITOCR characteristic achieving minimal relay operating time and coordination failure.