Variable tripping time differential protection for microgrids considering DG stability

Coordination between protective device fault clearing times and distributed generation (DGs) stability is essential to avoid nuisance tripping of DG. In this paper, a multi-agent protection scheme based on a variable tripping time differential protection scheme (VTDPS) is proposed for micro-grid protection capable of operating in both grid-connected and islanded mode. Variable tripping time is used for coordination with laterals protective devices without the need for additional current measurement and data transmission. Each agent is composed of primary, backup, and bus protection layers, with their relevant VTDPS, that achieves main and backup protection for a microgrid. Moreover, the critical clearing time (CCT) curves of DGs and tripping time curve of VTDPS are analyzed in each feeder zone and a relevant formula is developed to check the CCT constraint for the proposed multi-agent VTDPS. The presented method is simulated on a modified CIGREE microgrid benchmark system with synchronous DGs. The obtained results prove the capability of the proposed protection scheme to prevent instability of DGs in both modes of operation.