An Effective Protection Coordination Scheme for Networked Microgrids Based on Nonstandard Tripping Characteristics of DOCRs

Design of protection schemes in multiple interlinked microgrids is a difficult task, owing to changes in operating topology of the system. A microgrid can function in island mode, or can operate in conjunction with another autonomous microgrid, or get connected to another grid interfaced microgrid or operate directly with the main grid. Such changes in operating philosophy can vary the short circuit currents widely depending on the status of point of common coupling (PCC) in a system of multi-microgrids environments. A non-standard tripping characteristic (NSTC) of directional overcurrent relays (DOCRs) with four controllable parameters viz. TMS, PS, Beta, and Alpha is proposed for networked microgrids (NMG), which reduces the relay operating time and improves coordination among the relay pairs. The relay settings are optimally found using a new hybrid combination of wild horse optimizer and nonlinear programming (WHO-NLP) approach. All the possible operating modes of network microgrids are identified and a unique relay setting are designed. The proposed protection coordination scheme (PPCS) is validated on two 15-bus microgrid, forming as network microgrid, and on real time digital simulator (RTDS) platform using NSTC feature of DOCRs, in a software in loop (SIL) environment. Extensive test cases are validated, considering operational and topological changes of NMG for several grid connected and islanded events, variations in DG power output, DG outage contingencies, performances under high resistance fault, and bolted faults conditions due to various fault types to ensure the robustness of the proposed method in providing secure and reliable protection of NMG.