Adaptive and resilient protection coordination in smart microgrids enabled by a directional fault current limiter

Power systems, particularly distribution networks, are currently facing increasing challenges due to rising energy demand and the growing complexity of modern grids. The integration of Distributed Generators (DGs) within Microgrids (MGs) introduces additional fault current contributions, which significantly modify short-circuit levels. This change can result in miscoordination problems in existing Overcurrent Relay (OCR)-based protection systems, including both maloperation and delayed tripping. In addition, the increased fault current contribution from MGs may exceed the design limits of equipment such as circuit breakers (CBs), exposing them to higher electrical stress and increasing the risk of failure to operate properly. Consequently, these issues collectively degrade the overall reliability and security of the power system. To overcome these challenges, installing a Directional Fault Current Limiter (DFCL) between the microgrid and the upstream network is considered an effective mitigation approach. The DFCL helps in controlling fault current levels and improving protection coordination. However, determining the optimal impedance setting of the DFCL is a complex task, as it must simultaneously ensure proper coordination among OCRs and reduce short-circuit levels to enhance system reliability. In this context, this paper proposes an analytical methodology for determining the optimal DFCL impedance based on Thevenin equivalent impedance analysis. The proposed approach provides a straightforward impedance sizing procedure that restores overcurrent relay coordination following distributed generation integration while maintaining system reliability. Unlike optimization-based methods, the proposed analytical approach determines the required DFCL impedance directly from the equivalent Thevenin impedance without iterative computation, making it computationally efficient and suitable for real-time engineering application The methodology is validated under different operating scenarios involving synchronous generators, wind turbine generators, and photovoltaic systems. The results demonstrate that the proposed approach effectively restores relay coordination, limits excessive fault currents, and enhances the reliability of microgrid protection without requiring adaptive protection schemes or additional relay installation.