Co-Optimization of Protection Coordination and Power Quality in Microgrids Using Unidirectional Fault Current Limiters

This paper proposes a framework to simultaneously optimize the power quality (PQ) and protection coordination (PC) levels in a distributed energy resource (DER)-enabled microgrid with multiple connection modes. Introduction of DERs in a grid increases the fault currents. Different connection modes for these DERs lead to varying fault levels. Both phenomena jeopardize the PC. A solid-state unidirectional fault current limiter (SSUFCL) is used as an interface between the microgrid as downstream and the utility as upstream networks, which limits the downstream contribution to the upstream fault currents for increasing the PC and PQ margins, while it is disabled in the case of downstream faults to improve the PQ. In this way the complete coordination between upstream and downstream networks will be achieved. To simultaneously optimize the PC and PQ, the time and current settings of the over current relays (OCRs) and the characteristics of the SSUFCL are set. The optimization formulation of the PC problem is extracted. Fuzzy optimization technique is used to make a compromise between PQ and PC after objectives' fuzzification. Genetic algorithm is used to find the proper characteristics of the SSUFCL as well as the best settings of OCRs.