Due to the swift expansion and the deployment of distributed generation, protection systems of active distribution networks are more expected to be fast. In loop-based active distribution networks, directional overcurrent relays (DOCRs) are caught in different chains. These chains stand as the severe obstacle to follow fast-response protection, which remains a significant challenge. In this paper, to overcome this challenge, a fast protection scheme is proposed to break the chains in the corresponding loops by deploying auxiliary DOCRs. The most effective constraint associated with each chain is relaxed during the coordination process. Then, the auxiliary relays are employed to play the backup roles instead of conventional backup relays in the relaxed constraints. To avoid the misoperation of relays in the proposed scheme, low bandwidth communication links are suitably employed. Furthermore, the auxiliary relays are optimally placed and adjusted. The proposed approach demonstrates a mixed-integer nonlinear programming model which is tackled by particle swarm optimization (PSO) algorithm. Detailed simulation studies are carried out to verify the performance of the proposed approach.
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