*1.1. Background*

One of the major technical challenges facing the microgrid (MG) is the relay coordination for the different operations of the network. The different operation modes of MG result in frequent variations, thereby leading to changes in the fault current magnitude and its direction. The protection scheme needs to guarantee the optimal operation in all network scenarios, which include meshed and radial configurations as well as the connected grid and islanded modes of MGs. In case of a fault in the grid connection mode, the fault current will be fed from distributed generators (DGs) and the utility grid. On the other hand, when the grid is in islanded mode, the supply of the fault current will result from DGs as the only connection present in the MG [1,2]. These changes in the fault current have a significant impact on the basic requirements of the protection system such as speed, reliability, and sensitivity, which then result in some protection issues such as delay in tripping, false tripping, loss of mains, and blinding of protection [3,4]. In recent years, the coordination problem of overcurrent relays (OCRs) has obtained wide attention in the

**Citation:** Abeid, S.; Hu, Y.; Alasali, F.; El-Naily, N. Innovative Optimal Nonstandard Tripping Protection Scheme for Radial and Meshed Microgrid Systems. *Energies* **2022**, *15*, 4980. https://doi.org/10.3390/ en15144980

Academic Editors: Najib El Ouanjli, Saad Motahhir and Mustapha Errouha

Received: 19 May 2022 Accepted: 5 July 2022 Published: 7 July 2022

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protection system research area. These studies are focused on addressing and preserving the protection schemes' reliability in interconnected MGs to acquire the optimal configuration for protection approaches in MGs. Optimal relay coordination methods regarding whole previous studies can be divided into five methods, namely, optimization techniques, new constraints, dual-setting protection schemes, new or modifying objective functions, and nonstandard characteristics (NSCs) [3]. Figure 1 illustrates the classification of OCR coordination methods. The mutual goal among these methods is to obtain the optimal OCR coordination. Initially, the optimization techniques played an essential role in the optimal OCR coordination, starting with simple ways and passing through nature-inspired algorithms and advanced artificial intelligence [3].

**Figure 1.** Overcurrent Relay Coordination Methods.
