## 2 Background of Microgrids
### Definition and Classification of MGs
DC MGs utilize simpler power electronic devices and control methods, making their management and operation easier, particularly in coordinating ESSs within the infrastructure. Unlike AC MGs, DC MGs are not affected by issues related to reactive power or frequency control, such as circulating reactive currents or harmonic distortions. Synchronizing DC MGs with the main AC grid only requires voltage magnitude adjustments, whereas AC MGs need to consider voltage magnitude, frequency, and phase shift between each PCC. Furthermore, when integrating different DG sources, DC MGs demonstrate relatively higher energy efficiency due to shorter transmission distances and more effective utilization of ESSs. However, one drawback of DC MGs is their higher capital cost, despite offering lower operating costs and occupying smaller physical space compared to AC MGs [1, 8, 64]. Figure 2 illustrates the structure of a hybrid microgrid including PV power plants, wind turbines, and fuel cell stack. In such an MG infrastructure, DERs are integrated over the islanded DC and AC busbars, as seen in the figure. The power flow between islanded subgrids of MG and the utility grid is managed via the interface of power electronic converters, which can manage bidirectional power flow. The direction of power flow between load and sources is determined using generation potential of DERs. If there is excess generation capacity, energy storage devices such as fuel cell stacks or batteries are charged. Hybrid MGs are built with the intention of increasing network efficiency overall. As a result, they have been designed to minimize conversion stages, boost reliability, minimize the number of interfacing devices, and lower energy costs. The various literature has reported on a lot of research on the use, safety, and stability of DC, AC, and hybrid MGs [1; 59; 71; 72; 73; 74; 75; 76; 77; 78]. MGs provide increased reliability and flexibility to the utility grid during fault conditions and grid problems. They can continue operating even when the utility grid is down, thanks to their ability to generate power at close proximity to loads. MGs reduce the demand on the grid and utilize their ESSs as a feedback system to support the utility grid. However, the reliability of the microgrid can be a concern for the utility grid if the microgrid protection is not properly ensured. Common protection devices such as fuses, reclosers, and circuit breakers, which are typically installed at the distribution level, may not provide adequate protection for the microgrid. This is due to the significant difference in short-circuit capacity between the utility grid and the microgrid or any DG source within the microgrid. Coordinating and arranging protection devices becomes challenging due to this capacity inequality. Microgrid protection research has gained more attention, and various novel techniques have been developed and improved. To ensure commercial and industrial use, MGs should undergo testing and verification based on safety recommendations. Currently available directional overcurrent, distance, and differential relays used in transmission and distribution networks are being utilized in MGs. Although these relays were not specifically developed  
Figure 2: Schematic diagram of a hybrid microgrid. for MGs, they offer more sensitive protection infrastructures to handle challenging short-circuit levels in MGs. Despite the differences in fault characteristics and configurations between MGs and utility grids, it is expected that these relays will operate successfully in microgrid protection, similar to their performance in the main grid infrastructure [60; 79].