## 2 Background of Microgrids
### Definition and Classification of MGs
The general control infrastructure, as depicted in Figure 1, manages power flow control and acts as the interface between the MGCC and the distribution network. This control interface ensures the  
Figure 1: The block diagram of a detailed microgrid architecture with controller infrastructures. distribution of power at the desired levels by controlling the entire microgrid infrastructure. Communication channels are required between the different control layers depicted in the schematic diagram of the microgrid in Figure 1. The secondary and emergency controllers function as central controllers (CCs), while the local controllers within the MGs are referred to as distributed controllers. The general control processes may take minutes to an hour to complete. The general controller sends control signals to the central level controllers and other controllers connected to the distribution systems. Furthermore, the CC has the ability to quickly coordinate the actions of local and secondary controllers (SCs) within the microgrid. The SC mechanism can respond to system faults or command requests within seconds. Local controller (LC) systems are intended to function autonomously and in accordance with specified event timetables [47, 65, 66, 67]. The detailed introduction of LCs, SCs, general controllers, and central and emergency controllers are presented under MG control sections in the paper. Microgrid architectures are typically implemented in three types: AC, DC, and hybrid topologies. These architectures are based on the concept of integrating various microsources and loads into a single entity, which can be seen as a dispatchable prosumer within the power system. AC MGs gained recognition as a crucial component for integrating DGs and ESSs while providing power autonomy from the utility grid. This led to significant advancements in AC microgrid research over the past decade. The use of power electronics in MGs can be traced back to the control of parallel inverters in uninterruptible power supplies (UPSs). Alongside the development of AC MGs, there have been advancements in DC MGs. In 2004, [PERSON] et al. reported one of the earliest research prototypes of a DC microgrid with a capacity of 10 kW. They highlighted the advantages of simpler control and superior efficiency and reliability compared to their AC counterparts. These developments in AC and DC MGs have contributed to the dynamic progress in microgrid research, enabling the integration of DGs and storage systems while enhancing efficiency and reliability in power systems [68]. Subsequently, further research and studies expanded on the concept of DC MGs, demonstrating their versatility in various applications such as telecommunications systems, data centers [69], distributed RESs, ESSs, and residential uses [70]. DC MGs have emerged as a distinct research field, particularly with the proliferation of consumer electronics, integration of RES, and the growth of the electric vehicle (EV) market along with its associated charging infrastructure. In an AC microgrid, RESs and loads are connected to a single AC bus. However, AC MGs face challenges in terms of control and operation, which have been identified as their main drawback. Overall, the development of DC MGs has offered promising alternatives and solutions to address the limitations of AC MGs, opening up new possibilities for efficient and effective energy management in various applications and sectors. Additionally, Figure 1 illustrates a typical AC microgrid structure where both AC and DC resources are connected to the AC busbar and integrated with the utility grid through a static switch. AC MGs can be classified into three types based on the distribution system architecture: single-phase, three-phase three-wire, and three-phase four-wire. In contrast, DC MGs offer several advantages over AC MGs in terms of reliability and efficiency. They can be connected to various distribution energy resources and exhibit robust stability against external factors.