## 2 Background of Microgrids
### Definition and Classification of MGs
Military MGs often integrate RESs, energy storage, and advanced control systems to enhance operational capabilities, reduce fuel consumption, and improve energy security. They play a vital role in supporting mission-critical operations, emergency response, and disaster recovery efforts. Remote MGs are deployed in isolated or remote areas that lack access to the main utility grid. These MGs provide energy independence to communities, facilities, and infrastructure in remote locations. Remote MGs typically rely on a combination of RESs, such as solar, wind, and hydro, along with energy storage systems and backup generators. They enable reliable and sustainable electricity supply in off-grid areas, supporting essential services such as lighting, communication, healthcare, education, and water pumping. Remote MGs can significantly improve the quality of life and economic opportunities for remote communities. These different types of MGs cater to specific needs and priorities based on their intended application and location. Each type has unique challenges and considerations in terms of design, operation, and integration with existing infrastructure [60; 62]. The microgrid architecture consists of power sources, such as DERs and RESs, equipped with microgrid controllers, as well as the assets that ensure utility connections. Intelligent-power electronic devices are essential for controlling the power conversion processes between MGs and generation sources. A microgrid control infrastructure is established with centralized and distributed controllers. The central controllers are connected to the microgrid central controller (MGCC), which improves and enhances the operation of the microgrid. The MGCC determines power demand, optimization conditions, and load capacities while considering auxiliary services of the distribution system. Control signals are sent to controllable field loads and microgrid controllers to implement the defined optimization and operating scenarios. If needed, noncritical and flexible loads can be disconnected from the grid. Real-time measurements of active and reactive power are performed. MG controllers interact with central and load controllers in the overall distributed control strategy, aiming to supply maximum available electricity to the grid while considering market conditions. This technique is enhanced to address MGCC issues in systems with numerous DG sources and locally made decisions. Figure 1 illustrates a generic representation of the control techniques applied to a microgrid configured with various source and load types. In this scenario, the microgrid concept is managed by a microsource control system (MCS) where load controllers (LCs) handle controllable loads defined by load models. Each microgrid infrastructure is connected to a central controller (CC) through a distribution management system (DMS) or distribution system operator (DSO). These controllers handle medium-voltage (MV) and low-voltage (LV) controls in architectures with multiple MGs. The MGs consist of several layers and control loops, similar to typical utility grids. This hierarchical control scheme addresses fundamental infrastructure and dynamic interface requirements while enabling integration through central and distributed control systems [60; 63; 64]. To integrate DGs and RESs within a microgrid infrastructure, various control systems are implemented. The local control section includes primary control systems such as voltage and current control loops. The secondary control is responsible for regulating frequency and mean voltage deviations that occur due to load or source fluctuations. It also manages local auxiliary services. The central and emergency control layer handles protection and emergency control protocols to ensure the reliability of the microgrid. Emergency control techniques estimate faults by implementing protective and regulatory measures. The coordination between distribution networks and MGs is established through the general control, which ensures the efficient operation of MGs.