Today, power grids around the world face problems such as the gradual depletion of fossil fuel resources, low energy efficiency, and environmental pollution, which has recently led to the local generation of power from renewable energy sources at the distribution and high voltage distribution levels. This type of power generation at low voltage levels is distributed generation (DG) and its sources are known as distributed energy resources (DERs) [1]. According to the definition provided in the IEEE standard no. 1547.2-2011, “Any distributed generation can consist of a power generator and an energy storage system” [2]. According to the US Department of Energy, DGs are small generators that can be combined with energy storage systems and operated in grid-connected or island mode to improve power system performance and meet local needs. Following the expansion of the use of DGs in the power system, distribution networks have changed from passive mode with one-way power flow to active distribution networks with two-way power flow. In passive networks, the power distribution is one-way and from the transmission network to the distribution network, while the presence of distributed generation sources has caused two-way power distribution in the power networks. On the other hand, the presence of DGs in the power system has made the control of these networks a major challenge. Control of the DG systems in a modern distribution network requires the use of intelligent control methods as well as changes in the structure of distribution networks, such as the integration of distributed generation resources and energy resource management ^[3][4]. These changes led to the creation of microgrids in active distribution networks, opening up a major research field comprising sustainability, energy management, reliability, power quality, and market participation.

Increasing numbers of distributed energy sources are connected to the distribution network and the evolving smart grid. It is possible to integrate DERs to the grid through microgrids. Microgrids provide a coordinated method to facilitate the penetration of DGs into the power system and increase its reliability. Microgrids can operate in two modes: independently, or connected to the grid. They usually work in parallel with the grid, but there are cases where the microgrid is intentionally or unintentionally disconnected from the main grid and acts as an island. In order to reconnect an islanded microgrid to the grid, a synchronization procedure is necessary. DGs have less generation capacity and are operated at lower cost than large centralized generators, which power the conventional grid. The connection of DGs to low voltage networks have benefits, which include: reducing environmental pollution, increasing the efficiency of electricity generation, improving power quality of electricity supplied to customers, reducing losses in distribution networks, improving feeder voltage profiles, and releasing network capacity ^[4][8].

2. Low Voltage AC Networks

2.1. Low Voltage AC Networks

Distributed generation units are usually connected to the main grid via power electronic converters. For example, a wind turbine produces AC output power that can either be connected directly or via AC/DC/AC converters to the main grid. The low voltage AC networks can be connected to wide area networks through transformers and AC loads can also be connected directly to the network. However, DC loads require power electronic converters to connect to the AC network. On the other hand, solar photovoltaic arrays have DC output power and are connected to AC networks through inverters. 

3. Low Voltage DC Networks

2.2. Low Voltage DC Networks

Low voltage DC (LVDC) microgrids are being increasingly deployed for electricity supply to industries and and commercial buildings. In the future, it is expected that DC distribution systems will be operated alongside the AC systems to feed all DC electrical appliances and machines. The operation of these parallel systems will be optimally controlled by an energy management system (EMS). Solar PV systems deployed in modular scale are beneficial for DC power generation. In addition, where the primary energy source to the LVDC network is an AC generator, AC/DC converters become essential. Additional elements in the LVDC network include energy storage and DC loads. 

4. Wind Turbines

2.3. Wind Turbines

Wind energy is one of the oldest renewable sources that has gained acceptability in more recent years. The efficiency of energy conversion in wind turbines is improved through maximum power-point tracking control methods. Recent research topics in wind energy conversion include fault identification and isolation, fault-tolerant control, and fault-ride through operation. The older types of wind turbines are fixed-speed devices. They work with squirrel cage induction generators, and require a soft starter to prevent inrush currents. However, modern wind turbines are variable speed equipment, whose real-time operation is more stable and spread over low to high wind speed operations. This is facilitated by power electronics driven by smart control algorithms that convert variable-frequency generator output to grid-compliant frequency ^[5][9].

5. Photovoltaic (PV) Units

2.4. Photovoltaic (PV) Units

A photovoltaic cell, commonly called a solar cell, is a transducer for the direct production of electricity from solar radiation. When sunlight shines on a PV cell, a potential difference occurs between the negative and positive electrodes, causing current to flow between them. Several PV cells, arranged in series and parallel, make up solar panels or arrays ^[6][10]. PV systems are commonly applied in home, commercial, public, and agricultural electrical systems. These systems can serve as independent energy sources or grid-connected systems. As the higher numbers of PV systems are connected to distribution lines, they can provide grid ancillary services. In grid-connected mode, electrical power from the PV system is injected into the main grid through inverters, which match the voltage amplitude and frequency of the PV system with grid voltage. Photovoltaic power plants are connected to the main grid in a centralized or decentralized manner and support the grid by preventing voltage drop of the distribution network. In the stand-alone mode, offgrid locations can be conveniently electrified ^[7][8][11,12].

6. Energy Storage Systems

2.5. Energy Storage Systems

Due to the fact that the production capacity of renewable energy sources is a function of atmospheric and climatic conditions, the proper performance of a microgrid depends on the correct operation of energy storage equipment. These systems play an important role in balancing power supply and demand, and also support the stability of microgrids. The surplus energy produced by renewable sources is stored in energy storage systems and used when there is a shortage of production. Integration with renewable energy sources to increase power efficiency is one of the most important goals of using energy storage systems in grid-connected microgrids. However, in island microgrids, energy storage systems are used to improve power quality and increase reliability. The energy storage control system regulates charge and discharge cycles according to the microgrid loading conditions. This system is also required to maintain the charge status of the storage system within the allowable range ^[9][10][13,14]. 

7. Microgrid Operation Modes

2.6. Microgrid Operation Modes

In general, microgrids can operate in both grid-connected mode and island mode ^[11][15]. However, in some situations, such as the application of distributed generation systems in power supply to remote areas, island operation mode is the only option available. In general, the operation in island mode can be due to the low power quality of the main grid, the price and conditions of the electricity market, and the unavailability of the main grid due to a fault. In grid-connected mode, voltage and frequency support is provided by the main grid, and therefore distributed generation sources are controlled solely for the purpose of power supply. However, when a microgrid operates in island mode, at least one of the interface converters of the distributed generation sources must operate in grid-forming mode (voltage control), while in grid-connected conditions it operates in grid-following mode (current control).