Contenido

Son necesarias arquitecturas y herramientas para gestionar el flujo de energía de distintos tipos de fuentes hacia la red eléctrica. Por tanto, la microrgrid se puede clasificar en tres topologías:.

Microgrid AC

As for the AC type microgrid, the power supplies with AC output are interconnected with the AC bus through the AC/AC converter which will transform the variable AC frequency and voltage into an AC waveform with another frequency at another voltage. While power supplies with DC output use DC/AC converters for connection to the AC bus.[9]​.

DC Microgrid

In DC microgrid topology, power supplies with DC output are connected to the DC bus directly or via DC/DC converters. On the other hand, power supplies with AC output are connected to the DC bus through the AC/DC converter.[9]​.

Hybrid microgrid

The hybrid microgrid has a topology for the AC and DC output of the power supply. Additionally, the AC and DC buses are connected to each other through a bidirectional converter,[10]​ allowing power to flow in both directions between the two buses.[9]​.

Campus environment or institutional microgrids

The approach to campus microgrids is to aggregate existing on-site generation to support multiple loads located in a narrow geographic area,[11]​ where an owner can easily and quickly manage them[12]​.

Community microgrids

Community microgrids can serve thousands of customers and support local energy integration (electricity, heating and cooling). Some homes may have some renewable sources that can meet their demand as well as that of their neighbors within the same community. It can also have centralized or distributed energy storage. These microgrids can take the form of an AC and DC microgrid (as explained in the typologies) coupled together through a bidirectional power electronic converter.[13]​[14]​.

Remote off-grid microgrids

These microgrids are never connected to the macrogrid and instead operate in island mode at all times due to economic issues or geographic position. Typically, an “off-grid” microgrid is built in areas that are very far from any transmission and distribution infrastructure and therefore have no connection to the utility grid. Operating in a remote area that is dominated by renewable sources will reduce the levelized cost of electricity production over the life of such microgrid projects.[15]​.

Several independent microgrids can supply large remote areas, each with a different owner. Although these microgrids are designed to be self-sufficient in energy, intermittent renewable sources and their unexpected variations can cause unexpected power shortfalls or excessive generation in those microgrids. This will cause an unacceptable voltage or frequency deviation in the microgrids. To remedy such situations, it is possible to interconnect such microgrids provisionally to a nearby one to exchange power and improve voltage and frequency deviations. This can be achieved by a power electronics based switch after proper synchronization of two power electronic converters. Determining the need to interconnect neighboring microgrids and finding the appropriate microgrid to couple with can be achieved through optimization or decision-making approaches[16].

Military base microgrids

These microgrids are being actively deployed with a focus on physical and cyber security for military installations to ensure reliable power without relying on the macrogrid.[1]​[17]​.

Commercial and industrial (C&I) microgrids

This category of microgrids is maturing rapidly in North America and East Asia, specifically in the world's leading technological powers; However, the lack of standards limits it worldwide, which is why Europe has barely experimented with this category of microgrid yet.[18]​ The main reason for the installation of an industrial microgrid is due to the security of the energy supply. There are many manufacturing processes where a power supply disruption can cause large revenue losses and prolonged start-up time.[14]​ Industrial microgrids can be designed to supply near-zero emission circular economy industrial processes and can integrate combined heat and power (CHP) generation, powered by both renewable sources and waste processing.[19]​[20]​.

Local generation

One of the first fundamental components for the microgrid are the different types of generation sources that supply electricity, heating and cooling to the user. These sources are divided into two main groups:

• - Thermal energy sources such as natural gas or biogas generators or microcombustion of heat and power.

• - Renewable generation sources such as wind and solar turbines.

Consumption

The second fundamental component of a microgrid consists of consumption, which simply refers to the elements that use electricity, heat and cooling, ranging from individual devices to the lighting and heating systems of buildings, shopping centers, etc. In the case of controllable loads, electricity consumption can be modified according to the demands of the network.

Energy storage

In microgrid, energy storage can perform multiple functions, which is why it is another of its fundamental components:.

• - Ensures power quality, including frequency and voltage regulation.

• - Smoothes the production of renewable energy sources.

• - Provide backup power for the system.

• - Play a crucial role in cost optimization.

• - E Implements chemical, electrical, pressure, gravitational and heat storage technologies.

On the other hand, when multiple energy storages with various capacities are available on a *microgrid, it is preferred to coordinate their charging and discharging so that a smaller energy storage is not discharged faster than those with larger capacities. Likewise, it is preferred that a smaller one not be fully charged before those with a larger capacity. This can be achieved under coordinated control of energy stores based on their state of charge. If multiple energy storage systems are used (working on different technologies) and controlled by a single supervisory unit (an energy management system) and hierarchical control based on a master-slave architecture can ensure the best operations.[21]​.

Point Common Coupling (PCC)

Finally we have the PCC which is the point in the electrical circuit where a microgrid is connected to a main grid. They also highlight a series of microgrids that do not have a PCC and are called isolated microgrids that are generally present in remote sites where an interconnection with the main network is not feasible due to technical or economic limitations.[20]​.

Advantages

A microgrid is capable of operating in grid-connected and autonomous mode and handling the transition between the two:.

• - In grid-connected mode, ancillary services can be provided by trading between the mircrogrid and the main grid. There are also other possible sources of income.

• - In autonomous mode, the real and generated power within the microgrid, including that provided by the energy storage system, must be in balance with the demand of local loads.[21]​.

Microgrids offer an option to balance the need to reduce carbon emissions with continuing to provide reliable electrical power in periods of time when renewable energy sources are not available. They also offer the security of being reinforced for severe weather and natural disasters by not having large assets and miles of above-ground cables and other electrical infrastructure that must be maintained or repaired after such events.[22]​.

Additionally, you can switch between these two modes due to:.

• - Scheduled maintenance.

• - Degraded power quality.

• - Shortage in the main network due to failures in the local network or for economic reasons.

By modifying the flow of energy through the components of microgrids, they facilitate the integration of renewable energies, such as photovoltaic, wind and fuel cell generations, without the need to redesign the national distribution system. Modern optimization methods can also be incorporated into the microgrid energy management system to improve efficiency and economics.[23]​[24]​.

Challenges

Microgrids, and the integration of DER units, introduce a series of operational challenges that must be addressed in the design of control and protection systems, with the aim of ensuring that current levels of reliability are not affected and the benefits of Distributed Generation (DG) units are fully exploited. Some of these challenges arise from assumptions normally applied to conventional distribution systems that are no longer valid, while others are the result of stability problems that were previously only observed at the transmission system level.[22]​ The most relevant challenges in the protection and control of microgrids include:.

1. • Bidirectional energy flows. The presence of distributed generation (DG) units on the grid at low voltage levels can cause reverse power flows that can lead to complications in protection coordination, unwanted power flow patterns, fault current distribution and voltage control.[22]​.

2. • Stability problems. Interactions between the control system of DG units can create local oscillations, requiring small perturbation stability analysis. Additionally, transition activities between grid-connected and islanded modes of operation in a microgrid can create transient instability. It has been shown that direct current (DC) microgrid interface can result in a much simpler control structure, more efficient power distribution, and higher current carrying capacity for the same line ratings.[23]​.

3. • Modeling. Many features of traditional schemes, such as the prevalence of three-phase balanced conditions, mainly inductive transmission lines and constant power loads, are not true for microgrids and, consequently, the models must be reviewed and updated.

4. • Low inertia. Microgrids exhibit a low inertia characteristic that makes them different from bulk power systems, where a large number of synchronous generators ensures a very large inertia. This phenomenon is more evident if there is a significant proportion of DG units with power electronic interface in the microgrid. The low inertia of the system can cause serious frequency deviations in island mode operation if a proper control mechanism is not implemented. Synchronous generators operate at the same frequency as the grid, providing a natural damping effect on sudden variations in frequency. Other options include controlling the battery energy storage to balance the frequency.[23]​.

5. • Uncertainty. Operating microgrids involves addressing a lot of uncertainty, which is something the economical and reliable operation of microgrids depends on. Load profile and weather are two uncertainties that make this coordination more complicated in isolated microgrids, where the critical balance of supply and demand and typically higher component failure rates require solving a tightly coupled problem over a long time horizon. This uncertainty is greater than that of bulk energy systems, due to the reduced number of loads and correlated variations in available energy resources.[25]​.

Modeling tools

To plan and install microgrids correctly, an engineering model is necessary. There are multiple simulation and optimization tools to model the economic and electrical effects of microgrids. A widely used economic optimization tool is the Distributed Energy Resources Customer Adoption Model (DER-CAM) from Lawrence Berkeley National Laboratory.[26] Another is Homer Energy, designed by the National Renewable Energy Laboratory. There are also some electrical and power flow design tools that guide microgrid developers.[23]​ The Pacific Northwest National Laboratory designed the publicly available GridLAB-D tool and the Electric Power Research Institute (EPRI) did the same with OpenDSS. A European tool that can be used for electrical, cooling, heating and process heat demand simulation is EnergyPLAN from Aalborg University in Denmark. The open source network planning tool OnSSET has been implemented to investigate microgrids using a three-level analysis starting with settlement archetypes[22].

It should be noted that control and protection are difficulties for microgrids, since all auxiliary services for system stabilization must be generated within the microgrid and low short circuit levels can be a challenge for the selective operation of protection systems. An important characteristic is also the proportion of multiple useful energy needs, such as heating and cooling in addition to electricity, because it allows the substitution of energy carriers and greater energy efficiency thanks to the waste heat that is intended for heating, hot water and cooling.[18]​.

With respect to the control architecture of microgrids, or any control problem, there are two different approaches that can be identified:.

• - Centralized. It relies on a large amount of information transmission between the units involved before a decision is made at a single point. Implementation is difficult as interconnected power systems typically cover widespread geographic locations and involve a huge number of units.[24]​[26]​.

• - Decentralized. Each unit is controlled by its local controller without knowing the situation of the others. A compromise between those two extreme control schemes can be achieved by a hierarchical control scheme consisting of three levels of control:

1. Primary. It is designed to satisfy a series of requirements. To stabilize the voltage and frequency, plug and play capability is provided for DERs and sharing the active and reactive power between them, without any communication link. It is also used to reduce circulating currents that can cause an overcurrent phenomenon in power electronic devices. In conclusion, it provides us with the setpoints for a lower controller which are the voltage and current control loops of the DERs. These internal control loops are called zero-level control.[27]​

2. Secondary. It has a sampling time of seconds to minutes (slower than the primary one), which justifies the decoupled dynamics of the primary and secondary control and facilitates their individual designs.[28]​ The primary control setpoint is given by the secondary control in which, as a centralized controller, it restores the voltage and frequency of the microgrid and compensates for deviations caused by load variations or renewable sources. It can also be designed to meet power quality requirements, for example, voltage balancing on critical buses.[23]​[27]​

3. Tertiary. It is the last and slowest level, which considers economic concerns in the optimal functioning of the microgrid (sampling time is from minutes to hours) and manages the flow of energy between the microgrid and the main grid. It often involves predicting the weather, grid rate and loads in the coming hours or days to design a generator dispatch plan that achieves better financial savings. More advanced techniques can also provide end-to-end control of a microgrid using machine learning (ML) techniques such as deep reinforcement learning.[27]​ In the event of emergencies such as blackouts (such as those currently being experienced in various parts of the world), this type of control can manage a group of microgrids interconnected to form what is called a "microgrid cluster", acting as a virtual power plant to continue supplying critical loads. During these situations, the central controller must select one of the microgrids to be the slack and the rest as PV and load buses according to a predefined algorithm and the existing system conditions. In summary, control must be in real time or at least at a high sampling rate.[26]​.

Advanced Microgrid Montgomery, USA

Montgomery County in Maryland (USA) is making important advances in sustainability and energy security through the use of microgrids that improve the resilience of the county, in order to keep its residents safe and provide necessary services even in the case of prolonged power outages such as those that are occurring in several municipalities in China in late 2021. The Advanced Microgrid solution, from Schneider Electric and Duke Energy Renewables, installed at the Public Safety Headquarters in Gaithersburg, has received the maximum PEER certification (Performance Excellence in Electricity Renewal). They have achieved that 33% of total consumption comes from solar energy, 66% from a local cogeneration system and the wiring network is 100% underground.

This project is part of a larger one, which has been initiated to implement two Advanced Microgrids, one at PSHQ and another at the Montgomery County Correctional Facility in Boyds (Washington), two facilities that require continuous access to power to ensure their security and operational integrity.[29]​.

Hajjah and Lahj, Yemen

The UNDP project “Enhanced Rural Resilience in Yemen” uses community-owned solar microgrids reducing energy costs by about 2 cents per hour. It won the Ashden Humanitarian Energy Awards in 2020.[30][31]​.

Île d'Yeu

A two-year pilot program, called Harmon'Yeu, was started in 2020 during the Coronavirus pandemic lockdowns to interconnect 23 houses in the Ker Pissot neighborhood and surrounding areas with a microgrid that was automated as a smart grid with software from Engie. The process led to the installation of 64 solar panels with a maximum capacity of 23.7 kW in five houses and a battery with a storage capacity of 15 kWh was installed in one house. Six homes store excess solar energy in their water heaters. A dynamic system distributes the energy provided by the solar panels and stored in the battery and water heaters to the 23-home system. The smart grid software updates power supply and demand at 5-minute intervals, deciding whether to draw power from the battery or panels and when to store it in the water heaters. This program was the first project of its kind in France.[32]​.

Les Anglais, Haiti

A wirelessly managed microgrid is installed in rural Les Anglais, Haiti. The system consists of a three-tier architecture:

• - Control and monitoring service. Cloud based.

• - Integrated local gateway infrastructure.

• - Mesh network of wireless smart meters. Implemented in fifty-two buildings in Haiti.

Non-technical loss represents a significant challenge when providing reliable electrical service in developing countries, where it often represents 11-15% of total generating capacity.[33]​ An extensive data-driven simulation on seventy-two days of wireless meter data from a 430-home microgrid deployed in Les Anglais investigated how to distinguish it from total energy losses, aiding in the detection of energy theft.[34]​.

Mpeketoni, Kenya

The Mpeketoni Electricity Project is a community-based diesel power microgrid system, which was established in rural Kenya near Mpeketoni. Due to the installation of these microgrids, Mpeketoni has seen tremendous growth in its infrastructure. This growth includes an increase in productivity per worker, in values ​​of 100% to 200%, and an increase in the level of income from 20 to 70% depending on the product.

Stone Edge Farm Winery

A micro-turbine consists of a series of components:

• - Fuel cell.

• - Multiple battery.

• - Hydrogen electrolyzer.

• - Store.

All this enabled for PV in Sonoma, California.[35]​.

Schneider Electric, Barcelona

Iberdrola has installed a microgrid at the Schneider Electric plant in Barcelona, ​​with 990 solar panels, 5 electric vehicle charging points and a battery system, with a storage capacity of 216 kWh. Schneider Electric has also managed to convert the plant located in Molins de Rei into a Zero CO2 factory. [36][37]​.

• - 100% renewable energy.

• - Cogeneration (combined heat and power - CHP).

• - Demand response.

• - Distributed generation.

• - Electricity generation.

• - Electrical network.

• - Network connection.

• - Microgeneration.

• - Photovoltaic Energy.

• - Vehicle to grid (V2G).

• - Wind energy.

• - Hydroelectricity.

• - News website, focused on the Anglo-Saxon world: «Microgrid Knowledge».

• - Website that goes into depth about: “AC-DC converters”.

• - PDF on: «energy treatment through microgrids».

Contenido

Son necesarias arquitecturas y herramientas para gestionar el flujo de energía de distintos tipos de fuentes hacia la red eléctrica. Por tanto, la microrgrid se puede clasificar en tres topologías:.

Microgrid AC

As for the AC type microgrid, the power supplies with AC output are interconnected with the AC bus through the AC/AC converter which will transform the variable AC frequency and voltage into an AC waveform with another frequency at another voltage. While power supplies with DC output use DC/AC converters for connection to the AC bus.[9]​.

DC Microgrid

In DC microgrid topology, power supplies with DC output are connected to the DC bus directly or via DC/DC converters. On the other hand, power supplies with AC output are connected to the DC bus through the AC/DC converter.[9]​.

Hybrid microgrid

The hybrid microgrid has a topology for the AC and DC output of the power supply. Additionally, the AC and DC buses are connected to each other through a bidirectional converter,[10]​ allowing power to flow in both directions between the two buses.[9]​.

Campus environment or institutional microgrids

The approach to campus microgrids is to aggregate existing on-site generation to support multiple loads located in a narrow geographic area,[11]​ where an owner can easily and quickly manage them[12]​.

Community microgrids

Community microgrids can serve thousands of customers and support local energy integration (electricity, heating and cooling). Some homes may have some renewable sources that can meet their demand as well as that of their neighbors within the same community. It can also have centralized or distributed energy storage. These microgrids can take the form of an AC and DC microgrid (as explained in the typologies) coupled together through a bidirectional power electronic converter.[13]​[14]​.

Remote off-grid microgrids

These microgrids are never connected to the macrogrid and instead operate in island mode at all times due to economic issues or geographic position. Typically, an “off-grid” microgrid is built in areas that are very far from any transmission and distribution infrastructure and therefore have no connection to the utility grid. Operating in a remote area that is dominated by renewable sources will reduce the levelized cost of electricity production over the life of such microgrid projects.[15]​.

Several independent microgrids can supply large remote areas, each with a different owner. Although these microgrids are designed to be self-sufficient in energy, intermittent renewable sources and their unexpected variations can cause unexpected power shortfalls or excessive generation in those microgrids. This will cause an unacceptable voltage or frequency deviation in the microgrids. To remedy such situations, it is possible to interconnect such microgrids provisionally to a nearby one to exchange power and improve voltage and frequency deviations. This can be achieved by a power electronics based switch after proper synchronization of two power electronic converters. Determining the need to interconnect neighboring microgrids and finding the appropriate microgrid to couple with can be achieved through optimization or decision-making approaches[16].

Military base microgrids

These microgrids are being actively deployed with a focus on physical and cyber security for military installations to ensure reliable power without relying on the macrogrid.[1]​[17]​.

Commercial and industrial (C&I) microgrids

This category of microgrids is maturing rapidly in North America and East Asia, specifically in the world's leading technological powers; However, the lack of standards limits it worldwide, which is why Europe has barely experimented with this category of microgrid yet.[18]​ The main reason for the installation of an industrial microgrid is due to the security of the energy supply. There are many manufacturing processes where a power supply disruption can cause large revenue losses and prolonged start-up time.[14]​ Industrial microgrids can be designed to supply near-zero emission circular economy industrial processes and can integrate combined heat and power (CHP) generation, powered by both renewable sources and waste processing.[19]​[20]​.

Local generation

One of the first fundamental components for the microgrid are the different types of generation sources that supply electricity, heating and cooling to the user. These sources are divided into two main groups:

• - Thermal energy sources such as natural gas or biogas generators or microcombustion of heat and power.

• - Renewable generation sources such as wind and solar turbines.

Consumption

The second fundamental component of a microgrid consists of consumption, which simply refers to the elements that use electricity, heat and cooling, ranging from individual devices to the lighting and heating systems of buildings, shopping centers, etc. In the case of controllable loads, electricity consumption can be modified according to the demands of the network.

Energy storage

In microgrid, energy storage can perform multiple functions, which is why it is another of its fundamental components:.

• - Ensures power quality, including frequency and voltage regulation.

• - Smoothes the production of renewable energy sources.

• - Provide backup power for the system.

• - Play a crucial role in cost optimization.

• - E Implements chemical, electrical, pressure, gravitational and heat storage technologies.

On the other hand, when multiple energy storages with various capacities are available on a *microgrid, it is preferred to coordinate their charging and discharging so that a smaller energy storage is not discharged faster than those with larger capacities. Likewise, it is preferred that a smaller one not be fully charged before those with a larger capacity. This can be achieved under coordinated control of energy stores based on their state of charge. If multiple energy storage systems are used (working on different technologies) and controlled by a single supervisory unit (an energy management system) and hierarchical control based on a master-slave architecture can ensure the best operations.[21]​.

Point Common Coupling (PCC)

Finally we have the PCC which is the point in the electrical circuit where a microgrid is connected to a main grid. They also highlight a series of microgrids that do not have a PCC and are called isolated microgrids that are generally present in remote sites where an interconnection with the main network is not feasible due to technical or economic limitations.[20]​.

Advantages

A microgrid is capable of operating in grid-connected and autonomous mode and handling the transition between the two:.

• - In grid-connected mode, ancillary services can be provided by trading between the mircrogrid and the main grid. There are also other possible sources of income.

• - In autonomous mode, the real and generated power within the microgrid, including that provided by the energy storage system, must be in balance with the demand of local loads.[21]​.

Microgrids offer an option to balance the need to reduce carbon emissions with continuing to provide reliable electrical power in periods of time when renewable energy sources are not available. They also offer the security of being reinforced for severe weather and natural disasters by not having large assets and miles of above-ground cables and other electrical infrastructure that must be maintained or repaired after such events.[22]​.

Additionally, you can switch between these two modes due to:.

• - Scheduled maintenance.

• - Degraded power quality.

• - Shortage in the main network due to failures in the local network or for economic reasons.

By modifying the flow of energy through the components of microgrids, they facilitate the integration of renewable energies, such as photovoltaic, wind and fuel cell generations, without the need to redesign the national distribution system. Modern optimization methods can also be incorporated into the microgrid energy management system to improve efficiency and economics.[23]​[24]​.

Challenges

Microgrids, and the integration of DER units, introduce a series of operational challenges that must be addressed in the design of control and protection systems, with the aim of ensuring that current levels of reliability are not affected and the benefits of Distributed Generation (DG) units are fully exploited. Some of these challenges arise from assumptions normally applied to conventional distribution systems that are no longer valid, while others are the result of stability problems that were previously only observed at the transmission system level.[22]​ The most relevant challenges in the protection and control of microgrids include:.

1. • Bidirectional energy flows. The presence of distributed generation (DG) units on the grid at low voltage levels can cause reverse power flows that can lead to complications in protection coordination, unwanted power flow patterns, fault current distribution and voltage control.[22]​.

2. • Stability problems. Interactions between the control system of DG units can create local oscillations, requiring small perturbation stability analysis. Additionally, transition activities between grid-connected and islanded modes of operation in a microgrid can create transient instability. It has been shown that direct current (DC) microgrid interface can result in a much simpler control structure, more efficient power distribution, and higher current carrying capacity for the same line ratings.[23]​.

3. • Modeling. Many features of traditional schemes, such as the prevalence of three-phase balanced conditions, mainly inductive transmission lines and constant power loads, are not true for microgrids and, consequently, the models must be reviewed and updated.

4. • Low inertia. Microgrids exhibit a low inertia characteristic that makes them different from bulk power systems, where a large number of synchronous generators ensures a very large inertia. This phenomenon is more evident if there is a significant proportion of DG units with power electronic interface in the microgrid. The low inertia of the system can cause serious frequency deviations in island mode operation if a proper control mechanism is not implemented. Synchronous generators operate at the same frequency as the grid, providing a natural damping effect on sudden variations in frequency. Other options include controlling the battery energy storage to balance the frequency.[23]​.

5. • Uncertainty. Operating microgrids involves addressing a lot of uncertainty, which is something the economical and reliable operation of microgrids depends on. Load profile and weather are two uncertainties that make this coordination more complicated in isolated microgrids, where the critical balance of supply and demand and typically higher component failure rates require solving a tightly coupled problem over a long time horizon. This uncertainty is greater than that of bulk energy systems, due to the reduced number of loads and correlated variations in available energy resources.[25]​.

Modeling tools

To plan and install microgrids correctly, an engineering model is necessary. There are multiple simulation and optimization tools to model the economic and electrical effects of microgrids. A widely used economic optimization tool is the Distributed Energy Resources Customer Adoption Model (DER-CAM) from Lawrence Berkeley National Laboratory.[26] Another is Homer Energy, designed by the National Renewable Energy Laboratory. There are also some electrical and power flow design tools that guide microgrid developers.[23]​ The Pacific Northwest National Laboratory designed the publicly available GridLAB-D tool and the Electric Power Research Institute (EPRI) did the same with OpenDSS. A European tool that can be used for electrical, cooling, heating and process heat demand simulation is EnergyPLAN from Aalborg University in Denmark. The open source network planning tool OnSSET has been implemented to investigate microgrids using a three-level analysis starting with settlement archetypes[22].

It should be noted that control and protection are difficulties for microgrids, since all auxiliary services for system stabilization must be generated within the microgrid and low short circuit levels can be a challenge for the selective operation of protection systems. An important characteristic is also the proportion of multiple useful energy needs, such as heating and cooling in addition to electricity, because it allows the substitution of energy carriers and greater energy efficiency thanks to the waste heat that is intended for heating, hot water and cooling.[18]​.

With respect to the control architecture of microgrids, or any control problem, there are two different approaches that can be identified:.

• - Centralized. It relies on a large amount of information transmission between the units involved before a decision is made at a single point. Implementation is difficult as interconnected power systems typically cover widespread geographic locations and involve a huge number of units.[24]​[26]​.

• - Decentralized. Each unit is controlled by its local controller without knowing the situation of the others. A compromise between those two extreme control schemes can be achieved by a hierarchical control scheme consisting of three levels of control:

1. Primary. It is designed to satisfy a series of requirements. To stabilize the voltage and frequency, plug and play capability is provided for DERs and sharing the active and reactive power between them, without any communication link. It is also used to reduce circulating currents that can cause an overcurrent phenomenon in power electronic devices. In conclusion, it provides us with the setpoints for a lower controller which are the voltage and current control loops of the DERs. These internal control loops are called zero-level control.[27]​

2. Secondary. It has a sampling time of seconds to minutes (slower than the primary one), which justifies the decoupled dynamics of the primary and secondary control and facilitates their individual designs.[28]​ The primary control setpoint is given by the secondary control in which, as a centralized controller, it restores the voltage and frequency of the microgrid and compensates for deviations caused by load variations or renewable sources. It can also be designed to meet power quality requirements, for example, voltage balancing on critical buses.[23]​[27]​

3. Tertiary. It is the last and slowest level, which considers economic concerns in the optimal functioning of the microgrid (sampling time is from minutes to hours) and manages the flow of energy between the microgrid and the main grid. It often involves predicting the weather, grid rate and loads in the coming hours or days to design a generator dispatch plan that achieves better financial savings. More advanced techniques can also provide end-to-end control of a microgrid using machine learning (ML) techniques such as deep reinforcement learning.[27]​ In the event of emergencies such as blackouts (such as those currently being experienced in various parts of the world), this type of control can manage a group of microgrids interconnected to form what is called a "microgrid cluster", acting as a virtual power plant to continue supplying critical loads. During these situations, the central controller must select one of the microgrids to be the slack and the rest as PV and load buses according to a predefined algorithm and the existing system conditions. In summary, control must be in real time or at least at a high sampling rate.[26]​.

Advanced Microgrid Montgomery, USA

Montgomery County in Maryland (USA) is making important advances in sustainability and energy security through the use of microgrids that improve the resilience of the county, in order to keep its residents safe and provide necessary services even in the case of prolonged power outages such as those that are occurring in several municipalities in China in late 2021. The Advanced Microgrid solution, from Schneider Electric and Duke Energy Renewables, installed at the Public Safety Headquarters in Gaithersburg, has received the maximum PEER certification (Performance Excellence in Electricity Renewal). They have achieved that 33% of total consumption comes from solar energy, 66% from a local cogeneration system and the wiring network is 100% underground.

This project is part of a larger one, which has been initiated to implement two Advanced Microgrids, one at PSHQ and another at the Montgomery County Correctional Facility in Boyds (Washington), two facilities that require continuous access to power to ensure their security and operational integrity.[29]​.

Hajjah and Lahj, Yemen

The UNDP project “Enhanced Rural Resilience in Yemen” uses community-owned solar microgrids reducing energy costs by about 2 cents per hour. It won the Ashden Humanitarian Energy Awards in 2020.[30][31]​.

Île d'Yeu

A two-year pilot program, called Harmon'Yeu, was started in 2020 during the Coronavirus pandemic lockdowns to interconnect 23 houses in the Ker Pissot neighborhood and surrounding areas with a microgrid that was automated as a smart grid with software from Engie. The process led to the installation of 64 solar panels with a maximum capacity of 23.7 kW in five houses and a battery with a storage capacity of 15 kWh was installed in one house. Six homes store excess solar energy in their water heaters. A dynamic system distributes the energy provided by the solar panels and stored in the battery and water heaters to the 23-home system. The smart grid software updates power supply and demand at 5-minute intervals, deciding whether to draw power from the battery or panels and when to store it in the water heaters. This program was the first project of its kind in France.[32]​.

Les Anglais, Haiti

A wirelessly managed microgrid is installed in rural Les Anglais, Haiti. The system consists of a three-tier architecture:

• - Control and monitoring service. Cloud based.

• - Integrated local gateway infrastructure.

• - Mesh network of wireless smart meters. Implemented in fifty-two buildings in Haiti.

Non-technical loss represents a significant challenge when providing reliable electrical service in developing countries, where it often represents 11-15% of total generating capacity.[33]​ An extensive data-driven simulation on seventy-two days of wireless meter data from a 430-home microgrid deployed in Les Anglais investigated how to distinguish it from total energy losses, aiding in the detection of energy theft.[34]​.

Mpeketoni, Kenya

The Mpeketoni Electricity Project is a community-based diesel power microgrid system, which was established in rural Kenya near Mpeketoni. Due to the installation of these microgrids, Mpeketoni has seen tremendous growth in its infrastructure. This growth includes an increase in productivity per worker, in values ​​of 100% to 200%, and an increase in the level of income from 20 to 70% depending on the product.

Stone Edge Farm Winery

A micro-turbine consists of a series of components:

• - Fuel cell.

• - Multiple battery.

• - Hydrogen electrolyzer.

• - Store.

All this enabled for PV in Sonoma, California.[35]​.

Schneider Electric, Barcelona

Iberdrola has installed a microgrid at the Schneider Electric plant in Barcelona, ​​with 990 solar panels, 5 electric vehicle charging points and a battery system, with a storage capacity of 216 kWh. Schneider Electric has also managed to convert the plant located in Molins de Rei into a Zero CO2 factory. [36][37]​.

• - 100% renewable energy.

• - Cogeneration (combined heat and power - CHP).

• - Demand response.

• - Distributed generation.

• - Electricity generation.

• - Electrical network.

• - Network connection.

• - Microgeneration.

• - Photovoltaic Energy.

• - Vehicle to grid (V2G).

• - Wind energy.

• - Hydroelectricity.

• - News website, focused on the Anglo-Saxon world: «Microgrid Knowledge».

• - Website that goes into depth about: “AC-DC converters”.

• - PDF on: «energy treatment through microgrids».