A microgrid is a local electrical system with clear limits. It works as one unit that can be controlled. It can operate connected to the main power grid or work separately. Microgrids may be linked together or function alone, without being connected to the larger power system. Very small microgrids that serve one building or a single electrical load are sometimes called nanogrids.

A grid-connected microgrid usually works in sync with the main power grid but can separate from it and operate independently in "island mode" when needed. This helps keep the power supply safe within the microgrid and allows it to provide emergency power by switching between connected and island modes. This type of grid is called an islandable microgrid.

One type of microgrid controls small power sources at the level of a single home or building: the nanogrid. Some nanogrids use open-source hardware to provide solar power for small systems, like ambulances or even individual devices. While direct current (DC) systems are usually more efficient, nanogrids can also use alternating current (AC) to work with common devices.

A stand-alone microgrid has its own power sources and includes a system to store energy. These are used in places where bringing power from a main energy source is too far or too expensive. They help provide electricity to remote areas and small islands. A stand-alone microgrid can combine different energy sources, especially renewable energy, to work together.

Managing and protecting microgrids is challenging because all support services needed to keep the system stable must come from within the microgrid. Low levels of short-circuit current can make it hard to operate protection systems properly. A key feature of microgrids is their ability to provide more than just electricity, such as heating and cooling, which helps use energy more efficiently by using waste heat for heating, hot water, and cooling needs.

Definitions

The United States Department of Energy Microgrid Exchange Group describes a microgrid as a group of connected electrical devices and energy sources within clear electrical limits that function as a single unit that can control how it interacts with the main power grid. A microgrid can connect to or separate from the main grid to operate either while connected to the grid or independently, like during a power outage.

The Berkeley Lab explains that a microgrid includes systems for generating and storing energy that can power a building, school, or community when not connected to the main electric grid, such as during a disaster. A microgrid that can disconnect from the main power grid at the "point of common coupling" (PCC) is called an "islandable microgrid."

An EU research project describes a microgrid as a system that includes low-voltage electrical networks with energy sources like small turbines, fuel cells, and solar panels, along with storage systems like batteries and flywheels, and flexible electrical loads. These systems can operate either connected to the main grid or disconnected from it. If managed properly, these systems can improve the performance of the overall power network.

Electropedia defines a microgrid as a group of connected electrical devices and energy sources within defined electrical limits that form a local power system operating at low or medium voltage levels up to 35 kV. This system acts as a single unit that can operate either connected to the main grid or independently.

Microgrid Knowledge describes a microgrid as a self-sufficient energy system that provides power to a specific area, such as a college campus, hospital, business center, or neighborhood.

A stand-alone microgrid, also called an "island grid," operates only without connection to the main power system and cannot link to a larger electricity network. These are often used in remote areas or on islands and are sometimes called "mini grids" in some countries. A "nanogrid" is a system for a single home or building, and multiple nanogrids can be connected to form a microgrid that allows power sharing between them.

Campus microgrids focus on combining existing energy sources on a site to supply power to multiple buildings in a small area, making it easier for the owner to manage them.

Community microgrids can serve thousands of people and support the use of local energy, such as electricity, heating, and cooling. In a community microgrid, some homes may have renewable energy sources that can power their own homes and nearby homes. These microgrids may also have storage systems, and can be designed with alternating current (AC) or direct current (DC) systems connected through special power devices.

Some microgrids are not designed to connect to the main power grid and instead operate independently all the time because of cost or location. These "off-grid" microgrids are often built in areas far from existing power lines. Studies show that using renewable energy in these remote areas can lower the cost of electricity over time. In some cases, off-grid microgrids may be linked to the main power grid, but this requires careful planning.

Large remote areas may have multiple independent microgrids, each with its own owner. While these microgrids are designed to be self-sufficient, unpredictable energy sources like wind or solar can cause power shortages or excess energy. Without storage or smart controls, this can lead to electrical problems. To fix this, microgrids can temporarily connect to nearby microgrids to share power and stabilize the system. This can be done using special power devices after checking the system's stability.

Remote off-grid microgrids are often small and built from scratch, allowing them to use modern energy practices and innovations. These microgrids are commonly powered by renewable energy and use smart controls at the customer level, which is harder to implement in larger power systems due to older infrastructure.

These microgrids are being used in military areas to ensure reliable power without depending on the main grid. They are growing quickly in North America and parts of Asia, but the lack of clear standards limits their use globally. Industrial microgrids are often built to ensure a steady and reliable power supply, which is critical for manufacturing processes where power interruptions can cause financial losses. These microgrids can support eco-friendly industrial processes, use renewable energy and waste-to-energy systems, and include storage to improve efficiency. Some microgrids are also managed by large companies to improve reliability or reduce costs.

Topologies

Architectures are needed to control how energy from different sources enters the electrical grid. Microgrids can be divided into three types based on how they connect power sources to the grid:

For microgrids using alternating current (AC), power sources that produce AC are connected to the AC bus using an AC/AC converter. This device changes the AC's variable frequency and voltage to a different frequency and voltage. Power sources that produce direct current (DC) are connected to the AC bus using a DC/AC converter.

In microgrids using direct current (DC), power sources that produce DC are connected directly to the DC bus or through a DC/DC converter. Power sources that produce AC are connected to the DC bus using an AC/DC converter.

Hybrid microgrids support both AC and DC power sources. These microgrids have both AC and DC buses connected by a bidirectional converter, which allows electricity to move in both directions between the two buses.

Basic components

A microgrid uses different types of power sources to supply electricity, heat, and cooling to users. These sources include gases like natural gas, biogas, or hydrogen, often used in gas engines for combined heat and power (CHP) or combined cooling, heat, and power (CCHP) systems. They also include renewable energy sources that depend on weather conditions, such as wind turbines or solar panels.

In a microgrid, "consumption" refers to the use of electricity, heat, and cooling. This can include individual devices or larger systems, such as lighting and heating in buildings or commercial centers. Some types of energy use, called controllable loads, can be adjusted based on the needs of the network.

Energy storage in a microgrid has several important roles. It helps maintain power quality, regulates frequency and voltage, makes renewable energy output more steady, provides backup power, and helps reduce costs. Energy storage can use various technologies, including chemical, electrical, pressure, gravitational, flywheel, and heat storage. When multiple storage devices of different sizes are used, it is best to coordinate their charging and discharging so that smaller devices do not discharge faster or charge fully before larger ones. This coordination can be managed by monitoring their charge levels. If multiple storage systems are used and controlled by a single energy management system (EMS), a master/slave control system can improve performance, especially when the microgrid operates independently.

The Point of Common Coupling (PCC) is the location where a microgrid connects to the main power grid. Microgrids without a PCC are called isolated microgrids. These are often found in remote areas, such as distant communities or industrial sites, where connecting to the main grid is not possible due to technical or financial reasons.

Advantages and challenges

A microgrid can operate in two ways: connected to the main power grid or working alone (stand-alone). It can switch between these two modes as needed. When connected to the main grid, the microgrid can provide additional services by trading energy with the main grid. It can also earn money through other methods. When working alone, the power produced inside the microgrid, including energy from storage systems, must match the power needed by local users. Microgrids help reduce carbon emissions while still providing reliable electricity when renewable energy sources, like wind or solar, are not available. They also protect against damage from severe weather or natural disasters by avoiding large power lines and equipment that are hard to repair after such events.

A microgrid may switch between these modes due to planned maintenance, poor power quality, problems with the main grid, or for cost reasons. By adjusting how energy moves through its parts, a microgrid can include renewable energy sources, such as solar panels, wind turbines, and fuel cells, without changing the national power system. Modern methods can be used in the microgrid’s control system to improve efficiency, cost savings, and ability to handle challenges.

Microgrids and the use of small, local energy sources (called distributed energy resources or DERs) create challenges in how they are controlled and protected. These challenges must be addressed to keep the same level of reliability and to use the benefits of local energy sources fully. Some challenges come from old assumptions about power systems that no longer apply, while others are due to stability issues that were once only seen in large power systems. Key challenges include:

• Bidirectional power flows: Energy from local sources can move in both directions in the system, causing problems with protection systems, unusual power patterns, and voltage control.
• Stability issues: Control systems for local energy sources can cause small, repeated power fluctuations. Switching between connected and stand-alone modes can also cause sudden instability. Some studies show that using direct current (DC) systems in microgrids can simplify control, improve energy efficiency, and carry more power with the same equipment.
• Modeling: Traditional power systems often assume balanced power conditions and predictable behavior, but microgrids may not follow these rules, so models must be updated.
• Low inertia: Microgrids have less ability to resist sudden changes in power compared to large power systems, which rely on many large generators. This can cause large changes in power frequency during stand-alone operation unless proper control systems are used. Synchronverters (a type of inverter) can mimic traditional generators to control frequency. Other methods include using batteries or flywheels to balance power.
• Uncertainty: Microgrids must handle unpredictable factors, like changes in energy demand or weather. This is especially hard in isolated microgrids, where balancing supply and demand is critical. These uncertainties are greater in microgrids than in large power systems because they have fewer users and more variable energy sources.

To plan and build microgrids correctly, engineers use modeling tools. Many software programs help study the economic and electrical effects of microgrids. One example is XENDEE, a platform used by the U.S. Department of Defense. Another tool is DER-CAM, which helps plan energy systems economically. A free tool is SAMA, which uses advanced calculations to design solar-powered systems. Another tool is HOMER, developed by the National Renewable Energy Laboratory. Tools like GridLAB-D and OpenDSS help design electrical systems. In Europe, EnergyPLAN is used to study energy, cooling, heating, and industrial needs. A tool called OnSSET helps plan microgrids by analyzing data from different regions, such as in Bolivia.

Microgrid control

The way microgrid control systems are designed can be divided into two main types: centralized and decentralized. In a centralized system, all units share a lot of information before a single decision is made. This is hard to do because power systems often cover large areas and involve many units. In a decentralized system, each unit is controlled separately without knowing what other units are doing. A middle ground between these two systems is a hierarchical control method with three levels: primary, secondary, and tertiary.

The primary control has these goals:
• To keep voltage and frequency stable
• To allow devices to connect and work together easily, sharing power without needing communication
• To reduce extra currents that could damage equipment

Primary control sets the targets for lower-level controllers, which manage the voltage and current of devices. These lower-level systems are called zero-level control.

Secondary control works more slowly than primary control, taking seconds or minutes to collect data. This allows the two systems to operate independently. Secondary control adjusts the targets set by primary control to restore voltage and frequency, fixing issues caused by changes in power use or renewable energy sources. It can also help meet power quality standards, such as keeping voltage balanced at key points in the system.

Tertiary control is the slowest level, focusing on saving money by planning how energy is used. It considers factors like weather, energy costs, and future power needs to create plans for how generators should operate. Advanced methods, like machine learning, can also help control the entire microgrid.

During emergencies like blackouts, tertiary control can connect multiple microgrids to form a "virtual power plant" that keeps essential services running. A central controller chooses one microgrid to lead and others to follow based on rules and current conditions. Control during these times must happen quickly or at least very fast.

The IEEE 2030.7 standard provides a framework for microgrid control with four parts:
a) Device-level control (like managing voltage and frequency)
b) Local area control (like sharing data)
c) Supervisory control (like planning energy use)
d) Grid layers (like connecting to the main power system)

Many complex control methods exist, making it hard for small microgrids or home users to manage energy systems. Upgrading communication systems and data tools can be costly. Some projects use simple, ready-made tools, like Raspberry Pi, to lower costs and simplify control.

Examples

A zero-emission microgrid in Calistoga, Napa County, California, provides electricity to about 5,000 people. The microgrid is owned by Pacific Gas & Electric Company and powered by the Calistoga Resiliency Center. This center is a first-of-its-kind project that combines a lithium-ion battery energy storage system with onsite liquid hydrogen and hydrogen fuel cells. These technologies can supply power to Calistoga for up to 48 hours.

The United Nations Development Programme (UNDP) project "Enhanced Rural Resilience in Yemen" (ERRY) uses solar microgrids owned by local communities. This system reduces energy costs to 2 cents per hour, compared to 42 cents per hour for diesel-generated electricity. The project won the Ashden Awards for Humanitarian Energy in 2020.

A two-year pilot program called Harmon'Yeu began in spring 2020. It connected 23 homes in the Ker Pissot neighborhood and nearby areas to a smart microgrid using software from Engie. Sixty-four solar panels with a peak capacity of 23.7 kW were installed on five homes, and a battery with 15 kWh storage was placed on one home. Six homes store extra solar energy in their hot water heaters. A system manages energy from solar panels, the battery, and hot water heaters to supply all 23 homes. The smart grid software updates energy supply and demand every 5 minutes, deciding when to use energy from the battery, panels, or store it in hot water heaters. This was the first such project in France.

A wirelessly managed microgrid is used in rural Les Anglais, Haiti. The system has three parts: a cloud-based monitoring and control service, a local gateway, and a network of wireless smart meters at over 500 buildings.

Non-technical loss (NTL) is a major problem in developing countries, where it often accounts for 11–15% of total electricity generation. A study analyzed 72 days of wireless meter data from a 430-home microgrid in Les Anglais to help distinguish NTL from other power losses, improving detection of energy theft.

The Mpeketoni Electricity Project is a community-based diesel-powered microgrid in rural Kenya near Mpeketoni. These microgrids have increased infrastructure growth in the area, including a 100–200% rise in worker productivity and a 20–70% increase in income levels, depending on the product.

A winery in Sonoma, California, uses a system that includes a micro-turbine, fuel cell, multiple batteries, a hydrogen electrolyzer, and solar panels.