A microgrid is a local electrical grid with defined electrical boundaries, acting as a single and controllable entity. It is able to operate in grid-connected and off-grid modes. Microgrids may be linked as a cluster or operated as stand-alone or isolated microgrid which only operates off-the-grid not be connected to a wider electric power system. Very small microgrids are sometimes called nanogrids when they serve a single building or load. A grid-connected microgrid normally operates connected to and synchronous with the traditional wide area synchronous grid (macrogrid), but is able to disconnect from the interconnected grid and to function autonomously in "island mode" as technical or economic conditions dictate. In this way, they improve the security of supply within the microgrid cell, and can supply emergency power, changing between island and connected modes. This kind of grid is called an islandable microgrid. One version of a microgrid implements control of small-scale distributed generation (DG) at a single house/small building level: the nanogrid. Modular open-source hardware DC nanogrids have been developed to provide solar photovoltaic power for any small-scale system such as an ambulance or even down the device level. Although DC systems generally are more efficient, nanogrids can also be AC to make them compatible with more mainstream devices. A stand-alone microgrid has its own sources of electricity, supplemented with an energy storage system. They are used where power transmission and distribution from a major centralized energy source is too far and costly to operate. They offer an option for rural electrification in remote areas and on smaller geographical islands. A stand-alone microgrid can effectively integrate various sources of distributed generation, especially renewable energy sources (RES). Control and protection are difficulties to microgrids, as all ancillary services for system stabilization must be generated within the microgrid and low short-circuit levels can be challenging for selective operation of the protection systems. An important feature is also to provide multiple useful energy needs, such as heating and cooling besides electricity, since this allows energy carrier substitution and increased energy efficiency due to waste heat utilization for heating, domestic hot water, and cooling purposes (cross sectoral energy usage).
Definitions The United States Department of Energy Microgrid Exchange Group defines a microgrid as "a group of interconnected loads and distributed energy resources within clearly defined electrical boundaries that acts as a single controllable entity with respect to the grid. A microgrid can connect and disconnect from the grid to enable it to operate in both grid-connected or island-mode." The Berkeley Lab defines: "A microgrid consists of energy generation and energy storage that can power a building, campus, or community when not connected to the electric grid, e.g. in the event of a disaster." A microgrid that can be disconnected from the utility grid (at the 'point of common coupling' or PCC) is called an 'islandable microgrid'. An EU research project describes a microgrid as comprising Low-Voltage (LV) distribution systems with distributed energy resources (DERs) (microturbines, fuel cells, photovoltaics (PV), etc.), storage devices (batteries, flywheels) energy storage system and flexible loads. Such systems can operate either connected or disconnected from the main grid. The operation of microsources in the network can provide benefits to the overall system performance, if managed and coordinated efficiently. Electropedia defines a microgrid as a group of interconnected loads and distributed energy resources with defined electrical boundaries, which form a local electric power system at distribution voltage levels, meaning both low and medium voltage up to 35 kV. This cluster of associated consumer and producer nodes acts as a single controllable entity and is able to operate in either grid-connected or island mode. Microgrid Knowledge defines a microgrid as a "self-sufficient energy system that serves a discrete geographic footprint, such as a college campus, hospital complex, business center or neighborhood."
A stand-alone microgrid or isolated microgrid, sometimes called an "island grid", only operates off-the-grid and cannot be connected to a wider electric power system. They are usually designed for geographical islands or for rural electrification. In many non-industrialized countries, microgrids that are used to provide access to electricity in previously unelectrified areas are often referred to as "mini grids". Nanogrids belong to a single home or building and the interconnection of multiple nanogrids forming a network (microgrid), facilitating the sharing of power between individual nanogrids.
Campus environment/institutional microgrids The focus of campus microgrids is aggregating existing on-site generation to support multiple loads located in a tight geographical area where an owner can easily manage them.
Community microgrids Community microgrids can serve thousands of customers and support the penetration of local energy (electricity, heating, and cooling). In a community microgrid, some houses may have some renewable sources that can supply their demand as well as that of their neighbors within the same community. The community microgrid may also have a centralized or several distributed energy storages. Such microgrids can be in the form of an ac and dc microgrid coupled together through a bi-directional power electronic converter.
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