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.What are Microgrids and Why are They Becoming so Popular? Enchanted Rock, March 2023 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. [https://www.ijert.org/research/nanogrid-an-introduction-IJERTV10IS030002.pdf International Journal of Engineering Research & Technology:Nanogrid: An Introduction ]
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'.Microgrids and Vehicle-Grid Integration. Berkeley Lab. Retrieved 21 June 2022
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.N. S. Ghiasi, R. Hadidi, S. M. Sadegh Ghiasi and S. Ghaseminejad Liasi, A Hybrid Controller With Hierarchical Architecture for Microgrid to Share Power in an Islanded Mode, in IEEE Transactions on Industry Applications, vol. 59, no. 2, pp. 2202-2209, March-April 2023, doi: 10.1109/TIA.2022.3218273.
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.
Remote off-grid microgrids These microgrids are generally not designed or intended to connect to the macrogrid and instead operate in an island mode at all times because of economic issues or geographical position. Typically, an "off-grid" microgrid is built in areas that are far distant from any transmission and distribution infrastructure and, therefore, have no connection to the utility grid. Studies have demonstrated that operating a remote area or islands' off-grid microgrids, that are dominated by renewable sources, will reduce the levelized cost of electricity production over the life of such microgrid projects. In some cases, off-grid microgrids are indeed incorporated into a national grid or 'macrogrid', a process that requires technical, regulatory and legal planning.
Large remote areas may be supplied by several independent microgrids, each with a different owner (operator). Although such microgrids are traditionally designed to be energy self-sufficient, intermittent renewable sources and their unexpected and sharp variations can cause unexpected power shortfall or excessive generation in those microgrids. Without energy storage and smart controls, this will immediately cause unacceptable voltage or frequency deviation in the microgrids. To remedy such situations, it is possible to interconnect such microgrids provisionally to a suitable neighboring microgrid to exchange power and improve the voltage and frequency deviations. This can be achieved through a power electronics-based switch after a proper synchronization or a back to back connection of two power electronic converters and after confirming the stability of the new system. The determination of a need to interconnect neighboring microgrids and finding the suitable microgrid to couple with can be achieved through optimization or decision making approaches.
Because remote off-grid microgrids are often small and built from scratch, they have the potential to incorporate best practices from the global electricity sector and to incorporate and drive energy innovation. It is now common to see remote off-grid microgrids being largely powered by renewable energy and operated with customer-level smart controls, something that is not always easy to implement in the larger power sector because of incumbent interests and older, pre-existing infrastructure.
Military base microgrids These microgrids are being actively deployed with focus on both physical and cyber security for military facilities in order to assure reliable power without relying on the macrogrid.
Commercial and industrial (C&I) microgrids These types of microgrids are maturing quickly in North America and eastern Asia; however, the lack of well-known standards for these types of microgrids limits them globally. Main reasons for the installation of an industrial microgrid are power supply security and its reliability. There are many manufacturing processes in which an interruption of the power supply may cause high revenue losses and long start-up time. Industrial microgrids can be designed to supply circular economy (near-)zero-emission industrial processes, and can integrate combined heat and power (CHP) generation, being fed by both renewable sources and waste processing; energy storage can be additionally used to optimize the operations of these sub-systems. Microgrids can also be anchored by a large commercial retailer with a large quantity of generation for resiliency or economic reasons. By the mid-2020s, hyperscale data center operators began deploying purpose-built battery microgrids at building scale as a mechanism to time-shift grid load without altering compute schedules. Verrus, a venture of Sidewalk Infrastructure Partners, designed data-centre buildings around a per-building battery microgrid intended to shift load in response to grid-price and grid-stress signals rather than the workload schedule.
Topologies
Architectures are needed to manage the flow of energy from different types of sources into the electrical grid. Thus, the microgrid can be classified into three topologies:
AC microgrid Power sources with AC output are interfaced to AC bus through AC/AC converter which will transform the AC variable frequency and voltage to AC waveform with another frequency at another voltage. Whilst power sources with DC output use DC/AC converters for the connection to the AC bus.
DC microgrid In DC microgrid topology, power sources with DC output are connected to DC bus directly or by DC/DC converters. On the other hand, power sources with AC output are connected to the DC bus through AC/DC converter.
Hybrid microgrid The hybrid microgrid has topology for both power source AC and DC output. In addition, AC and DC buses are connected to each other through a bidirectional converter, allowing power to flow in both directions between the two buses.
Basic components
Local generation A microgrid presents various types of generation sources that feed electricity, heating, and cooling to the user. These sources are include gaseous fuels such as natural gas, biogas or hydrogen typically as [gas engines] in combined heat and power (CHP) or combined cooling heat and power (CCHP) configuration and/or intermittent renewable generation sources such as wind turbines or solar PV units.
Consumption In a microgrid, consumption simply refers to elements that consume electricity, heat, and cooling, which range from single devices to the lighting and heating systems of buildings, commercial centers, etc. In the case of controllable loads, electricity consumption can be modified according to the demands of the network.
Energy storage