Microgrid
A microgrid is a local electrical grid with defined electrical boundaries, acting as a single and controllable entity.[1] It is able to operate in grid-connected and in island mode.[2][3] A 'stand-alone microgrid' or 'isolated microgrid' only operates off-the-grid and cannot be connected to a wider electric power system.[4] Very small microgrids are called nanogrids.[5]
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.[6] In this way, they improve the security of supply within the microgrid cell, and can supply emergency power, changing between island and connected modes.[6] This kind of grids are called 'islandable microgrids'.[7]
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.[1] They offer an option for rural electrification in remote areas and on smaller geographical islands.[4] A stand-alone microgrid can effectively integrate various sources of distributed generation (DG), especially renewable energy sources (RES).[1]
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).[8]
Advantages and challenges of microgrids[edit]
Advantages[edit]
A microgrid is capable of operating in grid-connected and stand-alone modes and of handling the transition between the two. In the grid-connected mode, ancillary services can be provided by trading activity between the microgrid and the main grid. Other possible revenue streams exist.[38] In the islanded mode, the real and reactive power generated within the microgrid, including that provided by the energy storage system, should be in balance with the demand of local loads. Microgrids offer an option to balance the need to reduce carbon emissions with continuing to provide reliable electric energy in periods of time when renewable sources of power are not available. Microgrids also offer the security of being hardened from severe weather and natural disasters by not having large assets and miles of above-ground wires and other electric infrastructure that need to be maintained or repaired following such events.[39][40]
A microgrid may transition between these two modes because of scheduled maintenance, degraded power quality or a shortage in the host grid, faults in the local grid, or for economical reasons.[40][41] By means of modifying energy flow through microgrid components, microgrids facilitate the integration of renewable energy, such as photovoltaic, wind and fuel cell generations, without requiring re-design of the national distribution system.[41][42][43] Modern optimization methods can also be incorporated into the microgrid energy management system to improve efficiency, economics, and resiliency.[39][44][43][45]
Challenges[edit]
Microgrids, and the integration of distributed energy resource (DER) units in general, introduce a number of operational challenges that need to be addressed in the design of control and protection systems, in order to ensure that the present levels of reliability are not significantly affected, and the potential benefits of Distributed Generation (DG) units are fully harnessed. Some of these challenges arise from assumptions typically applied to conventional distribution systems that are no longer valid, while others are the result of stability issues formerly observed only at a transmission system level.[40] The most relevant challenges in microgrid protection and control include:
Examples[edit]
Hajjah and Lahj, Yemen[edit]
The UNDP project “Enhanced Rural Resilience in Yemen” (ERRY) uses community-owned solar microgrids. It cuts energy costs to just 2 cents per hour (whereas diesel-generated electricity costs 42 cents per hour). It won the Ashden Awards for Humanitarian Energy in 2020.[64]
Île d'Yeu[edit]
A two-year pilot program, called Harmon’Yeu, was initiated in the spring of 2020 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. Sixty-four solar panels with a peak capacity of 23.7 kW were installed on five houses and a battery with a storage capacity of 15 kWh was installed on one house. Six houses store excess solar energy in their hot water heaters. A dynamic system apportions the energy provided by the solar panels and stored in the battery and hot water heaters to the system of 23 houses. The smart grid software dynamically updates energy supply and demand in 5-minute intervals, deciding whether to pull energy from the battery or from the panels and when to store it in the hot water heaters. This pilot program was the first such project in France.[65][66]
Les Anglais, Haiti[edit]
A wirelessly managed microgrid is deployed in rural Les Anglais, Haiti.[67] The system consists of a three-tiered architecture with a cloud-based monitoring and control service, a local embedded gateway infrastructure and a mesh network of wireless smart meters deployed at over 500 buildings.[29]
Non-technical loss (NTL) represents a major challenge when providing reliable electrical service in developing countries, where it often accounts for 11-15% of total generation capacity.[68] 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 NTL from the total power losses, aiding in energy theft detection.[69]
Mpeketoni, Kenya[edit]
The Mpeketoni Electricity Project, a community-based diesel-powered micro-grid system, was set up in rural Kenya near Mpeketoni. Due to the installment of these microgrids, Mpeketoni has seen a large growth in its infrastructure. Such growth includes increased productivity per worker, at values of 100% to 200%, and an income level increase of 20–70% depending on the product.[70]