Microgrid

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 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. resources in grid-connected mode]] ===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=== In a microgrid, energy storage performs multiple functions, such as ensuring power quality, performing frequency and voltage regulation, smoothing the output of renewable energy sources, providing backup power for the system, and playing a crucial role in cost optimization. Energy storage may be achieved by a combination of chemical, electrical, pressure, gravitational, flywheel, and heat storage technologies. When multiple energy storage devices with various capacities are available in a microgrid, it is preferred to coordinate their charging and discharging such that a smaller energy storage device does not discharge faster than those with larger capacities. Likewise, it is preferred that a smaller storage device does not get fully charged before those with larger capacities. This can be achieved under a coordinated control of energy storage devices based on their state of charge. If multiple energy storage systems (possibly working on different technologies) are used and they are controlled by a unique supervising unit (an energy management system - EMS), a hierarchical control based on a master/slaves architecture can ensure best operations, particularly in the islanded mode. ===Point of common coupling (PCC)=== This is the point in the electric circuit where a microgrid is connected to a main grid. Microgrids that do not have a PCC are called isolated microgrids which are usually present in remote sites (e.g., remote communities or remote industrial sites) where an interconnection with the main grid is not feasible due to either technical or economic constraints. ===Advantages=== 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. 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. 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. 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. Modern optimization methods can also be incorporated into the microgrid energy management system to improve efficiency, economics, and resiliency. ===Challenges=== 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. The most relevant challenges in microgrid protection and control include: Bidirectional power flows: The presence of distributed generation (DG) units in the network at low voltage levels can cause reverse power flows that may lead to complications in protection coordination, undesirable power flow patterns, fault current distribution, and voltage control. Stability issues: Interactions between control system of DG units may create local oscillations, requiring a thorough small-disturbance stability analysis. Moreover, transition activities between the grid-connected and islanding (stand-alone) modes of operation in a microgrid can create transient instability. Recent studies have shown that direct-current (DC) microgrid interface can result in a significantly simpler control structure, more energy efficient distribution and higher current carrying capacity for the same line ratings. Modeling: Many characteristics of traditional schemes such as the prevalence of three-phase balanced conditions, primarily inductive transmission lines, and constant-power loads, do not necessarily hold true for microgrids, and consequently, models need to be revised. Low inertia: Microgrids exhibit a low-inertia characteristic that makes them different to bulk power systems, where a large number of synchronous generators ensures a relatively large inertia. This phenomenon is more evident if there is a significant proportion of power electronic-interfaced DG units in the microgrid. The low inertia in the system can lead to severe frequency deviations in island mode operation if a proper control mechanism is not implemented. Synchronous generators run at the same frequency as the grid, thus providing a natural damping effect on sudden frequency variations. Synchronverters are inverters which mimic synchronous generators to provide frequency control. Other options include controlling battery energy storage or a flywheel to balance the frequency. Uncertainty: The operation of microgrids involves addressing much uncertainty, which is something the economical and reliable operation of microgrids relies on. Load profile and weather are two uncertainties that make this coordination more challenging in isolated microgrids, where the critical demand-supply balance and typically higher component failure rates require solving a strongly coupled problem over an extended time horizon. This uncertainty is higher than those in bulk power systems, due to the reduced number of loads and highly correlated variations of available energy resources (the averaging effect is much more limited). ==References==

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