The Microgrid Concept
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Many research groups around the world are pioneering various µGrid concepts, also written as microgrids, as an alternative approach for integrating small scale distributed energy resources (DER of < approx. 1 MW) into low-voltage electricity systems. Many other terms are in common use to describe similar concepts, e.g. virtual power plants, minigrids, smart grids, smart distribution networks, embedded generation, distributed or dispersed generation.
Traditional approaches to embedding generation at low-voltages focus on minimizing the consequences for safety and grid performance of what are assumed to be a relatively small number of individually interconnected DER, for example implying, that they must instantaneously disconnect in the event of system outage. In other words, permitted local control of devices is very limited, and they can function independently, or islanded, only under special circumstances, e.g. during blackouts after the local system is fully isolated from the macrogrid. By contrast, µGrids would be designed to operate semi-independently, usually operating connected to the macrogrid but separating (islanding) from it, as cost effective or necessary for reliability or other objectives.
A µGrid is a semiautonomous grouping of generating sources and end-use sinks that are placed and operated for the benefit of its members, which may be one utility “customer,” a grouping of several sites, or dispersed sites that nonetheless operate in a coordinated fashion. The supply sources may include reciprocating engine generator sets, microturbines, fuel cells, photovoltaic and other small-scale renewable generators, storage devices, and controllable end-use loads. All controlled sources and sinks are interconnected in a manner that enables devices to perform the µGrid control functions unnecessary for traditional DER. For example, the energy balance of the system must be maintained by dispatch, and non-critical loads might be curtailed or shed during times of energy shortfall or high costs. While capable of operating independently of the macrogrid, the µGrid usually functions interconnected, purchasing energy and ancillary services from the macrogrid as economic, and potentially selling back at times.
Three key potential features of the µGrid are:
1. its design around total system energy requirements
2. its provision of heterogeneous level of power quality and reliability to end-uses
3. its presentation to the macrogrid as a single controlled entity
Design Around Total System Energy Requirements
Feature 1 implies that to the extent economic or desired for environmental purposes, the µGrid shares heat and power and optimal recovery of waste heat by combined heat and power (CHP) devices. While small scale thermal generation of electricity is unlikely to be competitive with central station generation, the dramatically improved prospects for useful waste heat recovery, especially in absorption cooling systems, can tip the economic scales towards DER. The arrangement of µGrids evolves from the need to optimize the overall energy system of the enduses, and since transportation of heat is typically more limiting than transportation of electricity, the location of heat loads is likely to dominate.
Provision of Heterogeneous Level of Power Quality and Reliability to End-Users
Feature 2 suggests a central goal of µGrids concerns tailoring PQR to the requirements of enduses, a starkly different principle than the provision of universal consistent service quality, which is the goal of macrogrids. The µGrid is built and operated so that critical loads are protected and high power quality is ensured where it is necessary, while other loads are served with PQR commensurate with their importance and/or reschedulability. The provision of heterogeneous PQR can improve overall reliability of critical equipment while lowering costs because of the sacrifice of non-critical ones.
Presentation to the Macrogrid as a Single Controlled Entity
Feature 3 concerns the µGrid’s presentation to the surrounding distribution grid as a single controlled system, akin to a current customer, or conversely to a small embedded generation source. The µGrid architecture ensures that its electrical impact on the distribution grid is not only as a good citizen that does no harm but also potentially as a model citizen, adding benefits to the distribution system such as reducing congestion, offsetting the need for new generation, supplying local voltage support, and responding to rapid changes in load levels. Nonetheless, the key characteristic of a µGrid is the existence of local control independent of the macrogrid. This control could be implemented by various technologies and be of variable complexity, but its existence defines the µGrid.