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Article Micro grids: different structures for various applications Tom Loix, KULeuven February 2009

Distributed Generation

Distributed Generation www.leonardo-energy.org

Focus The term micro grid is mentioned more and more in publications on distributed generation. It generally means a collection of consumers, generators and potential energy storage entities connected together and operated as a small grid which is connected to the main grid, but capable of operating a self-sufficient island. The micro grid is typically linked to the grid with the help of a switch, which allows it to work both in grid-connected as well as island mode. However, island operation is not currently permitted in the majority of countries.

Active distribution grids The aforementioned definition of a micro grid is a term that covers many possible implementations. This article attempts to give a concise overview of the different structures one encounters with micro grids. The choice of a certain structure is usually tied into its place in the grid, where the micro grid is to be connected, who the grid’s owner is and its principal aim. At present, as in the past, the largest part of electricity production occurs in large central power stations linked with the transmission grid. The distribution grid was a passive grid, without sources, linked to and supplied by the active transmission grid. Little or no distinction was made between the different types of consumers connected to the grid. In the case of demand exceeding supply, loads are disconnected to maintain the grid frequency and parts of the grid can be suffer blackouts. Clients requiring a highly reliable energy supply must install a UPS system, which shares several basic principles with the micro grid philosophy: local energy (a battery or a diesel generator) is used, which can work both Figure 1: Residential micro grid in linked in a grid as well as in an island state. Bronsbergen, Netherlands [1] These days, there is a growing trend for active distribution grids, where distributed sources account for an important part of the electrical production. These sources are linked with the distribution grid (at medium or low voltage). In the case of network problems, it is still standard to de-energise certain loads and disconnect distributed sources. The sources should, however, be able to support the grid. This is important if a large amount of the production occurs in the distributed way. For the future, the possibility of micro grids continuing to work in island mode in the case of a fault on the electricity grid is being considered. Microgrids could even offer grid support. Demand Side Management allows the option of shutting off particular loads (e.g. certain household machines) in the case of demand exceeding supply. Also, consumers could be classified into different classes, with different levels of power quality (e.g. critical load, which sets high requirements on the availability of the mains voltage and the pollution of it, against non-critical loads) and controllability.

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Micro grids: different structures for various applications www.leonardo-energy.org

A Utility micro grid The first type of micro grid is a so-called utility micro grid. This is (part of) a feeder for a distribution grid, with local energy sources and consumers. This type of micro grid can facilitate a large-scale introduction of distributed sources and can locally receive the growth of the user’s power (either completely or partially), so that congestion problems can be avoided or reduced. A utility micro grid can also deliver ancillary services to the grid, for example the local delivery or absorption of reactive power and the guarantee of a very good power quality for (some of) the local users. The principal objectives for the implementation of this structure are the reduction in impact of grid faults on local users (as the micro grid can, if Figure 2: Micro grid in San Lorenzo, Ecuador necessary, work in island mode, independent of the rest of the grid) and the simplification of [2], an example of a remote micro grid connecting distributed sources. This can be applied both in urban as well as rural areas.

Industrial or commercial micro grids Industrial or commercial micro grids form a second class of micro grid structures. These are typically a collection of critical and/or sensitive loads requiring high power quality and reliability. Typical examples are a data centre or a university campus, but also a shopping centre, a factory, an industrial installation or even a residential neighbourhood. The principal aims of this variety are an increase of the power quality, better reliability and also frequently energy efficiency compared to the electricity grid. Potentially, different loads can be further sub-divided into groups within the micro grid according to the required grade of power quality and reliability. The micro grid can switch over to island operation in the event of a grid fault, during maintenance, periods of poor power quality, or when grid energy prices are high.

Remote micro grids The third and final type of micro grid is the so-called remote micro grid. For the provision of energy in remote areas, developing countries and (geographic) islands, locally available energy sources, often renewable, are usually chosen. Combined heat and power (CHP) may be used. An autonomous micro grid is a good network structure for such cases, where it can also be possible to connect this micro grid to the electricity grid in the future. There are often problems with extending the electricity grid to these remote areas, and it is necessary to work with a pure island grid. It is thus very important that the local generator is adequate and its energy production is sufficiently reliable to ensure that the local consumers receive the highest possible availability in the energy provision. Failing this, it is possible that certain loads would have to be disconnected on an irregular basis to guarantee the stability and correct operation of the net. The application of energy storage can help the spread of this type of micro grid to a large extent.

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Distributed Generation www.leonardo-energy.org

Summary Of course, the emergence of such micro grids brings many changes. In a vertically integrated market, where generation, transmission and distribution assets are in the hands of a single entity, optimum decisions over the placement and connection of distributed sources can be sought relatively easily. When the distribution operator is no longer owner of the distributed sources, as can be the case in a free electricity market, conflicts can occur between the wishes of the grid operator and those of the owner. The owner is primarily interested in the profit of his installation, determined by the aggregated energy production, whereas the grid operator makes his plans with regard to the operation and expansion of the grid and the resulting necessary investments, principally from the perspective of the (maximum) energy. Measures must thus be taken to ensure that the distributed sources do not look only at the profits, but also contribute to attaining an optimal operation of the entire distribution grid.

References [1] [2] [3]

S. Cobben, “Bronsbergen: The First Micro Grid in the Netherlands”, Kythnos 2008 Symposium on Micro Grids, Greece, June 2nd, 2008. X. Vallvé, “PV-Hybrid Micro Grids for Rural Electrification: Field Experience”, Kythnos 2008 Symposium on Micro Grids, Greece, June 2nd, 2008. J. Driesen en F. Katiraei, Design for Distributed Energy Resources, IEEE Power & Energy Magazine, Volume 6, Issue 3, May-June 2008, pp. 30-40.

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