Active distribution grids

Active distribution grids

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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.

Figure 1:  Residential micro grid in Bronsbergen, Nederland [1]

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 linked in a grid as well as in an island state. 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.