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Electricity networks can be split into two major subsections: transmission networks and distribution networks.
The transmission network usually consists of high to very high voltage power lines designed to transfer bulk power from major generators to areas of demand; in general, the higher the voltage, the larger the transfer capacity. Only the largest customers are connected to the transmission network.
Transmission network voltages are typically above 100 kV. The networks are designed to be extremely robust, so they can continue to fulfil their function even in the event of several simultaneous network failures. Failure of a single element, such as a transformer or transmission line, is referred to as an 'N-1' event, and transmission systems should be capable of withstanding any such event. More complex cases of simultaneous failures of multiple elements (for example, the failure of a transmission line when a parallel line has been disconnected for maintenance), can be termed 'N-2' or similar. Transmission systems should also be capable of withstanding any such credible combinations.
Transmission consists mainly of overhead lines. Although underground lines offer the advantage of being less visually intrusive and raising less environmental objections, they incur higher initial investment costs and have a lower transmission capacity.
Transmission systems are operated by transmission system operators (TSOs), or independent system operators (ISOs). Responsibility for constructing or owning the physical network may belong to other organisations.
Transmission systems are actively managed through power system control centres, also known as dispatch centres. Balancing power entering and leaving the high voltage network, and reconfiguring the network to cope with planned and forced outages, is a 24-hour activity.
The European grid (Figure 4.1) is divided into five synchronous regions and five relevant organisations: NORDEL (Organisation for the Nordic Transmission System Operators), BALTSO (Cooperation Organisation of Estonian, Latvian and Lithuanian Transmission System Operators), UKTSOA (United Kingdom Transmission System Operators), ATSOI (Association of Transmission System Operators in Ireland) and UCTE (Union for the Coordination of Transmission of Electricity). Each of these organisations coordinates the TSOs involved at both operational and planning stages. The creation of the future European Network for Transmission System Operators (ENTSO-E) will provide a new framework aimed at facilitating coordination between the different areas.
Figure 4.1: Different Synchronous Regions in Europe
Source: UCTE Transmission Development Plan 2008
Distribution networks are usually below 100 kV and their purpose is to distribute power from the transmission network to customers. At present, with the exception of wind and other renewable power plants, little generation is connected to distribution networks, but this is changing rapidly, for example in Germany and Denmark.
Generation connected to distribution networks is often termed “embedded generation” or “distributed generation.” Distribution networks are less robust than transmission networks and their reliability decreases as voltage levels decrease. For example, a connection at 33 kV could expect to lose only a few minutes of connection per year on average, whereas a low-voltage connection at 230 V for an individual domestic consumer in a rural area would, on average, expect to lose at least an hour. As with transmission networks, distribution networks are operated (in some cases also owned) by Distribution System Operators (DSOs).
There is very little 'active' management of distribution networks. Rather, they assume a ‘fit and forget’ philosophy, in other words they are designed and configured on the basis of extreme combinations of circumstances (for example, maximum demand in conjunction with high ambient temperatures, which reduce the capacity of overhead lines), to ensure that even in these extreme circumstances the network conditions experienced by customers are still within agreed limits.
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