Island network
An island network , English microgrid ( "microgrid" ), is a locally demarcated electricity network , which consists of only one or a few electricity plants , supplies a spatially narrow area and has no direct electrical connection to other electricity networks.
The integrated network must be distinguished from the island network . However, the transition may be fuzzy, as a real network structure can be built in larger island networks. From a technical point of view, an island network is separated from the interconnected network in that the secondary control in an island network is only designed to keep the network frequency constant . In interconnected networks, the secondary control also takes on the task of keeping the transmission capacity on so-called coupling lines , that is, interconnected lines between individual network segments, in certain areas.
Smaller and autonomous power supply systems, such as the power supply in remote mountain huts or the electrical energy supply on ships , do not constitute power networks and are referred to as island systems or on-board networks .
Types of island networks
An island network can be a real island where it does not seem economical to connect to the power grid on the mainland, or it can be independent power grids on the mainland. In West Berlin , for example, there was an island network at the time of German division . Large national island networks exist nowadays mainly in developing countries as well as states that do not want to join forces with neighboring states or cannot join for physical reasons. For example, the power grid in Fairbanks , Alaska , is operated as an island grid with no direct connection to neighboring North American power grids as part of the North American Electric Reliability Corporation . The reason lies in the large spatial distances and the sparsely populated region, which does not economically allow interconnected operation with neighboring North American power grids.
Some traction power grids have certain similarities with island grids, even if there are links to the public power grid via traction power converter stations . Special traction current networks such as the Mariazellerbahn are also linked to the integrated network via the Erlaufboden power plant in converter operation and are therefore not to be understood completely as an island network.
It is also possible that not all lines of a voltage level are directly connected to one another in a power grid, but are only connected to other lines of the same voltage level via lines of subordinate voltage levels. Such conditions can arise from problems with network expansion. An example of this is the 380 kV line between Lübeck-Siems and Lübeck-Herrenwyk, which is only connected to the other 380 kV lines in Central Europe via 110 kV and 220 kV lines.
Supply and operation of island networks
Constructs
Small island networks and island systems are often operated with diesel generators . - If no fuel can or should not be used, the electricity is generated exclusively from renewable energies. We recommend (any) combinations of biogas systems , fuel cells , photovoltaic systems , solar inverters , small hydropower plants and wind turbines . In order to secure the energy supply, accumulators , battery storage power plants or flywheel storage systems are also integrated in some island networks . Renewable energies offer the advantage that they are low in emissions . Remote islands in particular benefit when no fuel has to be procured.
Problems
Possible disadvantages of island networks are the reduced reliability, the increased frequency and voltage fluctuations and the high costs of maintaining power reserves. Costs for power reserves arise, for example, when several redundant turbines in large power plants have to run in partial load operation with poor efficiency .
Put to the test
On November 30, 2019, the Bordesholm utility company (VBB) checked how reliably they could continue to deliver electricity to the community (almost 8,000 inhabitants) despite a supraregional disruption. The village was separated from the European network for an hour and thus became an experimental island network. Its own components are private combined heat and power plants and solar systems as well as a biogas system. The system is supplemented with a battery storage system made up of 48,000 small rechargeable batteries, "just like those used in cell phones". The consumption during the test was 2.5 megawatts , of which 1 megawatt was provided by the battery storage system. The switchover happened in a few milliseconds; the 50 Hertz AC frequency was more stable than in the network system.
Demarcation
An island grid must be distinguished from an intelligent power grid , even if similar technology is used under certain circumstances. Intelligent power grids are roughly summarized as the networking and intelligent control of energy consumers and producers in a power supply network.
Island network with high-voltage direct current transmission
There are also power grids that are connected to a larger power grid by means of high-voltage direct current transmission (HVDC) and are sometimes referred to as an island grid despite this electrical connection. These are all power grids on islands such as Gotland . If such a connection has been implemented, it is often the only technical option for coupling to another power grid. Such a connection increases the security of supply and, if necessary, enables the export of electrical energy. However, a pure supply from the HVDC is only possible with an HVDC with self-commutated converters .
literature
- Adolf J. Schwab: electrical energy systems . 2nd updated edition. Springer, Berlin et al. 2009, ISBN 978-3-540-92226-1 .
credentials
- ↑ Mahmoud, Magdi S .: Microgrid: Advanced control methods and renewable energy system integration. Oxford 2017, ISBN 978-0-08-101262-8 .
- ↑ Michael Görmann: No power failure in the event of a power failure. In: Kieler Nachrichten December 2, 2019, page 15.
- ↑ What is the difference between microgrids and smart grids? | ZHAW Institute for Environment and Natural Resources IUNR. Retrieved on September 22, 2018 (German).