Together with its project partner, the Danish transmission system operator Energinet, 50Hertz has planned the first offshore interconnector in the Baltic Sea by linking the two national offshore wind farm connections.
The so-called Kriegers Flak Combined Grid Solution (CGS) connects the Danish region of Zealand with the German state of Mecklenburg-Western Pomerania. The transfer capacity is 400 megawatts (MW). Construction was rolled out end of 2016/beginning of 2017. Commissioning/Energisation and start of test phase “Trial Operation” took place in December 2020. During Energisation the German and Danish grid will be connected together.
Germany currently operates two offshore wind farms in the Baltic Sea, Baltic 1 (48 MW) and Baltic 2 (288 MW). Denmark built the offshore wind farm Kriegers Flak (600 MW). The Kriegers Flak (Denmark) and Baltic 2 (Germany) wind farms are less than 30 kilometres apart. The interconnector was established by connecting both wind farms by means of two submarine cables.
The frequencies of the Danish and German transmission systems use a slightly different phase. That is why they need to be matched at the interface. This is enabled by means of two serial voltage source converters (VSC). A converter transforms the alternating current (AC) from the Nordic interconnected system to direct current (DC). Another converter transforms this direct current back to alternating current - only now adapted to the synchronous area of continental Europe.
This so-called back-to-back converter has been installed in Bentwisch, near Rostock. The construction of the double converter on land is more favourable and allows easier maintenance than the construction and operation of the installation on an offshore platform. This was provided for in the original concept.
Due to the different voltage levels of the Danish and German offshore wind farms (150 to 220 kilovolt), a transformer is also required. However, this has been installed on the Danish offshore platforms.
In order to implement the energy transition, the infrastructure of the German and also the European energy industry needs to be adapted. In the future, ever more flexible installations, interconnectors as well as storage units will be required in order to balance the fluctuations in the production of energy by wind turbines and solar panels.
Especially the transmission grids play a key role in this regard: extra high-voltage lines form the backbone of the power supply in Germany and Europe. Increasingly, electricity is generated where demand is highest, but where there are optimal climatic and geological conditions.
At the same time, however, the high level of security of supply and the grid stability should be guaranteed. In order to ensure both, electricity exchange between neighbours is also a tried and tested method. The Combined Grid Solution - Kriegers Flak functions exactly as such a bridge between Germany and Denmark and ensure the additional exchange of several hundreds of megawatts in electricity between our countries. As a result, the interconnector will further increase the security of supply of the German transmission grid. Moreover, the new connection facilitates the integration of regionally generated renewable energy in Germany and in eastern Denmark and thus increases its consumption.
Grid expansion projects such as CGS close the gaps in the European interconnected grid and contribute to the further development of the common European energy market. The European Commission has therefore classified CGS as a project of common interest (PCI). The project was financially supported by the European Energy Programme for Recovery (EEPR).
The Combined Grid Solution is a hybrid system. On the one hand, it serves as the grid connection for the offshore wind farms, on the other hand it connects the transmission grids of Denmark and Germany and, thus, allows for the transnational exchange of electricity. In case of strong wind, the full capacity of submarine cables can be used to feed the wind power generated offshore into two grids via one single system. If there is relatively little wind on the Baltic Sea, however, the remaining capacity can still be used for the exchange of electricity between the two countries. Consequently, the platform and submarine cables are optimally used, regardless of the wind on the high seas. This system does not yet exist anywhere else in the world.
In order to control this complex system, a newly developed Master Controller for Interconnector Operation (MIO) with the latest information technology has been installed in the Transmission Control Centre in Berlin/Neuenhagen.
The Master Controller acts as the ‘brain’ of the entire system. It calculates the available capacity of the interconnector (wind forecasts), prevents overload by controlling the converter and/or the wind farm, maximises the feed-in from the wind farms through active system control (936 MW are possible), controls the exchange of electricity between the grids of both countries by means of the converter. In this way, the infrastructure of the wind farm connections and the interconnector, mainly submarine cables, can be utilised to 100 per cent which also lowers the prices for the consumer.
The extension of one of the two Kriegers Flak substation platforms at sea was required for the interconnector project CGS. The cables from all the wind turbines in the wind farm are connected in the transformer station at the transformer platforms. The voltage is transformed from 33 to 150 or 220 kilovolts (kV) for efficient further transport.
Operating a transformer platform at sea is very challenging. Since sea air is very humid, the hall of the transformer platform must be particularly well air-conditioned and dehumidified. This creates almost the same climatic conditions as on land. Offshore transformer platforms are constructed in a similar way to gas-insulated switching stations on land. The transformer platform is additionally reinforced so that it is well prepared for the harsh weather conditions at sea.
The extension of one of the transformer platforms with the additional transformer serves as an interface between the German 150 kV grid and the Danish 220 kV grid.
Together with the Danish transmission system operator Energinet, 50Hertz implements the first offshore interconnector in the Baltic Sea connecting two national offshore wind farm connections. In 2017, both project partners started the construction work.
The legal basis for Combined Grid Solution - Kriegers Flak (CGS) is the Federal Requirements Plan Act (BBPIG) of 31 December 2015. In the Federal Requirements Plan Act, the project is listed as number 29. The basis for the Federal Requirements Plan is the confirmed Grid Development Plan for Electricity for 2024 which was established in 2014. This plan is based on a ten-year period for which the national Grid Development Plan (GDP) was developed and subject to public debate - see also netzentwicklungsplan.de or netzausbau.de.
Especially the balancing function for production and consumption across state borders is new and brings Europe even closer together in the future. The conversion of the electricity grids in the context of the energy transition has reached the European level and will be decisive for the security of supply in Europe.
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