50Hertz and Energinet inaugurate Combined Grid Solution

In: CablesConvertersGridInterconnectorsSubstationsWindfarms
50Hertz and Energinet have inaugurated the world’s first hybrid offshore interconnector, integrating German and Danish offshore wind farms. The Combined Grid Solution (CGS) connects two offshore substations in the Baltic Sea to each other as well as to the existing onshore connections of the offshore wind farms. As a result, the CGS can bring offshore wind power to Denmark or Germany and can also be used for cross-border energy trading.

The CGS is a technological innovation that connects the grid connections of offshore wind farms to for a bridge between Germany and Denmark. This includes the grid connections of the
Baltic 1 and Baltic 2 wind farms (Germany), which are operated by 50Hertz, as well as the grid connection to Kriegers Flak wind farm (Denmark), that is currently under construction. Two submarine cables 25 km length with a capacity of 200 MW each have been laid between the offshore substations of Baltic 2 and Kriegers Flak, linking both grid connection systems together.

Stefan Kapferer, CEO of 50Hertz, said: "With this project, 50Hertz and Energinet are pioneering the efficient integration of offshore wind farms into the cross-border European electricity market. It offers us several effective options to balance the frequency and voltage of our transmission grids at all times, to deal with the integration of renewable energy sources with more flexibility and to increase the stability of the overall system. Therefore the CGS is an essential puzzle piece of our strategy to cover 100 percent of the electricity demand in our grid area, over the course of a year, with renewable energy by 2032. In addition, the project shows the high significance of the European Baltic area for the energy sector. This importance is also reflected in the Baltic Sea Offshore Wind Declaration, which was signed by the neighbouring states in late September."

As the transmission grids of eastern Denmark and Germany are not synchronous, the construction of a double converter (back-to-back converter) at the Bentwisch substation near Rostock was necessary for this interconnector. Through the converter, the arriving alternating current (AC) is converted into direct current (DC) and immediately into alternating current again. This procedure ensures a smooth flow of electricity from Denmark to Germany and vice versa.

Once the integrated Kriegers Flak wind farm has been commissioned in 2021 as planned, it will provide power supply to 600,000 households in the Zealand region. It has a capacity of 600 MW and will be Denmark’s largest wind farm.

Thomas Egebo, CEO of Energinet, said: "The Combined Grid Solution is not only a very important milestone in reaching Denmark’s ambitious goal of a 100 percent green power system in 2030. The groundbreaking project also delivers an important building block for decarbonizing the rest of the society. Wind and PV must in the near future replace petrol in our cars, oil, coal and gas in our private and district heating. And power from wind and solar will in the long run be the main ingredient in production of green hydrogen, green gasses, jetfuels, for industry, shipping and aviation. Therefore, the Danish Parliament has decided to build energy islands in the Baltic Sea and the North Sea and surround them with large scale offshore wind farms. The innovative CGS-solution combines offshore wind and interconnectors and is a leading example for energy islands and an efficient and fast green transition."

The CGS consists of both hardware and software components. The Master Controller for Interconnector Operation or MIO, which was developed with partners, acts as the “brain” of the system. This digital control unit, located in 50Hertz’ Control Centre in Neuenhagen near Berlin, functions to harmonise both the requirements of the electricity market and the generation of electricity that depends on the wind conditions on the Baltic Sea.

The MIO’s most important task is the optimal exploitation of the interconnector and simultaneously ensuring the prevention of an overload of the line as well as the operating facilities in the substation. In this process, the MIO controls the market-based exchange of electricity between Denmark and Germany, not only on the basis of forecasts, but by ensuring the necessary voltage stability and system balance in case of physical deviations in real time. For this, the MIO can rely on the double converter in Bentwisch as well as on the
Baltic 1 and Baltic 2, and in the future also on the Kriegers Flak wind farm, which will be connected and in action by then.

The double converter also fulfils an important function for grid stability, so that more and more renewable energies can be safely integrated into the overall system. For the transmission of electricity through alternating current lines, a so-called reactive power is required to co-exist with the active power. Until now, this reactive power was mainly provided by conventional power plants. The double converter is able to provide a section of the needed reactive power compensation for the north-eastern part of the 50Hertz grid area.

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