© 50Hertz / Manfred Vogel

The helicopter is headed out over the open sea. It is taking course for the transformer platform of the Kriegers Flak wind farm, which may look tiny from above, but is of huge significance. It is here in the midst of the Baltic Sea that Europe is growing further together. Below the seabed, there are cables connecting up the Danish and the German electricity grids. This connection was recently completed as part of an innovative project undertaken by the two TSOs 50 Hertz Transmission (Germany) and Energinet (Denmark). The result is the world’s first hybrid offshore interconnector, i.e. the first connection linking offshore wind farms to more than one country. The sea cable, which goes by the name of Combined Grid Solution or CGS, is two times 25 kilometres long and links up not only the two transformer platforms in the Baltic Sea, but also the offshore wind farms to the onshore grids. In other words, it makes it possible for electricity to be exchanged between the Danish and German grids. The project is a world first, which makes the CGS a template for future offshore electricity grids, of which several are being planned.

How to make two into one

At the heart of the new system is an extension of one of the Kriegers Flak platforms, where the Danish and the German grids meet. The new Danish Kriegers Flak offshore wind farm and the two existing German Baltic 1 and Baltic 2 wind farms are part of the project. Once Kriegers Flak becomes operational, which is to happen in 2021, it will supply some 600,000 households on the Danish island of Zealand with electricity. Its output of 600 megawatts will make the wind farm Denmark’s largest. Whenever there is little or no wind in the Baltic Sea region, meaning that little offshore wind power can be generated, the platform and the sea cable will mainly be used to exchange electricity between the two countries. That way, the cables can be used to a capacity of up to 100%. This will also reduce prices for consumers.

What may sound simple was in fact very tricky to achieve. After all, the Danish and the German transmission networks do not work in synch. This meant that a special back-to-back converter had to be built and installed to allow for the exchange of electricity. This converter is now located in the Bentwisch transformer station near Rostock in Germany. On the Danish side, the connection leads to the transformer station in Bjæverskov.The back-to-back converter transforms the AC arriving from both sides into DC and then immediately back into AC, but in a form that works for the respective grid system. As a result, electricity can flow unhindered between the two countries and is available to the European electricity market. In other words, the Combined Grid Solution not only helps feed in renewables from offshore wind farms into the grid, but also improves grid stability and energy security within the German and Danish power networks. The cost of the German-Danish project, which is being supported by the EU, is approx. €300 million.

A super brain called 'MIO'

In order to control the complex processes involved in electricity trading and the transport of wind energy output, the converter needs an electronic 'brain'. A special new technology called Master Controller for Interconnector Operation (or MIO for short) has therefore been developed for this purpose. With the help of innovative system management strategies, MIO ensures the right voltage, efficient utilisation of the submarine cable and protection of the entire technology against overload – all in real time. To do this, it relies on the double converter in Bentwisch and the German wind farms Baltic 1 and Baltic 2. In future, it will also use the Danish wind farm Kriegers Flak. This novel technology enables renewable energy to be integrated into the overall system and made available to the European markets.

Thanks to their expertise in sophisticated technologies, Danes and Germans have been able to work together to turn this project into reality. They are thus demonstrating good neighbourliness and have a clear perspective of what they want to achieve: an electricity supply that is covered 100% by renewable energy. The Combined Grid Solution brings both partners a big step closer to this goal. But why is this so important?

Will electricity grids like those on land also become available at sea?

As we move into the future, the Baltic Sea will be of great importance for the development of offshore wind in Europe. WindEurope estimates that out of a total installed capacity of up to 450 gigawatts of offshore wind in Europe by 2050, the Baltic Sea will account for about 85 gigawatts. In the long term, the new solutions could create an electricity grid in the North Sea and Baltic Sea similar to that on land. The European Baltic Sea countries now want to promote the development of offshore wind energy by working more closely together. At the end of September 2020, they therefore signed a joint declaration, the Baltic Sea Offshore Wind Joint Declaration of Intent (in German only) establishing their plans. The initiative aims to strengthen cooperation based on a joint working group.

Offshore wind energy is also a priority of the German Council Presidency. The EU needs 360 gigawatts of renewable offshore energy to become climate-neutral. This is stated in its Offshore Renewable Energy Strategy, which was published on 19 November 2020. Together with its neighbouring countries, Germany wants to promote further cross-border offshore projects in the future.

In his opening speech, Federal Minister for Economic Affairs and Energy Peter Altmaier praised the Combined Grid Solution, calling it a 'European flagship project for cross-border cooperation in the field of offshore wind energy which can go on to play an important role as we work to establish a climate-neutral Europe'. He went on to emphasise that the German Council Presidency is therefore also focusing on creating a supportive EU framework that has two key aspects in view: the effective transport of wind power and additional cross-border trade on these lines.

Use of hybrid interconnectors set to develop

The Director of Internal Energy Market at the European Commission, Catherina Sikow-Magny, also predicts a great future for hybrid offshore interconnectors. Promising support in a statement on the opening of the Combined Grid Solution, she said: 'In order to support this development, we will create the necessary regulatory framework'. This support is crucial for the project to work: The current EU electricity market rules require 70% of the line to be opened for electricity trading; however, in a hybrid project like the Combined Grid Solution, this would lead to restrictions on wind power transmission. The European Commission has therefore granted Germany and Denmark an exemption.

Soon the hybrid interconnector between Germany and Denmark will no longer be the only one in the Baltic Sea. Other projects, to be completed by 2030, are already being planned: a set of Danish energy hubs (Danish project in the North Sea and Baltic Sea), North Sea Wind Power Hub (between the Netherlands, Germany and Denmark), WindConnector (between the Netherlands and the UK) and Nautilus between (Belgium and the UK).