The Krieger’s Flak interconnection between Denmark and Germany takes in offshore wind farms and gives a glimpse of a much more connected electricity future

DOUBLED UP
By connecting two offshore wind farms to form an interconnector, Denmark and Germany save on additional infrastructure to increase the capacity flowing between the two countries

FUTURE VISION
The link provides know-how and a starting point for more complicated cross-border links incorporating offshore wind, which could become

KEY QUOTE
If we had done it in the old-fashioned way you would only have one place to send the renewable power

Dubbed the Combined Grid Solution (CGS), an interconnection that links the German and Danish power systems, boosting the reliability of supply to customers in both countries, also takes in two offshore wind farms: Krieger’s Flak at 600 MW in Danish waters and Baltic 2 at 288 MW in German seas. With the wind projects separated by just 30 kilometres, the transmission system operators (TSOs) on each side of the border, Energinet in Denmark and 50Hertz in Germany, saw an opportunity to link the electrical infrastructure of the two projects, provide new flexibility to the region’s grid systems, and better utilise the capacity of the cables involved. The benefits are lower costs resulting in cheaper electricity. The Combined Grid Solution was made possible by use of a back-to-back converter, located in northern Germany. It supports the connection of the asynchronous current power grids of eastern Denmark and Germany, despite their different technical attributes. The first of two voltage source converters transforms the alternative current (AC) from the Nordic system into direct current before the second converter transforms it back to alternating current but now matching the so-called continental Europe synchronous area. “Constructing the back-to-back converter on land is economically beneficial and allows for easier maintenance compared to the installation on an offshore platform,” Energinet notes.

The two TSOs had originally planned to install converter stations within Denmark, Germany and offshore, in a multi-terminal solution, says Hanne Storm Edlefsen, area manager at Energinet. The call for tenders to provide and install the hardware, however, called the feasibility of the configuration into question. The system was redesigned to be a mostly AC solution with back-to-back converters in Germany. Use of back-to-back converters is not uncommon where asynchronous systems are connected, such as the Lithuania-Poland link and the Mexico-United States link. Energinet expects capacity utilisation of the interconnector to be similar to that achieved for the 600 MW Kontek onshore link between the same grid bidding zones in Denmark and Germany, which has been in operation since 1996 and is due to be replaced. According to the European Network of Transmission System Operators for Electricity (ENTSO-E), in 2017 the Kontek link had an available technical capacity of 85.8%. In total, 2.6 TWh of electricity, roughly 48.5% of the technical capacity, was exported from Den- mark to Germany with 0.9 TWh going in the other direction. Had the offshore wind farms been connected to shore with dedicated cables, the links would only have been used to their full capacity during hours when the wind turbines produce at full capacity. Repurposing the wind farm-only cables to double-up as system interconnectors greatly boosts use of their capacity, improving overall cost efficiency.

The transfer of electricity along the connection is controlled by a so-called Master Controller for Interconnector Operation (MIO), which Storm Edlefsen describes as the brain of the operation. It receives information from Energinet and 50Hertz to decide how best to distribute the power across the whole network to help keep demand and supply in balance on both grids. As well as increasing the interconnection capacity between the two nations, the link also benefits the wind projects, providing them with two markets in which to sell their output without the need for a parallel interconnector alongside export cables to Denmark and Germany. “If we had done it in the old-fashioned way you would only have one place to send the renewable power,” Storm Edlefsen adds. Increasing volumes of variable renewable generation on the grid require more dynamic balancing of supply and demand than in the past. Additional transmission capacity, both in-country and cross-border, helps the balancing task. “The more interconnectors you have, the better connected you are to other countries with other production patterns, then it is easier,” Storm Edlefsen says. Denmark is already well connected with its Nordic neighbours, meaning it can take advantage of the combination of Norwegian hydropower and Finnish nuclear along with its renewables generation.

Decarbonisation of energy is raising electricity demand as transport, heating and cooling are electrified to gain access to renewable energy, necessitating more transmission capacity. “We see in the future, with more and more renewables, areas where we have to do more to build out the grid,” says Storm Edlefsen.

HUB AND SPOKE

The CGS link is also the first of its kind to combine both generation and interconnection into a single system but, for cross-water links, it could become the norm. “I know that there are some plans in other countries where they are looking at this. But this was the first version and we are already trying to improve things,” says Storm Edlefsen. Energinet sees the Krieger’s Flak interconnection leading to larger, more complicated networks of energy islands, combining the generation of several offshore wind projects before exporting the power to numerous markets.

“The whole concept is that you make an artificial platform, maybe it is an island but could also be a caisson, or even just a platform,” Storm Edlefson explains. “You have the spokes from the wind production around these new offshore artificial islands. The spokes are the lines into the island from the wind production. This creates a near-shore environment far out at sea where there is a lot of wind potential and nobody can see it. Then, you have the interconnections going into different countries also like Krieger’s Flak.” While the energy islands sound complicated, Storm Edlefson insists much of the technology is already available, especially once the Krieger’s Flak project is fully completed. “The most difficult thing is actually to get people on board and coordinate across governments and countries on the build-out. It is not a huge technological leap that we have to take.”

Although rapid development of much more interconnection is required for an electrified Europe, completion of major grid infrastructure projects can take up to 12 years. “It is not because we are that slow,” says Storm Edlefson. “It’s because it is a bottom-up approach.” Currently, TSOs begin the development of new links before discussions between national governments. With 2030 renewables targets looming, however, along with the transmission capacity required to reach them, TSOs are trying to engage governments earlier. “We are trying to do a top-down approach where we have the ministers and the governments meeting [to reach] an agreement. At the same time, we are working from the Energinet side with the TSOs to show whether the [business] case is good or not. This should hopefully make the timing at the beginning of projects like this shorter. “The whole governmental issues with authorities giving permits, looking into environmental issues and so on, all these things need to be thorough, of course they do, because these are difficult projects environmentally. But this could also be done more in parallel and not sequentially,” Storm Edlefson says.•

TEXT David Weston ILLUSTRATION Hvass & Hannibal