Wind farm in the Noth Sea

Cutting offshore costs

How efficient grid connections can industrialize wind energy

To make wind energy more affordable and facilitate the energy transition, it’s not just essential to increase the efficiency of turbines: The technology for the grid connections also has great potential to reduce installation and operational costs. Cutting down on the size, weight, and complexity of substation technologies, for instance, enables up to a 30 percent cost reduction, as many offshore wind projects are currently demonstrating.

By Sonja Fischer

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Larger turbine blades, better reliability, increased power capacity: Offshore wind has experienced many improvements to its efficiency and power rating over the last few years. It quickly developed from the most expensive renewable energy solution to one of the cheapest. Most projects are already subsidy-free. Recent studies show that by 2050, offshore wind energy costs could even be  49 percent lower. Given the goal to achieve a climate-neutral Europe, the cost reductions, and increased demand, wind is now key to the EU Commission’s renewable energy strategy. By 2050, Europe aims to expand offshore wind energy to 300 Gigawatt (GW) with investments of €800 billion.


In addition to upgrading the turbines themselves, recent innovations in electrical transmission and grid connections for offshore wind have had a great impact on cost as well, explains Andreas Barth, Vice President Grid Access at Siemens Energy: “Offshore wind farms profit from up to 30 percent reduction in installation and operation costs, thanks to design improvements that substantially reduce the size and weight of an offshore platform.”

Barth’s team has already connected 8,700 megawatts (MW) of offshore wind to the grid. Another 4,000 MW is under construction. Many of these connections are made with alternating current (AC) technology, the first choice for wind farms close to the shore with a typical output of several hundred to 1,500 MW – which is still the scenario for many projects in the North Sea. 

To reduce the total grid access cost of offshore AC connections, Siemens Energy focused on the core electrical equipment and removed several optional ancillary systems. The resulting offshore transformer module (OTM) has now replaced the conventional substation, cutting weight by 40 percent. It’s installed on similar or even the same design as a turbine foundation, either on top of a jacket or on a wind turbine generator monopile. This means less transportation and fewer installation vessels, faster operation, and decreased risk: easily a €10 million reduction in capital costs. The solution is modular and can be modified for each project’s needs: There can be one-, two-, or three-circuit solutions on single or multiple foundations, with power ratings over 1 gigawatt (GW).


Several projects in the North Sea have already been installed with the new technology. The latest is the Moray East offshore wind farm in Scottish waters; it’s the largest offshore grid connection in terms of transmission capacity and order entry that Siemens Energy has delivered to the UK to date. Moray East proves that the OTM is also appropriate for large wind parks: Several OTMs that are nominally rated in incremental 250-MW blocks were linked together to provide the required transmission capacity. The project will set a record for the cost of delivering renewable power after its completion in 2022: Moray East will generate power at GBP 57.50/MWh – half the price of power generated by other offshore wind farms under construction today.

HVDC: More efficient over long distances

However, as wind farms generation capacities increase and their distances to the shore become greater in the more developed wind markets, grid connections will need to be realized with high-voltage direct-current (HVDC) transmission technology. Over long distances, HVDC transfers larger quantities of electricity with lower energy losses. “HVDC technologies are typically more efficient than HVAC technologies for the transmission of electricity over long distances, because only active power needs to be transmitted,” Barth explains. “That’s why HVDC is the only solution that’s technically feasible for submarine cables more 100 kilometers long.”


In the German North Sea, Siemens Energy has already put five HVDC grid connections in operation for transmission systems operator TenneT. Two more are under construction. Together they bring 5.6 GW of wind power to the shore and then to seven million households. The HVDC PLUS technology implemented in these projects is less complex and extremely compact, which predestines it for use in sea-based applications. It also features network stabilization, which is key to reliably transmitting volatile wind power. 

Cutting down the cost of HVDC offshore platforms

The increase in HVDC’s power rating has also initially enlarged the size of the offshore platforms. The sophisticated technology housed in the platform requires five to ten decks. Crew quarters, helipads, and cranes are added as well, so that the platform can easily weigh up to 20,000 tons. This means a costly and complex installation and increased capital and operational costs. So Barth’s team challenged themselves: What if “cutting down” on facilities – which had already proven its worth in the OTM technology for AC – could be achieved on HVDC platforms, too?


Their development efforts led to a functional platform with more efficient substation technology. For instance, gas-insulated switchgear (GIS) for DC applications are now available for the HVDC platforms: GIS uses up to 95 percent less space than comparable air-insulated switchgear. In addition, a 66-kV direct connection from the wind farms to the offshore converter station eliminates the need for an offshore transformer substation. “For some of the current projects, we cut the weight of the offshore platform in half, which reduces at least 30 percent of the usual investment and operational costs,” explains Barth. 

“For some of the current projects, we cut the weight of the offshore platform in half, which reduces at least 30 percent of investment and operational costs.” 
Andreas Barth, Vice President Grid Access at Siemens Energy

Beyond borders: Connecting grids, tapping new markets

More cost-efficient HVDC offshore technology will also facilitate the development of an international grid infrastructure – in the form of grid connection hubs that help support the exploitation of wind energy. HVDC offshore wind-farm links to several jurisdictions are used as an interconnector, increasing the flexibility and utilization of transmission lines. Integrated grids can compensate for regional fluctuations in the production or consumption of electricity. Currently, Siemens Energy is realizing seven of these HVDC interconnector projects onshore and three offshore – for instance, Nemo Link between the UK and Belgium.


There’s also a trend to use the offshore connections to wind farms for energy trading between countries. That calls for a meshed offshore grid that uses multi-terminal DC systems. In the future, there’ll be more cross-border trading initiatives like Eurobar, where several European transmission system operators intend to interconnect their offshore transmissions systems.


With President Biden’s commitment to deploy 30 GW of offshore wind energy by 2030, the United States is the next big market, with many developing offshore clusters. The first HVDC PLUS connection in the U.S. has already proven its worth as a reliable, cost-efficient transmission solution for the entire San Francisco Bay area. And just recently, Siemens Energy signed a contract to provide the HVDC grid connection for the Sunrise Wind project off the New York coast; it’s the first offshore wind project in the U.S. to use the HVDC PLUS technology. And there’s more to come: “Our technology and expertise are in great demand in Germany, the UK, the Netherlands, and the U.S. – and in any other regions where large amounts of offshore energy have to be transferred to land over long distances,” says Barth.

November 22 , 2021

Combined picture and video credits: Siemens Energy, Siemens Gamesa


Sonja Fischer is a freelance journalist based in Germany. She specializes in technology, healthcare, and history topics and writes for diverse local and international publications.

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