Modular architectures and offshore islands to revolutionise grid connection

As the number of offshore windfarms grows, so new ways to connect them to the grid are being developed, including islands in the North Sea and less-costly modular connections with a reduced environmental impact

Recent weeks have seen progress in the development of future gird architectures in the Netherlands and other countries opting for offshore wind energy.

In March 2018, the Dutch government unveiled its Offshore Wind Energy Roadmap 2030, setting out plans for the further development of offshore wind energy between 2024 and 2030. The roadmap specifies which windfarm zones will be released for development in the Dutch sector of the North Sea, when this will occur, the projected generation capacity of these zones, and when they will be taken into operation. The windfarms will be in three windfarm zones: Holland Coast (West), North of the Wadden Sea Islands, and IJmuiden Far Offshore.

Transmission systems operator TenneT will connect the offshore windfarms and is preparing to develop innovative and cost-effective grid concepts for the offshore grid connections required.

Under the National Energy Agreement concluded in 2013, five offshore windfarms with a total capacity of 3.5 GW will be built in the Dutch sector of the North Sea in the period until 2023, in addition to existing windfarms that have a capacity of 1 GW. A coalition agreement provides for a further 7 GW increase in offshore wind energy capacity between 2024 and 2030.

TenneT said that, in its view, the best way to connect windfarm zones located relatively close to the Dutch coast to the grid is to use alternating current (AC) technology. The energy produced in these zones can be transmitted via a standard TenneT transformer platform connected to an onshore high-voltage station located near the coast via 220 kV AC cables.

Each of these standardised connection systems has a capacity of 700 MW and will also be used by TenneT to connect the Borssele windfarm zone (2 × 700 MW), the Holland Coast (South) (2 × 700 MW) and Holland Coast (North) (700 MW) windfarm zones.

With a view to further cost reduction and limiting the space required, the roadmap enables combined preparations for the connection of the northern section of the Holland Coast (West) and the existing Holland Coast (North) zone. The standardised 700 MW design will be optimised further for application in the Holland Coast (West) and North of the Wadden Sea Islands zones.

In TenneT’s view, direct current (DC) connections will be needed to connect offshore windfarms on a larger scale, over greater distances, in a cost-effective manner, as is the case with IJmuiden Far Offshore. It believes that continued innovation should enable the development of DC connection capacities of 1,200-2,000 MW. These higher capacities will then make it possible to integrate IJmuiden Far Offshore with fewer connections (less cabling) and with less impact on the environment.

As previously highlighted by OWJ, TenneT is working with relevant government departments to research the possibility of constructing an island in the IJmuiden Far Offshore zone to accommodate a converter and transformer station, to which windfarms can then be connected. This approach could be more cost-effective than installing multiple large offshore DC platforms, it believes.

In addition to potential economic benefits, such an island would offer opportunities for converting wind power into hydrogen, facilitating a so-called ‘Wind Connector’, and creating port and maintenance facilities.

There are several potential benefits in this approach, not least that a Wind Connector to the UK would make it possible to use the infrastructure to connect windfarms and enhance integration in the international electricity market. This would significantly improve – potentially even double – the efficiency of the offshore grid and contribute to a further cost reduction in offshore wind energy. An island in the IJmuiden Far Offshore zone could also provide valuable experience for the creation in the longer term of an even larger energy island, further offshore, in the North Sea.

The Swedish Energy Agency has been commissioned by the Swedish Government to investigate ways to eliminating grid connection costs for offshore wind energy. The agency has chosen to examine two different models.

The first model entails moving the grid connection point to the offshore windfarm. This means that Svenska kraftnät (the Swedish National Power Grid) would be responsible for planning, construction and operation of the undersea cables and, hence, for all the costs entailed by the connection. Funding of the measure could be provided through an increased grid tariff.

However, the agency believes there are issues associated with this model. It could lead to a disparity between the onshore and offshore wind energy sectors, as onshore wind power and other electricity production facilities would continue to pay connection costs, but offshore wind would not. It is also the agency’s assessment that there would be no incentive for wind power producers to select locations that lead to cost-efficient connections if they are not obligated to pay any portion of the connection costs.

The second model entails the introduction of subsidies to wind power producers for a portion of the connection costs. The agency proposes that this support be limited to undersea cabling and associated transformers. This would create conditions comparable to those of onshore wind power. Another benefit of this model, in the agency’s estimation, is that the cost of the removal of connection costs would be limited while still retaining an incentive to connect windfarms in locations that are cost-effective. It is proposed that the subsidies under this model be financed through the introduction of a surcharge paid by all electricity consumers.

Removing costs may make Swedish offshore wind power more competitive in the European context, which – given that this would lead to an expansion of offshore wind power – could lead to increased profit margins in electricity production if there are rapid changes in the electricity market that cannot currently be predicted.

In Belgium, Elia has confirmed that all the main construction contracts for the planned modular offshore grid (MOG) are now in place. Development of the MOG remains on schedule and it should go live by the end of September 2019. The project will be fully operational in 2020.

With the MOG, Elia is developing an offshore electricity hub for four offshore windfarms to bring the electricity produced to shore in the most efficient way. It is the first project of its kind in Belgium and it will create opportunities for the further development of renewable energy in the North Sea. The MOG will allow windfarms connected to it to inject wind energy directly into the Belgian grid, even when there is a loss or failure of one of the offshore cables. Construction can be phased and synchronised to the time schedules of individual windfarms. It is also more cost-efficient and environmentally friendly. Using a MOG rather than a conventional solution facilitates a reduction in the length of cable by 40 km.

Detailed design of the platform forming the basis of the offshore switchyard will be undertaken by Ramboll. The contract awarded to the company encompasses the engineering work to bring the platform from a conceptual stage to a detailed structure, ready for fabrication. The jacket structure (bottom structure, anchored on the seabed) and the topside, will be fabricated by Heerema. The platform will be installed by Seaway Heavy Lifting in two separate campaigns. Installation of the jacket is scheduled for late 2018, with installation of the topside in 2019. The primary tubular steel components and the piles will be fabricated by EEW. The high voltage switching equipment and gas insulated substation will be delivered by Siemens. Dredging International will deliver and install the 85 km of 220 kV subsea cable. A contract for unexploded ordnance identification and disposal is due to be awarded shortly.

The platform will be located approximately 40 km from the coast of Zeebrugge. Two direct current 220 kV cables and one cable via the Rentel windfarm will link the platform with the Stevin substation in Zeebrugge, so that the electricity produced can be injected in the Belgian onshore grid.