TenneT has announced its ideas for supporting the further expansion of offshore wind energy: the use of Direct Current (DC) instead of Alternating Current (AC) technology and converter platforms or artificial islands to switch between the two.
Based on the Dutch ambitions in the field of increasing the sustainability of electricity production and the contribution of offshore wind energy to the targets for CO2 reduction in 2030, TenneT has examined a number of options to support this ambition.
Alternating Current for Wind Farms Close to Shore
According to TenneT the currently standardized connection method – based on AC technology – is the best solution for connecting wind farm zones located relatively close to the Dutch coast to the grid. The electricity generated at these wind farms is transported to a transformer platform, and then via 220 kV AC cables buried in the seabed to an onshore high-voltage substation located near the coast.
Each of these standardised connection systems has a capacity of 700 MW, and will also be used by TenneT to connect the Borssele wind farm zone (2 × 700 MW) as well as the "Hollandse Kust" (south) (2 × 700 MW) and "Hollandse Kust" (north) (700 MW) wind farm zones to the national high-voltage grid.
The onshore grid then transports the renewable electricity to the end-users. This connection system would also be usable for the other designated wind energy areas that are relatively close to the coast.
Direct Current Needed to Reach a Larger Scale
It is crucial for further cost reduction and thus the success of offshore wind to support a larger scale. In order to be able to realise larger amounts of offshore wind energy, wind farms will also be needed further offshore. In order to connect offshore wind energy on a large scale and further offshore in a cost efficient way, TenneT believes that the use of DC connections becomes necessary.
Offshore Converter Platform or Artificial Island
Further innovation of these connections should enable these DC connections to transport power up to 2000 MW. TenneT has investigated two options for this:
The first option: Using offshore converter platforms to convert the AC produced by the wind farms into DC for efficient transport to the onshore grid. In Germany, TenneT has gained experience in using DC connections in combination with converter platforms; seven such platforms are already in operation, while another two are under construction.
The second option is based on the construction of an artificial island instead of platforms. This allows the DC converters to be placed and maintained on solid ground. With sufficient scale, it is more advantageous to realise one artificial island instead of multiple platforms. This saves money and thus contributes to a further cost reduction for wind energy at sea.
WindConnector
In addition to the benefits mentioned above, an artificial island can also accommodate port and maintenance facilities and systems for the conversion of wind energy to hydrogen, and can facilitate a so-called WindConnector link to the UK.
A WindConnector is an electricity connection that not only makes it possible to transport offshore wind energy to the onshore grid of one country, but also interconnects the markets of different countries. This can take the form of a direct connection from a wind farm zone to a neighbouring country, or an interconnection between two different wind farm zones. In addition to transporting wind energy, such a configuration would also facilitate international electricity exchange. The infrastructure can be used in less windy conditions, and would therefore be utilised more efficiently.
Example of a situation using a WindConnector system, including an island hub (by TenneT).
Together with The Crown Estate, TenneT has performed a study examining the feasibility of a WindConnector link between the Netherlands and the UK. The results were published on 28 November.
Picture (top): Artist impression of an island-hub facilitating HVDC converters (by TenneT).
