On 14 May 2014, at the same time as the financial close of Gemini (in which all financial contracts with the investors were signed), the first pile of the LHVS was driven into the soil of the Eemshaven. A land station like this had never been built before, so there was no example. The Gemini Team had had to find it out for themselves. A unique challenge. Dennis Froeling explains.


Gemini is an offshore wind park, but with an important onshore component: ensuring that the electricity generated offshore reaches the Tennet grid properly and safely and can interconnect with the grid. ‘In themselves, the functional requirements were not new, but their translation in terms of the specific technology of this large wind park was. A lot of questions had to be answered therefore. For example: what are the technical requirements and which basic design is appropriate for those? How big are the transformers that will convert the electricity from 230 kv to 380 kv, so that it can be transmitted further along the high voltage cables?’ From the early stage, the principal contractor Van Oord, another Gemini shareholder, actively contributed ideas on the best solutions.

Building the land station

Building the land station in the Eemshaven. Here the energy generated from the wind park reaches the mainland. From there, it goes on onto the TenneT-grid.

Building in the swamp

Fortunately, the Gemini Team was not troubled by earthquakes in this area. However, the hydraulic conductivity of the soil is not very good here. Dennis explains: ‘In the past, this used to be a swamp. That posed extra challenges for the construction.’ In the autumn of 2015, the next milestone was reached; the two offshore stations and the land station ‘went live’. Before electricity is transmitted from the wind park to land, electricity first has to travel in that direction, as a sort of offshore current. That all went well.
Like many parts of the offshore wind park (such as the platforms and the cables), the LHVS is engineered with many redundancies. ‘The reliability of the system has to be top-level. For example, the air conditioning systems are built with duplicate components. The building is larger than would be necessary for the installation of the equipment. That way, Gemini retains some flexibility for the future.’




Three questions for Gineke van Dijk

When were you involved in Gemini?

‘The end of 2014 I had my first contact with Gemini. The contracts were already closed, but there were questions about its interpretation.’


What makes this project special for you?

‘Offshore wind is indispensable for achieving the sustainability goals and Gemini is therefore a very important project. TenneT was already committed as network manager at sea for newly constructed wind parks, but Gemini is a park that had to install its own cables from the park to the connection at our distribution center.
TenneT manager Gineke van Dijk was responsible for ‘joining’ the wind park to the Dutch consumer market. This posed new challenges but thanks to an intensive collaboration, the energy from Gemini is now successfully connected with the TenneT high voltage grid.
The park is located quite far from the coast, creating a big technical challenge. That made close cooperation between TenneT and Gemini necessary and led to intense, enjoyable contacts.’


What is your ‘Gemini’ moment, what memory remains the most?

‘During an open day a delegation from TenneT revieved a tour on the grounds of Gemini and the installation ship of Van Oord, the Aeolus. It was a beautiful day in autumn, and it was a special moment to witness the work on the ship, which was preparing the placement of the monopiles and would go out to sea the next day. The scope of the project became very visible at this moment. It’s nice to contribute to such a challenging project as TenneT, even if our role in Gemini was smaller than it will be in future offshore wind projects.

TenneT taking power further

TenneT manages the high-voltage grid in the Netherlands and large parts of Germany. TenneT transmits electricity at 110,000 volts (110 kV) and higher. With over 22,000 kilometres of high-voltage lines, the company crosses borders and connects countries. For all future projects the government has appointed TenneT as the operator of the Dutch offshore grid, but Gemini was an early project, which had to install its own offshore grid.
As the offshore grid operator, TenneT can realize its ambition to facilitate the transition to a more renewable energy supply as laid out by the government. Until 2023, TenneT will develop offshore projects as defined in the National Energy Agreement. This is TenneT’s ‘Programme Offshore Grid’. TenneT is committed to an intensive stakeholder consultation process concerning the development of the concept and realization of the offshore grid infrastructure.



The Land High Voltage Station

The Land High Voltage Station (LHVS) is
firmly rooted in the Eemshaven soil.
More than 400 piles have gone into the
ground (a former swamp area) to create a
solid foundation. The primary function of
the LHVS is to house the transformers which convert the energy from the wind park
(230 kV) to 380 kV. This electricity can then
be transferred to the Dutch high-voltage
grid, operated by TenneT.