Every system needs a proper backbone. For Gemini Wind Park, this is the grid between the Land High Voltage Station and the foundations of the turbines, placed in the North Sea. In between is an invisible but very important component: the cables. They transport the energy produced to land.

The Land High Voltage Station



The onshore part of the Gemini Wind Park operation may not look as spectacular as the offshore side but it is equally important to the proper functioning of the wind park. The Land High Voltage Station (LHVS) was built in the Eemshaven seaport in the northern part of the Netherlands. A station such as this had never before been built, so there was no example to be simply copied.
From the North Sea, the renewable energy generated in Gemini Wind Park must travel a long way. It becomes increasingly stronger as it approaches the shore, where its strength is decreased so that the energy can be used to power the appliances in people’s homes. Each turbine generates 690 volts of electricity.
The transformer in the turbine directly converts this electricity to 33 kilovolts. There are also seven to eight windmills in a loop with each other: their combined electricity goes to the Offshore High Voltage Station (OHVS), where a transformer converts it to 230 kilovolts. The electricity generated then travels along the 85-kilometre cable which comes ashore in the seaport of Eemshaven. The land station, filled with transformers, converts the electricity from the export cables into a higher voltage (380 kV) and connects the cables to the TenneT high-voltage grid so that electricity can be transported over longer distances.
The foundations for the OHVS consist of ‘jackets’ (tubular profiles measuring 1.4 m in diameter and 58 m long) with intervening links and bracings. The jacket topsides consist in turn of four levels, which serve as a housing for the transformers. Besides the high-voltage section, there are rooms for HVAC (cooling, heating), SCADA (control from the shore), battery storage and a backup diesel generator.

The Jackets | Offshore High Voltage Station



The jackets and the two Offshore High Voltage Stations form the heart of the Gemini operation in the North Sea. They were built with great care and then shipped out to sea. Great attention was paid to the quality and finish, to make sure the constructions will survive the harsh conditions.


In August 2015, Van Oord successfully installed the two Offshore High Voltage Stations (OHVS). The OHVS installations were built in Hoboken, Belgium, and then transported to Eemshaven seaport before their final destination in the North Sea. The main purpose of the OHVS is to house the electrical high-voltage and medium-voltage components which transform power supplied by the wind turbines in order to export the power to the onshore grid. The OHVS platforms convert the energy generated by the turbines into 230 kV of electricity.
The cable-laying process was divided into three main stages. First, laying the shallow water cable was performed between the Eemshaven seaport and the North Sea. Here, a great deal of focus was concentrated on laying the cables in a highly sensitive and unique area: the Wadden Sea (a UNESCO World Heritage Site). The tides make this a very dynamic area, so the cables had to be laid deep enough. After this area near land, the longest stretch was out into the wind park. Finally, the infield cables within the park were laid and connected to the turbines. Each part of this entire process demanded specialised technology and knowledge, provided by main contractor Van Oord.

The Cables | Cable laying



The type of cables used in the Gemini Wind Park have been applied in previous projects and are therefore regarded as ‘tried and tested’ technology. But what made the operation unique was the length of the cables: three sections of 95 km, 10 km and 103 km long, laid in a ring. That had never been done previously – not with this type of cable, capable of carrying 220,000 volts.