The Land High Voltage Station

THE BUILDING AND INSTALLATION OF

The Jackets | Offshore High Voltage Station

The Cables

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

2 - 12

←

→

THE BUILDING OF

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.

A personal view

TenneT

The LHVS

TenneT manages the high-voltage grid in the Netherlands and in 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 TenneT, Gemini was a new experience. Senior manager Gineke van Dijk explains.

In the development of the Gemini Wind Park, a great deal of attention was paid to the offshore construction of the wind park. But Dennis Froeling knows better than most that a great deal of work also had to be done ‘on land’. He is the Package Manager for the Onshore Substation. In other words, he makes sure that the electricity arrives on land safely and can then flow into the Dutch power grid. An introduction.

Take a closer look at the LHVS blueprint. Obviously, the land station provides a sturdy foundation for the transformers which transfer energy from the wind park to the electricity grid. A great deal of attention is paid to continuous reliability.

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

9 - 12

←

→

THE BUILDING AND INSTALLATION OF

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.

Paint!

Every building requires maintenance. On land it is usually not a problem; just erect scaffolding and get to work. At sea, it’s not that easy. And above all, it’s very expensive. That’s why the paintwork at Gemini Wind Park has been given special attention. QA/QC coordinator Folkert Tazelaar explains why.

Working safely

Getting to work is important for everyone but especially for those who work 85 km off the Dutch coast, in the inhospitable environment of the North Sea. How to get to work safely and stay safe whilst operating on the Gemini OHVS and turbines?

Onshore in Antwerp

The two OHVS for the Gemini Wind Park are the centre of the offshore operation. They were assembled in the Hoboken wharf area of Antwerp and are several storeys high, the size of an average apartment building. Van Oord outsourced the construction of the offshore stations to the FICG consortium, a partnership between Cofely Fabricom, Iemants and CG. Fabricom was the lead manager of this consortium.

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

3 - 12

←

→

THE BUILDING AND INSTALLATION OF

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.

The cables

The infield cables

Shallow water cable

690 > 380

Depending on the location of a wind turbine in the wind park, the infield cables vary. The more energy the cables must transport, the larger their dimensions. After the infield cable had been laid onto the seabed, a remotely operated vehicle (ROV) was deployed to initiate the burying of the cable.

When you insert a plug in a socket, you probably know that 230 volts will come out. But how do those volts get there? From the North Sea, the renewable energy generated by Gemini Wind Park travels a long way. It becomes increasingly stronger, until it ultimately decreases so that it can be used to power the appliances in our homes.

An explanation.

Every structure has a ‘crucial detail’. At the Gemini Wind Park, that might be the offshore cables which connect the park to the land. A total of 208(!) km of cable, each with a diameter of approximately 30 cm.

The export cables were carefully laid deep within the ‘Wad’, or Wadden Sea. The cables were installed at a depth that keeps them completely out of harm’s way.