A GLIMPSE INTO THE CABLE

The power is safely packaged

The Gemini cables have been applied before and therefore constitute ‘tried and tested’ technology. But what made the operation unique was its length. Three sections, 95, 10 en 103 km long in a ring; that was never done before. Not with this sort of cable, carrying 220,000 volts. The energy generated by the turbines runs through three conductors, which are surrounded by a plastic layer to insulate the 220,000 Volts from the earth.
Rubber filling is used in order to create a fine round whole of the three cores that together make up a triangular cable. This filling also keeps the fibre optic cable, which connects the stations and the turbines, in place. All data on the functioning of the wind park is transmitted along this cable. This cable, too, is encased in order to protect it from external influences. The outside of the cable is protected with steel armouring to shield it from fishing nets or pieces of rock that might fall onto the cable. A lot off focus was put on laying the cables in the ‘Waddensee’ (Unesco Heritage) and in the seabed.

 

DIGGING THE TRENCHES

The Nexus – built specifically for Gemini – transported 5,000 tonnes (up to 40 kilometres) of electrical cable on a massive carousel. The cable was then carefully unreeled and laid on the seabed with the laying crew on board controlling the cable slack required to manage seabed unevenness and fluctuations.

The infield cables

During the operation, the Jan Steen assisted with the special machine attached to the bow of the ship that dug the trenches in the sea bed into which the infield cables were laid.

 

WORKING IN ‘THE WADDEN SEA’

Shallow water cable lay

Another complication was that the cable had to go through the Wadden Sea area, a nature reserve with international UNESCO status. These natural considerations were very important. But the geological situation also had a role to play: the seabed in that area is continually on the move – it’s a dynamic system. Therefore, the cables had to be laid at a safe depth.

 

The location for Gemini Wind Park ended up being quite far out to sea, between a shipping route and a military exercise area. That meant the turbines were out of sight of the beach on the island of Schiermonnikoog; if visible, that would no doubt have attracted a great deal of public protest. An added advantage was that the wind is much stronger here. On the other hand, it meant that the export cables to Eemshaven had to be very long.

 

FROM 690 VOLTS TO 380 KILOVOLTS

How the electricity travels

Spools and condensers are used to minimise electricity loss during transport. Gemini uses smart techniques to get the 600 megawatts that are generated in the wind park onto the grid. That has to be built up gradually. For delivery reliability, it helps that Gemini Wind Park has configured a number of facilities in duplicate: two OHVSs, each with two 180-megawatt transformers and two export cables.
The electricity that is generated in the Gemini Wind Park has a long way to travel and goes through different processes along the way. The turbine generates electricity at 690 volts. The transformer in the turbine converts it to 33 kilovolts on the spot. Then there are seven to eight windmills in a loop with each other: the combined electricity goes to the OHVS, where the transformer converts it to 230 kilovolts. Then the electricity travels along the 85-kilometre cable that comes ashore in Eemshaven. There, we have a land station full of transformers, where the electricity is converted to 380 kilovolts so that it can be transported over longer distances.
And a few kilometres further on, Gemini has reserved a connection point with Tennet, the grid operator. After which it enters the homes – in reverse order– as 230 volts.

The journey of the Gemini-wind power depicted in one small scheme. Starting from the single turbine and finally arriving at the Dutch powergrid – and from there on onwards to the thousands of households that receive this clean and renewable energy.

Voltage

Voltage

Time

 

DC Voltage

 

AC Voltage