THE TECHNICAL CONCEPT
OFFSHORE CONVERTER PLATFORMS
The offshore grid integration systems connect offshore wind farms to the mainland. Direct current (DC) technology is used to transmit the power. This makes it possible to transport large volumes of energy over long distances with very low losses. However, the wind turbines themselves generate alternating current (AC). In order to convert this into direct current on site, each system requires what is known as a “converter”. This comprises various components such as transistors, diodes, capacitors and coils. Converters located in the North Sea have to withstand wind and weather, which is why they are housed inside a wind- and weather-proof platform. These converter platforms are also equipped with cranes and a helipad to facilitate maintenance and repair work.
The converter platforms are located far from the coast out on the high seas. However, due to their size, we construct them on land. A number of different methods can be used to install the platforms out at sea. How we proceed with the projects depends on the local conditions as the projects progress.
SUBMARINE AND UNDERGROUND CABLES
The electricity is transmitted from the offshore platforms to the coast by means of submarine cables. These are laid safely and in as environment-friendly a manner as possible in the seabed. Amprion employs various tried and tested laying techniques. Particularly when working in ecologically sensitive areas such as the Wadden Sea, we take special care to ensure we apply environment-friendly installation methods.
To cross under the East Frisian islands and the dyke on the North Sea coast, we use the horizontal directional drilling (HDD) method. This method enables us to realise longer crossings without having to remove the soil. Experts speak of a “closed” construction method. The only points at which we require space for setting up construction site equipment is at the start and destination points of such a route. The disadvantages of closed methods include the higher costs involved with such special solutions and the limited range.
On the mainland, we will lay underground cables from the dyke to the respective grid interconnection points. Building underground cable connections that are as cost-effective and soil-friendly as can be is a challenge. Amprion is collaborating with research institutes, industry experts and trade associations to overcome this challenge. Today, we have the expertise to lay underground cables using a number of different construction methods. Both closed and open construction methods are possible. In the open construction method, the soil is excavated layer by layer, with each layer being deposited separately at the side of the trench and later backfilled in reverse order. Whether an open or closed construction method is used depends, among other things, on the respective soil and groundwater conditions, as well as the landscape – for example, if rivers or motorways have to be crossed.
ONSHORE CONVERTER STATIONS
The transmission network in Germany predominantly uses alternating current technology. This is why we need another converter station near each of the legally defined grid interconnection points to convert the incoming direct current to alternating current. The electricity transmitted can then be fed into the 380-kilovolt alternating current grid by means of an alternating current line and the respective substation. From there, the electricity reaches the electricity customers via the transmission and distribution system. The core element of a converter station is the converter technology. Because these power electronics need to be protected from wind and weather, they are installed indoors.
You can find more information about converters here.
