Grid connection OST-6-1

The cable route for the OST-6-1 grid connection runs around 53 km from the eastern offshore substation via the western offshore substation to the planned landing point on the territory of the municipality of Dierhagen. From there on, the onshore route of around 35 km in length leads up to the new substation to be constructed in Gnewitz.

Offshore route

Between the offshore substations and the landing point in Dierhagen, a line route was chosen that minimises the impact on nature and the environment. That is why the length of the route is as short as possible and is bundled with existing and planned lines to the greatest possible extent. Sensitive nature reserves and protected biotopes, such as the Darßer Schwelle Site of Community Importance (SCI), are bypassed where possible. Where a bypass is not possible, the impact is limited to the extent required.

Landing area

The landing area is planned in the municipal territory of Dierhagen. As the existing corridor that was approved in the spatial planning for this area is already completely occupied by other cable projects, a different landing area is needed for OST-6-1. The study to find a suitable landing area must take into account both the technical feasibility and the minimisation of potential effects on people and the environment.

Onshore route

From the planned landing area of the submarine cable in the municipal territory of Dierhagen, the electricity generated at sea will be transported to the new substation to be constructed by means of a 35-km underground cable. On the land side, three 220-kV AC underground cable systems will be laid within an already approved cable corridor of around 12-15 km. To meet the expected development needs in the Baltic Sea, the construction of an empty or protective conduit system along the route is also planned.

The underground cable route, which is still being planned, should follow as straight a line as possible and is subject to a number of spatial planning, environmental and other criteria. Intersections with other lines, buildings, residential areas, streets and gas mains shall for instance be avoided as much as possible. Nature conservation areas and forests shall also be circumvented.

50Hertz is legally obliged to connect the wind farms in the Baltic Sea and manage the overall electricity system throughout the federal states of Berlin, Brandenburg, Hamburg, Mecklenburg-Western Pomerania, Saxony, Saxony-Anhalt and Thuringia. The establishment of the grid connection for projects in territorial waters follows § 17d (6) of the Energy Industry Act (Energiewirtschaftsgesetz, EnWG).

The OST-6-1 grid connection is subdivided into four primary permit sections.

Permit for offshore substations, offshore and onshore route

The offshore substations, the section in the territorial waters and the onshore cable route were approved in three separate planning approval procedures. The responsible authority was the Ministry of Economic Affairs, Infrastructure, Tourism and Labour of Mecklenburg-Western Pomerania.

The planning approval procedure is a comprehensive and detailed permit review that, among other things, also grants the building permit for the project, in this case including the spatial planning review. Simply speaking, the planning approval procedure mainly deals with the “where” and “how” (specifying the location, structures, technical execution) of the construction project. During the permit procedure, the relevant authorities and public-interest organisations, environmental associations as well as the public are formally involved in accordance with legal requirements. Moreover, 50Hertz ensured early public participation, also referred to as citizen dialogue, even before the start of the planning approval procedure (see also More transparency and public participation).

Substation permit

The 220-kilovolt (kV) alternating current cables are connected to the German extra high voltage transmission grid in a substation, the so-called point of common coupling. The necessary transformation of the voltage level from 220 kV to 380 kV takes place in this substation.

The Gnewitz substation was approved by the Rostock district under the Federal Immission Control Act (Bundes-Immissionsschutzgesetz, BImSchG) and forms the end point of the OST-6-1 grid connection as well as the connection to the existing grid, which is furthermore being upgraded as part of the grid reinforcement of the Rostock region. You can find more information about this here (German only).

50Hertz is laying submarine cables between the offshore substations and the landing point in Dierhagen. From there on, onshore cables lead up to Gnewitz.

Offshore cable laying

During offshore cable laying, the water depth and the condition of the seabed are the main factors determining which technology is used. The seabed of the Baltic Sea was shaped by the ice age and has different characteristics from region to region. In many places, it consists of soft materials such as silt, sand and peat, in other places of hard materials such as clay, marl and stony soil. In case of soft soil conditions, the submarine cable is jetted or ploughed into the seabed. These methods are particularly gentle from the perspective of environmental protection and nature conservation. If the seabed is very hard, stony or very peaty, other laying tools such as cutters or excavators need to be used. Following the laying, the cable is once more covered with soil from the seabed.

Landing area

To protect the sensitive coastal area, it is undercut using the closed construction method of horizontal directional drilling (HDD). As part of this process, around 1.2 km of underground channels are drilled between an offshore construction site and an onshore construction site at a depth of up to 20 m. The cable conduits are then pulled into these channels, followed by the submarine cables at a later stage. During closed construction, the cables are laid underground without the need to excavate the soil.

Onshore cable laying

From the landing point on the Dierhagen municipal territory, the cable route runs to the Gnewitz substation over a distance of around 35 km in length. For the main part of the route, cables will be laid using the open construction method. This involves digging cable trenches, laying cable conduits and backfilling. The cables are pulled in at individual points during a later step. The original soil is reused if possible and restored in separate layers in the order in which they were originally found.

To minimise the impact of the construction measures, roads, railways or conservation areas are crossed using the closed construction method. This involves horizontal directional drilling and microtunnelling.