Substations onshore/offshore

Substations form the connection between different voltage levels. To bring the electricity generated in the offshore wind farm to the consumer with as little loss of energy as possible, it is transmitted at various voltage levels. The submarine cable of 220 kilovolt (kV) alternating current is connected to the German extra-high-voltage transmission grid in a substation. The necessary transformation of the voltage level from 220 to 380 kV takes place in this substation.

Need and legal background

For the Ostwind 3 grid connection project, 50Hertz is planning to build a new substation between Stilow and Klein Ernsthof (both part of the municipality of Brünzow), where the current will be transformed from 220 kilovolts (kV) to 380 kV. As there is not enough space, the existing substation in the neighbouring municipality of Lubmin no longer offers the possibility to connect additional submarine cables. Furthermore, technical grid aspects play an important role here: the connection of an additional system to this site, which already has two offshore grid connections, would pose a larger risk of disturbances and system outages.

For the substation, the survey area in the municipalities Lubmin/Brünzow/Wusterhusen and Kemnitz was specified in Grid Development Plan 2030 (2019). Within this area, an extensive and careful examination of many different criteria will be performed to decide on the final location. The site should, for instance, be located outside of nature conservation areas and drinking water protection zones insofar as possible. Sufficient distance to residential and settlement areas shall also be taken into account. The traffic connection of the substation shall also be possible via suitable streets and rails. Moreover, the existing 50Hertz extra high-voltage grid, in this case a 380-kV overhead line, shall also be nearby, so that the substation can be connected to the 50Hertz grid.

Permitting

For the construction of the new substation in Stilow and the onshore cable route, 50Hertz is following a planning approval procedure (Planfeststellungsverfahren, PFV). The competent permitting authority is the Ministry of Economics, Infrastructure, Tourism and Labour of the state of Mecklenburg-Western Pomerania (WM M-V). A PFV is a detailed permitting check that does not concern the “whether” but the “how”, i.e. the specification of the site, the construction method and the technical execution of the substation.

Part of the PFV is the formal public participation that is laid down by law. The public agencies are involved in this process, such as the State Agency for Agriculture and Environment of Vorpommern (Staatliches Amt für Landwirtschaft und Umwelt, StaLU Vorpommern) As a rule, the PFV takes 12 to 24 months. The permitting process follows § 43 of the German energy industry act (Energiewirtschaftsgesetz, EnWG).

In the scope of the planning approval, the so-called tap point is also applied for. The tap point is the connection of the substation to the existing transmission grid of 50Hertz. This completes the integration of the wind farm into the German transmission grid.

In support of the formal public participation that is laid down by law, 50Hertz organises an informal public participation in the form of public dialogue. Citizen dialogue builds on 50Hertz's own experiences, not on legal stipulations. This difference is important, as the citizen dialogue ends where the formal public participation laid down by law begins (see also Transparency and public participation).

Construction method and technology

The purpose of a substation is to connect different voltage levels. In this case, the substation transforms the electricity from 220 kV to 380 kV and feeds it into the 50Hertz transmission grid.

The substation in Stilow will be built as an air-insulated switchgear. This means that normal air is used as the insulating medium between the live and de-energised parts within the installation. Such air-insulated switchgear require enough space to create sufficient distance and thus enough insulation between the technical equipment.

Accordingly, the larger part of the area on the premises of the substation in Stilow will accommodate the high-voltage equipment or the equipment that is an active part of the power line, the so-called primary technology.

Furthermore, there is a building in each substation in which the so-called secondary technology is situated. This technology functions as the brain of the primary technology. In other words, it protects and controls the grid connection system. The secondary technology acts as protection in the event of a fault and as a control system during normal operation, and is controlled from the 50Hertz Control Center.

Offshore wind farms generate high volumes of electricity, which then need to be transported to land, where they can continue on their way towards the consumer. On the high seas, substations (situated on platforms) are used to bundle the electricity generated in the wind farm and to transport it to shore by transforming it to a higher voltage level with as little loss of energy as possible.

Ostwind 3 is the third offshore project for the grid connection of the wind farm areas to the north-east of Rügen island (see also Ostwind 1 and Ostwind 2 ). Other than for previous 50Hertz grid connection projects, 50Hertz will not only plan, construct and operate the cable connection but also the offshore substation.

Approval

The electricity that is generated offshore by the future offshore wind farm is collected at the offshore substation and brought to the mainland by means of a 220-kV submarine cable.

For the first time, 50Hertz itself is planning, applying for and constructing an offshore substation as the project developer. The permit process is part of the planning approval procedure for the submarine cable in the German Exclusive Economic Zone (EEZ). The Federal Maritime and Hydrographic Agency (Bundesamt für Seeschifffahrt und Hydrographie, BSH) is competent for the permit. The BSH specifies the exact location of the offshore substation within area O-1.3. This area is also known as Windanker.

In principle, the planning approval decision contains a number of ancillary provisions to ensure that the construction and operation do not adversely affect the safety of the marine environment as well as ship and air traffic. In addition to the planning approval procedure, offshore substations in the German EEZ must also undergo a release process from the BSH.

Construction method and technology

The 50Hertz substation platform acts as the collection point for the electricity generated in the Windanker wind farm. Inside the wind farm, the wind turbines will be connected to 50Hertz’s offshore substation by means of 66-kilovolt (kV) submarine cables.

For the transmission to the point of common coupling on shore, the voltage of 66 kV from the Windanker wind farm will be transformed to 220 kV in the 50Hertz offshore substation. This is done by means of transformers and serves the purpose of transporting the electricity with a lower loss of energy. The 220-kV submarine cable will then establish the connection to the 50Hertz transmission grid on shore via the new substation to be built in Stilow in the municipality of Brünzow.

The 50Hertz offshore substation (OSS)

The 50Hertz offshore substation (OSS) consists of the so-called “topside” and a supporting structure or “jacket”. The topside is the structure above the waterline that accommodates technical installations such as transformers, switchgear and safety systems. The jacket carries the topside and is anchored into the seabed by means of long steel piles.

The electrical grid connection components are controlled by the Control Center (CC) in the 50Hertz grid. The operational exploitation of the offshore substation, such as maintenance measures or the monitoring of the firefighting or air-conditioning systems, is carried out by a control centre in the Offshore operating department of 50Hertz. Contrary to the offshore substations in the German North Sea, which at times are located hundreds of kilometres from the mainland, the 50Hertz offshore substation will be operated without staff because of its proximity to the coast. For medical emergencies, the 50Hertz offshore substation is also equipped with a helicopter landing pad.

The offshore substation, including the technical equipment, will weigh several thousand tonnes. It will be around 48 metres long and 24 metres wide. The platform will have a height of 36.5 metres above water level.

For the installation of the platform, the so-called heavy lift method is used. A floating crane ship first installs the jacked on the seabed. The jacket is anchored with extremely heavy steel piles. Later on, the topside will be lifted onto the jacket by the crane ship and connected to it.

Technology

The installations on an offshore substation are either primary, secondary or ancillary installations.

The primary technology essentially consists of the high-voltage conducting components, such as transformers, compensation coils, switchgear and the submarine cable.

The secondary technology on the other hand functions as the brain of the primary technology. In other words, it protects and controls the grid connection system. The secondary technology acts as protection in the event of a fault and as a control system during normal operation, and is controlled from the 50Hertz Control Center.

The ancillary installations concern installations that are necessary for the operation of the offshore substation. In addition to firefighting, air-conditioning and emergency power supply systems, this also includes safety systems, the helicopter flight deck as well as lighting systems, but also the pumps for the cooling systems.