Offshore grid development

How does offshore wind power get into the transmission grid?

The wind blows consistently and strongly at sea. As a result, offshore wind farms generate almost twice as much energy as comparable onshore plants due to significantly more full-load hours. The transmission system operators must feed the wind power generated offshore into the onshore electricity transmission grid. In the North Sea, primarily direct current (DC) technology is used for this. In the Baltic Sea, only alternating current (AC) has been used so far due to the different conditions, the existing route corridors and the shorter distances to the coast, but direct current technology will also be introduced here in the future. 

AC grid connection systems

Grid connection systems operating alternating current are installed between the transformer platform of the offshore wind farm and the onshore substation of the TSO. The electricity generated by the offshore wind farm is first combined on the transformer platform and raised in voltage. From here, the generated offshore wind power is “picked up” by the transmission system operator and transported to the onshore substation, where it is fed into the transmission grid. With this transmission technology, there is no conversion of the electricity from AC to DC or vice versa. Converter platforms or converter stations are therefore not needed. All wind farms in the Baltic Sea are connected directly to the AC grid because the distances between wind farms and the connection points on land are shorter than in the North Sea.

DC grid connection systems

In existing DC connection systems, offshore wind farms have so far been connected via 155-kV connection concept. The wind farms are bundled together into offshore wind farm regions. Each wind farm in this region has its own transformer platform, which is in turn connected to a central converter platform of the responsible TSO. The converter platform is also located offshore and converts the alternating current to direct current. A direct current cable runs from here to shore, where another converter station is installed. Here the direct current is converted back to alternating current and fed into the onshore transmission grid. A direct connection concept is in planning for future grid connection systems in which the cable lines of the offshore wind farm will be connected directly to the offshore converter platform of the TSO. This offers the advantage of reducing the technical and physical complexity in addition to cost benefits. If the distance from the wind farm to the converter platform is larger, the 155-kV connection concept will continue to be used.

The interaction of the Site Development Plan (SDP) and the Network Development Plan Electricity (NDP)

The decisions previously made in the Offshore Network Development Plan Electricity (O-NDP) are currently represented in the Network Development Plan Electricity (NDP) and in the Site Development Plan (SDP). In this way, the NDP and SDP form an integral and interwoven planning system.

The SDP addresses the geographic planning and timing for the wind power regions and power lines in North Sea and the Baltic Sea. Among other aspects, it defines the areas involved and the capacity to be installed. It also specifies the calendar year in which the planned offshore wind power plant and the corresponding offshore grid connection systems are to be commissioned. 

The NDP describes solutions for transporting the wind power through the North Sea and Baltic Sea onto shore. For example, specific grid connection points (GCPs) are identified for the offshore grid connections. In addition, the NDP identifies further planning options such as technological concepts, expansion scenarios or alternative grid connection points for the offshore connection.

Basis

For depicting the offshore grid development in the NDP 2037/2045 (2023), the TSOs took into account the Site Development Plan (SDP) 2023, for which the updating process was completed on 20 January 2023. However, the SDP 2023 lacks geographical and temporal specifications necessary to achieve the statutory expansion targets for offshore wind power for the years 2035 and 2045.

Considering the fact that offshore grid connection systems (OGCSs) take a long time to be realised, the planning horizon of the SDP 2023 at this time is insufficient for ensuring secure long-term planning for the implementation of OGCSs based on the NDP. In coordination with the Federal Network Agency, the present second draft of the NDP 2037/2045 (2023) is therefore largely based on the draft of the SDP from 1 July 2022. This applies in particular for the geographic planning of sites and border corridors as well as the planned completion dates for OGCSs after 2031/2032.

The SDP 2023 defines locations and timing for the connection of approximately 36.5 GW of installed generation capacity for offshore wind in the North Sea and Baltic Sea. Of this, 34.5 GW of generation capacity is planned by the end of 2031. Therefore, the TSOs have identified the need for additional OGCSs in the NDP to achieve at least 40 GW by 2035 and at least 70 GW by 2045 in accordance with Section 1 (2) of the Offshore Wind Power Act (WindSeeG). These requirements of the WindSeeG are intended to achieve the target of the German Climate Protection Act.

The approved Scenario Framework of the Federal Network Agency from 8 July 2022 assumes an installed generation capacity for offshore wind of 58.5 GW in the Baltic Sea and North Sea. Based on an installed generation capacity of about 30.5 GW in 2030 according to SDP, two offshore grid connection systems with a capacity of 2 GW each will be commissioned annually from 2031. Scenario A 2037 assumes an installed generation capacity for offshore wind of 50.5 GW. Compared with scenarios B 2037 and C 2037, this assumes a slower expansion of offshore wind power beyond 2030. In the year 2045, all scenarios assume an installed generating capacity for offshore wind of exactly 70 GW.

Installed generation capacity for offshore wind based on the approved Scenario Framework

* The installed generating capacity of the test field is not included as per the approved Scenario Framework from 8 July 2022

Source: Federal Network Agency – Approval of the Scenario Framework 2023 – 2037/2045

Expansion and investment volume for offshore grid connections

The OGCSs in the present offshore grid and the initial offshore grid are considered as given in the creation of the NDP, without re-examining their necessity. In the North Sea and Baltic Sea, the present offshore grid has a transmission capacity of 7,528 MW and the initial offshore grid 6,890 MW. The lengths of the routes amount to 2,038 km for the present offshore grid and 1,580 km for the initial offshore grid.

For the offshore expansion grid in the North Sea and Baltic sea, scenario A 2037 expects a length of 6,600 km with a transmission capacity of roughly 36 GW compared with roughly 9,300 km in scenarios B 2037 and C 2037 with a transmission capacity of about 44 GW. For long-term scenarios A 2045, B 2045 and C 2045, a transmission capacity of about 60 GW results in grid connection systems with a length of 13,300 km (excluding the initial grid). Even after 2037, eight more systems with a transmission capacity of 16 GW will be connected.

The NDP 2037/2045 (2023) was the first to examine national offshore interconnection and its benefits presented, as well as identify a project with two measures. Overall, national offshore interconnection is shown to be a grid-supporting and cost-efficient expansion measure to minimise long-range grid congestion.

The investments for the offshore grid connection systems in the offshore expansion grid are based on specific cost rates and are of a preliminary nature.

For scenario A 2037, the estimated investment volume for the offshore expansion grid amounts to around EUR 77 billion. Scenarios B 2037 and C 2037 require investments of about EUR 103.5 billion (including interconnection). For long-term scenarios A 2045, B 2045 and C 2045, the estimated investment volume is around EUR 145.1 billion (including interconnection).

Moreover, the investment volume for the offshore grid expansion measures of the initial offshore grid that are already being implemented amounts to around EUR 12.4 billion for all scenarios.

Based on the OGCSs confirmed by the Federal Network Agency in the NDP 2035 (2021), the additional need for 20 new OGCSs with a route length of approx. 8,455 km and an investment volume of EUR 86.7 billion was identified in this NDP 2037/2045 (2023).

Overview of the route lengths of the offshore expansion grid

* The procedure for determining the route length is explained at: www.netzentwicklungsplan.de/Zwu

Source: TSOs