Facts: Offshore connection to the grid

The challenge

Connecting offshore wind farms to the grid poses a new challenge to turbine and grid operators. In the initial phase, the still quite limited capacity of early pilot farms will probably make it relatively easy and inexpensive to solve the problem by using a conventional three-phase AC connection to the onshore grid system.

Greater capacities and distances from shore (beyond 50 kilometres) are factors that make it technically difficult to connect high-voltage three-phase current (Reactive Power) to the mainland grid. Therefore, subsequent phases will probably need to provide for high-voltage direct current (HVDC) transmission to land. However, converting alternating current to direct current at the wind farm and reconverting direct current to alternating current on land is technically more complicated and expensive than using a three-phase current connection.

Reducing costs for high-voltage direct current transmission
But the increasing density of power grids in Europe is presently contributing to greater advancement in HVDC technology, especially as a result of linking the Scandinavian, British and Irish grid networks to the European continent. So it appears that this technology may become available more economic in the future.

Grid connection system needed for expansion

At the moment there is no comprehensive plan for connecting offshore wind farms to the German grid. The Wadden Sea, a sensitive ecological system which lies between the area planned for offshore wind farms in the North Sea and the coast, will make it necessary to bundle cables to reduce impact to the environment. Laying many parallel cables through the Wadden Sea to the coast, as applications now still envisage, would lead to serious ecological impact of the national park.

If pilot projects show the technical and economic viability of offshore wind farms and their compatibility with nature conservation, then it also makes sense economically and from a grid operation perspective, to set up a bundled offshore cable system to support the expansion of offshore wind energy.

Grid capacity on land

Furthermore, it will be necessary to expand the high-voltage grid in coastal regions and probably the whole grid network in northern Germany. After all, wind energy must be harnessed and transported to consumers. A comprehensive plan is needed that takes into account the potential of an expanding offshore wind industry. On the one hand, this means accounting for the available capacity in the existing high-voltage grid, especially in coastal regions where the grid is very wide-meshed. On the other hand, the foreseeable strong growth of wind power on land must be incorporated into these plans, as it will reduce the capacity available for transporting offshore wind energy.

In single cases, bottlenecks could arise in grid capacity, probably starting in 2003 or 2004. To make sure first offshore pilot projects have the chance to get started in the near future, some of them must receive licensing before that time so that they can finalise feed-in contracts. The question of grid capacity is crucial to the rapid realisation of pilot projects and thus to the continuing development of offshore wind energy. The expansion of grid capacity on land requires very long periods of time for planning, licensing and construction.