The Unique Challenge of Developing Transmission for Offshore Wind

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Spurred by interest from states and developers, PJM and its stakeholders have begun to look at new and creative ways to connect offshore wind to the PJM system, undertaking the study of new processes to interconnect offshore transmission networks.

Offshore wind projects typically consist of offshore wind turbines, a subsea cable system, transition stations and high-voltage transmission lines to deliver wind power to the main grid. The potential buildout of offshore wind development may drive the need for flexibility to connect the offshore transmission facilities together in an “ocean grid.”

With typical generation interconnection projects, the generation developer bears the cost and responsibility to develop and construct its facility as well as the transmission needed to connect it to the grid. The developer/owner would then be eligible for interconnection rights for that project.

However, with the potential for more offshore wind projects, PJM and stakeholders are considering how PJM might study and integrate offshore transmission projects that would be developed separately from a generation project, in order to interconnect future ocean-based wind projects.

In other words, developers could be interested in building the underlying infrastructure to allow offshore wind projects to connect more easily, rather than requiring each generation developer to build its own transmission line for each new offshore wind project.

The Challenges of Connecting Offshore Wind

Relatively new to the U.S., offshore wind has been an energy source in Europe for close to 30 years. The first such project was installed off the coast of Denmark in 1991. 

Offshore wind power presents a unique set of technical, economic and regulatory challenges for planning and operations.

The U.S. federal government maintains jurisdiction over offshore wind resources on the Atlantic Outer Continental Shelf, which begins three miles from shore.

The Bureau of Ocean Energy Management (BOEM), which manages the responsible exploration and development of offshore energy and marine mineral resources on the U.S. Outer Continental Shelf, has developed a number of national and regional guidelines for renewable energy activities within its jurisdiction. These documents provide lessees, operators and developers with additional information on regulations, standards and regional requirements. The BOEM also has a Citizen’s Guide, detailing the nuts and bolts for the layperson.

Another consideration for potential wind farm placement is distance. A previous proposal in PJM failed, in part, because it was sited less than less than three miles off shore. Both fishermen and environmentalists expressed concerns about bird migratory patterns and boat safety that close to shore.

Source: Bureau of Ocean Energy Management
Source: Bureau of Ocean Energy Management

The Turbines

According to the BOEM, wind turbine generators are modified from onshore turbines to prevent corrosion and control climate. An offshore wind turbine foundation must be designed to withstand the harsh ocean environment, including high seas and hurricane-force winds.

Engineers consider site-specific conditions – such as the seabed and water depth – in planning offshore wind turbines. In addition, they build in servicing mechanisms and lightning protection systems.

Commercial-scale offshore wind facilities are similar to land-based wind facilities in their general set-up – tower, turbine and blades. Among the differences from land-based wind are larger turbines, bigger wind farms and the longer distance from the power system.

Most offshore wind facilities start with a substructure called the monopole – a large steel tube with a diameter of up to 20 feet. The piles are driven into the seabed at depths as much as 100 feet below the mud line to ensure the structure is stable.

To link to the power grid onshore, turbines connect to an electric service platform – similar to an oil rig platform – which serves as a collection point for the electricity and as a substation. High-voltage cables from the platform transmit the power to an onshore substation, where the power is then integrated into the grid.

The cables are typically buried under the seabed. This shields them from potential damage from fishing boats or the marine environment.