The American Wind Energy Association (AWEA), in a document on recommended practices for offshore wind, described transmission grid aspects to support the transfer of offshore power.
AWEA recently released recommended practices for using existing standards to plan, design, build and operate offshore wind facilities in compliance with federal and state regulations.
AWEA also said Oct. 11 that the recommended practices developed by its standards development board, along with the U.S. Department of Energy and the National Renewable Energy Laboratory (NREL), address structural reliability; manufacturing, qualification testing, installation and construction; equipment safety; operation and inspection; and decommissioning.
According to the document, AWEA and NREL began the process in October 2009 in light of industry and regulatory concerns that no set of guidelines and standards could be identified that addressed the complete design, deployment and operation of offshore wind turbines.
In discussing manufacturing and fabrication, the document noted that in an offshore wind facility, there are typically two categories of submarine cables: array cables and export cables. The total power from the facility is conducted from the electric service platform through export cables that can be routed directly to an onshore grid or to an offshore high-voltage direct current (HVDC) transmission grid.
An offshore electrical transmission grid is an extension of a land-based grid system that consists of an HVDC submarine cable system running parallel to the coastline adjacent to wind energy areas where offshore wind facilities are either located or proposed, the document added. Such an HVDC grid extension will generally consist of a series of offshore hubs and at each hub location, an offshore foundation supports an HVDC converter.
Ties to the onshore part of the grid consist of lateral HVDC submarine cable links routed to shore from the converter hub to interconnect to strategic locations in the onshore grid, the document added, noting that when using the offshore grid extended system to transfer offshore wind generation to shore, the HVAC export cable(s) from the wind facility’s electric service platform(s) would be routed to the hub closest to that electric service platform location.
The converter, which is part of the grid, converts the AC power to DC and the power is dispatched to where it is needed on the rest of the grid. HVDC converters are installed next to the onshore part of the grid where the power is again converted back to AC for end-user consumption on shore. This approach, the document added, allows for the networking of individual offshore wind facility production, which has the potential for improved overall system use.
Among other things, the document noted that cable installation associated with an offshore wind facility can be split into two groups: one for the inter-array cables, which connect the individual wind turbines in strings with the site electric service platform, and another for export cables, which run from the electric service platform and connect the offshore wind facility into the terrestrial grid.
Depending on the total output power of the wind farm facility, HVAC export cable systems need either a single three-core cable or multiple three-core cables for power transmission, which allows for the option of the cables to be installed and buried in a single operation using a jetting plow or injector, according to the document.
While it is more cost effective to simultaneously lay and bury the export cables, some cable designs cannot be buried by plow and the design of the cable needs to be considered when selecting the cable and installation method, the document said.
There are no offshore wind farms operating in the United States, but several proposals are underway, including by Deepwater Wind in Rhode Island and Energy Management’s Cape Wind Associates in Massachusetts. Additionally, the Atlantic Wind Connection, led by Trans-Elect Development Company and involving Google, Good Energies, Marubeni Corporation and Elia, is proposed as a high voltage, direct current subsea backbone transmission system that will be built over about 10 years, 12 miles to 15 miles off the coasts of New York, New Jersey, Delaware, Maryland and Virginia.