Offshore Wind Needs Major HVDC Transmission Expansion
A federal plan sets near- and mid-term goals for expanding transmission infrastructure along the Atlantic coast, including an interregional network of HVDC interlinks to bring offshore wind energy to shore.
The U.S. Department of Energy (DOE) and Bureau of Ocean Energy Management (BOEM) unveiled a 105-page transmission expansion plan detailing strategies to prepare the grid for increased offshore wind deployment in the Atlantic Ocean.
Offshore wind farm. Image used courtesy of Pexels (By Tom Swinnen)
In the near term, before 2025, the plan will start with updating transmission planning, siting, and permitting processes, identifying point of interconnection (POI) capacities and landfall locations, and establishing new equipment standards. From mid-decade through 2030, the agencies will work with states to formulate a regional offshore wind transmission network, standardize high-voltage direct current (HVDC) technology requirements, and prioritize routes along the outer continental shelf.
Then, from 2030 to 2040, the plan aims to develop a multi-terminal (MT) HVDC testing and certification center to ensure multiple substations can be connected to the offshore grid network. It will also establish minimum interregional transfer capacities and assign offshore substations and cables as shared infrastructure.
Interregional Meshed-Connection Transmission
Interconnection reliability is increasingly important amid the ongoing ramp-up in offshore wind project announcements and development activities in the Atlantic. A regional interconnection network would ensure that high volumes of new capacity can operate smoothly without causing increased strain on the grid.
The DOE and BOEM’s proposed offshore wind topology, which includes four interregional HVDC interlinks, reflects a hypothetical transmission build-out by 2050. Image used courtesy of DOE (Page 2, Figure 1)
While radial generator lead lines and shared lines are needed to deploy offshore wind in the Atlantic, the BOEM and DOE are pushing for additional reliability studies on four interregional HVDC interlinks aiming to minimize production costs and cable distances.
HVDC transmission cables, designed to send an electrical current flow in one direction, are the most efficient method to carry power over long distances from plants 35 miles or more from the coast. They lose less power than alternating current (AC) lines traditionally used in the grid, where the voltage and direction of the current change continuously.
According to a technical report from the European Commission, a 1,000- to 2,000-kilometer AC transmission cable with 3 gigawatts (GW) of power and a voltage of 800 kilovolts produces 6.7-10% losses, while a 6.4-GW DC cable with the same length and voltage loses 3.5-5%. With reduced capacitive losses compared to HVAC, HVDC cables can operate well underground and underwater, making them ideal for integrating wind resources.
However, HVDC systems require converter stations that are more costly and complicated than conventional substations. The DOE and BOEM proposed an interregional meshed-connection topology that’s configured to leverage points where congestion could be eased while avoiding co-use conflict areas. Many near-term offshore wind projects will use radial designs, representing an opportunity for an interregional set of MT-HVDC interlinks and converter stations to transfer power between states. MT-HVDC systems have more than two transmission lines linked together, sending power between multiple terminals.
Massachusetts’ 800-megawatt Vineyard Wind 1 project installed its offshore substation in mid-2023. The site’s 62 turbine generators are expected to come online by the end of the year, making it the first utility-scale offshore wind farm in the U.S. Image used courtesy of Avangrid
For example, one of the four proposed interlinks would establish a connection between Massachusetts, New York, and New Jersey, based on an analysis of interconnections at existing substations near Boston, Long Island, and New Brunswick. Another would link Maryland and North Carolina, informed by substation evaluations in Lusby and Greenville.
More details are forthcoming, as the National Renewable Energy Laboratory is currently wrapping up its two-year study to quantify the economic, reliability, and resilience impacts of multiple transmission scenarios. The study, expected to be completed by the end of this year, will also evaluate various transmission topologies (a meshed network, radial lines, or backbones), including the benefits of a coordinated transmission system compared to radial generator lead lines linking each offshore plant to shore.
Barriers to Offshore Transmission Development
A significant transmission infrastructure expansion is required for large-scale development along the Atlantic coast. Onshore systems will require upgrades to transfer large amounts of energy to load centers from the coast. Otherwise, wind farms more than 10 miles from shore may need to deliver power farther inland to access high-voltage transmission infrastructure.
Components of an offshore wind transmission system (top) and an HVDC transmission system (bottom). Image used courtesy of DOE (Page 10, Figures 5 and 6)
Workforce gaps are another critical concern, as the industry faces a shortage of experienced transmission engineers, manufacturing workers, technicians, and material transport drivers. The BOEM and DOE recommend expanding training programs and targeting new areas of the workforce, such as workers transitioning from fossil fuel industries. Universities and other educational institutions could also introduce new courses focused on HVDC transmission and offshore power systems.
The plan also cited component shortages for transmission equipment. Large power transformers face a limited supply of raw materials for grain-oriented electrical steel, while HVDC system shortages include DC switchgear and filters, cables, and converters.
Permitting issues and insufficient port infrastructure and installation vessels present additional challenges. The plan aims to reduce complexities in the transmission siting process with studies evaluating potential POI and routes to avoid multi-use conflicts and maximize offshore infrastructure’s throughput capacity.
In the longer term, current interconnection practices will need to be overhauled so that cable capacity and landfall sites minimize environmental impacts and consider future system requirements.
Meeting Demand for Wind Power
These plans anticipate a significant scale-up from today’s current offshore wind capacity. The federal government aims to deploy 30 GW by 2030, then 110 GW by 2050. Only two projects are operational today, totaling 42 megawatts (MW). However, state clean energy incentives have spurred a pipeline exceeding 40 GW, including sites along the Atlantic coast. Another 932 MW is under construction, according to the DOE. The BOEM has approved four commercial-scale offshore wind projects and plans to review at least 16 more by 2025, totaling over 27 GW.
Since 2021, companies have announced nearly two dozen offshore wind shipbuilding projects and over $3 billion in investments for 13 ports and 12 manufacturing facilities—all supporting increased demand across the supply chain. Two major developments are nearing completion today: The 800-MW Vineyard Wind 1 project near Massachusetts (serving over 400,000 homes) and New York’s 130-MW South Fork Wind (about 70,000 homes) are set to come online by the end of 2023 as the country’s first official utility-scale offshore wind projects.