Offshore Transmission

Introduction

Offshore wind generation is becoming a mainstream electricity generation source. In Europe, U.S., and Asia-Pacific regions there are ambitious goals for offshore wind. At present, in 2023, the total installed offshore wind capacity exceeds 50 GW. This is a significant amount of generation that is expected to grow rapidly across the world as countries look to diversify their energy mix and reduce carbon emissions.

For example, the U.S. alone has set a target of 30 GW of offshore wind generation by 2030 with several northeastern coastal states having already procured over 17 GW. With advancements in offshore wind generation technologies and larger turbines, the energy yield is expected to increase with decreasing costs. However, the cost reduction will be possible only if the transmission grid is accessible to offshore wind generation developers.

Transmission Grid: Present Day

Divya Kurthakoti

The transmission grids in most parts of the world are legacy lines and were not built to accommodate large-scale dispersed renewable generation including offshore wind generation. In most countries and regions, the coastal transmission infrastructure is either non-existent or sparse. Since many of the future offshore wind plants approach the size of nuclear power plants (GW scale), the need for high-voltage transmission is even more pronounced. In the U.S., offshore wind generation projects are connecting radially, and there is enormous competition to find the closest feasible transmission grid where a gigawatt-scale generation plant can be connected and operated. In a large-scale buildout, this approach leads to sub-optimal solutions for getting generation from multiple offshore wind plants to the consumers. These factors are driving several system operators, states, the federal government, and policy makers to reconsider the current approach of generation developers finding onshore interconnection points. Rather, there are several studies that have been conducted which show the benefits of an offshore transmission grid enabling a more robust and flexible interface between the wind plants and the onshore transmission system.

Future Outlook: Offshore Transmission

An ocean transmission grid as a backbone may become a reality in many parts of the world. Europe is ahead here and is developing massive High-Voltage DC (HVDC) transmission grids. A graphical representation of an offshore transmission grid is shown in Figure 1.

Figure 1: Offshore Transmission

Figure 2 shows a multi-terminal HVDC offshore transmission grid being designed and built in Europe. Offshore transmission is becoming a reality with Europe leading in development and governments like the U.S. enabling growth through policies, incentives, etc. as well as maturing HVDC technology applications. There is still abundant work remaining for large-scale mainstream development of offshore transmission to become a reality including development of offshore grid codes, maturing DC system protection and selectivity, and optimizing offshore transmission technology and designs.

Figure 2: Offshore Multi-terminal HVDC Transmission

One of the interesting, related technologies that HVDC offshore transmission grids would enable is the maturation of grid-forming controls. Grid-forming controls in very broad terms refers to converter controls that can operate without external reference. They can do fast voltage and angle control, like a synchronous machine. Most converter controls, by design, need a stiff reference (mostly from synchronous generators today) to operate and control the power from renewable energy sources. In the future, grid-forming controls, with similar and more advanced frequency regulation functionalities, may need to be explored for offshore wind connected to HVDC transmission systems. Some interesting research and development topics which will help accelerate the adoption of offshore HVDC wind plants include (i) coordination of grid-forming control between HVDC converters at point of connection on land and HVDC converters at the offshore connection point, and (ii) definition of potential grid-forming control needed to be provided by offshore wind plants. HVDC and long-term coordinated transmission planning are critical enablers to unlocking the huge potential of offshore wind energy and accelerating the decarbonization of the energy supply.

Divya Kurthakoti
Senior Lead Specialist, Ørsted