Until recently, developments in offshore electricity generation and the need for long distance power transmission, planned spacing between power cables has largely been a function of economic requirements and grid connection constraints (both key factors in routing decisions). Historically, subsea power transmission links were limited to a few miles in length linking islands to the mainland grid or between points on the mainland where a submarine route across a bay or estuary was economically and technically viable. Such short links are typically concentrated at the most convenient point for connection to the transmission grid and, where strong enough or reinforced, additional cables could be concentrated into that area to meet growing demand.
The distance constraints on subsea HVAC (high voltage alternating current) transmission are now being widened with new technology but still remain limited in efficiency to about 100 km. Developments in HVDC (high voltage direct current) transmission have allowed for longer transmission systems. As the development of offshore transmission of electricity increases, the need for consideration of effective and efficient use of space and routing becomes increasingly important. This is not only to take account of other users of the ocean, the seabed or offshore resources, but also for the security and integrity of the installed assets and of electricity supply.
This report seeks to identify the factors determining spacing between power transmission cables. While the focus of the report is the development of the offshore wind power transmission around the United States, the findings and conclusions may be considered relevant when determining spacing between cables for other purposes and indeed, between cables and other seabed assets.
In compiling this report it has been identified that there is not an extensive U.S. library of pertinent studies, guidance, data or information and it has been necessary to draw on the experience of other nation states and industries where relevant experience can inform the study.
The objective of this report is to provide a technical reference informing the regulator and developers and provide best practice guidance in respect of cable spacing for the developing offshore wind industry in U.S. waters.