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TX Completes Major Submarine Cable Research Project

Transmission Excellence (TX) has recently completed a major 2-year study of offshore cable ratings for the Offshore Wind Accelerator, a research programme administered by the Carbon Trust on behalf of a consortium of offshore wind developers.

This study examined the behaviour of offshore cables in environments that can act as “bottlenecks” that restrict their power-carrying capacity. These environments included J-tubes, cable protection systems, offshore burial at greater than usual depth, and drilled landfalls. For all of these use cases simple formulae describing the thermal behaviour of the cable were developed. For J-tubes and cable protection systems these formulae were verified using full scale tests with actual wind farm cable. For landfalls the formulae were checked against measured temperature patterns on in-service cables.

Using these formulae it was then possible to build a model that simulated the temperatures inside a cable over a period of decades, using multi-decade hourly weather data from the MERRA database (operated by the American space agency NASA) and seabed temperature data from the EU’s Copernicus database. This in turn allowed us to show how wind farm developers can compute the economically optimum size of wind farm that can be accommodated on any export cable, making use of the fact that the wind does not blow continuously.

Our results showed that the economically optimum design involves a wind farm which is substantially larger than the continuous capacity of the cable that connects it to shore. The potential saving from using the optimum design, relative to a wind farm that is limited to the continuous capacity of its export cable, is up to £40m per GW of offshore wind.

A further part of the project made recommendations for how cable temperatures should be monitored in order avoid the risk of damaging cables that are operated well above their continuous capacity.

The study was undertaken by TX in conjunction with Southampton University, who carried out the laboratory tests using their facilities, and who contributed their extensive knowledge of cable thermal modelling. The results are summarised in a joint paper which has been presented at the 2018 IET Renewable Power Generation conference.

[download paper]