Improved insulation quality in HVAC and HVDC subsea cables

One of the world?s leading manufacturers of high voltage subsea power cables is Nexans Norway. Nexans has won contracts and delivered cables all over the world thanks to long-term research and development. There is an increased demand for the subsea cables to transmit more and more energy, to be installed on greater depths and over longer distances. Higher voltage on the cables and more demanding applications require even better electrical insulation and high quality in all aspects of production. In a new project funded by the Norwegian Research Council, Nexans Norway will, together with SINTEF and one foreign partner, develop new knowledge and technical solutions in all steps of the production chain. The goal of the project is to improve the electrical insulation even further and thus increase the reliability of the subsea cable system. The expected innovations in the project are believed to be very important for the large future power transmission networks at sea. The project focuses on fundamental understanding of the criticality of different kind of contaminants and means to remove them from the insulation system. In addition to fundamental research, full scale cables will be produced and tested. The long term properties of these cables will also be studied in this project. There will also be a student from NTNU who will perform the master thesis in the project.

Subsea high voltage AC and DC cables are essential for integration of renewable energy resources such as offshore wind power, realization of subsea grids in the North Sea and reliable exchange of balancing power with Europe. In order to increase the energ y transfer, the voltage rating of the cables must be increased. This puts higher demands on the insulation system, usually made from crosslinked polyethylene, of the cables. For a reliable high performance cable insulation system, cleanliness is a promine nt feature, as contaminations reduce the lifetime and reliability of the cable system significantly. 95% of the contaminations in the polyethylene granules are organic, typically material fragments of e.g. polyethylene or stabilisers with different degree of oxidation. These fragments can be embedded as particles in the raw material (granules). They can also be generated during cable production during long extrusion runs. The key challenges to further increase the cleanliness are to identify the critical organic contaminations and their properties, and how to avoid the introduction of contamination in the cable insulation during all steps of production. In this project, critical contaminations generated during material or cable manufacturing will be expl ored by material characterization and numerical modelling. The electric pre-breakdown of contaminated insulation will be investigated. Based on these results, the critical contaminations will be identified and their properties revealed. Also different te chniques to remove contaminations during cable production will be evaluated. The gained knowledge will provide the foundation to improve the cable quality by assessing optical methods and improved extruder technologies. This will bring forward new funda mental knowledge for the Norwegian cable industry which is essential in order to develop reliable high power subsea transmission polymeric cables with lower cost.