Aluminium in High Voltage Subsea Cables

High voltage (HV) subsea cables are important for transmission of renewable energy, as offshore wind mill farm. Traditionally, the conductor of such cables has been made of cupper. Aluminium is a cheaper alternative, but aging phenomena has been identified, if moisture present, that might limit the lifetime of high voltage cables with such conductor. In this project work has been performed to understand better central mechanisms connected to water penetration to mitigate such to avoid effects of water. The goal is to produce cable with lower cost with acceptable life time. Originally, this was primary a medium voltage (MV) land cable project dedicated Nexans dept., Namsos. The project has been transformed to apply for HV subsea cable project, Nexans Halden dept. in regard of the consequent higher complexity. Today?s HV subsea cables have three components to mitigate water penetration; lead sheathing to hinder radially water penetration and swelling tape and conductor sealing material, a flexible conductive rubber material to be applied on each layer in conductor and at last swelling tapes to be applied in-between lead sheathing and cable core and in-between conductor and insulation system to limit axial water ingress. Otherwise, the water penetration around conductor is restricted by good adhesion of the semiconducting materials on the conductor in case of no use swelling tape on conductor. An important property that contribute to limit lifetime when alumina is used, different from cupper, is the degree of contact with other materials. With this background, basic knowledge is crucial in regard of water penetration and electric properties, in other words, to improve life time of cable. Good contact is needed to achieve adhesion between semiconducting material and conductor to mitigate water penetration at damage from for instance anchors or trawls. However, when good contact between these materials is established and water is present there are possibilities corrosion and consequently some porous zones in inner semicom material are likely to be created something that in next instance can initiate water treeing, over time leading to break down. Consequently, important project work has been to achieve better understanding of adhesion between aluminium and inner semicon polymer layer. After drawing the surface of conductor strands have variation in smoothness and amount of rest of drawing oil. We have explored how variation surface properties impact the contact and adhesion here since a better understanding is crucial in regard of how to best produce cables and consequently this activity has had a priority before an eventual costly subsea cable for full size testing could be produced. Rigg for testing of the effect of different surface treatment of aluminium and test method have been developed. Semiconducting materials are tested against different prepared samples of aluminium including the impact of two different drawing oils. Interesting and unexpected results have been achieved. The knowledge and aspects in regards of production methods this is already applied in commercial subsea cable projects. Paper abstract for NORD-IS 19 has been accepted. Another important activity has been the research on properties of swelling tapes. That tape retain most its electrical properties if swelling is most wanted. Inside the construction the tape will meet variation in pressure something that has impact on ability to swell i.e. absorption of water and thus electrical properties. Thus, variation in properties at different pressures together with different amount of moisture, with salts as well, have been explored. Such tapes contribute also to mitigate of corrosion on aluminium by absorption of water. Student has performed master thesis by working on this. Methods for measurements are developed and verified. New and crucial knowledge has been achieved something that will give basis for publication, planned spring 2019. As mentioned, water penetration is mitigated axially in conductor by the application of flexible rubber-based material. The adhesion, to achieve water tightness, is important here as well. Whatever method, the water tightness is critical property of HV subsea cable. The aging process between aluminium and semiconducting screening material is result of water ingress or condensation at temperature variation. These phenomena have earlier been studied by Nexans Halden. Furthermore, Nexans Halden, the competence centre for HV subsea cable, has much knowledge and experience regarding mitigation of axial water ingress in cable cores. This is continuously a field of exploration and improvements. This project has explored and summed up the status of all the R&D based knowledge on effect of water in cable in regard of axial water sealing, aging on HV subsea cable at application of aluminium conductor.

Dette forskningsprosjektet har hatt aktiviteter som er knyttet sentralt inn i Nexans' kompetanse og kjernevirksomhet. Oppnådde resultater, både på målinger på svellebånd og adhesjon halvledermateriale og overflateegenskaper på aluminium har medført undersøkelser og resultatet som er viktige og som har gitt svar på aktuelle problemstillinger. Nexans leverer for tiden store kabelprosjekter i milliardklassen hvor selv "mindre" resultater kan gi særdeles viktige bidrag for å oppnå ønsket kvalitet. Dette med å oppnå best mulig vanntetting er en stadig tilbakevendende problemstilling siden nye problemstillinger stadig dukker opp. En gjennomgang og status av kjent kunnskap er derfor av god nytte. Dette gjelder likeledes aldringseffekter ved bruk av aluminium.

Summary: This project will bring forward new fundamental knowledge for the Norwegian cable industry which is essential in order to develop reliable high power subsea and underground transmission cables with lower costs, and power cables for more demanding applications such as deep water offshore wind installations. Previously performed R&D projects sponsored by Norwegian Research Council have observed phenomena that may limit the life time of wet high voltage cables with Al conductors. In this work, financial support from the Norwegian Research Council is essential to establish fruitful co-operation between Norwegian and international cable industry companies, SINTEF Energy Research and NTNU.