The main purpose of this project has been to develop wet design for high voltage (HV) XLPE insulated power cables. Cables with copper conductors and aluminium conductors have been researched. Salt water has been used as ageing medium. A test setup for performing wet ageing tests of cables at high electrical stress in salt water has been developed. A methodology for measuring salt ion concentration in the insulation system has been developed.
A wet design cable is a design that allows water diffusion into the electrical insulation system. At the start of the project in 2019, medium voltage (MV) power cables rated up to and including 36 kV were usually of wet design, while a dry design, including a radial metallic water barrier, typically lead (Pb), was used for higher voltages operated at higher electrical stresses. The reason for this voltage class limit for wet designs is the increased risk of ageing of the XLPE insulation when exposed to high electrical stress in combination with humidity.
The two main advantages by using a wet design at higher voltage levels are reduced environmental impact and cost savings. Both derive from removing the metallic water barrier from the design. High voltage cables of wet design are also a very attractive solution for floating applications where a traditional metallic water barrier quickly will experience mechanical fatigue. A wet designed cable will also significantly reduce the weight of the cable and improve handling properties which will be favourable for installation considerations. Wet design cables will require fewer installation campaigns, which are a major time and cost element for long subsea cable projects.
Almost all previous knowledge of wet ageing of XLPE insulated cables has been based on ageing in tap water. It has been a general perception that salt ions do not migrate through the outer semiconductive layer/outer thermoplastic sheathing into the insulation system. In this R&D project, it was thus important to understand if salt ions diffuse into the insulation system, and possible influence of salt ions on the degradation of the insulation system during ageing. Long term wet ageing of full-scale cables was performed in salt water using identical ageing parameters as previous testing in tap water. Factory joints on long subsea cables are a necessity and were included on test objects subjected to long term wet ageing. Investigation of the influence of salt ions from the ageing was conducted by performing electrical tests on cables and factory joints after ageing and by analysing water tree growth in the insulation system. Moreover, increased frequency during ageing has previously shown to increase water tree growth in the insulation system and thereby the degradation. Part of the wet ageing tests on the cables was thus performed at increased frequency to study the influence on the insulation system in a salt water environment. The research performed in this project provides confidence in qualification of wet design cables up to and including 72 kV.
Previous studies have also shown that corrosion products on an aluminium conductor can act as initiation points for water trees in the insulation system and thus accelerate water treeing (stress induced electrochemical degradation (SIED)). Cables with aluminium conductor was also included in the long term wet ageing in salt water and subsequently characterised by electrical testing and water tree investigations.
In this R&D project it was also important to be able to accurately measure salt ion concentration in the insulation system. The feasibility of two commercial test methods for measuring salt ion concentration was investigated by laboratory manufactured polymer samples with a known salt content. One of the test methods showed promising results and can be used for measuring salt ion concentration in the insulation system accurately.
A purpose built test setup for performing wet ageing tests of cables at high electrical stress in salt water was also built in the project. For high voltage cables, the needed electrical stress during testing is very high, and there are several challenges related to performing such testing in salt water. This includes challenges such as preventing premature breakdowns at the terminations, and anomalies at the water surface due to non-radial temperature and diffusion gradients. Preliminary testing of cables at 70°C at high electrical stress demonstrated the validity of the built test setup.
The main result from WetCab is increased knowledge about wet ageing of XLPE-insulated HVAC cables. Specifically, a methodology for measuring ion diffusion has been developed, as well as short- and long-term testing of cables.
The results from WetCab are essential for the development of HVAC XLPE wet design cable systems (without lead sheath). Such cables constitute a key technology for the further development of renewable offshore power, and they can open the door to new markets such as interconnectors and electrification of platforms. Removal of the lead sheath is in itself a good measure to meet increasingly stringent requirements for the reduction of heavy metals. In addition, material costs will be reduced by 15-25%. Other benefits will be lower cable weight, better handling properties, and better mechanical performance, which can reduce the installation cost and -duration of subsea cables.
The WetCeb project will bring forward new and essential knowledge for the Norwegian cable industry to develop high voltage subsea transmission cables for more demanding applications such as deep water and offshore wind installations. In this project, the performance of superclean high voltage cable insulation systems will be verified by assessing the wet
ageing properties of models and full-scale prototypes. Numerical modelling and material characterizations will be used to assess the reliability and expected service lifetimes of new wet designed high voltage subsea cables. This knowledge will extend the use of wet cable designs beyond todays' acceptance criteria resulting in more efficient, lighter and thus cost optimal cable designs. Financial support from the Norwegian Research Council is essential to establish fruitful research co-operation between Nexans Norway, SINTEF Energy Research and NTNU.