Material considerations for submarine high voltage XLPE cables for dynamic applications

Special consideration is necessary choosing materials and selecting design for dynamic cables due to the mechanical stress, sometimes in combination with a wet design, experienced by the cable. For high voltage cables (>;36 kV) this is mainly related to the required (IEC 60840) metallic water barrier and for wet solutions the influence of water treeing and the associated accelerated electrical aging is of concern. Lead is commonly used as a radial water barrier for high voltage cables but the fatigue resistance is inferior to steel and most other metallic materials due to its low melting temperature and poor mechanical properties. For these reasons it is generally not considered suitable as a radial water barrier for dynamic power cables. Alternatives such as longitudinally welded copper sheaths have proved to be viable alternatives to lead and studies on non-metallic barriers have been performed which show promising results. For wet medium voltage designs or designs which allow some moisture ingress into the insulating material water treeing is often a concern. However, it has been shown that high relative humidity (>;70%) is necessary for water tree inception. By delaying the water ingress with sheaths and swelling tapes the time to reach critical humidity levels in the material can be increased from 1 month to several decades. The influence of hydrostatic pressure and compressive stress has been found to diminish water tree growth whereas tensile stress enhances it. For dc voltages there are indications that both humidity and mechanical stress can increase accumulation of charge in the insulating material. It has also been shown that moisture ingress will increase the conductivity which could influence breakdown strength negatively.