Abstract
The offshore wind sector as well as other renewable energy industries are in search for efficient and reliable solutions for both static and dynamic high voltage power cables. Wet design high voltage power cables can fulfil the need for cost-efficient solutions and put forward an appealing choice for dynamic cables. Using aluminium conductors in cables instead of copper can furthermore contribute to reducing the cost and weight of the cable. In the present study, a new aluminium alloy conductor with the same mechanical strength and fatigue properties as copper has been used to manufacture electric cables.
Several publications have expressed concern regarding the use of aluminium conductors in wet designs. The studies found that when using an aluminium conductor in a wet design XLPE cable, there is a risk of increased growth of vented water trees from the conductor screen due to the degradation of the conductor screen when liquid water is present at the conductor screen/conductor interface. This has been associated with local corrosion of aluminium that may cause the occurrence of porous structures in the semiconductor, known as Stress-Induced Electrochemical Degradation (SIED). However, this phenomenon has only been detected in aging studies or for service-aged cables when liquid water was introduced directly into a conductor with no strand fillers. No such phenomena have been observed in cables with an aluminium conductor when the insulation system was exposed to water only from the outside, as it would be the case in a normal operational scenario.
The present study focuses on further understanding if there is a risk related to the use of aluminium in wet design power cables by comparing a power cable with a standard aluminium conductor with a cable with an aluminium alloy conductor in a wet aging test. Cable sections were exposed to deionized water during isothermal ageing at 50°C and 90°C for 20 and 32 weeks, and temperature cycling between 75°C and 90°C for 20 weeks. After aging, the samples were investigated for local corrosion on the conductor strands and for degradation of the conductor screen.