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JDR: R&D effort led to its new 132 kV ‘wet’ array-cable
U.K.-based JDR Cable Systems, part of the TELE-FONIKA Kable Group, recently completed test qualifications for its new nextgeneration 132 kV subsea cables, which are available in both static and dynamic designs to serve fixed and floating offshore wind projects. Below, Joe Cole, JDR’s technology manager – power cables, discusses how a long-time technical hurdle was overcome.
WJI: What’s special about your newest 132 kV subsea cables, and how did that come to be?
Cole: Traditional 132 kV cables have always relied on a lead sheath to protect the insulation from water ingress. It provides a hermetically sealed barrier preventing water from diffusing into the insulation system within the cable, where it could accelerate the aging of the insulation, potentially causing premature failure. The lead sheath adds cost, weight, stiffness and installation restrictions. We have found a way to eliminate the need for that for our 132 kV inter-array cables.
WJI: What did you do?
JDR’s 132 kV wet-design inter-array cable passed tests at the Offshore Renewable Energy Catapult.
equipment to accommodate larger diameters, larger minimum bend radii and stiffer products. Also, for the dynamic cables, larger equipment was needed for extruding the sheathing for three-core cables. For dynamic cables, moving from 66 kV to 132 kV presented significant technical challenges. As the cables are inherently larger, the fatigue critical metallic components are exposed to increased mechanical stress during bending. To overcome this, extensive material and component testing was undertaken to understand and optimize the fatigue critical layers to ensure performance over the design life. The whole process took three years.
Cole: We have developed, designed and qualified “wet design” 132 kV cables which require no hermetic lead barrier layer, thereby reducing weight, cost and making these larger cables easier to handle. The development itself has been driven by market demand, similarly to the step change from 33 kV to 66 kV. As wind turbine generators (WTG) are getting larger and therefore more powerful, we are reaching the limit of efficient power transfer at 66 kV for intra-array cable applications.
WJI: What prevented your wet design for 33 kV and 66 kV cables from being used for the 132 kV design before this?
Cole: The key was the polymer used within the electrical insulation system. The insulation widely used for 66 kV wet design cables faces a phenomenon known as water-treeing, which is an aging mechanism. Water tree retardant (WTR) insulation uses a special formulation to slow this aging process caused by ingress of water into the insulation system. JDR worked with its polymer supplier to produce cable samples with the new WTR insulation material, which were then tested according to the CIGRE TB 722 aging regime, a two-year test to ensure the polymer can operate as a wet system.
WJI: Aside from the insulation, were there other technical hurdles?
Cole: Yes. JDR had to upgrade its Hartlepool cable manufacturing facility to allow for production of the larger and much stiffer dynamic versions of the new 132 kV cable technology. Upgrades were made to handling
WJI: Can you describe the testing process to shape product design and readiness for market deployment?
Cole: Following the materials and component testing, an extensive analysis was performed on the three-core cable to assess the design’s viability for deployment in floating offshore wind uses. The results of the analysis regime subsequently defined the “load blocks” used to form the 1.5 M cycle test; the number of cycles for each combination of tension and bend radius. Having a deep understanding of the material characteristics and implementing it in the analysis regime provides strong confidence in the success of the fatigue test and its applicability to the reallife conditions. This gives further confidence in the performance of the cable during its deployment offshore.
WJI: How important were your entity partnerships?
Cole: They were vital. ORE Catapult facilitated the majority of the qualification testing program as the independent test house, ensuring unbiased testing and reporting of the results of the product qualification. With DESNZ (BEIS) and the Offshore Wind Growth Partnership providing partial-funding support for the development, JDR was able to ensure the success of the R&D project and deliver a robust product aligned to the long-term needs of the offshore wind industry. With our new facility development nearly completed, JDR is now ready to meet the needs of the offshore wind industry, whether this is with array cables at 66 or 132 kV, or indeed if developers need to source AC export cables at voltages up to 275 kV in the next few years.