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The King of long distance
For decades, high-voltage direct current (HVDC) mass impregnated (MI) cables have been the preferred solution for long-distance subsea power transmission. We wanted to know why, so we asked Lars Lervik, Product and System Responsible Paper Cables in Nexans, to tell us more about the MI cables.
"MI cables are the undisputed champion when it comes to transmitting power over long distances," says Lars. He has been working as MI engineer in Nexans for more than 20 years, and he is crystal clear: "The success of the MI cables is based on expertise, experience, quality, and durability. To ensure the quality of the MI cable, robust design and well-proven manufacturing processes are required. The design of MI subsea cables is based on environmental data (maximum water depth, ambient temperatures, burial depth, thermal properties, etc.) and electrical data (voltage level, power transmission capacity, etc.). Consequently, the MI cable system design is tailor-made for each application and project. The conductor is made from copper or aluminum and consists of a circular center wire with concentric stranded layers of keystone-shaped wires, resulting in a very compact conductor with a smooth surface."
And here comes the tricky part: "The insulation system consists of lapped paper tapes impregnated with a high viscosity compound. Lapping the paper tapes is a delicate process and requires extreme precision and a lot of knowledge. A moisture-proof barrier, a lead alloy sheath, is then applied above the insulation system, and for mechanical and corrosion protection, a polyethylene sheath is applied. For mechanical strength, transversal reinforcement and steel wire armoring are applied. To achieve a torsion-balanced design, two layers of armor wires applied in opposite directions are used, and the armor is corrosion protected by bitumen compound and two layers of polypropylene yarn."
"This design has been used for decades. In 1977 Nexans installed the Skagerrak Interconnector subsea HVDC link between Denmark and Norway. The cable system has been in service for 40 years and is still in operation, proving the reliability and the longevity of HVDC MI cable systems. In fact, recently the cable had to be repaired due to external damage, and when we dissected it we were very impressed. After over 40 years in the sea, the cable was perfectly intact as if it had been produced yesterday!"
So, if this is such a great product, why don’t all cable manufacturers produce MI cables?
"Manufacturing of MI cables requires experience, special manufacturing equipment, and processes following the highest standards. Nexans’ cable factories in Halden(Norway) and Futtsu(Japan) are equipped with the necessary special manufacturing equipment for the paper lapping insulation (as mentioned above). The plants are also optimized for the manufacturing of long lengths of submarine cables, based on large turntables as intermediate storage between the different processes. The storage turntables for the finished cables can store 7,000 tons of cable in one length. The factories have deep water quays, so cables may be spooled directly from the onshore turntables onto the ships.“
If we could peek into a crystal ball, how is the future of MI cables?
“I would say that the MI cables have a brighter future than ever before. The world depends on more and more high voltage cables, longer subsea crossings and cables for deeper water. The need is also for land high voltage. We see an increasing trend in putting the high voltage transmission systems in the ground rather than in the air, and therefore MI cables are great suited for the future. There are no technical limitations for a maximum cable length of MI cables, thus these cables can be used for the longest links both on land and in the sea. We have qualified 600-kilovolt MI cables with a power rating of 1,100 MW per cable, giving a power transmission capacity of 2,200 MW in bi-pole configuration. This is impressive, but not surprising since we know what the cable can do and it's potential. So, if you want a safe and stable energy transmission system, I would recommend Nexans Mass Impregnated cables.“
Facts: HVDC MI cables from Nexans are used for transmission links worldwide, and interconnectors such as Sweden–Finland (Fenno-Skan), Estonia–Finland (EstLink 2), Norway–Denmark (Skagerrak) and Italy and Montenegro (Mon.Ita) employ this mature and well-proven technology from Nexans. It is also being used for the upcoming HVDC links between Norway and Germany (NordLink) and Norway and the United Kingdom (North Sea Link), where Nexans is a complete turn-key supplier.