Advanced Cable Design: Sumitomo

N O V E M B E R 2 0 2 5 | W W W. W I R E N E T. O R G

WIRE & CABLE

Advanced Design

WRAPUPS + wire Kraków + wire Southeast Asia

BONUS FEATURE:

AI & R&D ... has its time come?

O F F I C I A L P U B L I C AT I O N O F T H E W I R E A S S O C I AT I O N I N T E R N AT I O N A L

F E AT U R E 4 O F 6

Long Sumitomo R&D journey led to key DC XLPE cable advance Sumitomo Electric shared with WJI how it redefined submarine power transmission with a DC XLPE cable that reliably carries HVDC across long undersea distances. The advance—rooted in decades of research—faced challenges that included one deemed impossible yet ultimately led to a solution. Pictured below is the company’s 400kV submarine power cable: its newest cable is on the next page. tric breakdown and unlocked the possibility of true DC Sumitomo Electric’s (Sumitomo) ambitious journey to XLPE transmission at the voltages and lengths required produce a new DC XLPE submarine cable for intercon‑ for new interconnector projects. nector projects—technology essential for linking national Developing this insulation was only part of the answer. grids and transmitting large volumes of renewable power Because submarine cables may run for hundreds of km, over long distances underwater—was shaped by a series Sumitomo had to guarantee that the precise balance of pivotal R&D decisions made to overcome obstacles achieved in the lab could be reproduced at manufacturing encountered during testing and deployment. scale. That meant continuous monitoring at every stage— The specific goal was first set in the mid‑1980s, when extrusion, curing, cross‑linking, and spooling—plus Sumitomo engineers realized that a DC version of XLPE repeated mechanical, electrical and accelerated‑aging tests could transform long‑distance power transmission if the to catch flaws before installation. These stringent measures material barriers could be overcome. The decades‑long set a high bar but were critical for ensur‑ R&D effort, initiated in the mid‑1980s, ing no weak points would emerge once reached maturity in the 2000s and was the cable was laid. proven in full scale with projects such as Installation brought its own set Nemo Link, commissioned in 2019. of challenges. The company had The aim was to create a cable that could to refine how cables were handled, operate at much higher voltages, tempera‑ moved, and joined—especially at tures, and capacities than conventional landfalls and underwater connec‑ submarine cables—a critical step for tions. When the Nemo Link between expanding cross‑border power links and Sumitomo’s 400 kV HVDC submarine the U.K. and Belgium hit an unex‑ renewable integration—while still main‑ pected snag just three km from shore taining exceptional reliability and environ‑ power cable. during installation in 2018, Sumitomo mental safety. What made this so difficult quickly switched contractors and adapted its cable‑land‑ was that conventional XLPE insulation, well‑suited for AC ing methods. The project was completed on schedule service, repeatedly failed under DC conditions due to the despite logistical and marine obstacles. This ability to accumulation of space charges in the insulation. In simple manage change in real time, supported by preparation terms, space charges are stray electrical charges that slowly and scenario planning, proved as vital as any laboratory build up inside the insulation under DC voltage. Over time, breakthrough and demonstrated that R&D success also the buildup distorts the electric field and weakens the mate‑ hinges on execution in the field. rial from within. Eventually, the insulation breaks down The results are now visible in quantifiable, real‑world without warning, making standard XLPE unusable for performance. The cables maintained target voltage and high‑voltage DC transmission across undersea distances. current ratings over long stretches and at temperatures up Addressing this meant rethinking the material science and to 90°C, with no oil leaks or insulation failures. The addi‑ the production process, pioneering not only a new additive tive‑based XLPE insulation set a new standard for DC to disperse space charges but also new manufacturing and reliability, and the company’s process controls allowed installation techniques to ensure that every meter of cable, manufacturing at the scales required by major intercon‑ from the factory floor to the seabed joint, would perform nector projects. Facility operation rates such as the 99.8% flawlessly for decades. achieved by Nemo Link stand as testaments to the sound‑ Initial attempts to adapt AC XLPE cable as a DC solu‑ ness of the R&D—not to abstract ambition. tion exposed the problem: repeated breakdowns under Sumitomo Electric’s story is less about proclama‑ direct current, even in controlled laboratory conditions. tions and more about hard‑fought advancement: and Rather than abandon XLPE’s environmental and oper‑ maintaining the discipline needed to deliver consis‑ ational advantages, Sumitomo’s R&D teams systemati‑ tent, field‑ready performance. Each cable laid and grid cally investigated the underlying cause, conducting years connected stands as evidence that tough R&D deci‑ of tests with polymer blends and evolving manufacturing sions—pursued over more than 20 years and carried techniques. The breakthrough decision was to challenge through to manufacturing and installation—made a new industry orthodoxy—the long‑held belief that “pure” generation of global submarine power interconnectors insulation was always best. Engineers instead devel‑ not just possible but proven. oped and optimized a filler additive for polyethylene that Sumitomo’s new 525 kV HVDC submarine cable stands dispersed space charges within the cable insulation. This out as a true breakthrough in power transmission tech‑ innovation shifted the industry’s understanding of dielec‑

5 0 W I R E J O U R N A L I N T E R N AT I O N A L N O V E M B E R 2 0 2 5

W W W. W I R E N E T. O R G

F E AT U R E 4 O F 6

What’s special about Sumitomo’s new 525 kV power cable WJI asked Sumitomo Electric’s research and development department to discuss what makes its new 525 kV HVDC submarine power cable stand out. Below is their response. tion, and flexible splicing solutions—making it nology, raising the bar for both volt‑ adaptable for complex large-scale installation age and capacity in a marketplace that projects. These advances result in a cable that is rapidly turning to renewables and reduces losses, enhances operational efficiency, interconnectors. With the highest volt‑ and cuts maintenance needs for operators, all age rating ever achieved using crossof which are crucial for grid reliability and linked polyethylene (XLPE) insulation, sustainability. this cable system can transfer more As countries expand offshore wind farms than 2 gigawatts per circuit—enough and invest in cross-border energy transfer energy to power millions of homes or to meet climate targets, Sumitomo’s 525 kV link national grids across continents. cable becomes the backbone for high-capacity, The use of proprietary DC-XLPE long-range clean power delivery. Its efficient, compounds with advanced nano-fill‑ high-performance design directly supports the ers ensures the cable can handle high shift toward decarbonization and a resilient temperatures (up to 90°C), maximizing Sumitomo’s new 525 kV HVDC energy future. By combining advanced materi‑ thermal stability and minimizing loss, while maintaining reliable performance submarine cable. Sumitomo photo. als, proven manufacturing, and system reli‑ ability, this submarine cable system sets a new even under demanding load and marine world standard and highlights the critical role of conditions. innovation in enabling the new era of renewable electricity Passing long-term PQ test for 400kV system more transmission. Plus, it allows project developers to confi‑ than 10 years ago according to the international stan‑ dently plan for even larger interconnections in years ahead, dard CIGRE-TB496, Sumitomo’s cable proves itself with enabling flexible expansion and more robust global energy endurance and safety, backed by independent certifica‑ networks. tions. The engineering also includes seamless factory and offshore joints, transition technology for onshore integra‑

The Sumitomo engineer who made DC XLPE insulation work Shoshi Katakai, an ex-fellow of Sumitomo Electric’s Electric Wire & Cable Energy Business Unit, has spent his career on one relentless challenge: making XLPE insula‑ tion viable for high-voltage direct current. Since joining the company in the 1980s, he set out to achieve that goal, which was considered promising but unattainable.

The difficulty wasn’t just theoretical. Conventional XLPE, which had revolutionized AC power cables, failed under DC. Long lab tests showed the same result again and again: space charges built up inside the supposedly “pure” polyethylene insu‑ lation, weakening it until it broke down. From the standpoint of accepted industry knowledge, the outcome was unavoidable.

For years, develop‑ ment stalled, but Katakai refused to accept it.

Others clung to the Shoshi Katakai with his signature dogma of purity, but advance. he chose to experiment

W W W. W I R E N E T. O R G

with the opposite: adding something new. He began test‑ ing fillers and additives, searching for a material that, if distributed uniformly inside the insulation, could suppress the space charges. It was not glamorous research. Each candidate had to be analyzed, blended, cured and tested.

Progress was incremental, setbacks frequent.

Eventually, Katakai’s persistence paid off. He and his team developed a special filler material that dispersed evenly in XLPE and delivered superb DC insulation characteristics. The result was a stable, manufacturable compound capable of carrying HVDC reliably under the extreme stresses of submarine interconnectors.

“To solve space charge, we had to abandon purity and prove that an additive could make XLPE stronger. It was long, frustrating work, but that’s what made DC XLPE cables possible,” Katakai said.

That achievement—the world’s first DC XLPE— changed the industry. Today’s long‑distance intercon‑ nectors, quietly transmitting thousands of megawatts across sea floors, rest on the foundation Katakai discov‑ ered in the early 2000s by challenging orthodoxy with slow, stubborn, technical work until failure finally turned into proof.

N O V E M B E R 2 0 2 5 W I R E J O U R N A L I N T E R N AT I O N A L

51