Devotion to the Development of DC XLPE Cables

In 1979, J-POWER completed the first-stage submarine cable for the Hokkaido-Honshu HDVC Link, which connected Hokkaido with Mainland Japan, and started operation. However, the installed cable was the oil-impregnated insulation type. Thus, the company hoped to use the DC XLPE cable for expansion or renewal from the viewpoint of environmental protection. Mitsumasa Asano of J-POWER (at that time), who worked on the development with Sumitomo Electric, looked back on the situation and said, “I felt the passion of Sumitomo Electric’s development team members, who showed extraordinary perseverance to accomplish the project whatever it might take.”

The insight was right. After trial and error, the space charges were finally suppressed by adding a newly developed special filler material. R&D proceeded from Stage 1 (1988 to 1995), which aimed to attain 250 kV, to Stage 2 (1993 to 2001), which aimed to attain 500 kV. All the targets were achieved.

This coincided with the deterioration of the first cable installed for the Hokkaido-Honshu HDVC Link. In 2012, the DC XLPE cable was finally put to practical application for the first time in the world to meet the replacement needs.

“In the actual manufacturing operations, it was necessary to fabricate a long-distance cable. This posed difficulties in maintaining uniform quality compared to the prototype production. It was highly meaningful to hold discussions between the two companies about the quality control and manufacturing process,” said Asano.

Asano also remembered what impressed him during the installation work.

“The offshore cable had to be connected with the onshore cable manually. Accidents were likely to occur at this point in the cable construction project. This process required high engineering capabilities. Sumitomo Electric’s workers were reliable in terms of both skills and construction management,” said Asano.

The project was accomplished thanks to Sumitomo Electric’s overall capabilities: development engineers who perseveringly pursued outstanding specifications, in-depth quality control at the manufacturing site, careful construction work which took full advantage of the quality, and passion to attain high levels on all aspects.

* When the strength of an electric field that applies to an insulating material exceeds a threshold, the insulating material electrically breaks down, resulting in a rapid decrease in electrical resistance and allowing a large current to flow.

Let’s meet a legendary person who has focused on DC XLPE cables throughout his research career. Shoshi Katakai is affiliated with the Electric Wire & Cable Energy Business Unit. He has been devoted to R&D of DC XLPE cables since the 1980s, when he entered the industry.

This approach was considered to be applicable to the insulating material of DC power transmission cables. However, a new problem, which did not arise in the AC power transmission cables, emerged. Conventional XLPE attained the electrical breakdown strength far below the expectations for the DC voltage due to accumulation of the space charge, which was discussed above. In other words, AC XLPE cables made from pure polyethylene could not be used as they were for DC. Thus, Katakai came up with the idea of mixing an additive and decided to use a filler material. He made steadfast efforts to carefully identify potential filler components one by one to cope with high voltages and finally determined the components and blending to resolve the problem. Efforts to achieve further miniaturization and higher purification and distribution led to a special filler material. A material with superb DC insulation characteristics was completed by uniformly distributing the components in XLPE. This is the proprietary XLPE that underpins HVDC power transmission cables currently manufactured by Sumitomo Electric. The accomplishment was derived from doubting the conventional wisdom and seeking the truth with perseverance.

“Interconnectors have been expanded worldwide, but more of them are needed,” said Katakai.

The Japanese government has announced a plan to construct a next- generation grid to send electric power derived from renewable energy sources, such as offshore wind power, from Hokkaido to Tohoku and Tokyo as part of its clean energy strategy. In Europe, where the electric power market has been liberalized, construction of an international grid called “super grid” is underway as the first project of its kind in the world. The development of international interconnectors has been spurred by the growing need to introduce renewable energy. Full-scale efforts have been launched by regions and countries to achieve carbon neutrality. The supply of DC XLPE cables, which were developed by Katakai and other Sumitomo Electric engineers, will be accelerated for interconnector projects in and outside Japan.