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Special Feature 
Newsletter "SEI WORLD" Vol.455

Submarine Cable Projects in Europe

Europe is regarded as the largest market for submarine transmission cables. Let’s take a look at the submarine cable projects in Europe and the cables selected, in which J-Power Systems (JPS) Corporation of the Sumitomo Electric Group is involved.

DC XLPE Cable from JPS

In recent years, direct-current (DC) transmission lines have been actively built in Europe and elsewhere for their advantages in long-distance high-capacity power transmission. Most of these lines use oil-insulated OF cables,*1 but there are growing calls for solid-insulated XLPE cables for these lines in view of maintenance, economics and the environment. The insulation in an XLPE cable is cross-linked polyethylene (XLPE), which has a network molecular structure with greatly improved softening properties at high temperature. Compared with other insulation materials, XLPE can be used with smaller-sized conductors at higher temperatures.

XLPE cables have been widely used for AC power transmission, but with DC power have not delivered their full insulation performance due to the increased space charge*2 in the body of the insulating material. JPS developed a solution to this problem by uniformly dispersing a special compounding agent in the XLPE insulation, which dramatically improves the specific volume resistance of the insulating material and reduces the build-up of space charge in the insulation. The result is an exceptionally high-performance ultra-high-voltage DC XLPE cable, with such effective insulation that the cable can continuously operate at 90ºC, approximately 20ºC higher than its predecessor DC XLPE cable, and can also be used for transmission voltage polarity reversal*3 operation, both for the first time in the world.

*1
OF cable: A cable that uses oil-impregnated insulation paper as an insulating material
*2
Space charge: Charge built up in an insulating material. When a large charge builds up, the space charge distorts the electric field in which it is placed and affects the insulation performance of the cable.
*3
Polarity reversal: Switching between positive and negative voltages (polarity) applied to a cable depending on the operating conditions of an interconnected DC power line

Project: Submarine Cable Installation between Kent, Southeast England and Zeebrugge, Belgium

This power transmission line construction project will interconnect Kent in Southeast England with Zeebrugge in Belgium, where an AC/DC converter station is planned to be built. The overall line length is 141.5 km (130 km under sea and 11.5 km on land). The project will be implemented under an EPC contract*4 that covers engineering, procurement and construction. The total project cost is approx. 70 billion yen.

The XLPE insulating material successfully developed by JPS offers the world’s highest performance and versatility. It has been used for an additional cable laid to complement the existing power transmission line interconnection between Hokkaido and Honshu islands of Japan, which commenced 250 kV/300 MW DC power transmission in 2012. The XLPE insulating material was selected for the project because the client highly valued the fact that the material can operate at high voltages of up to 500 kV. Meanwhile, in addition to the project described here, JPS has received two further orders for submarine cables in the UK.

*4
EPC contract: A construction contract for a project comprising engineering, procurement and construction

JPS will undertake the engineering, production, installation, pre-completion testing and commissioning of the world’s highest voltage DC XLPE-insulated cable (±400 kV) and will hand it over to the client in 2019.

Project: Submarine Cable for Offshore Wind Power in Germany

This project involves materials supply and installation work for a 155 kV AC three-core CV submarine cable (15 km in total over a 6.5 km route comprising two 332 MW transmission capacity channels) connecting the offshore substation of an offshore wind power station and an offshore AC/DC conversion station. Sumitomo Electric, jointly with Siem Offshore Contractors (SOC) of Germany, won an order for this project in 2012 from TenneT, a German transmission system operator. Since the German government decided to abandon nuclear power following the accident at the Fukushima Daiichi nuclear power station, there have been growing expectations for renewable energy in Germany. Accordingly, there is an urgent need to develop offshore wind power in the shallow North Sea where the winds are stable. Sumitomo Electric’s track record and reputation, combined with the installation expertise of its business partner SOC, were highly regarded by the client and helped win the order for this important line.

In addition to the XLPE cables, our MI submarine cables will also be in operation in Europe.

In January 2014, Sumitomo Electric won a contract from Italian wire manufacturer Prysmian Group for the manufacture of MI submarine cables* for a submarine power cable project connecting Montenegro and Italy. This project is being planned by Terna Rete Italia S.p.A, a subsidiary of the Italian company Terna S.p.A. It involves two poles of the interconnection (about 415 km of submarine route with depths of up to 1,200 m) made of a 500 kV DC submarine cable system with a rated power of 500 MW per pole (1,000 MW per two poles).

This project will encourage and facilitate the development of next-generation power interconnections among European countries, called the Supergrid. The Supergrid is intended to rejuvenate the European power market by introducing large-scale renewable energy systems, balancing the supply and demand for electricity, and increasing the efficiency of power transmission.

Sumitomo Electric will manufacture 115 km of MI 500 kV DC submarine cable to complete this project on time. Manufacturing of the cable has started in 2014 at Sumitomo Electric’s Osaka Works for delivery in 2016. The winning of this contract marks the Company’s entry into the market for international 500 kV long-distance large-capacity interconnecting lines.

MI Submarine Cable:This cable uses insulating paper mass-impregnated (MI) with high-viscosity insulating oil. While exhibiting excellent DC characteristics, MI submarine cables, unlike oil-filled (OF) cables, require no oil feeding equipment and so can be used for long-distance power transmission.

Interview with Project Members

Q: How do you lay a submarine cable?

Submarine cables are directly loaded on a specialized cable-laying vessel or on a freight ship, berthed alongside the factory, and then transported to the work site. Once at the site, the cable is carefully unwound from the specialized vessel and laid on the sea bed, while the ship’s location is being accurately controlled. Particularly important submarine cables are buried under the sea bed by using underwater machines such as a high-pressure water jet, plow and chain cutter, to ensure that the cable will not be disturbed by anchors or fishing gear.

Q: How much does the submarine cable that connects Kent, Southeast England and Belgium weigh and how thick is it?

It weighs about 45 kg/m and is about 14 cm in outside diameter.

Q: Once installed, how long does the submarine cable last?

Its design service life is 40 years.

Q: What do customers think of JPS’s XLPE cable?

Our customers highly evaluate our XLPE cable for their critical lines based on Sumitomo Electric’s solid track record, complete commitment to quality and the environment, as well as the cable’s advantages that are specific to Sumitomo Electric products, such as compactness due to its operating ability at higher temperature and two-way system operation capability compatible with polarity reversal.
Demand for long-distance power transmitting DC XLPE cable is expected to grow in line with the growing global markets for international interconnection lines and offshore wind power generation.


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