Nexans helps modernise vital high-voltage underground cable circuit in London

Nexans helps modernise vital high-voltage underground cable circuit in London

Nexans, the worldwide leader in the cable industry, is currently working on a multi-million Euro contract for UK Power Networks, one of the UK’s largest energy companies, to modernise a 11.5 km high-voltage (132 kV) underground cable circuit that links the Twickenham and Laleham substations in London. The project to replace the two existing paper-insulated external gas pressure cables, which date from the 1960s, with new XLPE (cross-linked polyethylene) insulated ‘pipe-type’ cables, is intended to prevent age-related failures of this vital connection and ensure London’s security of supply.

Nexans completed the first of the two project phases in August 2010 after just four months of installation work. This was thanks not only to the experienced Nexans installation team but also to the installation technology used. The job involved the use of more than 23 km of pipe-type cables comprising three high-quality, stranded high-voltage XLPE cables with a copper wire screen and aluminium laminated sheath protected by flat steel wire armouring. High-quality XLPE compounds ensured a very compact design, which enabled the cables to be drawn into the existing pipes which are less than 160 mm in diameter. This was not only the largest pipe-type cable project ever undertaken by Nexans’ Hanover plant in Germany, the cable dimensions were also record-breaking, with a cross-section of 3 x 630 mm² making them the largest pipe-type cables ever manufactured by Nexans.
Reducing civil engineering costs and simplifying permission procedures
Nexans Pipe-type cables are designed for the modernisation of inner-city cable infrastructure where paper-insulated external gas pressure cables need to be replaced. The pipe-type cables are drawn into the existing steel pipe that contained the old cable, which minimises road digging and excavation work. Traffic disruption is largely avoided and local residents are only minimally affected. This is achieved thanks to the highly compact design of the Nexans pipe-type cables: the high-quality XLPE compound provides the required insulation with relatively thin walls. Following installation, the pipes do not need either gas-pressurisation or monitoring, which reduces operating costs.
Replacement of  ageing high-voltage cables has reached the halfway stage
In the modernisation process, the pipe-type cable is pulled in section by section. The 11.5 km circuit in London comprises more than 20 individual sub-sections with cable lengths of between 400 m and 500 m. The first of the two systems was replaced this year and has been successfully commissioned. The second, parallel system, is due to be replaced in the summer of 2011.
Contingency plan for the highly stressed cable section

Nexans devised a contingency plan to minimise failures during modernisation. Had the old, parallel section failed during the installation of the first, new cable, the defective sub-section could have been replaced by a pipe-type cable. Transition joints specially manufactured for this project are designed to connect the ends of the XLPE cable to the ends of the external gas pressure cable, allowing the old system to be restored. Jörg Niehoff, Nexans project manager in Hanover said: “The ability to prevent power outages is particularly important on this connection because while one cable is being installed, the other remains under load. The weak points in old cables mean that there is always a risk of failure”.

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