Transmission & Distribution

The TEN-E plan achieves another first

1 November 2008

The world’s longest undersea high voltage cable has been officially opened. A major part of the EU’s TEN-E grand plan for comprehensive European interconnection, it has already proved profitable beyond expectations.

TenneT, the Netherlands’ electricity grid operator, and its Norwegian counterpart Statnett have officially opened the Norway to Netherlands NorNed interconnector, at 580 km and 700 MW capacity the longest and most powerful subsea high voltage cable in the world. It is roughly twice the length of the previous reord holder, the Basslink between the Australian mainland and Tasmania.

The reliability of power supplies in Norway and the Netherlands has been substantially increased with the commissioning of the new interconnection, say the link’s operators, but the cable is primarily a trading route, aimed at reducing price fluctuations and exploiting off-peak price advantages, and has already shown a profit, since it started operating in May, that exceeds its first year forecast.

According to its operators, between opening for business on 6 May until September NorNed transported 1.7 million MWh of hydropower from Norway to the Netherlands and 0.1 million MWh of electricity in the other direction. Between July and September the transport capacity of the cable for import was auctioned at an average price of r39.29/MW, and for export at an average price of r1.42/MW. Its r70 million income to date has already exceeded the first year forecast of r64 million.

NorNed cost nearly r600 million and will help the Netherlands to manage peak power loads during the day by importing hydropower from Norway. It also offers the Netherlands with an alternative to fossil-fired generation, and TenneT expects CO2 emissions in the country to be reduced by about 1.7 million tonnes per year. Norway on the other hand will be able to trade its hydro-generated electricity surplus in a bigger market, and reduce its reliance on hydro (that is, on rainfall and snowmelt) which currently supplies more than 90 % of the country’s needs.

The HVDC substations for NorNed – the first direct link between the two countries – was supplied by ABB. The project took a total of ten years to complete – the last three of which were for the actual installation – and required 24 permits from four different countries. It consists of two converter stations – in Eemshaven, Netherlands and Feda, Norway – linking the cable, 420 km of which lies in shallow waters (less than 50 m depth) and 160 km in deeper waters (up to 410 m depth).

‘NorNed is a landmark feat of engineering that brings Europe even closer to the goal of creating a reliable, continent-wide electrical network with low environmental impact’ said Peter Leupp, head of ABB’s Power Systems division. ‘It is a significant contribution to the quality and reliability of Europe's power supply.’

The interconnector

The link has an operating maximum power capacity of 700 MW and an operating current of 824A. The northern section consists of two

450 kV cables in 310 km runs of mass-impregnated-paper-insulated cable from ABB Norway, in up to 410 meters of water. A two-core section supplied by Nexans completes the shallower southern 270 km. The cable has been designed to last for 40 years, although it is expected to remain in operation for longer. To date, no undamaged submarine cable has failed simply because of its age. In most cases ageing cables are replaced because they no longer offer the required capacity.

Initially, the NorNed link was intended to provide a transmission capability of 600MW, corresponding to a maximum conductor temperature of around 40°C. However, studies have shown that there is sufficient margin to allow for an additional 100 MW capacity even during the warmest summer periods, making the effective continuous operational rating of the NorNed link 700MW.

An unusual challenge for the cable engineering team was that for the Dutch and German section of the project the customer had specified that both cables be supplied as a twin-core design within a single outer sheath, a design that reduces the magnetic field enough to eliminate interference with maritime navigation equipment and communications. The twin-core cable runs for a distance of 270km out from the Dutch coast and then splits into two conventional single-core cables.

Market coupling

The link is basically a market coupler. Norwegian generation is almost entirely based on hydropower, which means that below-normal precipitation can adversely affect its power system. Power production in The Netherlands is based on fossil-fuel-fired thermal generation. The interconnection allows price-driven power trading between the two countries and increases the reliability of the supply.

In the longer term cycle, in dry periods in Norway the link will be mainly used to import power, whereas in wet periods surplus power will be exported to the Netherlands. The interconnection will also reduce CO2 emissions because it will cut the use of peak-load fossil-fuelled generators. Instead, it will cut the load on high efficiency baseload generators by exporting power from Norway during the daytime and importing power at night. It is also expected that the link could boost the development of wind power in the Netherlands by helping to compensate the effect of wind fluctuations.

Construction snags

The original completion date of end-2007 was delayed for five months when line testing in December 2007 revealed a break in the cable about 270 m from the Dutch coast. Although no details have been given out, mainly for contractual reasons, this is just where the joints between the single core and double core sections are located. Soon after that was fixed, tests unearthed a second break further north. The delay was caused mainly by persistent unfavourable weather conditions during the first quarter of the year.

Dutch grid operator TenneT tracked down the exact location and replaced the defective section of cable but with the causes of the faults still to be acknowledged, the decision about who will eventually pay for the fix was not determined. That remains the case to this day, and it is perfectly possible that it will never be discovered whether the fault is attributable to the manufacturer or to an accident during cable laying, a hazardous process that has a good track record but is not expected to be perfectly fault free.

And this kind of repair is an expensive process. The cable cannot be repaired on the seabed, so once the fault is located it must be cut, raised to the repair vessel and a new section added to allow for the extra distance the cable is now covering. Then a joint at each end of the new section must be completed, and the cable relaid. Potentially this is the most expensive part of the process, because the longer cable must be re-routed and protected, and it may not be possible simply to lay it on the seabed.

New links

Norway already plans more lines, to Germany and Denmark, adding to a growing underwater web linking Europe’s grids. In January this year Statnett and German grid operator E.On Netz started a feasibility study, due to report any day now, on the 700 to 1400 MW Nord.Link line which is estimated to cost $1.1 billion to $2.2 billion. A Norway - Germany cable is listed in the EU-supported TEN-E programme for trans-European electricity networks which aims for improved market integration in Europe.

At same time, TenneT and the UK’s National Grid were beginning a 1000 MW, 260 km line between Isle of Grain, in SE England, and Maasvlakte, near Rotterdam. Various other European subsea lines are planned. Work has begun on a 420 km cable between mainland Italy and Sardinia, 1600 m deep in the Tyrrhenian Sea. And significant as NorNed is, it wasn’t the first subsea link in the Nordic zone – the 75 km cable between Finland and Estonia – Estlink, also an ABB project – was completed nearly a year ago.

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