TRANSMISSION & DISTRIBUTION
Closing the Baltic ring28 July 2006
Two new European interconnectors to be brought into operation before the end of 2007, NorNed and Estlink, will strengthen considerably the links between the Scandinavian market and northern Europe and for the Baltic states provide a degree of independence from the Russian power system. One of the two, Nor-Ned, chalks up two firsts. It is the world’s longest underwater high voltage power transmission link, and the world’s first twin high voltage cable with a single converter.
The European Union’s long-term goal is to create a common energy market in which power supply between nations is secure, sustainable and commercially competitive. Interconnections between grids increase grid reliability generally and help to prevent blackouts. The EU is seeking to build more interconnections to create a single integrated electricity market and to increase cross-border electricity exchange. Interconnections should also enhance local industry’s competitiveness and benefit the consumer by increasing competition and driving down electricity prices.
In 2003 the Trans-European Network of Energy (TEN-E – see panel on page 15) identified several high-priority interconnections that were considered essential to securing power supplies and enabling cross-border energy trading between member states and neighbouring countries of the EU.
They include new or improved power links across the Alps, Pyrenees and Balkans, and over the North, Baltic and Mediterranean seas. A key requirement of the TEN-E directive is the integration of renewable sources of energy into the TEN-E network.
Towards the end of 2004, the contracts for two of these priority power interconnections – the 700 MW NorNed link that will join the Netherlands with Norway and the 350 MW Estlink between Estonia and Finland – were awarded separately to ABB. The links will be made using transmission and cable technologies originally introduced by ABB – HVDC, and HVDC Light.
In December 2004 ABB announced that it had received the go-ahead to proceed with the Nor-Ned project, a high-voltage direct current (HVDC) transmission link officially rated at 600 MW to connect the power grids of Norway and The Netherlands at Feda and Eemshaven. On completion the 580 km long link will be the longest underwater high voltage cable in the world.
The contract, originally awarded in 2000, is worth $270 million to ABB and is with the two state-owned power grid companies, TenneT in The Netherlands and Statnett in Norway. The three-year project started in January 2005 and is scheduled for completion in December 2007. Under the terms of the contract ABB will supply ‘state-of-the-art’ HVDC converter stations, to be manufactured in Ludvika, Sweden, and the greater part of the underwater cabling. Nexans will supply the balance of cabling. The Nexans element of the NorNed link is scheduled for completion in the summer of 2007. The interconnector is expected to enter service in the fourth quarter of 2007 or in early 2008.
International power trading is well established in northern Europe, and Scandinavia in particular, owing to the large number of HVDC power links created over the past 40 years including the Konti-Skan link between Sweden and Denmark, the Swepol link between Sweden and Poland, the Skagerrak link between Norway and Denmark, the Baltic cable between Sweden and Germany, and the Kontek link between Denmark and Germany.
Statnett and TenneT will each own 50 % of the new link, which is estimated to cost €550 million in all. The project adds a new link to the many that already exist between Scandinavia and the rest of northern Europe. Indeed, there is already a route to The Netherlands through Denmark and Germany, but the new link is needed to provide a direct connection to the Dutch hub and, primarily, to increase capacity to counter bottlenecks on the existing lines and higher losses on the older ones. It should also improve electric power supply reliability in both countries and reduce price fluctuations.
The link will be a dipole, 450kV/0/450kV, with a single point earth ground and therefore no sea return. This latter configuration has to some extent been discredited by the experience of Basslink, a 300 km undersea connector which reverted to a low voltage line return design after failing to demonstrate satisfactorily the environmental aspects of the sea return.
The grounded dipole arrangement allows higher power transmission levels compared to the alternative of one HV cable with one LV return, as in the latter case the voltage would be limited to 500 kV. The challenge, say ABB was to use two cables without a LV return to their full capacity and to do it economically. In the chosen configuration the cost penalty of a twin cable is alleviated by the single point grounded converter solution. This is the world’s first +/- 450 kV cable with a single converter. To date, all such links have employed a converter for each line.
The first 420 km, reckoning from the Dutch end, will be supplied by ABB, the initial 270 km being a twin cable (two cables in single armour) and the remaining 150 km single cables. The final 160 km of single cables, in the deeper water, will be supplied by Norwegian/French cable maker Nexans who will also supply a shorter length of cable to bring the link ashore in Norway. The Norwegian end of the route shelves steeply, and the permitting authorities will only allow twin cable near land in shallow water.
This section will also be 450 kV HVDC cables, to be laid in water depths up to 410 m. “We’ve a lot of experience of manufacturing deep-sea cables. That’s why we’ve been chosen to supply the section of the NorNed link that’s to be laid in the Norwegian Trench”, says Yvon Raak, Nexans’ executive vp for the European area. The contract is worth around 51 million Euros to Nexans.
The 450 kV line rating confers a useful capacity margin on this 500 kV rated cable. A larger margin is incorporated at the substations, which are rated at 600kV. The cable can carry 800 MW, intermittently, so there is a large margin here too.
The anticipated line loss of 3.7 % breaks down as 1.5% in the substations and 2.2% in the cable.
Commercial logic of NorNed
Viewed as a power trading route (as distinct from its role as a promoter of power reliability) the NorNed is more direct than existing routes, it is lower loss, and it avoids the weaker structure between Poland and Germany and a route through Lithuania and Poland which is problematic owing to political difficulties between those two countries.
During the daytime, power will head south from the Nordic hydropower plants: during the night, north, from the north European coal fired plants which work best as base load meeting a constant demand. Hydropower as the regulator is even more effective when it is allied to greater windpower levels, the long term unavailable backing up the short term unavailable: so one can envisage a time when best regulation will see German wind turbines extensively used to drive Norwegian water uphill to its storage reservoirs.
The NorNed is an open access international link, based, with the prospect of trading fees, on sound commercial lines. It is the first such link to be built. From the start, the grid operators will be able to derive a direct financial benefit from trading, a novel situation which is very promising. Compare this to, for example, the Estlink, which is owned by five parties, two in Finland plus the operators in Estonia, Latvia and Lithuania, and needs a 100 % usage rate by its owners to show a return.
Norwegian generation, being almost entirely hydropower, is strongly affected by yearly variations in rain and snow fall. Power production in the Netherlands is based on fossil fuel-fired thermal generation. The interconnector will therefore improve the reliability of the power systems in both countries as well as ensuring that power can always be delivered from a lower-cost area to a higher-cost area. From year to year, such variations are mainly governed by the level of precipitation in Norway, where supply security is compromised by the variability of its dry and wet seasons beyond the capacity of the storage to compensate.
Engineering the link
The link will have a maximum capacity of 700 MW (continuous) and an operating current of 824A and will consist of two cables at +/- 450kV with respect to ground. This enables the current and cable losses to be kept low, but requires a higher converter voltage.
Mass impregnated non-draining cable, a type well established for submarine HVDC applications, has been selected for the both sections. The ABB component is to be manufactured at the company’s Karlskrona factory in Sweden and consists of a copper conductor insulated with layers of paper impregnated with a high viscosity oil. The cable requires no external pressurisation, and even if is completely severed only a very small amount of the insulating medium will escape – hence the term ‘non-draining’. Sheathing is with lead alloy while mechanical protection is provided by steel tape and steel wire armouring. A cable like this would have a diameter of around 120 mm and weigh approximately 40kg/m, but cables for this type of HVDC project are not supplied ‘off-the shelf’. They have to be fully designed, type-tested and approved for the specific application. ABB is already well advanced in the design and testing process.
TenneT and Statnett expect to benefit from the very low transmission losses of the contracted solution. At a projected 3.7 % this is about half the figure typical of such systems and is worth several million euros a year to both utilities in transmission revenues. The link should also encourage the development of wind power in the Netherlands where there is a strong consumer movement for green power but considerable variability in wind strength.
An unusual challenge is that for the Dutch and German section of the project the customer has specified that both cables be supplied as a twin-core design within a single outer sheath.
The twin-core cable will add a great deal of complexity to the manufacturing process at Karlskrona, especially in cable handling which will require special rollers and guides. Similarly, specially designed handling equipment will be needed on the cable laying vessel. To cut down cable jointing costs the manufacturing operation is geared up to the manufacture and out-loading of long continuous lengths of cable – up to 150km of single-core and 75km of twin-core MIND cable – which minimises the need for jointing operations over long cable interconnections. Five field joints will be required before the NorNed cable enters deeper waters after approximately 420km.
After laying on the sea bed the cable will be entrenched and buried by a water jetting process. Close to the Dutch coast it will be buried to a depth of 3–5m, and then to 1m along the rest of the route. Along sections of the route where the nature of the sea-bed prevents trenching the cable will be protected by rock dumping.
The NorNed link will cross a part of the North Sea much traversed by earlier cables and pipelines, some long abandoned. In fact, there are around 20 cable and pipeline crossings. The usual method is to install additional protection for the existing cable or pipeline, lay the cable on top and then add a further layer of protection for both the new cable and the existing cable/pipeline.
Increased operational rating
Initially, the NorNed link was intended to provide a transmission capability of 600MW, with a projected 4% transmission loss. When designing power cables the general practice is to identify the thermally critical points along the cable route and choose a conductor cross-section that provides sufficient transmission capacity when the cable is utilised to its thermal limitation. The original 600 MW rating would correspond to a maximum conductor temperature of around 40°C.
However, the thermal limitation for MIND cables is a conductor temperature of 50-55°C, which incorporates a sufficient margin to allow for an additional 100 MW capacity even during the warmest summers. So the effective continuous operational rating of the NorNed link will be 700MW.
ABB is designing the NorNed cable to last for a 40-year life, although it could remain in operation for longer. In most cases ageing cables are replaced or superseded not because they are no longer functional, but simply because they no longer offer the required capacity.
Much is being made of the link’s green credentials, allowing the import of ‘green’ hydropower from Norway during the day when demand is high, and the export of excess capacity from thermal power stations during the night when demand is low. This model achieves several targets: it enables the Netherlands to run fossil-fuelled power plants efficiently at an optimal and constant base load, and it integrates renewable hydropower from Norway with the Dutch and European transmission grids.
These measures are expected to yield a reduction in CO2 emissions of almost 1.7 million tons a year. If CO2 emissions trade at €30 ($36) a tonne on the EU’s Emissions Trading Scheme (as they were in mid-July 2005) the reduction has a value of around €50 million ($60 million) a year.
The ETS is regarded by the EU as a vital component of the battle against climate change. It is the first international trading system for CO2 emissions in the world, and covers some 12 000 installations representing close to half of Europe’s CO2 emissions. It is designed to help the 25 member states of the EU (and Norway and Switzerland) achieve emissions compliance at lower cost.
Estlink, the €110 million transmission link between the power grids of Finland and Estonia – and therefore of the three Baltic states – was formally launched on April 29, 2005 in the Estonian capital, Tallinn, at a signing ceremony between representatives of contractors ABB and the new company formed to own and operate the interconnector, Nordic Energy Link. It is the first interconnection between the Baltic and Nordic electricity markets and is intended primarily to supply the Nordic sector. It is anticipated that 2 TWh pa will be transported via the cable.
The partners in the Estlink project and shareholders in the company are the three Baltic power utilities – Eesti Energia, Latvenergo, Lietuvos Energija – with Pohjolan Voima and Helsinkin Energia of Finland. Completion is scheduled for November 2006.
The importance of the project lies, primarily, in the improved security of electricity supply in the Baltic States and in reducing the dependence of their power systems on Russia. The European Commission gave the go-ahead for the project in April 2005, approving an exemption which allows users of the cable to be charged for the construction costs instead of adding the costs to the domestic tariffs of the main grids. The exemption had already been approved by electricity market regulators in Estonia and Finland. ABB is committed to deliver the high-end technology in record time - in this case less than 20 months.
Estlink is about 100 km long, 70 km of which is underwater cable and the rest underground cable (9 km in Estonia and 20 km in Finland). It will use ABB’s high-voltage direct current (HVDC) Light technology, linking Harku 330 kV substation (Estonia) and Espoo 400 kV substation (Finland).
ABB’s cable technology is described as ‘environmentally friendly’ because it ‘uses compact converter stations, incorporates oil-free, lightweight cables and [being DC] avoids the generation of electromagnetic fields’.
A contributor to energy trade between the Nordic countries and the Baltic states, the link is also a guarantor of the reliability of supply. The cable would allow black restart of Estonia´s power system after a major failure.
Extensive interruptions of the power supply, of a nature that would require restarting the whole system, have not happened to date. However, the system operators wish to be prepared for such an eventuality, considering the recent blackouts in other parts of Europe that have left large parts of Moscow, southern Sweden, Copenhagen and Italy without electricity. As a rule , interconnectors between countries would not allow restarting their power systems, but a special technology designed by ABB allows such restarts.
The company has prepared a power network analysis describing the adjustments to the Estonian-Finnish link that would enable it to meet the needs of Estonia´s power system.