Pole 1 of the Konti-Skan link, which spans the Kattegat strait between Sweden and Denmark, allowing the two countries to exchange power, is nearing the end of its design lifetime. It is operated by Svenska Kraftnät, which owns and operates Sweden’s transmission network with responsibility for the national electricity grid of 400 and 220 kV power lines, and Eltra, its Danish counterpart, which owns and operates the 400 kV transrnission network for Jutland and Funen in western Denmark and is responsible for the overall security of supply, including connections to neighbouring countries.

Eltra and Svenska Kraftnät have undertaken to strengthen the existing link by upgrading it to 308 MW while simultaneously improving its reliability and availability. This will be accomplished by replacing two 1960s vintage mercury arc converter stations, one at each end of the connector, with thyristor-based technology and adding more sophisticated controls. Only the submarine cable and overhead lines will remain. The contract covers the complete refurbishment of the converter station at Vester Hassing, on the Danish side of the Kattegat, and the building of a new station to the same specification at Lindome on the Swedish side, requiring an extension of around 25 km to the overhead lines from Stenkullen where the link comes ashore. Power is transmitted into the 90 km link at 285 kV DC.

The fact that Alstom’s T & D division (which will by January be part of Areva) won the r40 million contract in the face of competition from ABB and in ABB’s own back yard is a sign, it believes, that it is re-emerging as one of the three major players in the T & D world market. It is in fact Alstom’s first HVDC project in Scandinavia. And the company is looking for increased business in the region as the level of power trading among the Baltic states increases. Much of Sweden and Norway’s power is hydro sourced, and recent cold winters and dry summers have reduced electricity generation, increasing the need to import power.

Under the turnkey contract, due for completion in October 2005, Alstom will design and construct the two converter stations and manufacture and install all their equipment ­ 50 Hz switchyards, converter transformers, control systems, thyristor valves, DC filters/smoothing reactors, harmonic filter bays and capacitor banks. It will also undertake commissioning and technical training.

Thyristor valve

At the core of the new installations is the latest version of Alstom’s HVDC thyristor valve. These greater-power-density valves use series-connected, fully protected thyristors, each with 8.5 kV rating and of 125 mm diameter. This represents a 60 % increase on the 100mm, 5.6 kV earlier model. They will be controlled through Alstom’s Series V digital control and protection system, offering fully redundant operation, including monitoring and alarm capability, a proven system recently employed for the

500 MW back-to-back HVDC interconnector at Sosarom in India.

Improved performance

The refurbishment should revitalise the power exchange link, allowing each country to support the other by trading more energy to better balance out power shortfalls and surpluses. The new control technology is expected to deliver system security and reliability by obviating recalibration caused by ‘drift’ and eliminating the unreliability, large outage times and high maintenance costs of the mercury arc system.

The capacity of pole 1 will on completion be equal to the existing Konti-Skan 2 pole, at 380 MW, while the addition of a bi-pole control system will enable high level control of the existing Konti-Skan 2 pole as well as the new pole to minimise current flow in the sea electrode, an environmentally desirable feature. The control system also enables fast control of the power exchange through both poles, to benefit the two AC systems under emergency conditions. Plans for the future include full automation of both of the Konti-Skan 1 converter stations, enabling unmanned operation to be implemented.