Potential for offshore transmission

The wind farms of Näsudden on the Swedish island of Gotland are being linked to consumers in the north through a specially designed "light" high voltage direct current (HVDC) cable. With 37 MW of wind power installed at Näsudden, the existing grid is constructed for a maximum capacity of only 17 MW. The new "light" cable, with a capacity of 50 MW, is expected to solve the problem and create the opportunity for more development in the area. The cable, installation of which is now complete, will feed power to the north of the island. It was laid by international electrical engineering company Asea Brown Boveri (ABB) and Gotland utility GEAB.

HVDC cables have so far been used only to transmit large amounts of power over long distances. By using new power electronics, ABB hopes to make the technology feasible and competitive for transmitting power from far smaller plant, down to 50 MW-100 MW. To date HVDC systems have used thyristors to convert DC to AC and vice versa -- such as those used in the cable that connects Gotland to the mainland grid. The HVDC "light" cable uses voltage source converters with silicon-based Insulated Gate Bipolar Transistors. The technology takes less space and has several technical advantages, ABB claims, including weak grid benefits. The total cost of the link, including transformers and switchgear, is SEK 180 million, compared with about SEK 120-150 million for a conventional AC link. But it bears extra costs because it is an R&D project. With time the cost should drop to no more or even less than an AC cable.

If the technology proves economically competitive, it could become an option for large wind farms offshore and in desolate areas, says ABB. Today's variable speed turbines, which use AC-DC-AC converters, could be simplified to AC-DC converters. The next step could be to use DC generators in the turbines. This year ABB has introduced a new HV generator, which has been installed in a hydro power station in northern Sweden. The way is now open for a similar option for large offshore wind farms of the future, where HVDC generators transmit power by HVDC cables directly to transformer stations on land that feed AC into the grid. In that case, reasons ABB, wind turbines of tomorrow need neither converters nor transformers.

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