“uåX˜äŠÊ˜·³Ç

The first complete project of 2003 was online in January, a little-noticed 17.2 MW plant using four Bonus 2.3 MW turbines and four Vestas 2 MW units, which just get their feet wet on the edge of the Lim fjord off the small Rønland peninsula near Jutland's west coast. It was built by two local wind co-operatives.

In February, a community owned 23 MW offshore wind plant of Bonus 2.3 MW turbines was completed near the island of Samsø in protected waters east of the Jutland peninsula. By June, utility Elsam finished building its 10.6 MW offshore prototype testing plant in a basin at the harbour of Frederikshavn with four units from three different manufacturers: two Vestas 3 MW units, a Nordex 2.3 MW and a Bonus 2.3 MW turbine.

debating the future

The biggest news was the completion of the 165.6 MW Nysted wind farm off the coast of Rødsand in December (“uåX˜äŠÊ˜·³Ç, September 2003). The plant, owned by utility Energi E2, uses 72, Bonus 2.3 MW turbines. It is the second of what was once to be five public offshore demonstration projects -- the first being the 160 MW Horns Reef station in the North Sea, which Elsam built in 2002.

The fate of any such future offshore demonstration plant is in parliamentary debate, after the government's initial tender for a third plant was laughed back to the drawing table by the Danish wind industry for what were seen to be uneconomic and unreasonable requirements (“uåX˜äŠÊ˜·³Ç, December 2003). A revised EU-wide tender for a 150 MW extension to Horns Reef is expected to be issued this year, but little other activity is planned.

The end of the year also brought a rather fizzled victory for NEG Micon, which has tried since 1999 to win approval for a small, nine-turbine wind station off the harbour of Grenaa on the Jutland east coast. A public protest group managed to cut the installed capacity of the project by a third before the national energy complaints board put an end to the appeals and closed the case. The end result was a permit for NEG Micon to build three 2.8 MW turbines.

Learning curve

If the point of the offshore demonstration projects is to learn by doing, then Elsam, Vestas and ABB are being taught a hard lesson at Horns Reef. After problems with the wind plant's transformers, Vestas is now removing all 80 transformers supplied for each 2 MW turbine by ABB Power Transformers and replacing them with technology from Germany. About two-thirds will come from Siemens, with Starkstrom-Gerätebau (SBG) supplying the remainder, according to Jens Bonefeld of Elsam Engineering.

Bonefeld says the breakdowns were due to a combination of factors. First, the thickness of the insulation used on the transformers' windings varied greatly -- and although the transformers passed routine tests, it was not enough to secure safe operation, he says. This factor combined with a ventilation system that sucked the salty, damp sea air from outside directly over the transformers. The units also run at the highest voltage level normal for their type, says Bonefeld. Due to the offshore and windy location of the turbines, they can routinely go from standstill to full power and back again quickly. "It was all those factors together that caused the problems," says Bonefeld. The new transformers will be more protected from direct contact with outside air, he adds.

The Horns Reef power station lies 14-20 kilometres from the Jutland west coast in the North Sea. Vestas declines to talk about the transformer problem, but Danish engineering newspaper Ingeniøren reports the cost of the retrofit will run up to several million Danish kroner. Whether Vestas or ABB is to pay the bill, and who is to compensate Elsam for the resulting electricity production loss, remain unanswered questions. The several-tonne heavy transformers, which convert the 690 kV electricity from the generator to the grid's 33 kV, are to be exchanged with help from the turbines' internal cranes.

previous failure

It is not the first time transformers have failed in offshore turbines. At Middelgrunden -- the 40 MW plant near the Danish capital of Copenhagen using 20, Bonus 2 MW machines -- 11 Siemens transformers were exchanged in 2001. Danish utility Energi E2, which owns ten of the affected turbines, says it learned from the experience before building Denmark's other big offshore plant, Nysted. Here, the transformers were also supplied by ABB, but are of a different type than those used at Horns Reef. They have also been placed in the bottom of the towers, where the environment is dry and controlled and where they are not subject to vibrations, according to Ingeniøren.

In calculating possible total cost of the Horns Reef repairs, Ingeniøren notes that the removal and exchange of just one transformer at Middelgrunden cost between DKK 400,000 and 600,000 (EUR 53,700-EUR 80,600), though that also included the cost of an offshore crane, which Vestas does not need.

A special research project, funded by the eastern Denmark transmission system operator Elkraft System, has been launched to pin down the weak points in the troubled offshore transformers, in co-operation with Danish Technical University, Energi E2 and ABB Denmark.