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Two European wind turbine manufacturers, Micon of Denmark and Tacke Windtechnik of Germany, reported blade failures on 600 kW turbines last month, while three 500 kW machines from Dutch company NedWind failed in heavy winds, with sections of their glass fibre blades coming down in the process. The blades on the Micon machines were supplied by Dutch company, Aerpac, while Abeking and Rasmussen of Lemwerder in Germany supplied the failed Tacke blade. NedWind, which usually makes its own blades, declined to confirm if the damaged blades were also made by the company.

A first series of ten sets of Aerpac 19.2 metre blades are faulty, Micon reports. The company's Dieter Fries says routine checks have revealed cracks in blades installed on two 600 kW turbines, in Denmark and Germany. The ten sets are installed on five turbines in Germany, two in the Netherlands and three in Denmark. "Our specialists think the cracks could have arisen through the unusually strong winds in January," says Fries.

According to Aerpac's Rob Roelofs the failures were caused by "aeroelastic instabilities -- in short, unknown vibrations." He says such vibrations can occur on large machines when braking or turning out of strong winds. "Opposing forces can occur of which we don't know enough yet. I think it has to do with scale and flexibility of the turbine. A new problem that comes while turbines are getting larger," adds Roelofs. "It is not only our problem. It is a problem for the whole wind industry that has to be solved."

Roelofs says there is no question about the quality of the Aerpac 19.2 metre blade. About 60 of the blades, of glass fibre reinforced epoxy, have been made, of which 50 are running on Micon turbines and machines from German company, Hanseatische. Current fatigue and load testing of a 19.2 m Aerpac blade by the Danish Test Station for Wind Turbines at Risø for Det Norske Veritas has given excellent results, says Roelofs. The blade was picked at random from the production line and could withstand loads up to 2.5 times the design load.

For reasons of safety, Micon has reduced the shutdown wind speed of all ten turbines with blades from the pre-series production run to 16 m/s. Over the next few months all the affected turbines will have their Aerpac blades replaced with blades from LM Glasfiber of Denmark, bringing their rotor diameter to 43 metres, states Fries. This change should allow a significantly better performance in stronger winds as well as a considerable reduction in noise, he adds.

German Tacke Windtechnik reports that a 21 metre blade was totally written-off on one of its TW 600 kW machines at Carolinensiel in Wangerland in a January 9 storm. An unusually strong gust caused two thirds of the blade to break off. The turbine came to an immediate standstill, says Tacke. Examination of the blade by both Abeking and Rasmussen and Germanischer Lloyd revealed faulty construction. A film used to roughen the areas to be glued had not been removed from the blade shaft, preventing a proper join of the two halves of the blade. Subject to the stresses of a particularly strong blast, the weak blade split and broke off. Tacke dismantled the machine house and tested the components at its headquarters. No other damage was found and the turbine has been reassembled and is back in action.

Lastly, storm force winds in the Netherlands on January 22 wreaked havoc in the Vokerak wind farm near Willemstad, operated by utility PNEM. Three 500 kW turbines with rotor diameters of 40 metres were in the process of braking and turning out of the wind when parts of the blades broke off and fell close to the towers. Estimated damage is NLG 750,000, says Ruud Kool from PNEM. The turbines, certified by CIWI, had not yet completed commissioning. Eleven other NedWind 500 kW machines on the site, with 35 metre rotors, were not damaged

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