Undamaged after strike, lightning protection

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Two blades hit by lightning in Denmark last year remained totally undamaged despite registering currents of 31,000 and 27,000 amperes apiece. The blades are installed on the Nordtank 1.5 MW prototype wind turbine at Tjaerborg and are equipped with LM Glasfiber's "lightning protection system," reports the blade manufacturer. Inspection of the 29.2 metre blades after they were struck revealed no more than small burn marks on the surface of the receptor; even the silicon packing around the receptors remained intact, according to LM's head of development, Peter Grabau.

LM's lightning protection consists of a receptor built into the tip of each blade from where electricity is conducted to the hub. From here the wind turbine's built-in lightning conductor system dissipates the electricity. The system has been developed by LM together with Denmark's Technical University and Technical Institute (DTI).

"Normally such a severe hit would cause serious pressure damage, but in this case no damage is visible," says Kaj Olsen of DTI. "The tip still locks tight and the blades remain very quiet. In this case if they hadn't been protected they would have suffered damage. Our investigation reveals that the system has passed its first test -- at any rate when dealing with normal lightning."

According to Grabau, the system has been laboratory tested in England and Germany and is approved for strikes of up to 200,000 ampere. So far, of 800 blades equipped with LM's receptors, four have been hit. No damage has been registered. LM will now use data gathered by its lightning protection system to continue its lightning research.

Also in Denmark, wind turbine manufacturers Vestas and Bonus report good experience with their lightning protection systems. Lars Budtz at Vestas says that out of 1650 Vestas blades with built-in lightning protection, only one has been destroyed after taking a strike, one required repairs and on a third, burn marks had to be wiped away. Out of 2600 blades without lightning protection, 14 have been destroyed. Bonus has around 600 blades with its own protection system built into LM blades. No damage after lightning strikes has ever been recorded, says the company's Søren Winther.

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