March 3, 2014 | 19
Lightning strikes have been known to incapacitate wind turbines by destroying their blades. But while most tall structures are prone to lightning strikes, wind turbines seem to be especially susceptible. Recently scientists captured high-speed footage of these strikes, and they discovered that the wind turbines may in fact be the architects of their own demise: the nature of the turning turbine helps to cause these strikes.
Typically when lightning strikes a tall object, the strike is initiated from the cloud. A channel of negatively charged plasma, called a negative downward leader, moves from areas of negative charge in a storm cloud down toward a positively charged building, tree or wind turbine. As the negative leader nears the structure, it induces a positive upward leader, which jumps up to meet the negative leader. The connection forms a current, and the bright lightning flash we observe is actually to the result of a shock wave flowing up the connected channels, called a return stroke.
In the case of these wind turbines, positive upward leaders are generated from the turbine blades in the absence of a negative downward leader from the clouds above. These upward leaders are the zigzagging lines that grow up toward the sky in the beginning parts of the video (below).
In the paper describing their findings, published online February 6 in the Journal of Geophysical Research, the researchers describe the phenomenon they think is responsible for this lightning. When tall objects build up a positive charge underneath a negatively charged storm cloud, they form a cloud of positively charged ions, which helps to dissipate the electric field around them. But, says Oscar van der Velde, a researcher at the Polytechnic University of Catalonia and a co-author of the paper, “if your blade can escape this cloud of ions, then the field will remain high. And if the field is high enough, you can trigger a real lightning flash.”
In the video—which takes place over just 150 milliseconds—the lines that stay illuminated have formed currents with the clouds above. Still, there is no bright flash and thus no return stroke, van der Velde says: “If you get a return stroke… it saturates the image.” This is just as well for the turbines because the return stroke is the most damaging part of a lightning strike. Yet “even discharges without return strokes can cause progressive damage to the turbine materials, ultimately leading to their failure,” van der Velde points out.
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