Lightning attaching to a turbine will always find its way to the ground. The current will usually find the path with the least resistance, which in the case of a wind turbine is through the Lighting Protection System (LPS) of the blade, to the hub and then through the tower to the ground. In this scenario, the blade and turbine will not endure any damage and normal operation can continue.
However, as Wind Power LAB observes in the industry, in some cases of lightning, the LPS fails to attract the strike and the current needs to find an alternative path to the ground. These cases can include disconnects in the LPS, or random attachment outside of the LPS, which amounts to 2% of all lightning strikes hitting the turbines according to International Electrotechnical Commission (IEC). Moreover, lightning outside the design parameters of the LPS is also likely to cause a damage to the blade.
Lightning strikes and the Lightning Protection system
When a lightning attaches outside of the LPS to the blade glass fibre reinforced laminate, it encounters a material with high electrical resistivity. The lightning current carries a large amount of energy and overheats the connection area causing burn mark in the attachment point. Depending on the parameters of the lightning it can also damage the epoxy matrix and the fibres of the composite and cause delamination in the region of the attachment. Most commonly it will be limited to the BIAX layers of the laminate and appear as a peeling stripe of several centimetres’ width with +/- 45 degree orientation. In some cases, the damage could be of even greater extent and cause TE debonding and delamination of structural layers.
Characteristica of lightning strikes
Long strokes, or rapid successive strokes from negative lightnings will transfer a lot of energy into the laminate. The heat generated from the lightning traveling through the resistive composite will move the laminate above its flashpoint and cause the resin to start burning. That damage mechanism will appear as soft glass fibre mats due to the resin evaporating.
Short burst lightning strikes are more likely to cause debonding on the TE/LE and delaminations with varying extent and severity, based on the peak current. The pressure increase will cause stress to the blade shells and result to failure in the weakest points, often being the TE and LE bond lines.
Mitigating the risk
From wind industry knowledge it is evident that lightning can cause damages to blades in a variety of ways. We believe it is important to mitigate lightning risk through a systemized data driven approach. Detecting a lightning damage before it has a chance to develop can be the difference between a blade repair and blade replacement. In Wind Power LAB, we have encountered and investigated numerous lightning damages and obtained knowledge on which lightning types are higher risk for the blade integrity. We have implemented that knowledge into our LASSIE lightning risk management tool. If you require any assistance with the mitigation of lightning defects on your wind farm, feel free to reach out to our blade and lightning experts.
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