Deficient standards

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Existing lightning protection standards for wind turbines laid down by the International Electrotechnical Commission (IEC) are not being interpreted with enough rigour to account for today's taller turbines and the installation of them in windy areas prone to thunderstorms.

When estimating the number of expected direct strikes on a wind turbine at any given location, the IEC standards use a formula based on the local strike density per square kilometre, the collection area measured in square kilometres and an environmental factor compensating for the morphology of the terrain. The standard, however, does not take into account the use of tall modern turbines when applying its environmental factor.

As a result, the impact of lightning on new wind turbines in the field is greater than presumed and proper account is not being taken of the effect on a wind turbine's control system of the large number of small lightning leaders starting on blade tips that do not constitute a real strike. These small leader formations only dissipate charge into the air, but the related current will easily reach tens of kilo amps, affecting the control system in the wind turbine and its surge protection devices (main article).

At lightning rich locations in the US, strike density can easily be over 20 strikes a year per square kilometre, even without wind turbines erected. Once a series of tall metal towers are in place, the site's exposure rate becomes far higher.

In general, most statistical lightning strike data are taken from downward initiated, cloud-to-ground lightning -- and all specifications for wind turbine lightning protection are based on downward initiated lightning. The standard requires that the wind turbine withstand up to 200 kA with a specific energy of up to 10MJ/ohm. The charge related to such a lightning impulse will be around 100 coulombs.

These parameters have been the steering factor for many years, but tall turbines have introduced a new phenomena: most strikes are upward initiated. To deal with this change, a new focus is required on both interception efficiency and the dissipation capacity for multiple strikes. No longer is it sufficient to design protection systems which are only efficient against one large strike.

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