The Influence of Lightning Induced Field Distribution on the Pitch-Control-System of a Large Wind-Turbine Hub

Lightning strikes pose a major threat to wind turbines as facilities are increasing both in number and in height. When lightning strikes a wind turbine blade, the lightning current is conducted to ground via the hub. The hub’s electronic components make it very sensitive to high currents and electromagnetic fields. While simple solutions exist to mitigate the threat of lightning current on the control electronics, shielding the electronics from the induced electromagnetic fields is much more difficult. This is particularly true for magnetic fields. The method-ofmoments will be used and a numeric model of the hub and its components is created in order to specify the magnetic fields inside the hub. Model validation is done using magnetic field measurements performed inside a large wind turbine hub. A surge current of 1.3 kA was injected into the hub for these measurements. Currents up to 200 kA may arise during a lightning strike, and the model has been extrapolated to fit these conditions. In this case it is especially important to take the material properties of the hub into account. Because the hub under test is made of cast iron its magnetic properties must be considered. Skin-effect and nonlinear-phenomena must be taken into account for a proper model adaptation. A special device fabricated of cast iron is used in order to define the current’s influence on the given phenomena (this device is referred to as Equipment Under Test (EUT)). Using an impulse current generator, the influence of surge currents of up to 100 kA on the cast iron EUT is examined by means of field measurements. Applying these results to the method-of-moments analysis of the hub, the magnetic field distribution and the induced currents caused by a 200 kA lightning strike are presented.