Analysis of Lightning Transient Characteristics of Gravity Grounding Devices for Offshore Wind Turbines

When lightning current strikes the blade lightning arrester and travels through the down conductor and tower, it forms a wave process that impacts the impulse grounding resistance and ground potential rise of the grounding device. To study the transient characteristics of gravity grounding devices for offshore wind turbines under lightning strikes, a novel comprehensive model of offshore wind turbines was developed. This model integrates the Method of Moments (MoM) and Fourier transform, considering the complete path of lightning current from the wind turbine to the ground. The model includes blades, tower, and grounding device. The study examines the effects of varying seawater depths, silt layer soil resistivities, lightning current waveforms, and tower heights on the impulse characteristics of grounding devices during lightning strikes on wind turbine blades. The influence mechanism is analyzed using wave process theory. Calculations indicate that when the seawater depth is 0 m, the impulse grounding resistance significantly increases with the rise in soil resistivity of the silt layer. As the seawater depth increases, the influence of silt layer soil resistivity on impulse grounding resistance diminishes significantly. The shorter the lightning current wavefront time, the higher the potential rise of the grounding device. However, when the wavefront time of the lightning current is long, the oscillation attenuation of the grounding device's potential rise is not significant. The height of the wind turbine tower affects the oscillation frequency of the ground potential rise of the grounding device, with the oscillation frequency being inversely proportional to the tower height.