Computation Approach of Impulse Impedance of Wind Turbine Grounding System for Lightning First Return Stroke

Abstract:
This work presents a method to assess the impulse impedance of the wind turbine grounding system (WTGS) considering the adjustable lightning parameters α (adjusting the peak values) and β (adjusting the rise time T30) of the injected current representative of the first return stroke (FRS). The harmonic grounding impedance (HGI) is calculated using the full-wave electromagnetic software (EMS) FEKO(r) via the numerical Method of Moments (MoM) for a frequency range from 100 Hz up to 5 MHz, where the GS is buried in a frequency-dependent (FD) soil resistivity of 1,000 and 5,000 Ω·m. The transient ground potential rise (GPR) is computed as a function of α and β of the lightning current FRS. Afterwards, the impulse impedance (Zp) is estimated by two polynomial equations, one for each soil type, in terms of (α, β) using the Fitting Curve Toolbox in MATLAB(r). The results obtained by these equations are compared with those calculated by the recursive convolution method. Results indicated a significant impact on GPR waveforms obtained for distinct values of (α, β). The Zp computed by the proposed method has shown an excellent agreement compared to those assessed with the recursive convolution method. Among its benefits, the proposed method presents a more efficient computational process and can be integrated into Electromagnetic Transient (EMT)-type programs for transient analysis of wind turbines subjected to lightning strikes.