Transient response of grounding systems of wind turbines under lightning strikes Rodolfo Araneo, Giampiero Lovat and Salvatore Celozzi DIAEE - EE Division University of Rome ”La Sapienza” Rome, Italy Abstract—The paper focuses on the transient analysis of grounding systems of wind turbines. The analysis is carried out in the frequency domain and it uses a recently proposed hybrid approach based on circuit theory and Method of Moments, in order to fully account for resistive, inductive and capacitive couplings. The corresponding transient response is obtained by means of the Inverse Fourier Transform. A typical wind turbine grounding system arrangement based on ring electrodes is analyzed. Index Terms—Transient analysis, grounding impedance, light- ning strikes, nodal annalists, method of moments. I. I NTRODUCTION In recent years, the increase of clean power generation has been observed worldwide. Environmental concerns correlated with conventional energy supply and use have boosted the research for possible solutions, which today mainly include improved energy efficiency and increased renewable energy supplies [1]–[3]. Renewable wind energy has been widely applied as a mean to reach these goals and, consequently, wind turbine installations have recently increased, regarding also those regions where lightning activity is significantly more intense and lightning damage can be more common [4], [5]. Wind turbines are candidate victims for cloud-to- ground lightning mainly due to their special shape, complex construction and the fact that they are usually placed in isolated locations, at higher altitudes, where the isokeraunic level is often high. Relevant statistics indicates that fatal damages in blades, generators and specially in control circuits are caused either by direct lightning strokes to wind turbines or transferred overvoltages from nearby fault locations [6]. The grounding system of wind turbines is aimed at preven- tenting against excessive overvoltages and potential gradients that could damage parts of the wind turbine and at reduc- ing human hazards. However, the grounding system design presents several specific issues: • the grounding system of a wind turbine is generally much smaller than the grounding systems of the buildings of equal height; • wind turbines in wind farms are electrically intercon- nected; • the lightning protection level for a wind-turbine is much higher than that of a normal building having an equivalent foundation; • the locations where they are placed present generally poor grounding conditions, i.e., high soil resistivity. In this paper, we focus on the transient analysis of typical grounding systems of wind turbines. Low-impedance ground- ing system is a major prerequisite for an effective protection of wind turbines from lightning strikes. In addition, the transient analysis [7], [8] is important because the injection of high impulse currents into the grounding system leads to an increase of the grounding potential during the transient state, which can occur in a danger to humans, apparatus and systems. The transient analysis is performed in the frequency domain by means of a recently proposed hybrid approach [9], [10] based on circuit theory and Method of Moments (MoM), capable to accurately account for resistive, inductive and capacitive couplings. The corresponding transient response is obtained by means of the Inverse Fourier Transform (IFT). Other efficient approaches have been proposed in the past, based either on analytical aproximations or on numerical methods, like FD- TD, e.g. [11]–[14]. However, the proposed method, solving accurately the Sommerfeld integrals occurring in the Green’s functions of a stratified medium, appears to be very accurate and reliable, especially for the early-time response. II. GROUNDING SYSTEM ANALYSIS The physical problem of interest is illustrated in Fig. 1. The grounding system of the wind turbine is buried below ground and is subjected to a transient current generated by the lighting strike at a certain point (the inuence of wind turbine is here neglected). Most of the wind turbines manufacturers specifies grounding standards for the tower grounding. Typical grounding systems are arranged in a ring shape around the tower’s base and are connected with the tower itself through its foundation. A. Mathematical Model We assume that the conductors of the grounding network are completely buried in the earth that is considered stratified in N S planar layers, each characterized by a complex conduc- tivity σ i = σ i + jωε 0 ε r,i (with i =1,...,N S ). Non-linear phenomena in the ground are neglected. All the layers are assumed to have the vacuum permeability, μ 0 . In the following a time-harmonic variation of angular frequency ω, exp (jωt), is assumed. All the quantities, unless otherwise indicated, are represented in terms of their complex phasors. 978-1-4799-3226-9/14/$31.00  2014 IEEE Proc. of the 2014 International Symposium on Electromagnetic Compatibility (EMC Europe 2014), Gothenburg, Sweden, September 1-4, 2014 1080