# Evaluating the High Frequency Behavior of the Modified Grounding Scheme in Wind Farms
## 1 Introduction
The study of the possibility of serious indirect and direct lightning damages to electrical and electronic devices in wind farms is the main focus of [24]. The authors in [25] introduce a method for analyzing circuits based on vertical conductors to decrease the grounding resistance during lightning strikes in a pair of turbines. Despite the mitigation of steady state soil resistivity, there is still a great need to reduce the current and voltage transient behavior during lightning strikes. In [26], the authors examine the behavior of wind turbines through three different models that are based on constant resistance, nonlinear resistance and frequency-dependent resistance to obtain a proper grounding scheme for lightning strikes. The results indicate that, considering all transient states, a hybrid model needs to be applied to the system. Although this work is promising, it suffers from some limitations, and the details of this grounding design and the efficiency of the hybrid model are no longer being discussed. In order to avoid a lightning strike, [27] examines the geometrical shape of the grounding system in a normal wind turbine. A model that increases the maximum step voltage and transient impedance by improving the geometrical shape and some other grounding system factors is presented. However, the study does not consider the impact of soil ionization and the frequency dependency of the grounding system transient behavior. The authors of [28; 29] study direct and indirect strikes in a small wind farm. To facilitate their simulation, they assume that the grounding system is a nonlinear resistance rod and that soil ionization is the only effective parameter for grounding protection. The impact of a modified wind turbine grounding system in a wind farm with ten wind turbines was studied in [30] in order to examine the reduction of backflow current overvoltages, GPR and the limitation of surge arrester burnout. As the main objective in the design of a grounding system for a wind farm is to prevent step and touch voltages from exceeding the maximum allowable limits and these two parameters are affected by different Short-Circuit Calculation (SCC)procedures [31], compares three of these to not only calculate the response of the grounding system, but also check the violation of safety limits for step and touch voltages [32]. In this paper, we consider a transient voltage study of a wind turbine [29] that considers high frequency models for all relevant components of the pair of turbines, such as transformers, vertical rods, horizontal conductors, Surge Arresters (SAs) and Frequency-Dependent (FD) cables. The study aims to reduce overvoltages by modifying the typical wind turbine grounding scheme according to our Proposed Modified Grounding Scheme (PMGS). Specifically, this study simultaneously investigates the effects of soil ionization and of frequency dependency to obtain the proposed model. An analysis is then conducted for two case studies: a single wind turbine under direct and indirect lightning strikes and a pair of turbines consisting of two wind turbines interconnected via an FD underground cable. A restructured version of the Electromagnetic Transient Program (EMTP-RV) is used to calculate the high-frequency transients. The PMGS and the high-frequency-dependent components (i.e., transformers and surge arrester) enhance the protection of the pair of turbines against strikes by increasing their insulation strength. The rest of this paper is structured as follows. Section 2 presents the fundamental components of a wind turbine. Section 3 introduces the proposed modified grounding scheme. Section 4 analyzes the overvoltage transitions in the single wind turbine and the pair of turbines. Finally, Section 5 presents some conclusions.