# Evaluating the High Frequency Behavior of the Modified Grounding Scheme in Wind Farms
## 1 Introduction  
An ongoing concern about wind generators [1] and wind farms is how to protect them against harsh environmental conditions [2]. With the use of wind power generation growing rapidly, lightning damage must receive more attention [3; 4]. A lightning strike is a tremendously powerful phenomenon that can produce overvoltages in various components of a wind turbine [5; 6], which can easily spread across a network of turbines [7]. Wind turbines are tall, isolated towers consisting of sensitive devices, which makes lightning a real and persistent threat. One of the major consequences of direct and indirect strikes is Transient Ground Potential Rise (TGRP). When lightning causes a transient overvoltage, the electricity tends to take the path of least resistance to earth. As a result, the electrical potential is increased in both the grounding system and the soil, which makes the grounding system unreliable and reduces the wind tower generator system's reliability and safety [8; 9; 10]. Therefore, one of the effective ways to increase a wind tower's stability and reliability during lightning strikes is to analyze the transient behavior of wind turbine grounding systems [11; 12]. Since the grounding system provides safety for electrical installation, it is significant to conduct a broad research on earthing changes [13; 14] during fast transients such as changes in soil resistivity orchanges on the size and structure of grounding to improve its functionality in different parts of the power system [15]. This is especially important in mitigating the rate of overvoltage for wind turbines, as several studies have been carried out to design appropriate models in this regard. In [16], a multi-port transient model to enter the frequency-dependent responses of a wind turbine grounding system into time-domain codes is presented. A hybrid model based on circuit theory and the method of moments is another approach to increasing the reliability of the grounding system in a frequency domain [17]. The method presented in [18] is based on measuring and analyzing the Ground Potential Rise (GPR) around a wind turbine caused by different types of lightning strikes. The important parts of the simulated wind turbine grounding system are grounding mesh and foundation feet. The authors use the Laplace transform to investigate the efficiency of this method. However, in order to design an accurate and comprehensive grounding model with high reliability, it seems necessary to consider the high-frequency performance of the grounding system. A three-stage methodology, Modified Matrix Pencil Method (Modified-MPM) [19], is designed to incorporate large grounding systems' behavior into electromagnetic transient solvers. The results demonstrated that using the resistive model for the grounding system leads to a significant decrease in the lightning-generated overvoltages' computation. The authors in [20] used a field solver to model the tower foot grounding system during the lightning strike. Through sensitivity study and comparison between wideband modeling and constant resistance, they indicated that with the 19% difference in Max voltage, the wideband modeling would be able to generate the frequency-dependent model and take into account soil resistivity variation for utility companies. Studying the behavior of the grounding system in a broad range of frequencies through parameterized macro modeling is the focus of [21]. This method is much more cost-effective than using field solvers and therefore is accurate in predicting the behavior of the multidimensional system in both the frequency and time domain. Reviewing the above literature indicates that in order to have a reliable system, it is crucial to simultaneously study nonlinear soil ionization and frequency dependency [22]. Many studies have been carried out to design protection against strikes for wind power plants, but a few of these have considered a high-frequency model of the turbine's components in real conditions [23].