Wind Turbine Grounding System Research Articles

Time domain modeling of lightning transients in grounding systems considering frequency dependence and soil ionization

This paper presents a new model for analyzing transient grounding system behavior during a direct lightning strike. The modeling that we propose allows us to consider a simple grounding conductor buried horizontally or vertically, a grounding grid, wind turbine grounding system and grounding circuits for wind farms while including the aerial parts (towers and blades). This modeling is developed directly in the time domain taking into account the frequency dependence of the impedance and admittance of the wired conductors and of the electrical parameters of the soil (ρ, ε) and also considering the ionization of the soil. This formalism is based on transmission line equations, rational approximation by vector fitting and the Finite Difference Time Domain numerical method. The proposed formalism is verified considering both simulations and experimental results from available published researches. The Ground Potential Rise GPR in the grounding circuits of wind farms are discussed during the first stroke and also during the subsequent stroke whose frequency range generates more propagation of electrical waves along conductors. This formalism makes it possible to contribute to all knowledge/research of grounding systems.

Measured and simulated impulse responses of the grounding systems of a pair of wind-turbines connected by a buried insulated wire

• Prediction method of ground characteristics of wind turbines based on numerical electromagnetic field analysis method. • From the analysis, highly accurate transient grounding characteristics of wind turbines can be guessed. • It was possible to accurately acquire the characteristics of the lightning current flowing from the wind turbine to another wind turbine during a lightning strike. In recent years, it is becoming more common to replace old wind turbines and build new wind turbines with larger capacities in Japan. In the grounding design when replacing wind turbines, wind-turbine owners often utilize a part of the grounding electrodes of the conventional wind turbines to achieve lower grounding resistance with low cost. In addition, in order to make the grounding resistance value of the wind turbine lower than 10 Ω specified by IEC 61400–24, the grounding systems of multiple wind turbines are often connected. The ratio of the cost of constructing a grounding system of a wind turbine is not low, the cost is increasing year by year. Therefore, many companies involved in the design of grounding systems are beginning to use numerical electromagnetic field analysis methods in their designs in order to reduce costs. In this paper, measured and simulated impulse responses of the grounding systems of two single wind turbines and a pair of wind turbines connected by a buried insulated wire are introduced, and its characteristics are considered. In the analysis using the FDTD method, it is difficult to take into account the frequency-dependent characteristics of the soil because Maxwell's equations are time-differenced to formulate the equations used in the FDTD method. To construct a highly accurate analysis model, it may be necessary to build a model that takes into account the frequency-dependent characteristics of the soil. The method to construct such frequency-dependent model is also introduced in this paper.

Statistical Analysis of Lightning Flashes over Wind Parks in Greece

Currently, lightning phenomenon, mechanisms, and impacts on lives and infrastructures have been satisfactorily decoded and studied. Sound knowledge of lightning parameters is available in international literature. Yet, there are few studies referring to lightning statistics such as the number of flashes over an area, current amplitude distribution, etc., except for official documentation from national weather services, perhaps because of the stochastic nature of lightning. This work presents full recorded data for lightning flashes over wind farms distributed at the Hellenic territory. The data come from real time measurements at wind farm stations from 2011 to 2020 and concern number of CG flashes and lightning current amplitude. They are statistically processed and analyzed and contain useful information regarding the lightning characteristics of various geographic regions all over the country. Furthermore, the study displays data from field measurements of ground resistance at wind turbines and highlights techniques of designing and enhancing grounding systems of wind turbines for given lightning protection level (LPL). The present study, therefore, provides stakeholders with useful data and noteworthy conclusions about lightning occurrence and characteristics in Greece in order to make informed decisions on the various project stages, such as selection of the wind farm site, proper and in-depth risk assessment, and investment in safety measures for personnel and equipment.

Bare versus insulated conductors for improving the lightning response of interconnected wind turbine grounding systems

This paper investigates the lightning response of a pair of interconnected wind turbine grounding systems assuming their connection to be performed by a bare or insulated underground conductor. Typical first and subsequent stroke current waveforms are injected at one of the grounding systems and the transient response is studied for different soil resistivities considering frequency-dependent soil parameters. The ground potential rise (GPR) at the current injection point and the voltages transferred to the adjacent grounding system are calculated. GPR peak reductions are obtained using either a bare or insulated conductor, but the former is more effective. It is shown that when the wind turbine grounding systems are interconnected by a bare conductor, the GPR peak reduction is essentially due to the interconnecting wire. On the other hand, when the interconnection is made through an insulated conductor, the GPR reduction is related to the current that is partly diverted to the adjacent tower, especially for high resistivity soils. For an insulated interconnecting conductor and first lightning return-stroke currents, the adjacent grounding system also contributes to the GPR peak decrease.

Ground Potential Rise in Wind Farms due to Direct Lightning

This paper investigates the ground potential rise (GPR) developed in wind farms subject to direct lightning currents representative of first and subsequent negative downward strokes and return strokes associated with upward negative flashes. The wind farm grounding system is studied considering different numbers of interconnected grounding systems and for distinct values of soil resistivity taking into account frequency-dependent soil parameters. A wideband multiport model of the wind farm grounding system is obtained and implemented in a time-domain electromagnetic transient tool to include the aerial elements in simulations, namely the wind turbine blades and tower. It is shown that interconnecting more wind turbine grounding systems in parallel leads to a significant reduction of the low-frequency grounding resistance, whereas the decrease of the impulse impedance is much lower, especially for low-resistivity soils and negative downward subsequent strokes or negative upward lightning return strokes. It is also shown that the presence of the aerial components significantly affects the GPR in the response to lightning currents representative of negative downward subsequent strokes and return strokes associated with negative upward flashes, which have higher frequency content compared to negative first return strokes. The presence of the aerial components increases the GPR peak.

Effective Length of Counterpoises Connected to Wind Turbine Foundation

Many failures of wind turbine blades and electrical equipment due to powerful lightning strikes have occurred. The lightning-induced wind turbine failures that have occurred in recent years can be classified into two main categories. The first is a mechanical component failure. The second is a failure or malfunction of electric or electronic equipment inside or around the wind turbine due to an increase in the potential of the wind turbine grounding system or the electromagnetic induction caused by the lightning strike. In the latter case, the equipment often fails owing to the increase in the potential of the grounding system, the method of suppressing the potential rise by laying a grounding wire vertically or horizontally to the wind turbine foundation is adopted. We propose an empirical expression for the effective length of counterpoises connected to a large grounding electrode such as a wind turbine foundation, and verify the expression using finite-difference time-domain (FDTD) method.

Assessment of grounding grid for enhancing wind turbine service sustainability

In this paper, a wind turbine service sustainability is achieved by assessing grounding grids under steady-state and lightning-based transient conditions. An effective grounding grid is designed concerning IEC61400-24 standard, where Taif city, Saudi Arabia is considered a farming place. However, high soil resistivity is a challenge during the design. To accomplish the study, apparent resistivities are measured using Wenner method, in the field. A framework of multi-layer soil modeling is performed using the Sunde model, and it is optimally solved using the genetic algorithm. Using the optimal stratified soil, the wind turbine grounding system is assessed considering grounding resistances, electric potentials, and step voltages. The break-point effect is evaluated considering the designed grounding system. Then, the lightning-based transient study is investigated for both the healthy grid and break-point considerations. COMSOL Multiphysics and ATP/EMTP were used to accomplish simulations and modeling. The results ensure the effectiveness design of the assessed grounding system.

A Comparative simulation on the grounding grid system of a wind turbine with FEA software

The mathematical models proposed in the literature are used to determine the grounding resistance value for electrical installations. These models are mathematical expressions that change according to grounding type and the mechanical properties of the grounding system elements. Besides, the inclusion of nonlinear terms is another important issue in the modeling studies. For the modeling of nonlinear systems and the analysis of the faults that may occur, the finite element analysis (FEA) approach provides realistic results. In the present study, the simulation of a grid fault that may occur in the grounding system of a sample wind turbine (WT) is modeled by 2D electrostatic solver in FEA software. The simulation results prove that the findings are similar to the realistic faults in the grounding system.

Analysis of the lightning impulse and low-frequency performance of wind farm grounding systems

Previous analysis of the lightning response of wind turbine grounding systems has often considered only one unit. This paper investigates the impulse and low-frequency performance of isolated and interconnected grounding systems of wind turbines, subjected to lightning currents representative of first and subsequent negative downward strokes and return strokes associated with upward negative flashes. The system behavior is studied for different lengths of interconnecting conductor between adjacent grounding systems and for distinct values of soil resistivity considering frequency-dependent soil parameters. It is shown that in the case of lightning currents with shorter front-times, the interconnection improves the impulse response of grounding, reducing the GPR peak, only for soils of higher resistivity, whereas in the case of currents with longer front-times, the interconnecting improves the impulse response in all cases. The relative improvement of the impulse response when the adjacent grounding systems are interconnected becomes more significant with the increase of the soil resistivity. In all cases, interconnecting the grounding systems strongly reduces the low-frequency grounding resistance seen by each individual wind turbine earth termination.

Factors Determining the Effectiveness of a Wind Turbine Generator Lightning Protection System

As a result of the growing supply and demand for wind power, wind turbine generators are increasingly being installed at suboptimal sites that have high soil resistivity and high incidence of lightning strikes. This means that lightning protection systems for wind turbines are becoming a critical component of wind farm design. Not only do effective lightning protection systems ensure the safety of the physical wind turbine structure and human operators, they also protect the electrical and control systems installed inside wind turbine generators and safeguard the lives of human operators. This paper presents a framework to assess the effectiveness of wind turbine lightning protection systems at the wind farm design phase. Performing the analysis at this early stage reduces lightning-induced downtime, which leads to increased energy yield. Our results show that the grounding system plays a critical role in the wind turbine lightning protection system. For this reason, we also analyze various influential parameters of a grounding system design. We present results from full-wave electromagnetic simulations of the complete wind turbine grounding system, including the foundation.

A comprehensive analysis of the effect of frequency-dependent soil electrical parameters on the lightning response of wind-turbine grounding systems

The impact of frequency-dependent soil electrical parameters on the lightning response of typical wind turbine grounding systems is investigated. It is shown that the frequency dependence of soil parameters is responsible for decreasing the ground potential rise and, thus, the impulse impedance and impulse coefficient of the grounding systems. This effect is more pronounced for high-resistivity soils and fast current pulses. It is also shown that, considering typical dimensions of actual wind turbine grounding systems and soils with resistivity higher than 300Ωm, the impulse impedance seen by each single wind turbine is expected to be lower than the low-frequency grounding resistance.

Isolated vs. Interconnected Wind Turbine Grounding Systems: Effect on the Harmonic Grounding Impedance, Ground Potential Rise and Step Voltage

Wind turbines are very vulnerable to lightning strikes due to their height, sharp edges and remote locations often with high soil resistivity. In this paper we present numerical simulations of the impedance of a typical wind turbine grounding geometry. We analyze the influence of interconnecting grounding systems of different wind turbines. IEC TR61400-24 suggests interconnection of grounding electrodes of wind turbines through horizontal electrodes (in the form of insulated or bare conductors) to achieve low steady-state grounding resistance. The analysis takes into account the frequency dependence of the soil electrical parameters. We show that the low frequency grounding impedance can be reduced by a factor of two or more as a result of interconnecting grounding systems. However, the reduction is significantly lower at higher frequencies because of the interconnection wire’s inductance. We analyze the spatial distribution of the ground potential rise and step voltage in response to typical first and subsequent lightning return stroke current waveforms. It is shown that both, ground potential rise and step voltage can be significant along the wire, especially for high resistivity soil, and placing sensitive equipment near the interconnecting wire should be either avoided, or insulated wire should be used.

Transient behavior of wind towers grounding systems under lightning strikes

Wind turbines, because of their height and localization, are frequently subject to direct lightning strikes. Thus, the investigation on the performance of their grounding system is of paramount interest for the prediction of the potential threats either for people working in (or animals passing through) the wind farm area and for the power and control units installed in close proximity of the turbine towers. In this paper,we perform a comprensive study of the transient behavior of the earthing systems of wind turbines. The analysis is conducted in the frequency domain and an hybrid approach, based on circuit theory and Method of Moments, is adopted to fully account for resistive, inductive and capacitive couplings between elements of the ground system. The actual transient behavior is obtained by means of an Inverse Fourier Transform. The results, computed considering a typical wind turbine grounding system configuration, provide more insight on the nature of the early-time transient response of grounding systems and allow to draw up useful design guidelines.

Electrical Discharge and Its Impact on Drivetrains of Wind Turbines

Defects in drivetrains of wind turbines are typically very costly due to the difficulty in accessing the defective components for repair and maintenance. Electrical discharge has been identified as one of the key root causes to defects in wind drivetrains. Improving the reliability of key wind turbine components has been attracting a great research interest since the launch of the first wind turbine. In this paper, a study to understand the electrical discharge and its impact on wind drivetrains is presented. The root cause of electrical discharge in wind turbines is discussed. A field test is carried out on 1.5-MW doubly fed induction generator wind turbines in practical environments. Suggestions are provided on the design of the wind turbine grounding and protection system to improve turbine reliability and reduce defect rates due to electrical discharge.

Transient performance of interconnected wind turbine grounding systems

Transient performance of interconnected wind turbine grounding systems

Research of Mountain Area Wind Farm Grounding Scheme

Wind power has become one of the most potential and vital green energy sources. Location of the wind farm has a better wind resource，and it is also relatively open. Lightning accident in the wind farm happened occasionally, in order to avoid the situation, which impulse current damages the wind turbine, in the accident, lightning protection grounding system of wind turbine is essential .In this paper, according to the experience of Dongshan wind farm project , the method of how to design the wind turbines and booster station grounding grid as required, the calculation of ground resistance and specific measures to reduce grounding resistance of wind turbine grounding device are discussed.

Interfacing harmonic electromagnetic models of grounding systems with the EMTP-ATP software package

This paper proposes a methodology for interfacing the frequency-domain harmonic electromagnetic models of complex grounding systems (buried in horizontally stratified multilayer medium) with the EMTP-ATP software package, for the purpose of transient analysis of electrical power systems. It comprises three individual steps: (1) construction of the frequency-dependent nodal admittance matrix for the arbitrarily positioned system of conductors in horizontally stratified multilayer medium, (2) application of the vector fitting technique to this frequency-dependent nodal admittance matrix and (3) construction of the Foster-type networks from the results of the vector fitting and their interfacing with the EMTP-ATP environment. The proposed methodology is general and could be applied to different kinds of grounding systems, ranging from wind turbine grounding systems, to groundings of high voltage transmission line towers and towers carrying GSM base stations.

Electromagnetic Analysis of Interconnected Groundings at Wind Farm

Interconnection of the grounding system of wind turbines reduces steady-state grounding resistance, however, the transient behavior during a lightning strike to an interconnected wind turbine grounding system remains to be discussed. This paper investigates the influence of interconnection on the lightning surge response of wind turbine grounding system. Two different numerical methods, Numerical Electromagnetics Code (NEC-4) based on Method of Moments (MoM), and Virtual Surge Test Lab (VSTL) based on Finite Difference Time Domain (FDTD) method, are used for lightning surge analysis of wind turbine grounding systems. Comparison of results using NEC-4 and VSTL are made to clarify the usefulness of these methods for transient analysis of a large grounding system. The influence of soil parameters, front time of injected current, and the type of a conductor employed for interconnection, on the response of interconnected wind turbine grounding is discussed.

Transient analysis of grounding systems for wind turbines

The paper deals with transient analysis of grounding systems wind turbines. A typical wind turbine grounding system arrangement based on ring electrode is analyzed. Special attention is focused to the influence of additional vertical and horizontal electrodes, respectively. Influence of grounding wire placed in cable trench on the transient behavior is studied in detail. The mathematical formulation in the frequency domain is based on the thin wire antenna theory and the related set of Pocklington integro-differential equations. The corresponding transient response is obtained by means of the Inverse Fourier Transform (IFT). The set of Pocklington integro-differential equations is solved by the Galerkin-Bubnov Indirect Boundary Element Method (GB-IBEM) featuring the use of isoparametric elements. A number of illustrative computational examples are presented in the paper.

Wind farm grounding system design for transient currents

Wind turbines are often very high structures that are usually installed in high keraunic level areas. The keraunic level is the number of storm days per year. Therefore, wind farms are very vulnerable to lightning discharge. The damage due to a lightning strike can be reduced if the high current is quickly conducted to the ground. To date, wind turbine grounding system designs have been based on prior experience, without accurately studying transient grounding system behavior. In this work, typical wind farm grounding system geometries are analyzed in the context of lightning strikes.