Soil Ionization Research Articles

Galerkin boundary element method for simulating lightning response of grounding grid in horizontal multilayered soil model considering soil ionization effect

AbstractThis work proposes a novel mathematical model based on the Galerkin time‐domain boundary element method for accurately calculating the lightning current distribution and lightning impulse response of the buried substation grounding grid, in a multi‐layer horizontal layered soil model, by taking into account the soil ionization effect. To improve computational efficiency, the quasi‐static complex image method and its closed form time‐domain Green's function have been introduced into the model that has the ability to analytically calculate the mutual inductance coefficient between the branch currents of any two conductor segments and the mutual resistance coefficient between the leakage currents. The Galerkin time‐domain boundary element method proposed in this work can simulate the transient lightning impulse response of a substation grounding grid buried in the multi‐layer horizontal layered soil.

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.

Lightning Transient Impulse Problem of Substation Grounding Grid in Multilayer Horizontal Layered Soil Considering Soil Ionization Effect and Frequency Characteristics

Lightning Transient Impulse Problem of Substation Grounding Grid in Multilayer Horizontal Layered Soil Considering Soil Ionization Effect and Frequency Characteristics

Lightning Response Transient Characteristics of Substation Grounding Grid in Multilayer Horizontal Stratified Earth Model Considering Soil Ionization Effect

This article proposes a new time-domain partial element equivalent circuit method mathematical model. The mutual impedance matrix between the potential on the discrete node and the leakage current on the conductor connected around the discrete node is adopted. To reduce the amount of calculation, the matrix equation that minimizes the unknown quantity is derived and adopted. The model can accurately calculate the lightning current distribution and lightning impulse response in a horizontal multilayered soil model considering the soil ionization effect. To improve computational efficiency, the quasi-static complex image method and its closed form of Green's function in the time domain are introduced into the model.

Transient analysis of earthing electrodes considering soil ionization phenomenon under lightning impulse condition

Transient analysis of earthing electrodes considering soil ionization phenomenon under lightning impulse condition

Analysis of grounding system transient with consideration of soil ionization and mutual coupling effect

The grounding system of an electrical substation provides safety to both operator and equipment. The lightning current waveform has a major influence on the dynamic performance of ground electrodes. If lightning current is large enough, soil will be ionized, which will have a great effect on the transient performance of the grounding system. Many numerical models have been developed to evaluate the transient performance of grounding system, still it is a challenging task to analyze the impulse behavior of grounding system efficiently. In this paper, the transient behavior of grounding systems with consideration of nonlinear and dynamic effects of soil ionization is presented. Grounding systems such as vertical rod, horizontal conductor and grounding grid buried in the homogeneous soil are considered as case study. Proposed model is the combination of circuit theory approach and method of moment with consideration of frequency-dependent impedance and mutual coupling effect between the conductor segments. Self and mutual component of inductance and resistance are considered in the analysis. Initially, transient performance of grounding system is analyzed for single vertical rod, horizontal and square grounding electrodes, further it is extended for vertical rod (more than one), horizontal conductor and grounding grids with different configurations. The analysis of grounding system with the consideration of mutual coupling effect is compared with devoid of mutual coupling, and differences between them are evaluated. For validating the developed method, simulated results are compared with the results reported in the literature, and good agreements are found. Analysis shows that the grounding impulse impedance decreased with increasing the magnitude of injected current. Developed method will help to improve the modeling of simple as well as complex grounding system buried in homogeneous soil.

A Practical Method for Transient Analysis of Grounding Grids

A practical method is proposed in this paper for analysis of the lightning transient in the grounding grids. The circuit parameters of the grounding electrodes are evaluated by a set of analytic formulas, in which the soil ionization effect is taken into account. By means of the circuit parameters, the grounding electrode segments are represented by the π‐type circuits and a circuit model is built for the grounding grids. The lightning transient responses in the grounding grids can be calculated from the circuit model. The calculated results are compared with the measured ones to check the validity of the circuit model. In view of the actual distribution randomness, the lightning current amplitude is taken as a random variable and a statistical algorithm is further presented for statistical analysis of the lightning transient in the grounding grids. In the algorithm, the accumulative probability function is introduced to describe the random distribution of the lightning current amplitude. The lightning transient responses calculated in a large interval of the lightning current amplitude are weighted by the corresponding probability density. Then, the statistical values of the lightning transient responses can be obtained from the weighted sum. A numerical example is also given to discuss the difference between the statistical values and the nonstatistical ones calculated by the conventional method. © 2023 Institute of Electrical Engineer of Japan and Wiley Periodicals LLC.

Back Flashover Probability of the Overhead Power Line Insulation Taking into Account the Soil Ionization and Frequency Response Features

Back flashovers caused by a lightning strike on an overhead power line tower or overhead ground wire give rise to voltage surges dangerous for the insulation of power substation electrical equipment. A pulse back flashover turns into a short-circuit arc leading to the power line tripping. Calculations of voltage surges in a single–circuit 110 kV power line are carried out using the methods of the grounding devices theory, and the back flashover probability is estimated based on the curve of dangerous currents. The insulation back flashover probability for a power line tower with a ground wire in areas with high-resistance soil is determined by the grounding conductor surge impedance. It is shown that the analysis carried out with taking into account the soil ionization and frequency response features yields an essentially lower back flashover probability. The second ground wire placed under the power line wires reduces the inductance of the ground wires, thereby intensifying the spread of current through the grounding conductors of neighboring towers. This leads to a decreased tower voltage and an increased voltage induced on the wires, thus resulting in a decreased voltage across the insulation voltage and, hence, lower back flashover probability. The factors governing a positive effect from using grounding counterpoises of an overhead power line in areas with high-resistance soil are shown, and the conditions under which this effect is achieved are determined.

Modeling of Stray Currents From Metro Intruding Into Power System Considering the Complex Geological Conditions in Modern Megacities

The risks of power supply in power systems attributed to stray currents caused by denser metro lines are increasing in modern megacities. Factors such as multi-layered soil and multiple metro operating lines (MOLs) make it more difficult to analyze dc coupling between the metro and power system. This article proposes a model for dc coupling of the metro and power system considering the two systems’ spatial information, the actual multi-layered soil, and the non-linear ionization of soil caused by stray currents. This article presents a dc coupling model suitable for studying multiple metro lines and large-scale power systems in modern megacities, which has an average relative error of 6.3% compared with experimental results. The neutral current of the transformer in a substation is up to 43 A (normal operation requires it to be less than 6 A in amplitude) under the influence of multiple MOL. The non-linear ionization of the soil near the power substations’ grounding network will decrease the soil resistance, this also results in greater dc currents intruding into power substations.

Studying Direct Lightning Stroke Impact on Human Safety Near HVTL Towers Considering Two Layer Soils and Ionization Influence

A lightning strike is considered one of the most risky natural phenomena that can lead to human harmful and the surrounding soil layers. To tackle this issue, this article investigates the influence of direct lightning characteristics in terms of human body safety. Specifically, such investigation is carried out on the effect of resistivities of two-layer soils on human safety when lightning stroke hits the towers of the high voltage transmission lines (HVTLs). The merit of the proposed study is that the soil ionization phenomenon is taken into consideration. Further, the study focuses on the current passing through the human heart, when step and touch (contact) voltages are generated by grounding potential rise, caused by direct lightning strikes transmission tower and the produced potential rise that a person could be exposed. Also, studying the effects of peak current and time of lightning strokes are investigated. Additionally, the paper presents the effect of different reflection factors on human safety. For validation purposes, the ATP program is used in the simulation of the grounding system as well as the human body model. Numerous simulations were accomplished in order to examine the behavior of the current passing through with the human heart. Based on the simulation results, it was concluded that the soil characteristics have superior influences on the contact and step potentials and, accordingly, the survival threshold.

Modeling soil ionization effect based on experimental data

• Measurements of Soil Ionization. • Modeling Soil Ionization effect based on experimental data. • Means for estimating soil ionization on the response of grounding electrodes. • Ionization of soils subjected to uniform electric field. • Ionization of soils subjected to non-uniform radial electric field. Results of a huge number of experimental tests involving soil ionization developed by the authors were used in this paper for analyzing the phenomenon and developing means for predicting the effect on the response of grounding electrodes subjected to lightning currents. These results refer to samples of fifteen different soils (from about 100 to 10,000 Ωm at natural moist content) subjected to impulsive current waves exhibiting front times similar to those of first and subsequent return stroke currents. The tests considered conditions of uniform and radial non-uniform electric fields with intensities comparable to those resulting from the impression of real lightning currents on grounding electrodes. A curve was suggested for taking the effect of ionization on the lightning response of electrodes into account for soils in general, by means of an equivalent increase of the electrode radius as a function of the linear density of current dispersed to the soil.

HEM-TD: New Time-Domain Electromagnetic Model for Calculating the Lightning Response of Electric Systems and Their Components

A new electromagnetic time-domain model dedicated to calculating the lightning response of electric systems and their components is presented. It is formulated taking as reference the main features of the frequency-domain Hybrid Electromagnetic Model. A differential aspect of this model is the capability to efficiently address problems involving nonlinear effects, such as soil ionization around grounding electrodes, the surge-arresters operation and the Corona Effect in applications related to the lightning performance of transmission lines. In addition, it is able to consider frequency-dependent effects, for instance that of soil resistivity, with an accuracy compatible with engineering applications. The basic formulation of the model is shown along with the validation of the results it yields by comparison with reliable references, including experimental results.

Overview of Transient Simulations of Grounding Systems under Surge Conditions

The present paper gives an overview of modelling methods of standard grounding systems under surge conditions, using the non-uniform transmission line approach. The model presented considers the soil ionization and the frequency dependence of the soil parameters during the current transients. Furthermore, the representation of the non-linear response of the soil is made using a shunt time-variable resistance to simulate the behavior of the grounding resistance when a surge current flows through the system. The model development and analysis are made using ATP-EMTP/ATPDraw transient software.

An EMTP Extension for Computing Earthing System Transient Step and Touch Voltages

This paper presents a novel technique for computing dangerous voltages due to lightning transients imposed on earthing system, which is based on the use of the well-known ATP-EMTP software package. Earth surface transient potential distributions, as well as step and touch voltages computations, are performed through extending the widely used EMTP software package with a new post-processor (computer program), developed especially for that purpose. The earthing grid is approximated by the circular cross-section conductors. In numerical model, conductors are subdivided into segments (1D finite elements) and Clark's model with distributed constant parameters is then applied. Leakage conductance of conductor segments is modeled in EMTP as an additional lumped parameter. Analytical expressions for distributed and lumped segment parameters are derived using the average potential method. Due to the limitations of the EMTP, EM coupling between segments is neglected. The earth model is limited to the homogenous earth. Soil ionization effect is not accounted for, but could be incorporated, through some modifications of algorithms. Lightning surge model used is based on the Heidler's model of current source.

A simplified numerical modeling of the transient behavior of grounding systems considering soil ionization

The analysis of the transient performances of grounding systems during a lightning discharge is proposed in the literature by modeling in time or in frequency domain. The frequency modeling makes it generaly possible to take into account the electromagnetic effects observed during the rise time of the pulse while temporal modelling makes it possible to introduce the nonlinear phenomenon (soil ionization) observed following an injection of a very high intensity lightning strike. In this work we propose a modeling with a simplified implementation which takes into account the various electromagnetic couplings which arise after spatial discretization of the grounding system as well as the ionization of the soil. This new formalism is developed from the equations of nonuniform Transmission Lines (nuTL) and the Finite-Difference Time-Domain (FDTD) method. This simplified modeling is carried out solely by spatial discretization of the grounding system conductors, is done without loss of precision and with very low calculation times. Our calculation results are compared to those already published and carried out by simulation or by the measurements.

Transmission Line Dynamic Circuit Model for Effective Length of Ground Electrode Under Lightning Transients

The effective length of the ground electrode is one of the primary research interests of lightning grounding system design. In this article, using a practical numerical analysis, a new expression for estimating the effective length of the simple ground electrode is proposed. A new dynamic circuit model based on the transmission line is developed to derive the expression. The model considers the soil ionization, frequency-dependent soil parameters, mutual reactance, and skin effect. Several rigorous impulse test simulations are performed on different electrode lengths, soil properties, and lightning currents to estimate the effective length of ground electrode numerically. A new expression is arrived at using the curve-fitting and optimization technique from the relationships of the effective length with soil resistivity, impulse rise time, and current. The proposed dynamic model and the expression to obtain the effective length are validated by published results.

Soil impulse ionisation model based on dynamic changes of electric field

Soil ionisation has an important influence on impulse characteristics of grounding conductor. This paper introduced improvements to an existing soil ionisation model considering the influence of electric field on the change of soil resistivity, while soil resistivity is not directly dependent on soil electric field. The validity of improved soil ionisation model was verified by surge response of vertical conductor in uniform soil and horizontally stratified soil. Furthermore, the impulse experiment was carried out on horizontal conductor in soil with finite volumes. Based on experimental configuration, the corresponding calculation model was built, and calculated results were in good agreement with experimental results.

Analysis of soil impulse discharge characteristics based on optical‐electrical synchronous observation

The impulse performance of grounding electrodes highly depends on soil impulse discharge characteristics. The analysis of soil discharge dynamic process images is helpful to understand the impulse discharge mechanism of grounding electrodes. This paper designed an optical-electrical synchronous observation platform for simulate soil (SiO2 crystal) discharge, and discharge dynamic process images is obtained by high-speed camera. Through analysing breakdown delay time and thermal power before breakdown, it is found that soil impulse discharge is mainly an electrical breakdown process, and soil ionization intensity is positively correlated with the light intensity emitted by whole soil sample in breakdown channel development stage. The time from voltage application to the occurrence of ionization is defined as initial ionization delay time (IIDT). Using image processing technology, the change curve of optical image average grayscale value with time in the process of soil discharge is obtained, and a calculation method of IIDT is proposed by comparing the difference between average grayscale value curve and background image average grayscale value. Experiment results show that IIDT is negatively correlated with impulse voltage, particle size and water content, but not with salt content.

Experimental Investigation of the Lightning Impulse Behavior of Wet Sandy Soil

Soil ionization, that is, electrical discharges developing in the ground, may have a significant impact on the behavior of grounding systems under lightning currents, as it causes a reduction of the impulse ground impedance. This may have several implications on insulation coordination studies for power systems. The effects of soil ionization have not been completely clarified yet due to its complexity associated with soil characteristics and conditions, ground electrode configuration, lightning current characteristics, as well as difficulties in the accurate determination of discharge activity. This study deals with the experimental investigation of the lightning impulse behavior of wet sandy soil. An experimental setup and measurement procedure are introduced for this purpose. Experiments under lightning impulse voltages of negative polarity were performed on two wet sandy soils, both as obtained from their extraction sites, as well as after drying and manual addition of moisture; thus, the influence of the soil sample preparation procedure is evaluated. Results are discussed considering soil ionization inception, electrical breakdown of soil, impulse impedance, and soil electrical properties. A simulation model is proposed for the prediction of the instantaneous impulse impedance when soil ionization occurs; this model is suitable for time domain simulations in ElectroMagnetic Transients Program (EMTP)-type software.

Performance of Grounding Electrodes Under Lightning Strokes in Uniform and Two-Layer Soils Considering Soil Ionization

This article presented the characteristics of the grounding electrodes in uniform and two layer soils when subjected to lightning including the impacts of soil ionization with frequency, soil resistivity, and permittivity variations. The critical breakdown field strength of two layer soils with different values of the reflection factors was presented. The transient probabilistic grounding potential rise and the transient impedances of the horizontal and vertical electrodes were investigated in uniform and two layer soils including the effect of soil ionization with frequency. Finally, the influences of resistivity and permittivity variations which have substantial impacts on the electric field at the occurrence of the soil ionization were considered. Moreover, transmission line approach (TL) with the aid of ATP has been used to compute the transient grounding voltages. The results indicated that the reflection factor has a significant impact on the equivalent radius of the grounding electrode. It is also observed that the peak values of the lightning induced voltages decreases sharply when the soil ionization phenomenon considering the variations in soil resistivity and permittivity with the stroke frequency changes. Furthermore, the impedances of the burial electrodes that have negative reflection factors were lower than that in case of positive reflection factors.