Ground Potential Rise Research Articles

Study on corrosion fracture diagnosis method of grounding wire of tower grounding device

Tower grounding device provides the discharging channel for currents under lightning and fault. However, the grounding wire is prone to corrosion. Existing detection methods through measuring the grounding resistance to detect fracture, but it suffers from poor accuracy when the fracture grounding wire is inconsistent with the detection position. This paper proposed a simple, rapid and effective diagnosis method based on the ground potential rise (GPR). Firstly, the non-uniform corrosion characteristics of the grounding wire in soil are analysed and the corrosion deformation regularity of the grounding wire surface is obtained. Then the fracture model of the grounding wire is developed. The GPR variation feature and mechanism of the grounding wire under faults are studied. Finally, the simulation and experimental results show that the proposed method can accurately detect the fracture status of the tower grounding wire, which can provide a solid basis for diagnosis of the tower grounding devices.

Impulse Impedance and Effective Area of Grounding Grids

High values and uneven distribution of the transient ground potential rise (TGPR) at grounding grid conductors in cases of lightning can disrupt the system safety and reliability. To assess the transient performance of grounding grids under lightning currents, a parametric analysis was performed with a computer simulation of 13,978 cases of time-domain responses with a broad range of parameters. From this study, graphs of impulse impedance and a new formula for the effective area have been developed. The graphs are general, describe grids of any shape and size and depend on three parameters: grid conductor spacing, the resistivity of earth, and the zero-to-peak time of the lightning current pulse. Although the parametric analysis was based on a uniform soil model, the graphs can also be used for the two-layer soil model. In this paper, graphs and formula are used in a simple procedure to estimate the effectiveness of a mitigation measure to reduce the TGPR by increasing the grounding grid mesh density in a limited area around the current injection points. The results from the procedure are compared with experimental results, and the application and limitations of the procedure are illustrated by considering large grounding grids as examples.

A Discussion on Practical Limits for Segmentation Procedures of Tower-Footing Grounding Modeling for Lightning Responses

In this article, we evaluate the practical limits of length segmentation of tower-footing grounding modeling. The objective is to present a relation between computational efficiency and segmentation for transmission lines lightning performance studies. A physically consistent electromagnetic model based on field theory is applied. The segmentation process is designed to improve computational efficiency, in relation to the proposals suggested in the literature, and to maintain the precision of the results. The criteria considered acceptable for a given segmentation (in relation to a reference) are as follows: 1) the grounding potential rise (GPR) must have a peak value up to a maximum of 10% higher, 2) the lightning overvoltages peaks on the tower top (and insulator strings) must have a peak value up to a maximum of 5% higher, and 3) the GPR waveforms shall follow the same pattern (the same being valid for overvoltages). Soils with low (50–300 $\Omega {\cdot}$ m), medium (300–1000 $\Omega {\cdot}$ m), and high (1000–4000 $\Omega {\cdot}$ m) resistivities are considered. The relative permittivity is maintained at 10. The results illustrate that the optimum segmentation can be as great as 1000 times the radius of the electrode. This ratio is higher than the maximum segment sizes suggested by the technical literature. The computational gains are quite high (over 30 times faster).

Analysis the influence of corrosion layer on the grounding performance of grounding electrodes

The grounding electrode buried in soil is prone to corrosion affected by soil and leakage current. The surface of corroded grounding electrodes is wrapped with corrosion product, which affects the normal current dissipation process and weakens the grounding performance. The accurate analysis of the grounding performance with corrosion layer can lay a solid theoretical basis for the safety of the power system. In this study, the corrosion layer is regarded as a layer of conductive medium between the electrode and soil. The leakage current along the electrode and the charge on the interface are analysed by coupling point matching method with boundary element method. Finally, the leakage current, grounding resistance, ground potential rise (GPR) and the step voltage of the two typical electrodes with corrosion layer are investigated. The results show that the corrosion layer causes the reduction of the leakage current at the end of the electrode and the enhancement of the leakage current in the middle location. Furthermore, the corrosion layer causes the increase of the grounding resistance and GPR, and the decrease of the step voltage. In addition, the influence of the reduced radius on the grounding performance is much smaller than that of the corrosion layer.

Impact of Frequency-Dependent Soil Models on Grounding System Performance for Direct and Indirect Lightning Strikes

The goal of this article is to investigate the effect of frequency-dependent soil models on the performance of grounding electrodes subjected to lightning strikes. Several soil models are examined while accounting for the variation of soil resistivity and permittivity as a function of the lightning current frequency spectrum. The analysis is performed for a homogeneous soil and a two-layer horizontally stratified soil. The impact of the frequency-dependent soil parameters on the ground potential rise (GPR) of simple grounding electrodes subjected to lightning is analyzed and discussed. The analysis is performed in the frequency domain and in the time domain. A wind turbine and its grounding system are also considered in this article. Special attention is given to the case of indirect lightning, rarely mentioned in the literature. The GPR of the grounding electrodes is examined when the frequency dependence of the soil is taken into account and the lightning channel is located at close distances to the electrodes. Indeed, the level of induced electromagnetic fields caused by a nearby lightning channel can still be too high and potentially dangerous. The computations are performed using an efficient Method of Moments (MoM) numerical tool based on surface-wire integral equations for a stratified medium in the frequency range from dc to several MHz. Numerical results demonstrate that the frequency dependency of the soil parameters results in a decrease of the potential rise of the grounding electrodes, with respect to the case where the parameters are assumed constant.

Comprehensive Design of an Earthing System for Broadlands Hydropower Station Switchyard

Broadlands Hydropower Project (BHP) harnesses the last hydropower potential of Kehelgamu-Maskeli Oya rivers with an installed capacity of 35 MW and 126 GWh annual energy generation. Excavation of the powerhouse switchyard site exposed to a bedrock formation with highly weathered granitic gneiss beneath a thin layer of scum soil top, resulting in an irregular high soil resistivity profile. Therefore, the main purpose of this research is to design a safe and effective grounding system for the switchyard of BHP, which can carry fault current into the ground without exceeding tolerable ground potential rise, ensuring the desired operation of protective & control devices so that not to endanger human & equipment. Owing to the nature of non-uniform high soil resistivity and limited land space for extension, this has become a great challenge. Research used two approaches, guidelines of conventional IEEE 80-2000 standards and Finite Element Method (FEM). Initially, resistivity measurement was conducted covering the entire area of concern. A soil model was prepared using orthodox horizontally stratified two-layer soil model using Sunde’s graphical technique based on measured data. Then the grounding grid was designed adhering to guidelines given in IEEE 80:2000 standard and observed high overall grid resistance, eventually exceeding the tolerable step and touch potential levels. Thereafter a soil model was prepared based on FEM which facilitates plot of accurate and smooth surface voltage distribution over the entire switchyard area. Applying fault current to these discrete finite elements and based on the first principle of Kirchhoff’s current distribution balance, the localized voltage distribution has been developed for the entire area and plotted using a self-developed MATLAB computer program. FEM model can trace the points where the touch and step potentials exceed safe limits in two-dimensional stratified grid, estimation of voltage gradients at boundary areas, which all are unable to track using conventional IEEE method. Accuracy of the model can further be increased by reducing the size of the soil element. Finally, several sensitivity studies were conducted so as to optimize the BHP switchyard grid design ensuring safe grid operation.

Characteristics of Two Ground Grid Potentials After a Triggered Lightning Stroke

It has been determined that the discharge of lightning current to ground through grounding grid results in high potential at the grounding grid and other nearby grounding grids, which is very destructive. In the present paper, based on artificially triggered lightning technology, the characteristics of the ground potential rise (GPR), the transient effect at the active grid under the transient impulse of lightning current was analyzed. The impulse grounding resistance is found to decrease for large input currents. On the other hand, for small input currents, it usually increases. In addition, the characteristics of the transfer potential of the passive grid which is located 40 m from the active grid were analyzed. It was shown that the peak value of the transfer potential at the passive grid was approximately 5.1% of the peak value of the ground potential at the active grid during the return stroke. The achievements made in this study may contribute toward the optimization of the simulation model.

A matrix method for the study of interferences in transformers neutrals due to HVDC ground current injection

This paper discusses the occurrences caused during the commissioning of the ground electrode of the Porto Velho Converter Station, located at the North region of Brazil, and the methodology used to analyze and mitigate the problem.The converter station is composed of a group of agents and linked with two major power plants – Santo Antonio and Jirau, several 500 kV and 230 kV transmission lines and nearby substations. The main issue caused by an electrode at long distances is the DC current at transformers neutrals caused by ground potential difference between locations.The predicted current at the transformers neutrals are unacceptable, which main effect is the core saturation, resulting in audible noise, vibrations and excessive heating in the equipment. Due to the multi-agent nature of the station, the use of current blocking devices at third party assets are not an option.The paper will describe the investigation of alternatives, resulting in a site selection for a new electrode. We will emphasize the importance of a proper geologic survey and soil model, not only for the site selection but also for predict the ground potential rise, an effect which could extend for several kilometers.

Effects of Various Earth Grid Configurations on Ground Potential Rise Caused by Lightning Strike

Ground Potential Rise (GPR) caused by lightning strike is a potential hazard for electrical equipment inside an oil and gas refinery plant. In order to mitigate the risk, horizontal grounding grid is applied. The best mitigation is to install a grounding grid with mesh size as small as possible. This condition requires a high cost. In order to obtain the optimal mesh size, a series of simulation of a grounding grid with mesh size variations on GPR caused by lightning strike has been carried out. CDEGS software was used to observe the GPR with various mesh size from 6.5 x 6.5 m to 20 x 20 m. Simulation results show that the maximum transient GPR rises as the grounding grid mesh size is increased, while the GPR distribution throughout the grounding grid area does not change much for different mesh sizes. In the other hand, decreasing the grid size would mean that more conductors are required, hence the cost would increase accordingly. The result shows that grid sizes from 6.5 x 6.5 m up to 20 x 20 m have no significant difference in term of GPR. In term of cost, 10 x 10 m does not show significant difference with 20 x 20 m, on the other hand, there is a significant difference for grid sizes 1 x 1 m to 10 x 10 m. From the results, grid sizes between 10 x 10 m up to 20 x 20 m are still applicable as stated in Petronas Technical. To comply with proper GPR value, additional protection devices are needed to protect the electrical equipment from potential damage.Keywords –GPR, grounding grid, mesh size

Framework for optimal grounding system design concerning IEEE standard

This paper investigates the optimal grounding grid design using artificial intelligence techniques based on the empirical formula for grounding resistance (Rg), touch and step voltages (Et and Es), which are addressed in IEEE Std. 80-2013/Cor. 1-2015. The objective function is formulated based on the grid conductors’ material cost and the installation cost. Particle swarm optimization (PSO) and genetic algorithm (GA) are individually utilized to search for and confirm the global best solution of minimizing the grounding system cost with considering all operation constraints including the safety criteria. A modified objective function is constructed based on self-adaptive penalization utilized to account for constraint violations during the optimization process. The selection strategy for modifying the local best positions of particles and the global best position of the swarm is proposed to enhance the ability of PSO for fast convergence. Results proved that either PSO or GA settles on the global best solution, successfully. To perform the space domain investigation, the optimal grounding grid design is implemented using a finite element method, where the COMSOL Multiphysics program is selected to verify the settled optimal global solution. Using COMSOL, the grounding resistance and safety criteria are evaluated over the grid diagonally. The COMSOL results ensure the operating constraints satisfaction of the optimal grounding grid design. The performance evaluation reveals that the grounding potential rise limit, the available grid area, and the number of vertical rods greatly affect the optimization of the grounding system design. The results also illustrate a great significant effect of the fault current and upper surface resistivity on the optimal grid design.

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.

Analysis of frequency distribution of ground fault-current magnitude in transmission networks for electrical safety evaluation

Ground Potential Rise (GPR) evaluation under fault conditions is important for the evaluation of electrical safety risk to human beings who are present in and around electric installations. Current national and international standards recommend such evaluation is based on a set of ‘worst-case’ conditions that include a simplified fault current calculation procedure based on assuming a maximum value. Accordingly, this approach may lead to over-design of grounding systems in certain cases by overestimating individual risk. To address this and move towards a comprehensive probabilistic assessment of risk, a more detailed model of the electric network is required for fault current magnitude evaluation. This paper describes the application of the multi-conductor method to determine fault current distribution on transmission networks. First, a model of a portion of the UK transmission network is built and the fault current and its distribution among the phases, ground wire and ground is evaluated for three example fault positions. It is found that the position of the fault along the line, substation bus section status and proximity to generation affect greatly the current distribution. Then, a transmission model based on the national network is built, and the effects of system loading and generation on fault current magnitude were also considered. A complete frequency distribution of fault current magnitude is obtained and the results demonstrate the value of such description for the probabilistic assessment of human safety in grounding system design.

Performance of Large-Scale Grounding Systems in Thermal Power Plants Against Lightning Strikes to Nearby Transmission Towers

In spite of the contemporary interest in renewable power plants, thermal power plants are still inevitable. Various electric equipment and apparatus are grounded via a large-scale grounding system in thermal power plants. In this paper, the three-dimensional finite-difference time-domain method has been employed to study the performance of such a large-scale grounding system against a lightning strike to a nearby transmission tower. The study has emphasized how a nearby sea, which is utilized for cooling purposes in thermal power plants, influences the ground potential rise on the large-scale grounding system considering soil ionization. The results show that the distribution of the ground potential rise on the large-scale grounding system is quite dependent on the alignment of sea with the large-scale grounding system. In addition, the extent that soil ionization affects the ground potential rise is dependent on the distance between the struck tower and the large-scale grounding system.

On the Interconnections of HV–MV Stations to Global Grounding Systems

The interconnection of grounding systems of HV–MV stations via the armors of medium voltage cables, is herein analyzed to verify the effects on touch voltages in ground-fault conditions. The major contributions of this paper are two: the analysis of the impact of an HV ground-fault on a global grounding system (GGS), and the analysis of the parameters that may affect safety due to the interconnection between HV–MV stations and the GGS. The authors have analyzed cases when the connection of an HV–MV station to a GGS improves safety, and then may introduce hazards under ground-fault conditions. Two main issues are herein discussed: 1) the transfer of dangerous voltages to substations, due to ground-faults occurring at the HV–MV station; and 2) the reduction in the magnitude of the ground potential rise caused by ground-fault conditions at substations, due to the connection of their ground grids to the HV–MV station's grounding system. This paper, by examining various grid configurations, demonstrates that in some instances the inclusion of HV–MV stations in the GGS may reduce the level of protection against touch voltages, and that this depends on the following elements: the number of MV lines fed by the faulted station, the number of MV–LV substations per line, the value of the ground resistance of the substations, and the distance between the substations. This paper has practical relevance for both utilities distribution systems and industrial facilities supplied by the MV power grid.

Diagnostic analysis and protective measures of transient ground potential rise in a 1000 kv UHV substation

With the continuous expansion of power grid capacity and growth of voltage levels, the transient ground potential rise (GPR) caused by the switchgear operation has become great threaten to the safety of the grid. In the startup operation of 500 kV part of Taizhou 1000 kV ultra-high voltage (UHV) substation, the electromagnetic lock installed in the switchgear filed was broken by the GPR in case of switchgear operation. There are two possible transmission paths of the GPR, one is the ground copper bar connecting the ground and the other is the control cable connecting the control cabinet and electromagnetic lock. In order to find out the transmission path of the electromagnetic disturbance, the experiment is carried out to distinguish the two possible paths. The waveform of the GPR in case of switchgear operation from different measuring points are recorded and compared, the results are analysed by comparing the signal characteristics and the transmission path is eventually diagnosed. Finally, the protective measures are proposed by wrapping the control wires with the conductive metal cube to reduce the influences of GPR. The outcomes in the study provide beneficial methods to analyse the GPR disturbance and positive guidance for the protection of locally installed equipment.

Probabilistic Assessment of Ground Potential Rise Using Finite Integration Technique

The behavior of grounding systems against waves with high-frequency content such as lightning or very fast transient overvoltages is completely different from the steady state. This paper presents a more flexible model of a grounding system with respect to frequency dependency of electrical parameters of soil and moisture variations using the finite integration technique (FIT). The applicability and accuracy of the FIT have been verified against experiments as well as conventional methods such as method of moments, finite-difference time-domain method, and partial element equivalent circuit. In addition, the impacts of moisture variations of soil on resistivity and permeability have been quantified through Weibull distributions. The obtained probabilistic parameters of soil resulting from moisture uncertainty have been involved in the model to provide a more realistic model. The probabilistic ground potential rise indicated that the grounding system can be optimized up to 50% by accepting a low rate of risk depending on the soil moisture level. The provided insight is a beneficial approach, especially for a gas insulation substation, where the available space is limited for a grounding system.

Simulation of the electric field and the GPR resulting from vertical-driven rods earthing system in a multi-layers earth structure

A three-dimensional model for calculating the ground potential rise and the electric field distributed at the ground surface of an earthing system using driven vertical rods is presented. The electric field distribution is obtained by using Laplace's equation which is solved by finite element method. The presented model for simulating electrical field is a three-dimensional field problem with different types of earth models; one and two- layers earth models, in two different cases. The first case is represented by one driven rod while the second one is represented by three driven rods. The strength of the finite element method in this application is in its ability to allow a detailed representation of the types of the soil and different cases of the rod simulation. The new simulation model has been assessed through comparing the vertical field strength over the vertical driven rod surface, with that estimated one in other experimental and computational work. A primarily simulation for investigating the thermal effect is achieved and more investigation about this point will be presented in the future papers.

Analytic continuation as the origin of complex distances in impedance approximations

Engineering approximations of physical systems sometimes produce models in which real-valued model-based physical-distances are added to complex-valued distances and/or (for electrical systems), real-valued current/charge image intensities are replaced with complex-valued quantities. These models are arrived at often using ad hoc approximations that allow infinite integrals or series to be approximated in closed form. Arriving at accurate ad hoc approximations in a compatible analytic form is often the difficult step in the derivation of these approximations. In this paper, we show that this difficult ad hoc step can be replaced for many classes of functions with the use of analytic continuation via Padé approximants, along with some reasonable engineering judgement. We apply our approach to several existing approximations in the electrical engineering field (overhead transmission line impedance, underground cable impedance and Green’s functions used in ground potential rise calculations) and show that these approximations can be derived elegantly, without the need for grand leaps of insight, and provide a basis for both distance parameters and current/charge intensities that are complex-valued.

Investigation and Optimization of Grounding Grid Based on Lightning Response by Using ATP-EMTP and Genetic Algorithm

A large number of electromagnetic transient studies have been analyzed for finding the overvoltage behavior of power system. A grounding grid of power system is so important for reducing the effect of overvoltage phenomena during a short-circuit event. Two sections are important in grounding system behavior: soil ionization and inductive behavior; this paper focuses on the inductive manner of grounding grid. The grounding grid is considered as a conductor segment; each conductor segment acts as a grounding unit. In this paper, the transient methodology is introduced to investigate the lightning effect on grounding body at each point of grounding grid in normal and optimized conditions. Genetic algorithm is applied for regular and irregular grounding grid to obtain best values of mesh size with the lower ground potential rise (GPR) as compared with the normal condition for more safety. The grounding grid is a combination of inductance, resistance, and capacitance. This model is suitable for practical applications related to fault diagnosis. Several voltages on different positions of grounding grid are described in this paper using ATP-EMTP and genetic algorithm. The computer simulation shows that the proposed scheme is highly feasible and technically attractive.