Tower Grounding System Research Articles

Optimisation of lightning current discharge to the ground in electrical networks: Introduction of the proportionality coefficient (K)

The electrical network, essential to our society, frequently encounters disruptions from lightning strikes, resulting in material damage and power blackouts. Swift diversion of lightning currents to the ground is imperative to safeguard the grid. This study proposes a proportionality coefficient (K) to effectively distribute lightning current between grounding and network flow. The optimality of this coefficient depends on the tower grounding system resistances; lower resistances facilitate optimal distribution, enabling more current to flow to the ground. In the examination of the Djiri-Ngo power line in the Republic of Congo, grounding systems were optimised based on soil types. Three electrodes were used for clayey sand, while fifteen were employed for siliceous sand. Optimal coefficients were determined to be 0.86 for clayey sand and 0.81 for siliceous sand. These coefficients denote that 86% and 81% of the lightning current were directed to the ground, in contrast to non-optimal resistances (69% and 29% with a single grounding electrode). The experiments highlight the importance of adapting grounding systems to soil characteristics, rather than adhering to a uniform approach. Efficient diversion of lightning current to the ground is paramount for grid protection.

Evaluation of the Backflashover Performance of a 150 kV Overhead Transmission Line Considering Frequency- and Current-Dependent Effects of Tower Grounding Systems

Evaluation of the Backflashover Performance of a 150 kV Overhead Transmission Line Considering Frequency- and Current-Dependent Effects of Tower Grounding Systems

Analysis of overvoltages across line insulator strings considering the ground-wire and phase conductors corona

This paper assesses the influence of corona effect on the overvoltages developed across the insulator strings of overhead transmission lines due to lightning. The influence of corona is investigated considering lightning strikes to the tower top and shielding wires at midspan. A large range of soil resistivity and lightning currents peaks are evaluated in the analysis, and the stress across the insulator strings is evaluated applying the integration method. Simulations are carried out in the Alternative Transient Program (ATP/EMTP), wherein the corona effect is represented through the accurate Suliciu corona model, which was implemented using the MODELS interface and combined with J. Marti line model. Furthermore, it was also represented the wideband behavior of tower grounding system. Results showed that corona effect has a minor influence in the overvoltages across the insulator string for strikes to the tower top. However, it strongly influences the overvoltages across line insulators ensuing from a lightning strike to the shielding wire at the midspan, leading to a decrease in the peak value of the lightning current that causes line flashover.

Modeling of different tower grounding systems using hybrid continuous circuit-trapezoidal integration method

In order to protect the transmission lines against lightning strikes, it is important to investigate the tower grounding system. In this paper, a recent method called hybrid continuous circuit-trapezoidal integration method is used to calculate the impulse impedance of the grounding system. Moreover, conventional structures of the grounding systems have been simulated and the results show that, with the same wire length, the square with additional wire system has the least impulse impedance as compared to counterpoise, square and crow?s foot. Moreover, the effects of soil resistivity and lightning current rise-time on the impulse impedance of these grounding systems are investigated. It is concluded that the design of the grounding system depends on the geographic location of the site in terms of soil resistivity and isotropic characteristics of the area in terms of lightning current rise-time.

Impact of First Tower Earthing Resistance on Fast Front Back-Flashover in a 66 kV Transmission System

Lightning stroke on a transmission tower structure is one of the major reasons that results in high voltages at the tower arms due to the excessive lightning current flowing through the transmission tower to earth. The Surge voltage seen at the tower cross arm on the first tower close to a substation is the worst case. If this voltage is higher than the withstand level of the insulator string, the insulation of substation equipment will be exposed to transient over-voltage called fast front back-flashover (FFBF). The peak of this transient overvoltage is affected by the value of the system’s earthing resistance. This paper studies the effect of reducing the grounding resistance of both the surge arrestor (SA) and the first transmission line tower adjacent to a 66 kV substation on FFBF. Three case studies using PSCAD/EMTDC software are presented to simulate the variation of the potential sire at the substation equipment with different resistance values for the first tower and SA earthing resistance. The paper also addresses the economic protection system for solving the problem of transient overvoltage. The study proves that the proper design of the first tower grounding system enhances the safety of the system and reduces the cost for the grounding system to the minimum.

Improving tower grounding vs. insulation level to obtain the desired back-flashover rate for HV transmission lines

This paper aims to present a novel proposed formula, along with technical viewpoints, to solve the problem of transmission lines outage rate due to back-flashover phenomena in terms of soil resistivity, the number of disc insulator units in the string, and tower grounding system structure. Improving the tower grounding system (reducing the impedance) and developing the insulation level are appropriate solutions to prevent the occurrence of the back-flashover, dependent on many factors. To the best of our knowledge, the trade-offs between these two parameters, in respect of soil resistivity, to obtain the desired back-flashover rate (BFR) has not been presented via a compact formula yet. In this paper, it will be shown that in a specific soil resistivity and grounding structure, there is a minimum required insulator unit number and grounding conductor length to achieve the desired BFR, which could be calculated by the proposed formula, straightforwardly. Also, the economic selection between increasing the insulator unit number and developing the tower grounding system in terms of soil resistivity and BFR is investigated. The simulation procedure is coded with Matlab and linked to EMTP/ATP program to analysis the BFR. This method is done for a typical 150 kV overhead transmission line, however, it can be applied to any voltage level of transmission line and significantly facilitate the cost effective design of them.

Monte Carlo Evaluation of the Impact of Subsequent Strokes on Backflashover Rate

The paper deals with the impact of subsequent strokes on the backflashover rate (BFR) of HV overhead transmission lines (OHLs), assessed by means of an ATP-EMTP Monte Carlo procedure. The application to a typical 150 kV Italian OHL is discussed, simulating several tower grounding system arrangements. Subsequent strokes parameters are added to the statistical simulation variables: peak current, front time, time-to-half value, lightning polarity, line insulation withstand, lightning location and phase angle of the power frequency voltage. The input data are fed to an ATP-EMTP complete circuit model of the OHL, including line insulation, lightning representation and tower grounding system, the latter simulated by a pi-circuit model able to simulate the effects due to propagation and soil ionization, at modest computational costs. Numerical results evidence a non-negligible BFR increase (in relative terms) due to subsequent strokes: for spatially concentrated grounding systems the BFR increase approximatively vary in inverse proportion with the low frequency grounding resistance, whereas for spatially extended grounding systems the BFR increase depends on the grounding system behavior at high frequencies.

Protection of Buildings in the Vicinity of Transmission Towers Against Potential Rise Above the Ground Electrode—Study Case

This paper deals with those measures against overvoltage taken when protecting buildings occupied by people or animals. Such overvoltages can appear in the case of ground fault if the grounding system of the building is located within a potential funnel of a transmission tower's grounding system. This paper presents an entire concept regarding protection against any potential rise within the building. Computations of electrical potential on the grounding system having been performed using data obtained by measurements. This includes equipotential bonding, overvoltage protection of the building, and a transformer substation with metal–oxide varistors and gas-discharge arresters installed on the low-voltage line. A simulation model for the entire overvoltage protection based on known mathematical models has been merged within this paper. Adequate overvoltage protection elements were selected for protection against potential rises in the grounding systems of buildings on the basis of computation results.

Quick calculation of the grounding resistance of a typical 110 kV transmission line tower grounding system

A general practical method, based on the Finite Element Method modelling and a numerical analysis of the obtained results, was applied for the calculation of the grounding resistance of a complex grounding system frequently used for typical 110kV transmission line towers. An approximate formula for a practical evaluation of such a grounding system resistance, valid for the ranges of input parameters used in the Serbian transmission power system, is proposed. A new, more accurate formula for calculating the square loop grounding resistance is also given. The proposed method for generating approximate formulas intended for fast grounding resistance calculations is general and, therefore, can be applied to any type of complex grounding systems.

The Research on the Impulse Characteristics of the Extended Grounding Electrode in the Layered Soil

The composition of the tower's grounding system is always with several layers, for single-layer structure of the tower in high soil resistivity area, it is the use of partial replacement of soil to reduce the tower grounding resistance, also the analysis object is equivalent to the hierarchical model, Therefore, the study of layered soil structure tower grounding impulse characteristics is more representative. To study the impact properties of the layered structure of the soil grounding, In this paper, through the establishment a double-layer soil model structure, the impulse characteristics of the soil structure of grounding electrode is studied through simulation, resulting of the quantitative relationship between several high resistivity soils partially replacing low resistivity soil area size and impulse grounding resistance of elongation grounding. The results show that the impulse grounding resistance decreases as the replacement area of low resistivity soil increases and converges to a certain value, which indicates the impulse characteristics of extended grounding electrode in double-layer soil model increases with the expand of the upper level low resistivity zone and approaching that of single layer with low resistivity soil. The results have a reference value for use of local soil replacement method to reduce tower impulse grounding resistance in high soil resistivity areas and the determination of soil replacement plan.

Generalized pi-circuit tower grounding model for direct lightning response simulation

Simulation of direct lightning strokes to overhead line towers requires accurate modeling of tower grounding systems. When large-scale statistical simulations are needed (e.g. in backflashover rate assessment), detailed but computationally intensive models are not suitable. This paper presents a genetic algorithm-based procedure to synthesize a generalized pi-circuit model, aimed at simulating the impulse response of complex grounding systems, including non-linear soil ionization, for backflashover studies. The equivalent electrical parameters of the pi-circuit model are determined as functions of soil resistivity for some commonly used grounding systems; the pi-circuit model is then validated by time-domain ATP-EMTP comparison with a complete circuit model, obtaining very good results (worst-case errors do not exceed 3–4%). ATP-EMTP time-domain simulations of direct lightning strokes to overhead lines equipped with the pi-circuit model are also reported, showing an excellent agreement with results yielded by the complete circuit model as well as drastically decreased computation times (up to 3 orders of magnitude).

An ATP-EMTP Monte Carlo procedure for backflashover rate evaluation: A comparison with the CIGRE method

The paper presents the application of an ATP-EMTP Monte Carlo procedure for backflashover rate (BFOR) evaluations to HV overhead lines (OHL), equipped with three different types of spatially extended and/or geometrically involved tower grounding systems. The ATP-EMTP circuit model of the OHL includes detailed line insulation and lightning representation; grounding systems are simulated by a new and simplified model which reproduces the effects of propagation along the ground conductors and soil ionization phenomena. The statistical variables include lightning stroke parameters (polarity, peak current, front and tail times), lightning location, as well as line insulation withstand and phase angle of the supply voltage. An external software engine generates all the required statistically-oriented ATP-EMTP input data, sequentially launches and manages ATP simulations and finally post-processes the results. A good performance in terms of convergence and computation times is evidenced. Results are compared to those yielded by both the simplified and ‘complete’ CIGRE methods for backflashover rate calculations, showing the applicability of the proposed procedure to line configurations characterized by complex tower grounding arrangements.

Experimental Investigation of Impulse Characteristics of Transmission Line Tower Footings

Understanding the impulse response of tower grounding systems is a key factor in the determination of the lightning performance of transmission lines and the safety voltages near high voltage towers. This paper reports the results of an experimental study on the impulse characteristics of full-scale transmission line tower footings. The ground potential distribution both at the ground surface and below ground was examined under impulse currents and variable-frequency AC currents. Prospective impulse step and touch voltages were determined by measurement of the ground surface potential distribution near the tower base. The behaviour of the impulse resistance of the tower footings under highmagnitude impulse currents was investigated. The non-linear characteristics of the impulse resistance with peak current are demonstrated for the entire tower base and for single tower footings. The soil ionisation inception is found to be dependent on the rate of rise of applied current. The dynamic impulse resistance was used to describe the dynamics of soil ionisation.

Simplified HV tower grounding system model for backflashover simulation

The article presents the application of a simplified model for the transient simulation of an overhead line tower grounding system. The model, based on a Pi-equivalent circuit, also accounts for non-linear soil ionization phenomena. The ATP-EMTP backflashover study of a typical European 150kV overhead line using the proposed model is compared to one carried out by using a full circuit-based grounding system model, already validated by comparison with experimental tests and with simulation results yielded by field models available in literature. Results show an excellent agreement between Pi-circuit and full model, both in terms of lightning current values causing backflashover and time plots of insulator voltage and ground potential rise at tower foot. Moreover, the simplified model drastically reduces the computational resources (i.e. memory occupation and execution time) required when complex power systems must be studied in transient and non-linear conditions. Results show that the proposed model is suitable for massive, Monte Carlo-type statistical direct lightning simulations.

Backflashover simulation of HV transmission lines with concentrated tower grounding

For high voltage (HV) transmission lines (TLs) the backflashover phenomenon can occur when lightning strikes a tower or a shield wire (SW). In this paper, the backflashover phenomenon of typical HV-TLs has been studied. The simulation code, based on ATP/EMTP program, accounts for both the discharge process between the arcing horns of insulator strings, by means of a leader progression model and the non-linear transient behaviour of tower grounding systems consisting of vertical rod(s). The lightning current ranges, causing back flash in a 161 kV-50 Hz single circuit line with two grounded SWs, have been investigated in detail. Numerical results of the investigations are in good agreement with actual behavior of the studied TL.

Calculation of grounding resistance

This paper introduces an algorithm for the calculation of grounding resistance based on the boundary element method (or current simulation method). A series of computer results for different kinds of grounding systems is presented. On the basis of the computer results, an evaluation is made of commonly used formulas for grounding resistance and an expression is given whereby tower grounding systems can be designed.