Backflashover Rate Research Articles

Improvement of the lightning performance of transmission lines by combining conventional and non-conventional measures

This work discusses the use of combined measures for improving the lightning performance of transmission lines exhibiting high backflashover rate. It developed a methodology for assessing how the line performance is affected by installing underbuilt wires combined with the reduction of tower-footing resistance and the partial protection of the phases by surge arresters. The methodology can be applied to any transmission line. To develop a quantitative idea of the impact of the combined measures, a 230-kV double-circuit transmission line installed in a region of very high flash density and extremely unfavorable soil resistivity was used as reference. The range of efficient application of the tested solutions was determined as a function of tower-footing resistance for this line.

Optimization of overhead transmission lines insulation and grounding costs with respect to backflashover rate

In this paper, the optimization of transmission insulation and grounding is performed considering two backflashover rate (BFR) evaluation procedures widely applied in the industry. The optimization approach is based on the Lagrange formulation introduced by Hileman in late sixties. This method used the old AIEE procedure to evaluate the BFR of transmission lines. Since then a number of BFR evaluation are available to determine BFR rates outperforming legacy procedures. Recently, the Hileman’s optimization method was updated using the EPRI two-point method to evaluate the BFR. However, despite the step-by-step EPRI method being still widely used in industry, stochastic simulation with specialized programs is becoming standard. Thus, in this paper, the insulation and grounding optimization methodology is updated using more adequate and recent BFR evaluation procedures such as the IEEE Std. 1243-97 and the Monte Carlo EMTP-based methods. Technical and economic results were compared with the existing optimization approach based on the EPRI two-point method using the EPRI 345 kV transmission line as test system. Results show that IEEE and EPRI methods yield equivalent results. The stochastic-based method (MC-EMTP) is more optimistic showing total and marginal insulation and grounding costs below the IEEE/EPRI methods.

Analysis of Lightning Performance on 345 kV Transmission Lines Using Python Programming

One of main causes of interruption of electrical power supply is the lightning strike on overhead power transmission lines. The lightning performance of transmission line can be determined by value of shielding failure flashover rate (SFFOR) and back flashover rate (BFOR). The object of this study is to create a computer application to compute lightning performance on the transmission lines using Python programming. Pythons package tkinter used for program interface window. Application programming is done by using the concept of object-oriented programming (OOP) using Pythons keyword class. Validation shows that the application has applied the method correctly with a percentage error 0 % for SFFOR and 3.14 % for BFOR. The application can do analysis on the factors that affecting SFFOR and BFOR such as the effect of thunder day, tower foot resistance, and number of isolator disk. The results obtained in this study is computer application that can perform lightning performance analysis and analysis of factors that can affect it, such as thunder day, tower foot resistance and the number of isolator disk.

A Methodology for Optimal Design of Transmission Lines to Protection against Lightning Surges in Presence of Arresters

In this paper, a methodology for determination of the optimal value of protection design parameters, i.e. tower footing resistance, insulation strength, and surge arresters’ rating in the planning stage of transmission lines (TLs) is presented. This method calculates the shielding failure flashover rate (SFFOR) of TLs, based on Electro-geometric model (EGM) of TLs, and the back flashover rate (BFR) of TLs, based on the Monte Carlo method, in which the accuracy of the proposed methodology has been verified by comparing the resultant results with those obtained with the use of the IEEE FLASH program. The proposed method can be directly used to achieve the minimum lightning flashover rate (LFOR) of TLs by the minimum investment cost. Also, it can be used, indirectly, for determination of the appropriate value of the footing resistance, insulation strength and arresters’ rating to satisfy any target number of LFOR that might be specified by the utilities or standards.

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.

High frequency response of grounding electrodes: effect of soil dielectric constant

paper is a postprint of a paper submitted to and accepted for publication in IET Generation, Transmission & Distibution (GTD) and is subject to Institution of Engineering and Technology Copyright. The copy of record is available at the IET Digital Library.Abstract:Grounding electrodes have an important role in electric power transmission and distribution systems. They are used to prevent excessive hazardous voltages between metallic structures and ground in the case of system faults or lightning surges. It is important that they provide a low impedance path for the current in to the ground. The electrical properties of soil, which vary substantially with geographical location and time of year, affect the process considerably along with the properties of the grounding electrode itself, such as its dimensions.In order to have an accurate estimation of the developed overvoltages and the backflashover rate of the transmission lines due to a lightning strike, one has to take into account the effect of the value of the soil electrical parameters, such as the electrical conductivity and dielectric constant.This paper investigates the high frequency behavior of the grounding electrodes by solving a full-wave electromagnetic problem using the Finite Element Method (FEM). The focus is on taking into account the effect of the variation of soil relative permittivity which has been neglected in the previous studies of the grounding systems. This allows an evaluation of the response of grounding systems due to seasonal changes and specifically change of the water content of the soil, which would cause its electrical properties to vary significantly. This studydemonstrates the importance of considering the variation of relative permittivity of the soil especially in the modeling of electrodes buried in highly resistive soil.

Lightning Performance Evaluation of Italian 150 kV Sub-Transmission Lines

A significant majority of overhead transmission lines’ (OHLs) outages is due to backflashovers caused by direct lightning strikes: the realistic assessment of the lightning performance is thus an important task. The paper presents the analysis of the lightning performance of an existing 150 kV Italian OHL, namely, its backflashover rate (BFOR), carried out by means of an ATP-EMTP-based Monte Carlo procedure. Among other features, the procedure makes use of a simplified pi-circuit for line towers’ grounding system, allowing a very accurate reproduction of transient behaviours at a very low computational cost. Tower grounding design modifications, aimed at improving the OHL lightning performance, are also proposed and discussed.

Study on the Influencing Factors of Insulator on the Back-Flashover Lightning Withstand Performance of ±800 kV UHVDC Transmission Lines

The current research results indicate that the insulator’s insulation performance has a very important influence on the back-flashover lightning withstand performance of UHVDC transmission lines, especially for ±800 kV voltage level. However, it is not clear which factors will influence the insulation performance of the insulator, and the influencing mechanism is also not clear yet. To figure out this problem, the insulator’s insulation performance under different conditions has been deeply analyzed and considered to reveal the influence mechanism in this paper, such as the surface hydrophobicity, pollution degree, and the string type. Firstly, the insulator’s model is established using COMSOL software, and the lightning impulse voltage of insulator is calculated and verified with the corresponding experimental data. Then, the ±800 kV UHVDC transmission lines model is constructed using PSCAD software, and back-flashover lightning withstand level and back-flashover rate are calculated by considering the above lightning impulse voltage as the threshold of flashover. Finally, the back-flashover lightning withstand performance of ±800 kV UHVDC transmission lines is deeply analyzed based on different insulators. The simulation results demonstrate that the back-flashover lightning withstand performance of ±800 kV UHVDC transmission lines is obviously weakened with the increase of the pollution degree and slightly weakened with the decrease of the surface hydrophobicity. Considering the same pollution degree, the V-type string insulator has the least influence, while the II-type string insulator has the greatest influences on the back-flashover lightning withstand performance of ±800 kV UHVDC transmission lines. The research results are beneficial for providing theoretical basis for stable operation and reliable power supply of ±800 kV UHVDC transmission lines.

Closed-Form Expressions for the Estimation of the Minimum Backflashover Current of Overhead Transmission Lines

The minimum backflashover current, I <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">BF</sub> , of typical overhead transmission lines is estimated through ATP-EMTP simulations considering the tower ground resistance and AC voltage phase angle as parameters. Several single- and double-circuit lines are investigated with operating voltage from 66 kV to 765 kV and Basic Insulation Level from 325 kV to 1950 kV. The backflashover rate, BFR, of the investigated lines is obtained using an expression that accurately considers lightning incidence to the line for prospective currents higher than I <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">BF</sub> ; the relevant CIGRE expression underestimates BFR. BFR can be easily estimated using solely the phase angle values corresponding to zero AC voltage of the phase conductors. Based on the I <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">BF</sub> and BFR results, simple expressions are introduced for the calculation of an equivalent minimum backflashover current as a function of insulator length and tower ground resistance or impulse impedance. These expressions enable straightforward estimation of BFR without the need for time-consuming simulations or laborious analytical methods adopting simplifying assumptions.

Monte‐Carlo‐based simulation and investigation of 230 kV transmission lines outage due to lightning

Here, using the probabilistic evaluation based on the Monte Carlo method, back-flashover rate and shielding failure flashover rate of 230 kV overhead transmission lines in the western regions of Iran are evaluated. To such an aim, first, the number of thunderstorm days per year is collected from the reported weather information in order to determine the ground flash density. Then, using MRU-200 equipment, the tower-footing resistance of several towers is measured. Matlab® software is used in order to produce lightning surges considering its probabilistic nature and randomly distribution on the ground to evaluate striking distance based on the geometric model. Then, calculated parameters are transferred to EMTP-RV software by establishing a link to perform the transient simulation and report the results for modelled 230 kV transmission line. Finally, considering IEEE-1243 standard, it is shown that due to high ground flash density, using 230 kV tower with one shield wire is not sufficient to protect the line against lightning phenomena.

A New Method to Determine Incremental Costs of Transmission Lightning Protection Systems

This paper investigates the relationship between the cost associated to lightning protection systems (LPS) and the back-flashover rate (BFR) in high voltage transmission lines. The fundamental research question raised is how to determine the incremental cost of the mitigation measures associated to a prescribed back flashover rate. The interaction of tower supplemental grounding and tower insulation design is analyzed satisfying a given reliability level at minimum overall investment cost. A new optimization model relating back flashover phenomena to expenditure in LPS is presented expressly accounting the dynamic behavior of the tower footings being suitable to be applied using any existing BFR evaluation methodology. To illustrate the method, practical study case based on the step-by-step Anderson-EPRI method is presented. Despite the EPRI method has limitations, its simplicity is useful to illustrate the calculation of incremental cost curves of LPS without resorting to simulation software. Researchers are encouraged to apply the method using other BFR evaluation methodologies.

Constraints on the use of surge arresters for improving the backflashover rate of transmission lines

The effectiveness of using transmission line surge arresters (TLSA) under different conditions for improving the lightning performance of lines is assessed by means of computational simulation. These conditions comprise using TLSA only in critical towers and/or using devices only in certain phases, leaving one or two-phases unprotected. In addition to assess the effect on stricken towers, the transfer of overvoltage to unprotected adjacent towers is also addressed, considering different values of tower-footing impedance. In the analyses, lightning overvoltages across insulator of typical single-circuit 138-kV and double-circuit 230-kV transmission lines were calculated by using the Hybrid Electromagnetic Model (HEM) and flashover occurrence was assessed by using the Disruptive Effect method (DE), in each simulated condition. The results indicate that the operation of TLSA protects the own insulator and diminishes the backflashover risk at unprotected insulators. However, this risk remains high, depending on tower-footing impedance. Also, the risk of flashover at the unprotected adjacent towers due to the operation of surge arresters at the stricken tower was found to be significant in the case of all phases of the stricken tower protected, notably when the tower-footing impedance of the protected tower is high and that of the adjacent tower is low.

Evaluation of a Direct Lightning Strike to the 24 kV Distribution Lines in Thailand

This paper evaluates the effect of a lightning strike directly on the 24 kV distribution lines in Thailand, where such strikes are one of the main causes of power outages. The voltage across the insulator, and the arrester energy absorbed due to the lightning, need to be analyzed for different grounding distances of the overhead ground wire, ground resistance, lightning impact positions, and lightning current waveforms. Analysis and simulations are conducted using the Alternative Transients Program/Electromagnetic Transients Program (ATP/EMTP) to find the energy absorbed by the arrester and the voltages across the insulator. The results indicate that when surge arresters are not installed, the voltage across the insulator at the end of the line is approximately 1.4 times that in the middle of the line. In addition, the ground resistance and grounding distance of the overhead ground wire affect the voltage across the insulator if the overhead ground wire is struck. When surge arresters are installed, a shorter grounding distance of the overhead ground wire and a lower ground resistance are not always desirable; this is because they reduce the back-flashover rate and the voltage across the insulator if lightning strikes the overhead ground wire. However, lightning strikes to the phase conductor result in high arrester energy and the possibility that the arrester will fail. Furthermore, the tail time of the lightning waveform is a significant variable when considering the energy absorbed by the arrester, whereas the front time is important for the voltage across the insulator. In case lightning strikes directly on the connected point between the overhead lines and the underground cables, the distribution line system is protected only by the lightning arrester at the connection point. The overvoltage at the connection point is lower than the basic impulse level at 24 kV of 125 kV, but the overvoltage at the end of the cable is still more than 125 kV in case the cable is longer than 400 m. When the distribution line system is protected by the lightning arrester at both the connection point and the end of the cable, it results in overvoltage throughout the cable is lower than the critical flashover of insulation. This method is the best way to reduce the failure rate of underground cables and equipment that are connected to the distribution line system.

Effects of Lightning Channel Equivalent Impedance on Lightning Performance of Overhead Transmission Lines

Modeling of lightning return-stroke channel affects significantly the estimation of fast-front overvoltages arising across overhead line insulators, thus, also the lightning performance of power lines. This paper introduces a simplified method for estimating the minimum lightning currents causing shielding failure flashover and backflashover of overhead transmission line insulation, as affected by lightning channel equivalent impedance. The validity of the proposed methodology is demonstrated through ATP-EMTP simulations of typical 66 up to 765-kV overhead transmission lines with variable tower ground resistance. An assessment is made of the effects of lightning channel equivalent impedance on the lightning performance of overhead lines, that is, shielding failure flashover and backflashover rates. Lightning channel equivalent impedance values of 1000 and 400 Ω are suggested, respectively, for shielding failure and backflashover studies, based on the results obtained in this paper together with a discussion on the literature data regarding channel impedance.

Introducing novel risk-based indicator for determining transmission line tower's backflashover performance

Transmission line (TL) backflashover (BF) performance has been traditionally ascertained using a single number, the so-called backflashover rate, which is measured in number of BF events per 100km-years. This paper aims at presenting a novel indicator for assessing performance of high-voltage TL tower's ability for tolerating direct lightning strikes without provoking BF events. This indicator is also defined as a single number, which can be computed for any TL tower (by means of EMTP simulations) and measures, in a novel way, its tolerance against BF events. It is given in terms of the risk of the BF occurrence, which means it is statistical in nature and depends on the total sum of conditions governing the BF events. The BF risk, as an indicator, is obtained from the probability density function of the shield wire(s) incident lightning currents and the cumulative distribution function of the BF currents statistical distribution. Hence, it merges complete probabilities of obtaining lightning currents striking a tower with probabilities of those currents provoking BF events on that tower. This novel risk-based indicator can be related to the price of that risk and the associated investment costs, enabling the cost-effective optimisation solutions to the problems of TL arrester applications and station insulation coordination design. This seems appropriate, considering the fact that the investment in surge arresters and related protective measures is commonly perceived as buying insurance.

Application possibilities of special lightning protection systems of overhead distribution and transmission lines

Reduction of the overhead lines back-flashover rate (BFR) after application of special lightning protection systems is analyzed in this paper. The following lightning protection systems are considered: line arresters, underbuilt wires, additional shield wire and guy wires. Partial or combined efficiencies of mentioned lightning protection systems are analyzed. Overhead distribution (35 kV) and transmission (110 kV and 220 kV) lines BFR are estimated through the semi-statistical calculation procedure as suggested in international technical documents. System equivalent circuits in the EMTP-ATP software are created with carefully chosen models and parameters of elements. Pursuant to the estimated results, conclusions about application possibilities of different lightning protection systems are given.

Evaluation of the impact of different frequency dependent soil models on lightning overvoltages

This work assesses the influence of including the wideband behaviour of grounding systems in EMT-type programs on evaluation of transients resulting from direct lightning strikes to transmission lines. The grounding frequency behaviour is determined by using an accurate electromagnetic model and included in EMTP/ATP by means of an equivalent circuit derived from Vector Fitting technique. Furthermore, the impact of the frequency dependence of soil parameters on the lightning performance of transmission lines is addressed. It was found that representing the tower-footing grounding by a simple resistance can lead to significant errors in terms of grounding potential rise. However, for the overvoltages that appear across the insulator strings, the representation of the grounding system by a simple resistance leads to results whose accuracy is similar to those obtained using more complex representations which consider the wideband behaviour. Also, it was shown that the frequency dependence of soil parameters leads to a reduction of the grounding impulse impedance and causes a decrease of the backflashover rates, improving the lightning performance of transmission lines. In addition, the effect of considering the variation of soil parameters with frequency is more intense in the ground potential rise than in the overvoltages in the insulator strings.

Lightning performance of transmission lines: Assessing the quality of traditional methodologies to determine backflashover rate of transmission lines taking as reference results provided by an advanced approach

This paper presents a critical review of CIGRE and IEEE traditional methodologies for calculating the backflashover outage rate (BFOR) of transmission lines, considering their adopted simplifications. These methodologies, along with an advanced computational approach named HEM-DE, were applied to a real 138-kV line to determine its backflashover rate. The quality of the results produced by the methodology at each step of their calculation procedure was assessed and discussed, taking as reference the accurate results provided by the advanced approach, based on the application of the hybrid electromagnetic model (HEM) and disruptive effect model (DE). According to this assessment, in the 40-to-10-Ω range of tower footing grounding resistance, the CIGRE methodology overestimates outage rate of the line in relation to the reference values (98–21% larger in the 40-to-10-Ω range of tower footing grounding resistance). The IEEE methodologies underestimates the BFOR, yielding rates 47–23% lower in the same resistance range.

The effect of wide band modeling of tower-footing grounding system on the lightning performance of transmission lines: A probabilistic evaluation

This paper presents a probabilistic evaluation, based on Monte-Carlo method, to estimate back flashover rate (BFR) and shielding failure flashover rate (SFFOR) of overhead transmission lines (TLs). In the proposed approach, as opposed to the conventional methods, a wide band model of the tower-footing grounding system is adopted assuming the soil electrical parameters to be either constant or frequency dependent. The statistical parameters affecting the TLs’ outage rate are taken into account in the probabilistic studies. Simulations are done for a typical 400-kV transmission line which is modeled in EMTP.RV. From the simulation results, it is found that the BFR of transmission lines is markedly affected by the tower footing grounding system model. This effect is more pronounced when the soil electrical parameters are frequency dependent. It is also found that the SFFOR of the transmission lines is not significantly affected by the wide-band model of the tower-footing grounding system.

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.