Lightning Performance Of Transmission Lines Research Articles

An expeditious calculation approach to consider the influence of power frequency voltage on the calculation of the lightning performance of transmission lines: Validation for different peak current-front time correlation expressions and cumulative peak current distributions

The approach to consider the influence of the power-frequency voltage on the calculated lightning performance of transmission lines with voltage level from 69 kV to 500 kV by means of the 0.88-multiplying-factor applied on critical currents is validated for simulation conditions that assume the correlation between lightning peak current and front time, and several cumulative peak current distributions of negative first strokes. The obtained results confirm that the relation between critical currents calculated in the presence and in the absence of the power-frequency voltage follows the 0.88-factor for all investigated cases. The root mean square errors (RMSE) calculated for the critical currents vary from 3.20 kA to 7.37 kA and for the backflashover probability are not greater than 0.02 %, ensuring the generality of application of the proposed approach. The ratio between the backflashover rates obtained by the 0.88-factor approach and by the Monte Carlo method is around 1 for most of the simulated cases, confirming the quality of the results provided by such expeditious procedure.

Recommendations related to the Peak-current distributions in studies of lightning performance of transmission lines based on the range of critical currents associated with Backflashover

The impact of cumulative peak current distributions of first return stroke currents on the calculated lightning performance of transmission lines was addressed. The specific features of distributions obtained from measurements at instrument towers and of enlarged distributions including data of magnetic links, were analyzed. The result of their application to real transmission lines (of 138, 230, 345 and 500 kV and exhibiting different parameters) was determined, in terms of backflashover probability and rate. The use of different distributions resulted in great impact on the calculated performance. This impact depends overall on the range of critical currents, below 90 kA and above 100 kA, in which the behavior of the distributions is quite different. Based on the results, a set of recommendations were presented to support the user on the decision about the distribution to adopt, considering the specific features of the transmission line.

A Comprehensive Analysis on the Influence of the Adopted Cumulative Peak Current Distribution in the Assessment of Overhead Lines Lightning Performance

Backflashover rate (BFR) is strongly dependent on the cumulative peak current distribution (CCD) adopted in the calculations. An original aspect of the present work is that such dependence is simultaneously assessed in estimating the probability of the critical current being exceeded as well as in the annual number of flashes to the line. An IEEE brochure recommends that the distribution values that characterize the atmospheric characteristic of the region under study as accurately as possible be used. The objective of this article is to evaluate the impact of the use of different CCDs, related to several measurements carried out around the world, in the estimation of the lightning performance of transmission lines (TLs). Structures of 138, 230 and 500 kV were analyzed. In the simulations, representative curves of lightning associated with measurements taken at Monte San Salvatore (MSS), Morro do Cachimbo (MCS) and TLs in Japan (TLJ) were considered. The distributions recommended by the IEEE and by the CIGRE and the distributions of Berger obtained from MSS, MCS and TLJ were considered. The presented results indicate differences of up to 100% between the considered work distributions and the IEEE one for certain values of tower footing impedance.

Differences in the response of transmission lines subjected to the currents of negative and positive lightning flashes: Influence of ground terminations

The differences on the lightning response of transmission lines, whose towers are subjected to the impression of pulses of current of first negative return strokes and of positive flashes, are determined by computational simulation. It is shown that the amplitude of the resulting transient overvoltages across insulators per unit of impressed current is much larger for negative return strokes. As most of the traditional procedures for assessing lightning performance of transmission lines ignore these differences, their results can exhibit significant errors.

A New Approach for Considering the Effect of the Power-Frequency Voltage on the Calculated Lightning Performance of Transmission Lines

The effect of power-frequency voltage on the backflashover rate of 138 kV to 500 kV transmission lines was assessed by means of an elaborate computational approach, based on the Monte Carlo method. Accurate results were obtained using this approach and were taken as reference for assessing the quality of results of the main methodologies available in literature for considering this effect that significantly increases the backflashover probability. The results showed that the effect is more pronounced for low tower-footing grounding impedances and for transmission lines with higher voltage level. With the exception of the approach recommended by CIGRE Brochure 63, the other tested approaches showed to yield reasonable results. Two new alternatives were proposed in this work for considering the effect. Particularly, the work developed an impactful original result: the use of a simple 0.88-multiplying factor to correct the critical currents. This approach yielded top-quality results, exhibiting very small average error (1.2%) and lowest maximum error (9.2%) in relation to the reference results.

Stopping criterion for Monte Carlo method-based simulations of the lightning performance of transmission lines

• Lightning performance of 230- and 500 kV TL is assessed by computational simulations. • Technical-economic analysis of techniques to improve lightning performance is shown. • Installation of extended counterpoise wires and underbuilt wires is considered. • Decrease of 88%–94% in BFOR is obtained with the use of underbuilt wires. • Underbuilt wires are economically viable mainly for high resistivity soils. A stopping criterion to be applied on Monte Carlo-based computational simulations of the lightning performance of transmission lines in terms of backflashover is proposed. Considering typical 138 kV and 230 kV transmission lines as case studies and the application of an elaborated probabilistic calculation approach, the influence of the number of iterations of the Monte Carlo simulation on the calculated critical current, backflashover probability, error of the distribution and the dispersion coefficient is assessed. The obtained results leaded to the proposed stopping criterion that establishes limits for the error (5%) and the dispersion coefficient (0.35). Its application yields consistent results with the decrease on the number of required iterations up to 95% and higher than 90% for simulations related to the 230 kV and 138 kV TLs, respectively, when compared to simulations of reference with 20000 iterations.

Technical-economic analysis of conventional and non-conventional techniques to improve the lightning performance of transmission lines: Extended counterpoise grounding wires and underbuilt wires

• Lightning performance of 230- and 500 kV TL is assessed by computational simulations. • Technical-economic analysis of techniques to improve lightning performance is shown. • Installation of extended counterpoise wires and underbuilt wires is considered. • Decrease of 88%-to-94% in BFOR is obtained with the use of underbuilt wires. • Underbuilt wires is economically viable mainly for high resistivity soils. This work presents a technical-economic analysis of using extended counterpoise wires and underbuilt wires to improve the lightning performance of typical 230 kV and 500 kV transmission lines. The installation of underbuilt wires leaded to decreases up to 88% and 94% on the backflashover rate of the lines with high tower-footing grounding impulse impedance. Also, it is shown that the cost of installing one underbuilt wire combined with the extension of counterpoise wires converges to the cost related to the intervention only on grounding for increasing values of soil resistivity. A difference of only 3% between the costs of both techniques was indicated for the 230 kV TL installed over a soil with resistivity of 20,000 Ω.m. These aspects indicate the use of underbuilt wires as an economically viable solution for improving the lightning performance of transmission lines, mainly for high resistivity soils.

Assessing the influence of peak current distributions of first return strokes on the lightning performance of transmission lines: Instrumented tower distributions versus standard distributions

• The effect of peak current distributions on backflashover is shown. • Instrumented tower and standard distributions are adopted. • IEEE and CIGRE distributions overestimate backflashover for high voltage level TLs. • MCS distribution leads to largest BFOR probability for low insulation level TL. • A new formulation to reproduce high current region of MCS distribution is proposed. The influence of peak current distributions of first return strokes of instrumented towers and international standards on the calculated lightning performance of 138 kV and 500 kV transmission lines is evaluated. This performance, expressed by the backflashover probability, is determined by computational simulations using the Hybrid Electromagnetic model (HEM) and the Disruptive Effect method (DE). The performance is calculated considering the cumulative peak-current distributions obtained from direct measurements at the instrumented towers of Morro do Cachimbo (MCS) and Monte San Salvatore (MSS) stations and those provided by IEEE and CIGRE standards. According to the results, using the standard distributions yields larger backflashover probabilities of transmission lines of higher insulation level, in relation to those calculated using instrumented-tower distributions. Considering transmission lines of lower insulation level, the performances calculated using standard distributions are close to those obtained using MSS distribution, but yields lower backflashover probabilities in relation to those obtained using MCS distribution. The latter leads to backflashover probabilities about 40% and 47% larger than those obtained by using IEEE and CIGRE distributions, respectively, in the tower-footing impedance range of 20 Ω – 40 Ω.

Review of measures to improve the lightning performance of transmission lines

• Measures for improving the lightning performance of transmission lines. • Reducing the backflashover rate of transmission lines. • Techniques for improving TL lightning performance. • Threshold tower-footing grounding resistance for improved lightning performance. • Impulse impedance of tower-footing electrodes of transmission lines. This paper discusses fundamental aspects related to the measures for improvement of the lightning performance of transmission lines, considering the advantages and constraints on their application. The content reflects the experiences developed by the authors facing exciting challenges related to the improvement of the performance of real transmission lines installed under very unfavorable conditions. The work considers the approaches and the calculating procedure the authors adopted for providing the support required for decisions about the measure to be adopted. The results seem to be useful for academics and professionals involved with theoretical and practical aspects related to the lightning performance of transmission lines.

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.

EVALUATION OF ENERGY STRESS ON LINE ARRESTERS

Line Surge Arresters (LSAs) are efficient means for the improvement of the lightning performance of transmission lines. Determination of optimal LSA number, location and rating is important for the improvement of the reliability and availability of a transmission system. In selection of the LSA special attention should be paid to their energy stress which depends on complex interactions between the arrester locations, grounding, shielding and the local lightning environment. LSAs experience higher energy stress compared to station arresters, because the incoming surge to a station is limited by insulator flashover on the transmission line and impulse corona. In this paper calculations of energy stresses were carried out for a double-circuit 220 kV line with a single shielding wire. Parametric studies were conducted in which arrester discharge energy was a function of: time to half value of stroke current, number of towers with arresters, footing resistance, span length and angle of power frequency voltage. Arrester energy stress is analyzed in case of stroke to tower and shielding failure. From conducted analysis it can be concluded that energy stress on LSAs is lower for shorter span lengths. Tower footing resistance has only minor effect on the discharge energy. Arrester discharge energy strongly depends on time to half of the stroke current, number of towers with installed arresters and angle of power frequency voltage.

Evaluation of the Lightning Performance of Transmission Lines Partially Protected by Surge Arresters Considering the Frequency-Dependent Behavior of Grounding

This work evaluates the efficiency of installing surge arresters in only one or two phases concerning the lightning performance of transmission lines. The simulations were performed in the ATP software, taking as reference a Brazilian 138 kV transmission line, and considering the rigorous representation of the frequency-dependent behavior of the tower footing impedance. The electrical parameters of the soil were considered as either constants or frequency-dependent, in order to analyze the influence of this effect in partially protected lines. The results obtained have shown that the partial protection technique is highly efficient. Although it does not guarantee the protection of the unprotected phase(s), the probability of insulation breakdown is strongly decreased. It was also verified that the inclusion of the frequency-dependent effect in the soil parameters significantly reduces the backflashover rate estimates of the studied line when compared to the constant values situation.

Evaluation of the Impact of Including the Frequency-Dependent Behavior of Grounding Systems on the Lightning Performance of Transmission Lines and on Grounding Systems Design

Evaluation of the Impact of Including the Frequency-Dependent Behavior of Grounding Systems on the Lightning Performance of Transmission Lines and on Grounding Systems Design

Estimation of voltage waveform at top of transmission line tower struck by lightning of negative and positive polarity

The object of research is a circuit that simulates a lightning strike to a tower of 220 kV power transmission line, taking into consideration the reflection of a current wave from 10 nearest towers. Computation of the voltage arising at the top of the struck tower is necessary further to determine the lightning performance of transmission line by various methods. The lightning current has several maxima, in the case of a positive impulse polarity and, accordingly, several minima, in the case of a negative polarity, which are generally being called peaks. In addition, the lightning current impulse has a non-constant steepness in the entire area of current rise up to the first peak. The approximation of the real lightning current by simplified mathematical expressions cannot take into account all its real features. For a more detailed study of transient processes caused by thunderstorm activity, there is a need to use oscillograms of real lightning currents when modeling. The problem of determining the voltage at the top of the stricken transmission line tower was solved using circuit simulation. For an in-depth study of how the shape of the lightning current impulse affects the shape of the voltage at the top of the tower struck, digitized oscillograms of real lightning currents were used. The simulation was carried out for 7 negative lightning impulses with the first peak varying from –33.380 kA to –74.188 kA. In the case of positive lightning, 3 oscillograms were used with the first peak varying from +38.461 kA to +41.012 kA. The article shows that the shape of the front of the lightning current impulse and the amplitude of the first peak of the lightning current have a decisive effect on the maximum voltage value at the top of a power transmission line tower struck by lightning. The maximum voltage occurs precisely at the front of the current wave before the first peak of the lightning current. Therefore, the back flashover of the insulation from the tower to the phase conductor is most likely at a moment in time at the front of the current wave. By the time the maximum current is reached, the voltage at the top of the tower will be reduced by several tens of percent, compared to the maximum voltage at the tower, which occurs much earlier at the front of the current wave. The conducted research contributes to the development of methods for calculating the lightning performance of power lines and extends the scope of application of circuit simulation programs.

Alternative evaluation of voltage at top of transmission line tower stricken by lightning

The object of the research is a circuit that simulates a lightning strike to a tower of 220 kV power transmission line, taking into consideration the reflection of a current wave from 10 nearest towers. Computation of the voltage arising at the top of the stricken tower is necessary further to determine the lightning performance of transmission line by various methods. For Indian conditions, the average number of lightning strikes to this power line per 100 kilometers per year is about 77, which is a fairly high figure. As a rule, for the tasks of lightning protection, the lightning current is approximated by some analytical expression. In most cases, such expressions are various combinations of exponential functions. However, the waveform of real lightning currents on oscillograms differs significantly from the waveform attributed to them and approximated by relatively simple exponential expressions. For a more detailed study of transient processes caused by thunderstorm activity, there is a need to use oscillograms of real lightning currents when modeling. The problem of determining the voltage at the top of the stricken transmission line tower was solved using circuit simulation. To simulate the lightning current, digitized oscillograms of real lightning currents with peak values of –5.256 kA and –133.586 kA were applied. The article shows that the proposed approach gives a more accurate and visual representation of the transient process at the top of the stricken tower than the approximation of the lightning current by simple exponential expressions. Applying a simplified exponential description of the lightning current leads not only to a simplification of the nature of the transient process at the tower top, but also to an underestimation of the results to 8.8%. The selection of the equivalent circuit for the power line towers also affects the result. Representation of towers in the equivalent circuit with lumped inductances leads to slightly higher values compared to application of surge impedances in the circuit. In this case, the smaller the current amplitude, the greater the difference (8.6 % in the domain of low currents and 1.9 % in the domain of high currents). Since this leads to some reserve during the computations of lightning performance, it is recommended to use an equivalent circuit with lumped inductance for a transmission line tower. The conducted research contributes to the development of methods for calculating the lightning performance of power lines and extends the scope of application of circuit simulation programs.

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.

Hybrid continuous circuit‐trapezoidal integration method analysis of multi‐cross structure of grounding system

An essential requirement to improve the lightning performance of transmission line is the achievement of low impulse impedance of grounding system, which diminishes the occurrence of the external overvoltages. In this study, for the first time, a multi-cross structure used as transmission line tower-footing grounding system is investigated by means of hybrid continuous circuit-trapezoidal integration method (HCCTIM). It is discovered that the angle of minimum impulse impedance or the optimal structure of multi-cross grounding system is mainly dependent on the branch length compared to the effective length of a single horizontal conductor. Furthermore, in the current work, the trapezoidal integration method is used to solve the continuous circuit equations which elements are extracted from electromagnetic integral method, called HCCTIM. Coupling elements and soil ionisation are considered in this model. This method is an accurate, simple, and less time-consuming computational method. The comparison of the results with measurements, other recent simulations and CDEGS software, satisfactorily validate the proposed method.

Lightning Performance of Transmission Lines: Impact of Current Waveform and Front-Time on Backflashover Occurrence

The influence of the return-stroke current waveform and front-time on the occurrence of transmission-line backflashover was assessed via numerical simulation, using the data of two real lines as examples. The overvoltages resulting from distinct lightning current representations (triangular, CIGRE-concave, and double-peaked waveforms) were calculated using an electromagnetic model. The occurrence of backflashover was assessed applying different flashover criteria. Using robust criteria, in particular, the disruptive effect and leader progression models, leads to similar backflashover probabilities for all waveforms; although the probability resulting from these criteria could be different. Also, it leads to practically the same probability obtained using the median current front-time (3.8 μs) or by varying this parameter within realistic range (1.5–10 μs). This still holds true when a correlation between the front-time and peak current is considered. The conclusions are not valid when applying the voltage–time curve method, as the results yielded by this flashover criterion are affected both by the current waveform and the front-time.

Prototype of a measurement device for frequency dependent soil electrical properties

Studies show that grounding potential rise and lightning performance of transmission lines are significantly influenced by frequency dependent soil properties (resistivity and permittivity). In order to perform calculations of the transient grounding characteristics more accurately, additional experimental data for different soils is needed. Currently, field measurements of the frequency dependent soil properties can take a significant amount of time. This limits the amount of available experimental data. Thus, for conducting a large number of measurements, a convenient measurement device is needed. Several parameters of the measurement device for the frequency dependent soil electrical properties are discussed in the article (such as preferable generated waveform, the amplitude of the signal, input impedance of the voltage measurement probe, isolation between measurement circuits), and a prototype of the device is presented. A critically important factor for this kind of measurements is high impedance between the current and voltage measurement circuits. In order to achieve proper isolation between the circuits, the optically-isolated probe was made for the voltage measurements. An example of measurements with the dipole-dipole electrode array for high-resistivity soil is presented. Further improvements for the measurement device are also suggested in the article. With these improvements, the measurements can be almost as fast and simple as those made with a conventional soil resistivity meter.

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.