Performance Of Overhead Transmission Lines Research Articles

Evaluation of the backflashover performance of 150 kV and 400 kV double-circuit overhead transmission lines as affected by lightning attachment models and peak current distributions

The evaluation of the backflashover performance of overhead transmission lines requires the adoption of a lightning attachment model and a peak current distribution. These affect the estimated backflashover rate, BFR, of the lines. The present study aims to quantify the influence of lightning attachment model and peak current distribution on the BFR of double-circuit overhead lines. This is achieved by evaluating typical 150 kV and 400 kV lines. ATP-EMTP simulations are conducted to obtain the critical currents causing backflashover of line insulation. Then, the critical currents are used in lightning incidence computations to obtain BFR. The influencing parameters taken into account in this work are the low current and low frequency tower ground resistance, the location of lightning strike to the shield wire along the span, and the AC voltage phase angle, all affecting the critical backflashover current. Several lightning attachment models and peak current distributions are evaluated. Their influence on BFR is found to be considerable, especially for lower tower ground resistance values. The lightning peak current distributions yield more pronounced effects on BFR. The results of this study stress the need for computing BFR using a more accurate expression than the simplified one based on CIGRE TB 63.

Framework for assessing the performance of overhead transmission lines under wind-temperature effects

This paper presents a framework for evaluating the wind-induced resistance of overhead transmission lines (OTLs), aiming to overcome the limitations associated with disregarding temperature effects and relying on a single metric for failure assessment. The proposed framework factors in temperature influence, carefully considers applied loads and structural characteristics, and accurately calculates failure probability based on a specific case study. Meteorological data is initially collected to create a probabilistic model of wind speed and temperature distribution, accounting for correlation. Components and systems are graded based on structural characteristics and damage modes. The neural network surrogate model is trained with finite element computations for load-structural response sample sets. The surrogate model, in conjunction with logistic regression, generates samples to determine fragility. Finally, the total structural failure probability and monthly failure likelihood are computed by integrating the joint probability model and fragility. Analyses highlight temperature's significant impact on conductor-ground line safety, validating the proposed framework's rationale.

Hydrophobicity grade discrimination of high voltage composite insulators based on water trace image processing

At present, when the hydrophobicity (HC) of high-voltage composite insulators is graded, there is a certain probability that the noise point information will be lost in the denoising process of the water trace image, which makes the image features nonlinear and diversified, affecting the autonomous learning of the algorithm and resulting in low accuracy of the HC discrimination results. Therefore, a method based on water trace image processing for HC grade discrimination of high-voltage composite insulators was designed. The water trace of the high-voltage composite insulator was converted into a gray image, which was enhanced after smoothing and binarization. The dynamic threshold was used to segment the target and background, extract the image edge feature information, and construct a Bayes network to determine the HC level of the high-voltage composite insulator. The results of the performance test show that the average accuracy of the designed discrimination method is 97.89% for different grades, which verifies the reliability of the method in practical applications. The conclusion of this study is helpful to replace insulators in time, improve the safety performance of overhead transmission lines, and prevent accidents.

OVERVIEW OF FACTORS AFFECTING THE ESTIMATION OF LIGHTNING SHIELDING PERFORMANCE OF OVERHEAD TRANSMISSION LINES

This paper examines the risk of lightning stroke to overhead transmission line. The estimation procedure for the lightning performance of overhead power lines is based on the selected lightning attachment model, available lightning parameter statistics, the transmission tower design and voltage levels, type of overvoltage and other characteristics. In this paper the overview of factors affecting the estimation of lightning shielding performance of overhead transmission lines was performed. Among the factors that can affect the estimation accuracy, one can list insufficiently accurate data on the ground flash density in the area of interest and lack of complete data on statistical distribution of lightning current magnitudes. The paper shows that the influence of wind on the increase in the horizontal exposed distance of the phase conductor is not also taken into account. In this research traditional electro-geometric model was used for estimation of lightning performance of 220 kV overhead power line. Results obtained suggest that swing of suspension insulator strings caused by strong winds may lead to increased risk of lightning shielding failure during thunderstorm. Calculation performed for 3 kA lightning current magnitude shows that at swing angle equal to –1 degree, the horizontal unprotected distance of phase conductor increases by 3.1%, that corresponds to 5.240 m uncovered width. When the swing angle is increased to –5 degree, the uncovered width is increased by 15.8% that corresponds to 5.885 m uncovered width. It is proposed that an increase in the risk of lightning shielding failure as a result of wind load can be accounted by applying an appropriate correction factor in expressions for calculation of shielding failure rate, shielding failure flashover rate, etc. Proposed correction factor should account frequency and strength of wind in the area of transmission line route and depend on transmission line voltage level and tower design. Further efforts should be focused on obtaining and justifying the numerical values ​​of this correction factor.

Influence of a lossy ground on the lightning performance of overhead transmission lines

• Accurate lossy ground representation impacts lightning overvoltages simulation. • Carson's classic formulation may lead to inaccuracies for high-resistivity soils. • Accurate lossy ground representation in simulations leads to higher TLs outage rate. • An increase up to 15% in TL outages was found when ground is accurately represented. This paper investigates the impact of incorporating both displacement currents and frequency-dependent soil parameters in the calculation of the ground-return impedance for the determination of the backflashover rate of 138 kV and 230 kV overhead transmission lines due to direct lightning strikes. This is done by considering the Sunde's formulation for calculating the ground-return impedance, which is incorporated in a time-domain simulator using an alternative implementation of Marti's transmission line model. The obtained results show that the peak of the lightning overvoltages computed considering Sunde's formula, along with frequency-dependent soil parameters, leads to higher amplitudes in comparison with those simulated using the classical Carson's formula, which neglects the frequency dependence of soil parameters and assumes conductive currents in the soil much larger than displacement currents. This effect is more pronounced in high-resistivity soils and leads to an increase in the line backflashover rate within 4-6%, 7-12% and 9-15% for 1000 Ωm, 5000 Ωm and 10000 Ωm soils, respectively, considering 138 kV and 230 kV lines, and assuming a typical median first stroke lightning current. Analyses performed with slow and fast first stroke currents confirm the trend of increase of the backflashover rate with the consideration of Sunde's formula.

Optimal placement of surge arresters for transmission lines lightning performance improvement

Installation of transmission line surge arresters (TLSA) is one of the solutions found to improve the lightning performance of overhead transmission lines (OHTL). This work proposes a new multi-criteria methodology for the optimal placement of surge arresters on OHTL based on a multi-objective optimization algorithm coupled with ATP (Alternative Transient Program) software. Lightning performance calculation is performed considering the transfer effect of the lightning surge due to arresters operation. This transfer mechanism that can lead the adjacent towers to flashover is rarely mentioned in the literature. A case study considering a 230 kV transmission line with 231 towers and 105 km long operating in Brazil is presented. Compared to the real performance data, the solution found with the best cost-benefit ratio allows a reduction of 68% in the line outages with the use of 90 surge arresters (i.e., 0.86 TLSAs/km of line). This is practically 50% of the number of arresters calculated using the classical placement method based on the tower-footing impedance value. Regarding the transfer effect of the lightning surge, the results show that the performance levels and, consequently, the number of TLSAs to protect the line may be underestimated if this effect is not considered in the modelling.

A Stochastic Model for Evaluating the Lightning Performance of a −400 kV HVDC Overhead Line

A stochastic lightning attachment model for evaluating the lightning performance of overhead transmission lines is introduced. The proposed model considers the stochastic nature of lightning attachment by taking into account the lightning discharge branched and tortuous behavior. It integrates physical criteria on leader inception by adopting the Petrov and Waters model, as well as leader discharge progression considering also the competing interaction of multiple upward leaders. A generalized methodology combining stochastic modeling of lightning incidence with an electromagnetic transients (EMT) simulation model is proposed so as to estimate the backflashover and shielding failure flashover rates of HVDC overhead lines; an application to a monopolar −400 kV line sheds light on the basic parameters that affect the lightning performance of HVDC lines. The probability of shielding failure is higher than that obtained by the deterministic lightning attachment models. The stochastic nature of lightning attachment and the effect of lightning peak current distribution should be considered for the design and evaluation of the lightning performance of the emerging HVDC systems.

Enhancing the lightning performance of overhead transmission lines with optimal EGLA and downstream shield wire placement in mountainous areas: a complete study

Service unavailability of transmission lines, particularly in mountain areas due to the direct and indirect lightning strikes, is considered as a challenging issue within electric companies. Therefore, the efforts to mitigate the power outages caused by this phenomenon have been introduced. In this study, an attempt has been made to study the different types of externally gapped line arresters (EGLAs) considering the Pinceti model and discharge voltage curve. Based on the best selected EGLA, the installation effect of EGLA on the possibility of instance back-flashover in different phases of the transmission line is investigated in the presence of downstream shield wire. For assessment, a double-circuit 400 kV transmission line in a mountainous area is modelled in EMTP-RV software. Then, an extensive analysis is carried out to verify the effectiveness of this method. The obtained results show the acceptable transmission line direct lightning protection can be achieved by four EGLAs in upper and middle phases with downstream shield wire. Although employing the downstream shield wires in the transmission line imposes more costs, but by using them, the scale and capability of EGLAs can be reduced.

Corona inception field of typical overhead line conductors under variable atmospheric conditions

Corona inception field of line conductors is of major importance for assessing corona performance of overhead transmission lines. Using computational models that consider the ionization processes in corona discharge, the corona inception field of ACSR conductors typically utilized in overhead transmission lines is estimated. The interactive effects of line conductor geometry, voltage polarity as well as atmospheric conditions on corona inception field are assessed and discussed based on the ionization area around the conductor. It is shown that the corona inception field of bundle configurations can be satisfactorily approximated by that of the subconductor when considered single. Generally, the inception field of negative corona is slightly lower than that of positive corona and lesser affected by variations in atmospheric conditions. An empirical expression for the straightforward estimation of the corona inception field of line conductors has been introduced, accounting well for the effects of conductor geometry, voltage polarity and atmospheric conditions.

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.

Estimation of the shielding performance of transmission lines considering effects of landform, lightning polarity and stroke angle

Lightning is the main cause of transmission system outages, affecting reliability of power supply and resulting in economic losses. Shielding failure is an important issue in addressing the lightning performance of overhead transmission lines. In this paper an improved method based on the electro-geometric model is proposed to evaluate the shielding failure flashover rate (SFFOR) of transmission lines, considering the influences of landform, lightning polarity and stroke angle. In the improved method, the striking distances to phase conductors, shield wires and ground are properly differentiated by using recent equations developed to take into account other factors not only magnitude of lightning strokes. These factors include height of wires as well as lightning polarity. To reflect the effects of changes in terrain and height of wires on SFFOR, a method for identifying the topography and the relative position among the wires and ground is proposed. A 400-kV double-circuit transmission line section in complex terrain area was taken as example, and the results show that this method can reflect changes of the SFFOR along the span and can be useful to figure out which towers are easy to be struck by lightning along the entire transmission line. This research is helpful for the design and operation of overhead transmission lines.

Effects of Considering A Stroke Angle Distribution on Estimating the Shielding Performance of Overhead Transmission Lines

The shielding analysis method recommended by the IEEE Working Group on Estimating the Lightning Performance of Transmission Lines is based on the Electrogeometric Model and the concept of striking distance. The simplistic assumption of a vertically descending leader is adopted, although a statistical distribution for the stroke angle has been previously proposed in the literature. This paper aimed to investigate the influence of assuming both vertical and non-vertical flashes on the evaluation of the shielding failure flashover rates of overhead transmission lines. The study is based on a modified Electrogeometric Model built in a procedure implemented by the authors in MATLAB, in a comparative perspective with the well-known IEEE Flash program. Parametric studies are performed to determine the sensitivity of the outage rates with respect to the parameters of the transmission line, the striking distance equation and leader angle distribution.

Estimation of the shielding performance of overhead transmission lines: The effects of lightning attachment model and lightning crest current distribution

Shielding failure of overhead transmission lines is a major cause of transmission system outages, affecting reliability of power supply and resulting in economic losses. In this study the shielding performance of overhead transmission lines is evaluated with the aid of general expressions, derived by implementing in shielding analysis different categories of lightning attachment models. Thus, the effects of lightning attachment model, transmission line parameters and lightning crest current distribution on shielding failure rate of overhead transmission lines have been quantified. Alternative approaches to that proposed by IEEE Std 1243 for assessing the shielding performance of transmission lines, including computer simulations of lightning attachment, are evaluated. Shielding failure rate results are discussed and compared with field data reported in literature. For typical overhead transmission lines, an upper limit of the estimated shielding failure rate is 0.4% of the rate of lightning strokes to the line. More and reliable field data is needed in order to evaluate lightning attachment models with respect to the lightning performance of overhead transmission lines.

Probabilistic evaluation of lightning performance of overhead transmission lines, considering non-vertical strokes

This paper represents a developed procedure for the evaluation of flashover rates caused by both lightning vertical and non-vertical strokes on transmission lines, using a Monte Carlo method. The main goal of this paper is to increase the accuracy flashover rates through the accurate modelling of network components, such as footing impedance, the transmission tower and chain of insulators, and also simulation of lightning non-vertical strokes. Modelling network components, statistical parameters related to lightning and a problem-solving algorithm are described. Parametric studies using this procedure can also be performed to determine the sensitivity of the flashover rate with respect to some parameters of the return stroke and the transmission line. Simulation results obtained using MATLAB and the ATP/EMTP show that considering the lightning non-vertical instead of vertical strokes leads to a better evaluation of the lightning performance of transmission lines.

Analysis of Lightning Shielding Failure for 500-kV Overhead Transmission Lines Based on an Improved Leader Progression Model

An improved leader progression model (LPM) for analyzing the shielding failure performance of overhead transmission lines is established, and implemented in calculating the shielding failure rate and shielding failure flashover rate of 500-kV transmission lines. This paper discusses the inadequate parts of the existing LPM and presents improvements to meliorate the model for applications on extremely high voltage (EHV) transmission lines. The model is improved mainly in three aspects: 1) the background electric field produced by the lightning cloud is simulated by multielectrode configuration, 2) different inception criteria of upward leaders for shielding wires and phase conductors are chosen, and 3) the impact of power frequency operating voltage on the shielding failure flashover rate is considered. The improved model is applied to analyze the shielding failure performance of 500-kV transmission lines in China considering the flat and hilly terrain conditions. The shielding failure flashover rates by existing models and the improved model are calculated and compared with the operating data of 500-kV transmission lines. It is shown that the predicted result of the improved LPM is in accordance with the operating experience. The improved model is suitable for the lightning shielding assessment and applicable in the design of EHVAC transmission lines.

Artificial neural network-based software tool for calculating the lightning performance of high-voltage transmission lines

An artificial neural network (ANN) is addressed for evaluating the lightning performance of high-voltage transmission lines. Several structures, learning algorithms and transfer functions were tested to produce a model with the best generalising ability. Actual input and output data, collected from operational Hellenic high-voltage transmission lines, were used in the training, validation and testing process. The method is coded in a comprehensive software program to be used by electric power utilities as a useful tool for the design of electric power systems, as an alternative to the conventional analytical methods. The aims of the paper are to describe in detail the proposed ANN method and the developed software tool and to present the results obtained by its application to operational Hellenic transmission lines of 150 kV and 400 kV. The ANN tool's results are compared with results produced from a conventional method and real records of outage rate showing a quite satisfactory agreement. overvoltages in distributions lines (13) and for lightning protection of high-voltage transmission lines (14). The calculation of the lightning performance of overhead transmission lines presents many uncertainties owing to the random nature of the lightning phenomenon and the lack of reliable data. This is why all the existing methods for the calculation of lightning performance are based on empirical and approximating equations. ANNs can be an effective alternative solution to this problem, as they can present great accuracy and, in many cases, better results using only actual line data in the lightning performance calculations. Moreover, the efficient and economic implementation of ANNs with today's computer technology makes them very attractive tools. In this paper, an ANN method is proposed to identify the lightning performance of high-voltage transmission lines. By testing several combinations of different structures, learning algorithms and transfer functions, we have developed an ANN that presented the best generalising ability among all the other combinations. Actual input and output data, collected from operational Hellenic high- voltage transmission lines w ere used in the training, validation and testing process. The developed method was coded in a comprehensive software program and applied to several operational Hellenic transmission lines of 150 kV and 400 kV so that its accuracy could be validated. The results obtained are compared with those produced using conventional methods and with real records of outage rate.

Improvement of the lightning shielding performance of overhead transmission lines by passive shield wires

This paper addresses a numerical electromagnetic analysis of the influence of installing passive shield wires "extra ground wires" (EGW) underneath the bottom phase conductor on the lightning shielding performance of 765-kV, 3-phase, double-circuit transmission lines. The method of moment (MOM) is employed to model the whole structure in three dimensions except the lightning channel. The lightning channel is simulated by the well-known transmission-line model (TLM), where the influence of the lightning-channel-generated electric and magnetic fields (EMF) are taken into account. The lightning shielding performance with and without EGW is introduced and discussed, without ac energization and due to three distinct lightning strokes, namely, the negative first, the negative subsequent and the positive strokes. The produced voltages and currents in the phase conductors and ground wires as well as the voltage across the insulators are computed, with and without the EGW, due to different lightning current waveforms, locations and inclination of the lightning channel, and return-stroke velocities. The introduction of such EGW results in a significant improvement of the shielding effectiveness by reducing the lightning-produced voltages and currents in the phase conductors as well as the voltage across the insulators. Accordingly, much lower transient overvoltages can be expected with less inadvertent outages of the lines, where the probabilities of flashover and back-flashover decrease, and the reliability enhances.

An integrated backflashover model for insulation coordination of overhead transmission lines

The lightning performance of overhead transmission lines is measured with the probability of backflashover. This probability is affected by (a) tower type, (b) grounding arrangement, (c) soil resistivity and (d) lightning activity and lightning parameters of the area. This paper presents an integrated model of the transmission towers, tower grounding and overhead line conductors. The model is utilized in a Monte Carlo simulation for the purpose of predicting the lightning performance of a transmission line. This paper presents the models and the simulation method. Typical results for typical transmission towers and grounding arrangements are provided. The model reveals that the grounding arrangement as well as the surge impedance of the transmission tower are important parameters affecting line performance.

Effects of agricultural fires on the performance of overhead transmission lines

It is noted that the occurrence of fires under transmission lines is responsible for a great number of line outages in many countries. The insulation requirements for lines of several voltage levels were established on the basis of field investigations and laboratory tests under fire conditions. The results obtained and the statistics presented indicate that for actual AC transmission lines with rated voltage up to 500 kV, faults are mainly due to conductor-to-ground short circuit at midspan or phase-to-phase short circuit, depending on line configuration. Another problem arising from the burning is the contamination of the insulators due to the accumulation of particles (soot, dust) on their surface. Pollution severity in terms of ESDD (equivalent salt deposit density) measured on insulators coming from regions of sugar cane fields in Brazil has shown values of up to 0.1 mg/cm/sup 2/. >

Surge response of transmission towers

The lightning performance of overhead transmission lines is related to the surge impedance and wave propagation time of the line towers. Theoretical and experimental studies show that present models for the tower response are not adequate. New theoretical models for tower surge impedance are presented and supported with experiments.