Lightning Shielding Performance Research Articles

ELECTRICITY TRANSMISSION AND ENVIRONMENT: EFFECT OF WIND LOADS ON LIGHTNING SHIELDING PERFORMANCE OF OVERHEAD POWER LINES

In this paper the estimation of wind load effect on the lightning shielding performance of overhead power lines was performed. According to electro-geometrical model any phase conductor has horizontal exposure width where this conductor is not protected against lightning by the overhead ground wire. A typical double circuit 220 kV lattice power transmission line tower was considered. Obtained results demonstrate that in the presence of thundercloud in windy conditions unprotected distance of phase conductor may increase due to deflections of phase conductors. Geometric locations of the conductor attachment points on the suspension insulator string and the lower point of the conductor sagging were calculated in the range of wind pressure from 0 to 800 Pa. This allowed to determine the exposure width values of a 220 kV overhead power line upper phase conductor in the same range of wind pressure values. The results show that for a minimum lightning current of 3 kA, the unprotected distance increases by 4.323 times from 4.167 m to 18.013 m when the wind pressure increases from 0 to 800 Pa (from 0 to 36.140 m/s). For a minimum lightning current of 5 kA, the unprotected distance increases by 7.735 times from 2.825 m to 21.851 m when wind pressure and wind speed vary in the same range. Although the transmission line is reliably protected against lightning strikes with currents greater than 16 kA at wind pressure of up to 200 Pa (18.070 m/s), when the wind pressure increases from 300 Pa to 800 Pa (from 22.131 m/s to 36.140 m/s), the unprotected area increases from 4.752 m to 26.204 m. In Summary, the results show that the influence of wind load must be taken into account in the tasks of calculating lightning protection of overhead power lines. Further efforts should be focused on studying the lightning shielding performance of overhead power lines of higher voltage classes.

Research on the lightning shielding performance of strained angled tower for the [formula omitted]1100 kV UHVDC transmission project

Rigid jumper is usually used in the strained angled tower for the UHV transmission project, which makes the protection angle of the ground wire to the jumper end larger, thus the lightning shielding failure risk of the strained angled tower may increase. In order to study the lightning shielding performance of strained angled tower for the ±1100 kV ultra-high voltage direct current (UHVDC) transmission project, a numerical model for the simulation of lightning attachment to the strained angled tower was established based on the research results in the aspects of lightning downward leader, inception and propagation characteristic of upward connecting leader and so on. The maximum lightning shielding failure current and the shielding failure flashover rate (SFFOR) of rigid jumper were calculated under different line angles and overhead ground wire cross-arm lengths based on the proposed model. Results show that the lightning flashover of rigid jumper mainly occurs on the outer side while the rigid jumper on the inner side is in a good shielding state. When the cross-arm length of overhead ground wire on the outer side of J27104A1 tower with 90° line angle is greater than 21.7 m, the SFFOR of rigid jumper will be below 0.1 times/(100 km a), which can meet the operation requirement. The analysis provides technical references for the lightning protection design of strained angled tower for the UHVDC transmission project.

Improved Electrogeometric Model of UHV Transmission Line Based on Long Gap Discharge and Its Application

Based on long air gap discharge test data and lightning return stroke observation data, an improved electrogeometric model (EGM) considering terrain conditions is established and verified to analyze the lightning shielding performance of UHV transmission lines. The striking distance formula is modified as rs = 0.13 (I2 + 40I)0.814. In this paper, the lightning shielding failure rate of three-phase conductors of EHV and UHV transmission lines calculated by the improved EGM model is consistent with the lightning observation data of actual transmission lines in Japan and the scaled lightning discharge simulation experimental results of UHV transmission lines in plains and mountainous areas of China, which verifies the applicability of the improved EGM model to evaluate the lightning shielding performance of large-scale UHV transmission lines. The improved EGM model is applied to evaluate the influence of tower type and slope steepness on the shielding failure tripping rate of UHV transmission lines. The shielding failure tripping rate of the SZ322 tower is higher than that of the SZT1 tower, and the shielding failure tripping rate of UHV transmission lines is greatly affected by slope gradient and increases with the increase of slope gradient.

Model test on the effect of hillside on lightning shielding performance of ±1100 kV transmission lines

Lightning damage is one of the main causes of faults in transmission lines. To study the effect of terrain on the lightning shielding performance of UHV transmission lines, with negative 250/2500 μs switching impulse voltage, a large-sized model of the Z38102AL tower with the ratio of 1:12.5 for ±1100 kV UHV transmission lines is used to conduct the simulation tests in hillside in this paper. According to the pictures of discharge process, the discharge process of transmission line around the hillside area is analyzed. The results show that the ground potential rise in the hillside area changes the space electric field, and the attraction of the slope to the downward leader makes the transmission lines in the hillside area more vulnerable to lightning. The test results show that the equal probability curve of the shielding failure spatial region is inclined parabola curve, and the shielding effect of the slope is weak, the dispersion of discharge increases, which leads to the decrease of the difference in the attraction ability of the conductor voltage polarity to lightning. By comparing the test results with the plain model test, the results show that distribution of discharge is larger and the total shielding failure probability is higher in hillside area. Conclusively, under the influence of hillside slopes, the increase of exposure space and the change of space electric field caused by the rise of ground potential have a stronger effect on the shielding failure probability than the polarity effect of conductor voltage, which makes the difference of conductor polarity in hillside area smaller. It is found that the influence of voltage polarity of ±1100 kV conductor on the probability of shielding failure is stronger by comparing the results of ±1100 kV test and ±800 kV test.

Research on effect of lightning shielding level of EHVDC transmission line by installing coupling ground wire

Lightning shielding failure is the main reason causing lightning flashover of Jiangcheng EHVDC transmission line because of the designed ground wire protection angle. Based on the electric-geometry model, a simulation model including the coupling ground wire is established by using MATLAB. The effect of the coupling ground wire at different height and horizontal position on lightning shielding failure flashover rate is analysed. Results show that coupling ground wire can reduce the lightning shielding failure flashover rate and the lightning shielding performance increases with the increase of the height of the wire and the horizontal distance between the axis of the tower and the coupling ground wire.

Lightning Attachment to UHV Power Transmission Lines: Effect of the Phase Voltage

The self-consistent leader inception and propagation model is used to analyze the influence of the phase voltage on the attachment of lightning to ultra-high voltage power transmission lines (UHV-TLs). An UHV-ac line with shielding failures reported in the literature is used as a case study. It is shown that the length of upward leaders initiated from conductors and their striking distances are longer under positive voltages than when energized with the opposite polarity. Therefore, the fraction of shielding failures of each conductor changes significantly with the phase angle in ac lines. However, it is found that the overall effect of voltage on lightning attachment can also be limited by the electrostatic screening produced by shield wires and their leaders. This proximity effect mainly reduces the velocity of upward leaders launched from energized conductors. Therefore, the effect of voltage on the lightning attachment process cannot be generalized since it is strongly coupled to the proximity of shield wires and their associated leaders. Thus, the lightning shielding performance should consider case-to-case variations in the upward leader velocity in different UHV-TLs designs, given not only by the line voltage but also coupled to the proximity of other wires and their launched leaders.

Investigation on Shielding Failure of a Novel 400-k$\text{V}$ Double-Circuit Composite Tower

In this paper, the lightning shielding performance of a newly designed 400-k $\text{V}$ double-circuit composite tower is investigated. Based on a revised electrogeometric model (EGM) method, traditional shielding failure regions located at both sides of a traditional tower are no longer a big issue for the fully composite tower, due to its unusual $^{\prime }Y^{\prime }$ configuration. Instead, a new unprotected region exists in the tower center. The maximum lightning current that can lead to shielding failure and the shielding failure rate of the new tower are calculated. To verify results from the revised EGM method, a scale model test is conducted. Spatial shielding failure probability around the tower is calculated based on the ratio of discharge paths recorded in the test. Moreover, based on test results, the maximum shielding failure lightning currents are obtained. Analysis and results derived from the revised EGM method and scale model test have a small deviation, verifying the feasibility of these two methods in the lightning performance evaluation of the fully composite tower. Both methods confirm that the unusual negative shielding angle of $-\text{60}^{\circ }$ in the newly composite tower provides good lightning shielding performance. The feasibility of the novel composite tower is verified from the lightning protection point of view.

Experimental research on lightning shielding performance of large scaled UHV transmission line

In actual operation, the shielding failure rate of UHV transmission line is much higher than that in simulation, so it is crucial to study the lightning shielding performance by experimental research. The testing UHV transmission line model is scaled down in the ratio of 12.5 which is relatively large comparing to other testing models. Two kinds of negative switching impulse waveform were applied to HV rod to simulate the lightning descending leader. The probability of transmission line being stroked was calculated statistically by recording more than two thousand discharges and the factors that affecting the discharge probability were studied through contrast tests. The results show that the grounding condition, DC bias, the protection angle, the switching impulse voltages have an influence on the discharge probability, breakdown voltage and breakdown time. The spatial boundary of shielding failure area was determined in the middle of the span. The upper and lower spatial boundary and the distance between them increased with the increasing of the lateral distance between HV rod tip and transmission line model simultaneously.

A new estimation model of the lightning shielding performance of transmission lines using a fractal approach

The path of the lightning discharge shows characteristics of branching and tortuosity, and the lightning shielding failure of the transmission line also has statistical features. In this paper, an improved leader progression model using a fractal approach is introduced, which considers both the deterministic and stochastic features of the downward lightning leader. Based on the proposed model, the statistical features of lightning shielding failure on transmission lines will be analyzed. First, the fractal dimensions of the downward lightning trajectory in simulation are calculated and the value of breakdown probability constant η will be determined based on the fractal dimensions of natural lightning. Second, using the proposed model the shielding failure probability contour around the transmission line space is also obtained and used to describe the shielding failure zone and then the contour curve is applied to analyzing the characteristics of transmission lines that suffer from direct lightning strokes. Moreover, a method to estimate the lightning shielding failure rate of transmission lines is put forward, and the influence of the line height and the shielding angle of ground wires on the shielding failure rate of transmission lines are also investigated.

Discharge characteristics of 5 m long air gap under foggy conditions with lightning shielding of transmission line

The electric geometry model suggested by Armstrong and Whitehead was used to calculate the performance of lightning-shielded transmission lines. However, since the ultra high voltage (UHV) designed transmission lines were brought into 500 kV operation, it has become evident that their actual performance is different from the calculated predictions in terms of their actual faults, lightning observation results, etc. It is thought that the UHV designed transmission lines are subjected to foggy or rainy conditions when lightning strikes, because they often pass through mountainous areas at higher altitudes. However, according to the electric geometry model, the striking distance is determined by the lightning-stroke current value only, and the model does not consider the environment of discharge path for the relevant lightning strokes. These considerations suggest that foggy conditions may affect transmission line lightning shielding performance. In this study, discharge tests were conducted using a scaled-down transmission line with a 5 m air gap. The discharge point to the simulated conductor was analyzed in both dry and foggy conditions. Tests were also conducted with a DC bias applied to the simulated power lines, to take the operating voltage of the lines into consideration. These experiments have shown that the effect of fog on the discharge ratio to the conductors is negligible.

Magnetic Fields and Loop Voltages Inside Reduced- and Full-Scale Structures Produced by Direct Lightning Strikes

This paper presents a numerical electromagnetic analysis of magnetic fields and loop voltages inside reduced- and full-scale lightning protection systems (LPSs) structures resulting from direct lightning strikes. The method of moments 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 (TL model), where the influence of the lightning-channel generated electric and magnetic fields are taken into account. Three distinct LPSs were modeled, namely, reduced-scale model with return conductors (RSRC), reduced-scale model with lightning channel (RSLC), and full-scale model with lightning channel (FS). The computed results of magnetic fields and magnetic-field derivatives were verified versus some experimental results for the RSRC model. In addition, the scale factor for all the measured quantities were also checked as functions of the geometrical scale factor for the positive and the negative first stroke currents. The lightning shielding performance with and without bonding was investigated for three distinct lightning stroke types, namely, the negative first, the negative subsequent, and the positive strokes. The voltages and currents generated in loops located inside the struck FS LPS were computed with and without bonding and grounding resistance and for different lightning current waveforms, locations and inclination of the lightning channel, and return stroke velocity.

A study of EMF mitigation underneath power lines and lightning-shielding enhancement by extra ground wires

This paper presents a theoretical study of the influence of extra ground wires (EGW) on both the electromagnetic fields (EMF) mitigation underneath 765 kV, 3-phase, double circuit transmission lines and the lightning-shielding performance. The method of moments (MOM) combined with the transmission-line model is used to simulate both the aforementioned transmission lines comprising the towers and the lightning channel in three dimensions, where the influence of the channel radiated EMF is taken into account. For power frequency AC energization, the EMF at 1 m above the ground level and at the surface of the phase conductors, and the induced currents in both normally used top ground wires and EGW are computed for various phase arrangements and numbers of EGW. In addition, the lightning-shielding performance with and withont the EGW is introduced and discussed without AC energization. The induced 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 amplitudes and waveforms, and return-stroke velocities. The most advantageous merits of using the EGW are as follows. (1) The EMF underneath the overhead power lines are drastically reduced with a negligible effect on the conductors' surface gradient and the charging currents. This can be generally used to meet regularity limits, especially when converting the existing 3-phase double circuit transmission lines to 6-phase systems so that the power handling capability can be increased by up to 73% via increasing the operating voltage. (2) The lightning-shielding performance is significantly improved by reducing the lightning-induced voltages and currents in the phase conductors as well as the voltage across the insulators, i.e. decreasing the probabilities of flashover and back-flashover and enhancing the reliability.

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.