Tower Surge Impedance Research Articles

Power Line Tower Lightning Surge Impedance Computation, a Comparison of Analytical and Finite Element Methods.

The aim of this work is to compare methods for the computation of transmission line tower surge impedance, that are: traditional analytical method using simplified equations; and the Finite Element Method for the mapping of electromagnetic fields surrounding the tower[9]. In the latter were used two different software, the public domain FEMM, and the commercial software FLUX3D. The first is able to perform calculations based in a 2D representation of the problem while the second uses a 3D representation of the problem to perform computations. It is also verified the impedance variation along the tower height, phenomenon that most of the time is not considered, and that can provide a direct impact in the performance of transmission lines facing the atmospheric discharges. With this purpose it was used the software ATP-EMTP, where it was modeled a simple case who brought results for immediate comparison.

Overvoltages caused by direct lightning strokes to a hybrid overhead line

This paper presents an analysis of the overvoltages caused by direct lightning strokes to a hybrid overhead line with 138 kV, 13.8 kV, and 220 V circuits. The primary focus is on medium voltage line overvoltages and their dependence on key parameters, including ground impedance, soil resistivity, stroke current front time, conductor characteristics, lightning channel impedance, insulator capacitance, conductors' sags, and tower surge impedance. Using specific models for the transmission tower and high-voltage insulators, validated through laboratory tests and EMTP simulations, critical currents are determined for varying soil resistivity and impulse impedances. The analysis confirms the line robust performance against stroke-induced backflashovers.

Modeling guyed towers of transmission lines in the assessment of backflashover occurrence

This paper evaluates the modeling of guyed towers of transmission lines in EMT-type programs to assess the backflashover occurrence of transmission lines and proposes a new modeling approach to be applied for this kind of towers. Two approaches to model the guyed towers by the revised Jordan model were evaluated: (i) the approach of CIGRE Brochure 63 that suggests representing the tower surge impedance as the parallel of the surge impedances of the mast and the guyed wires, ignoring the mutual coupling among them, and (ii) a new approach assuming the surge impedance of all the conductors in the tower section, guyed and mast conductors, including their mutual coupling effect. The evaluations considered the results provided by the Hybrid Electromagnetic Model (HEM) as reference for assessing the quality of the approaches. The approach (i) provided results of lightning overvoltage significantly lower than the reference, HEM model, leading to an underestimation of the probability of backflashover of the line. On the other hand, the approach (ii) was responsible for the results closest to those provided by HEM, indicating its quality, and the recommendation of applying this proposed approach for modeling guyed towers in the evaluations of backflashover occurrence.

Mitigation of common mode failures at multi-circuit line configurations by application of line arresters against back-flashovers

Due to the limited number of corridors multi circuit line configurations are often applied. These overhead lines frequently consist of high towers that are subject to lightning strokes. In case of higher current amplitudes and higher footing resistances due to bad earthing conditions back-flashovers are caused leading to common mode failures and to severe outages. The paper describes investigations performed by means of computer simulations to identify the towers of a multi-circuit line consisting of voltage levels 380 kV, 220 kV and 110 kV that are endangered by back-flashovers of the 110-kV double-circuit lines. The footing resistance of towers of the targeted line section has been measured by an instrument at high-frequency. Influence of various factors on the back-flashover over 110 kV insulator strings has been studied by means of EMTP-ATP simulations. Different current waveforms of the lightning stroke have been used to represent the first stroke and subsequent strokes. The towers are represented by the models described in [3], [8]. Available flashover analysis methods [7], [8], [12], [13] like leader development method by Pigini et al and by Motoyama, and voltage-time integration method by Kind have been applied. The towers at which back-flashover is more likely to occur than at other towers are identified by the time integral of voltage according to Kind. Various factors like tower footing impedance, tower surge impedance and tower height are considered. Application of line a surge arrester is shown to be a successful mitigation technique to reduce the back-flashover rate of those 110 kV lines. The lightning overvoltage performance of surge arresters has been analyzed by means of digital simulations. Based on the results of investigations line arresters were installed on the towers in question. Since the installation no further common mode failure has been observed.

Back flashover mitigation for transmission tower by parallel additional multiple conductors from tower top to ground footage to alter the surge impedance

This research assessed the transmission tower’s surge impedance before and after paralleling the overhead ground wire (OHGW) made of twisted galvanized steel cable from topmost of the transmission tower to the tower ground footage. Laboratory scale lightning current impulse generator of 5 kA 8/20 μs lightning waveform was applied at topmost of the laboratory scale transmission tower. The height of the constructed transmission tower is 1.96 m, and the width of the transmission tower is 0.42 m. When the impulse current was applied, consideration parameters for estimating transmission tower’s surge impedance are the voltage and current obtained from each cross-arm. The value of surge impedance can be calculated form measurement using impulse voltage divider and Rogowski coil for lightning impulse current at the tower. The result of the surge impedance of the entire transmission tower before connecting the conductors is 91.58 Ω. When paralleling the OHGW conductors from 1 to 4 wires, transmission tower surge impedance is changed. When paralleling one conductor, the surge impedance of the laboratory scale transmission tower is 70.00 Ω. When two conductors were paralleled, the tower surge impedance value is 63.75 Ω. When three conductors are paralleled, the tower surge impedance is 62.50 Ω. And when four conductors were paralleled, the surge impedance is further reduced to 61.25 Ω. Therefore, by paralleling multiple conductors using this method, transmission tower’s surge impedance was able to reduce due to more current pathway via additional OHGW. Also, the voltage that appears at each cross-arm of the transmission tower was reduced and thus, lowering the back flashover phenomena.

A Study on an Impact of Tower Surge Impedance Against Lightning Surge Phenomena

Lightning strikes creates damages to the electrical power system utilization and equipment. To reduce the effect of lightning strikes in the tower transmission, the ground resistance of the tower must be kept to a low value of less than 5 Ω. This research is a study of tower surge impedance using additional conductor line connected by parallel two overhead ground wire at the top of the tower transmission line. The additional of ground wire which is connected from the top of the tower utilizes copper and aluminium alloy for the simulation. Calculation of tower surge impedance using finite element method by COMSOL in 3D simulation based on traditional electromagnetism equations. Surge impedance values were calculated, and it was found that the calculation results obtained with less than 0.5 % error when using finite element method compare with conventional numerical method. Aluminium alloy conductor was found to have an effect on reducing the overvoltage magnitude of lightning strikes at the tower transmission line when compare with copper conductor.

Integrated model of transmission tower surge impedance and multilayer grounding system based on full-wave approach

In this paper, a full-wave approach based on the method of moment (MoM) is proposed to investigate the harmonic impedance of a tower and its connected ground electrode in the frequency domain. The accuracy of the results is validated in comparison with NEC-4. The proposed numerical method is also employed for the evaluation of a full-sized HVDC tower harmonic impedance. The main contribution is the assessment of the harmonic impedance of a real tower with detailed geometrical information connected to the multi-layer grounding system. The validity of the transmission line method is evaluated through comparison with the results computed using the developed full-wave approach at the high frequency. In addition, the simulation results assure that a real tower’s harmonic impedance could be smaller than the value estimated for very simplified models at the high frequencies. When the full-wave method is applied and the precise model of the ground electrode is considered, the harmonic impedance of the tower in the frequency domain and consequently, the transient impedance in the time domain are different, while the grounding system is assumed to be a perfectly conducting plane. These differences can become very significant, especially close to the resonant frequencies. The harmonic impedance of power transmission towers is strongly influenced by the connected grounding system.

An Electromagnetic Model for the Calculation of Tower Surge Impedance Based on Thin Wire Approximation

When lightning strikes a transmission line tower or shield wires, electromagnetic waves propagate through the tower back and forth, increasing the voltage across insulator strings. This can eventually lead to a back-flashover (BF), which may cause damage to equipment or costly power outages. To calculate the over-voltages and predict the probability of a BF, an accurate model of the tower and its grounding system is needed in electromagnetic transient (EMT) type simulators. There are a number of theoretical models for the equivalent circuit of a transmission tower. However, they either are not accurate enough or they are derived for a certain type of transmission tower, which limits their applicability. Numerical electromagnetic analyses have less simplifications compared to the theoretical solutions and are by far less expensive than field measurements. They also have the flexibility to analyse any type of tower. In this paper, the direct method for the measurement of tower impedance is implemented by NEC4 and applied to a $\text{400}\text{-kV}$ double circuit tower with all its details. The process of obtaining the wire network of the tower used in this paper is completely automated and it can be applied to any other type of transmission tower. The results of the numerical simulations are compared to those obtained with existing tower models. The developed model in this paper is capable of considering all the details of the tower and including the finite resistance of the ground and grounding electrodes.

Analysis of tower surge impedance using the finite element method

In this work, the finite element method (FEM) provided computer generated results from electromagnetic fields, based on a 3D model of a transmission tower. The simulation of electric and magnetic fields enabled the calculation of the tower surge impedance. The surge impedance calculated from these results, uses traditional electromagnetism equations. Comparisons with measurements in a real tower allow for the validation of the proposed method, which resulted in surge impedance values with differences less than 0.5% in relation to the measurements. Additionally, the surge impedance from the proposed methodology is compared to results from analytical techniques, which resulted in differences of up to 60% in the values of surge impedance, in relation to the measurements. In order to verify the influence of tower surge impedance on tower voltages, a transmission tower was modeled using EMTP software. Under the influence of a surge current, the simulations provided tower voltages at strategic heights. Measurements of tower voltages made the comparison of these results possible in order to evaluate the consistency and accuracy of the tower model.

A tutorial about tower transmission models for the analyses and prediction of backflashovers

Lightning is one of a main causes of sudden outages in the power systems. When a lightning strikes the ground wires or directly to transmission tower, a backflashover may occur. It consists of an electric arc from the tower structure to the line phase conductors, causing transient voltage surges in the transmission line and may cause an outage. In this context, an electric parameter named tower surge impedance plays a key role for the prediction and estimation of backflashovers. Several authors have proposed models representing the transmission tower in order to estimate its surge impedance and measure overvoltages caused by the lightning on transmission lines. In this article, a brief historical survey on transmission towers modelling proposed by diverse authors, a description of the backflashover and the most common models for representing towers are presented.

Modelling of 132 kV Overhead Transmission Lines by using ATP/EMTP for Shielding Failure Pattern Recognition

Abstract Lightning overvoltage is the major cause of the transmission line interruption in Malaysia. It is the main reason that endangers the safety and reliability of transmission line system. The phenomenon is a dominant factor considered in designing substations and transmission-line insulation, therefore its investigation is important. This work modelled a 132 kV overhead transmission line for shielding failure pattern recognition by using ATP-EMTP software, as they are vital to the evaluation of lightning performance. The modelled transmission-line is divided into few parts: wires (shield wires and phase conductors), towers, cross-arms, insulator strings, tower-surge impedance and tower-footing resistance. The developed model is simulated with four different magnitudes of lightning-strike current on each phase conductor in order to investigate the shielding failure voltage pattern across the line insulation in the transmission line system. When the lightning- strike current is injected from upper phase to lower phase conductor, the magnitude of the maximum induced voltage across the insulator string at those phases has been decreased. On the other hand, as the magnitude of the lightning- strike current has been increased, the magnitude of the maximum induced voltage across the insulator string at each phase will be increased. Flashover occurs when the voltage induced across the insulator string at each phase is equal or exceeds the CFO voltage. These analyses have been done based on the waveforms obtained from the simulations on developed model.

Application of the insulator coordination gap models and effect of line design to backflashover studies

Any insulation coordination study must accurately (or as accurately as possible) express the type and magnitude of all overvoltages on the power system. If these overvoltages are higher than the rating of the equipment, they will result in damage to that equipment. This paper presents the application of the insulator coordination gap models and the effect of line design to the line performance in terms of backflashover rate. The models which are ranging from as simple as voltage controlled switch to as detail as leader progression model will carefully be evaluated. Sensitivity analyses on the effect of line designs such as variation in footing resistance, height of tower/conductor, tower surge impedance and soil resistivity will also be carried out in determining the backflashover rate and the probability of transformer damage by comparing the maximum voltage recorded at the substation entrance with the basic lightning insulation level (BIL). Results will then be compared in finding which model makes the analysis more or less sensitive to any design parameter.

A New Frequency-Dependent Surge Impedance Calculation Method for High-Voltage Towers

A new technique for the calculation of the surge impedance of high-voltage towers is presented. The tower is represented by a four-terminal network for which the surge characteristics are determined by solving the transmission-line equations. The boundary conditions of voltage at the top and base of the tower are determined using a numerical solution of the electromagnetic field on a physical model of the tower. The physical model is a multiconductor geometrical model incorporating the conductors forming the main body, the slanted conductors, and the horizontal cross-arms where necessary. In the process of computing the terminal voltages and currents, soil resistivity effects are also taken into account. Surge impedance values obtained using this method are in good agreement with both published analytical formulae and field experiments.

Numerical electromagnetic field analysis on measuring methods of tower surge impedance

The dependence of the tower surge impedance on the measuring methods is studied with the help of NEC-2. The tower surge impedance measured by the direct method varies dependent on the arrangement of the current lead wire. The arrangement of a vertical current lead wire and a horizontal voltage measuring wire gives about 20% higher value than that measured with a horizontal current lead wire in extension to a voltage measuring wire. The former gives about 150 /spl Omega/ for a typical double-circuit transmission tower and it is almost equal to the value given by the estimating method recommended by the IEEE Working Group. The tower surge impedance evaluated from the current splitting ratio at the tower top is more than 20% higher than that characterized by the direct method when the current is injected into the connecting point from a simulated vertical lightning stroke. On the other hand, the tower surge impedance evaluated by the refraction method is about 10% lower than that characterized by the direct method.

Surge impedance of independent transmission towers characterized by the direct method

Measurement of the surge impedance of a transmission tower by the direct method is studied with the help of NBC-2. The arrangement of the measuring wires in the experiment influences the result, and it is possible to correct the measured tower surge impedance to that for a standard arrangement with a vertical current injection wire and a horizontal voltage measuring wire. By reviewing various experimental results in this way, it is concluded that an independent double-circuit transmission tower has an impedance of about 150 Ω with the traveling wave speed in the tower being equal to that of light. This value can be applied to the case of a direct lightning hit on the tower top, regardless of the type of lightning, upward or downward. © 1998 Scripta Technica. Electr Eng Jpn, 122(4): 17–24, 1998

Experimental evaluation of a UHV tower model for lightning surge analysis

An experimental investigation was performed on a UHV tower model for the EMTP multiconductor calculation of lightning overvoltage at substations associated with back-flashover at an adjacent transmission tower. The various lightning surge response characteristics were measured on an actual UHV tower, and parameters of a multistory transmission tower model that can reproduce voltages across the insulator strings, voltages of the crossarms, and voltages of the power lines were determined. A value of 120 /spl Omega/ was determined as the surge impedance at each section of the multistory tower model, which closely agreed with the tower surge impedance measured for the UHV tower alone.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

鉄塔サージインピーダンスに関するジョルダンの式の誤りの確認

鉄塔サージインピーダンスに関するジョルダンの式の誤りの確認

Lightning Surge Response Of Transmission Towers

A sensitivity study of HV and EHV lightning performance shows that surge response characteristics of towers and lines are as important as the footing response. Tower travel time has a very large sensitivity for taller double-circuit lines. Existing methods for analysis of tower and line response have considerable uncertainty. This uncertainty is resolved by extensions to travelling- wave analysis combined with experimental verification. New equations for tower surge impedance and tower travel time are proposed to deal with the most common situation of a midspan stroke to the skywire.

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.

Tower Surge Impedance

Experimental and theoretical methods for determining the surge impedance of towers are presented. It is demonstrated that the values so obtained adequately describe the response of the tower when a transmission line is struck by lightning. A right cone representation for conventional double-circuit steel towers is proposed, and a theoretical analysis using field theory yields the surge impedance as Z= 60 ln (2/S) where S is the sine of the half-angle of the cone.