Tower Footing Research Articles

Estimation on settlement of precast tower footings along the Qinghai–Tibet Power Transmission Line in warm permafrost regions

In order to mitigate frost jacking of the foundations, most of the towers in permafrost regions along the Qinghai–Tibet Power Transmission Line from Golmud, Qinghai Province to Lhasa, Tibet Autonomous Region, China were installed on precast or cast-in-place footings. However, the monitoring data showed that these footings were suffering more from settlement rather than heave due to the settlement of the warm and ice-rich permafrost under the footings. In order to estimate the long-term settlement of these footings, a series of numerical simulations were conducted for predicting the changes in ground temperatures under the precast footings in the coming 50years, taking three mean annual ground temperatures (MAGTs) of permafrost (−0.5, −1.0 and −1.5°C), two moisture contents of foundation soils (20, 35 and 55%), two embedding depths of footings (3.7 and 5.8m), and the most intense scenario of climate warming (linear rate of +0.052°C/a) into account. Then, the settlement processes of the foundation soils were calculated by using the layer summation method in which the thaw consolidation of permafrost soils as well as the compression of plastic frozen soils under the footings were considered. The computation results show that due to climate warming and disturbances of the ground surfaces, major settlement of the foundation soils would occur after 20 or 30years of operation. Additionally, the compression of plastic frozen soils under the footings could considerably contribute to the total settlement of foundation soils. Furthermore, embedding depth of the footings would have a significant effect on both thermal and mechanical stability of the footings. When the embedding depth is set at 3.7m, thawing of the permafrost beneath the footings would always happen in warm (MAGT>−1.0°C) permafrost areas during the next 50years, and the settlement of foundation soils could exceed 20cm for all calculated conditions by the end of the 50th operating year. When it is 5.8m, however, thawing of the permafrost would take place only in very warm (MAGT>−0.5°C) permafrost areas, and the settlement of foundation soils would be less than 20cm by the end of the 50th operating year.

Relation of Influential Parameters to Back-Flashover on the Insulators in Transmission Tower

This paper focuses on behavior of over-voltage across the cross arm and back-flashover on the insulators with different front times of lightning waveshape. The simulations and analysis are performed using Alternative Transients Program/Electromagnetic Transients Program (ATP/EMTP) to investigate both over-voltage across the cross arm and insulators. The scheme under investigations is a part of Thailand electricity transmission and distribution system. The standard of lightning voltage waveshape (1.2/50 μs) will be compared with the other lightning voltage waveshape front time (4/50 μs) in this paper. The results indicate that the front time of lightning waveshape plays an important role for the back-flashover voltage across insulator as well as the tower footing resistance and the lightning current.

Development of an adaptive neural‐fuzzy inference system based meta‐model for estimating lightning related failures in polluted environments

Owing to the random nature of the lightning phenomenon, the statistical based methods such as Monte Carlo simulation are the best tool to perform the lightning-related studies. However, Monte Carlo simulation is complex and time consuming, especially, for larger networks where a lot of regions must be investigated, separately. Although some researches have been carried out to evaluate the lightning flashover outage rates based on artificial intelligence (AI) techniques, but, so far, any work has not been reported to perform a risk assessment of lightning surges by means of AI. This study presents a meta-model based on adaptive neural-fuzzy inference system (ANFIS) that can be utilised to perform the lightning-related risk analysis with a good accuracy. The inputs of ANFIS are tower footing resistance, critical flashover voltage and the rate of lightning occurrence; and its outputs include the failure risk of both the insulation and arrester. The trained ANFIS can be very efficient in terms of time and computational resources for the further analysis, like placement of arresters or insulation coordination studies, on the electrical network on which the training is performed. To demonstrate the effectiveness of the ANFIS meta-model, an optimisation method, based on genetic algorithm, has been implemented in order to find the optimum location of a specified set of line arresters on power network. Also, in order to consider the environmental condition, that is, pollution and altitude, a new method to determine the importance of each node of interest in protection is introduced.

Optimal design of tower footing device with combined vertical and horizontal grounding electrodes under lightning

The property of tower footing device under lightning is an important issue for lightning protection. This paper focuses on analyzing on the optimal design of the tower footing device under lightning. Firstly, the practical effects of three kinds of common applied driven rods under lightning are researched by field testing. It is concluded that in the testing condition, the non-metal driven rod takes the best effect while the metal driven rod takes the worst. Moreover, the optimal installation position of the driven rods and the optimal structure of the horizontal electrodes are analyzed by numerical simulation, respectively. Both of them are heavily affected by soil resistivity and size scale of the tower footing. In order to solve the two practical problems, quantitative optimal criterions are put forward, which can be referred to while determining the optimal design of the tower footing device under impulse. That is very meaningful for engineering application.

Relation Among Velocity of Lightning Travelling Wave, Lightning Stroke Current, and Tower Footing Resistance to Study Back-Flashover on the Insulators in Transmission Tower

This paper studies the impact of back-flashover on the insulators with various velocities of lightning travelling wave, the lightning stroke current, and the tower footing resistance. The impedance of transmission tower section such as top tower, upper phase, middle phase, and lower phase that is calculated with lightning velocity dependent parameters is also considered. The velocity of lightning travelling wave at 300 m/μs and 200 m/μs is compared to consider back-flashover including maximum voltage at the cross arm and voltage across insulators. The results is shown that the velocity of lightning travelling wave plays an important role for the back-flashover voltage across insulator as well as the tower footing resistance and the lightning stroke current.

Fault Current-Division Factor of Substation Grounding Grid in Seasonal Frozen Soil

The determination of the maximum ground fault current, which has a close relation to the maximum hazardous step-and-touch voltage, is essential for the safety of a substation. Seasonal frozen soil changes the grounding resistances of the substation grounding grids and the tower footing devices, which seasonally influences the maximum grid current. This paper first introduces the method to calculate the fault current-division factor, and then discusses the influence of the seasonal frozen soil layer on it by using the proposed method. Based on the cases of two substations connected by the transmission lines and without neutrals or other connection paths, the influence of several factors on the fault current-division factor, including the thickness of seasonal frozen soil, and the number of transmission lines with overhead ground wire connected to the substation grounding grid, is analyzed. It is reasonable to consider the influence of the seasonal frozen soil on the fault current-division factor in grounding grid design. According to the analysis, adding vertical grounding rods to the grounding grid and the tower footing grounding device can almost eliminate this seasonal influence.

Impact Assessment on the Ore Body under the Tower Footing of 500kV Transmission Line

Zhangjiaba-Enshi 500kV Transmission Line is a backbone network in the West to East Power Transmission Project of China, which passes through the factory area of Chongqing Sanxing Iron Mine Factory. According to solving the additional stress on foundation rock produced by tower load based on the Boussinesq theory and the redistribution of surrounding rock stress resulting from iron mining computed by using the calculation formula for redistribution stress on circular chamber in rock mass, this paper analyzes and demonstrates the impact of Zhangjiaba-Enshi Line N327 tower footing pressure mine on mining within the range of mining scheme, and determines that there is no mutual impact between the Zhangjiaba-Enshi Line N327 tower and exploitation of Sanxing ore horizon.

Lightning back flashover double circuit tripping pattern of 132 kV lines in Malaysia

This paper presents a study on flashover patterns due to double tripping using EMTP software. The simulation was performed on a 132kV line double circuit for economic installation of transmission line arresters (TLAs). The simulation results were validated with the field data of 132kV transmission line in Malaysia. A vertically configured double circuit transmission line was modelled and the effects of tower footing resistance, lighting current values, power frequency voltage and the coupling effect of the phase conductors study were taken into account. A good agreement between the simulation and field data was obtained. A specific back flashover phase of the double circuit line can be determined by considering the effects that have been mentioned. Through determination of the tripping pattern, a suitable installation of line arresters and mitigation of line tripping due to back flashover can be achieved.

Experimental Investigation of Impulse Characteristics of Transmission Line Tower Footings

Understanding the impulse response of tower grounding systems is a key factor in the determination of the lightning performance of transmission lines and the safety voltages near high voltage towers. This paper reports the results of an experimental study on the impulse characteristics of full-scale transmission line tower footings. The ground potential distribution both at the ground surface and below ground was examined under impulse currents and variable-frequency AC currents. Prospective impulse step and touch voltages were determined by measurement of the ground surface potential distribution near the tower base. The behaviour of the impulse resistance of the tower footings under highmagnitude impulse currents was investigated. The non-linear characteristics of the impulse resistance with peak current are demonstrated for the entire tower base and for single tower footings. The soil ionisation inception is found to be dependent on the rate of rise of applied current. The dynamic impulse resistance was used to describe the dynamics of soil ionisation.

Assessment of the backflashover occurrence rate on HV transmission line towers

SUMMARY An assessment of the backflashover occurrence rate on the high voltage (HV) transmission line towers is presented in this paper. It is based on a mathematical model that accounts for the transmission line geometry, keraunic level, statistical distribution of lightning current parameters, and correlation between statistical variables defining the lightning current waveshape. Furthermore, assessment of the backflashover occurrence rate on these towers accounts for the tower footing impulse resistance, lightning impulse insulation strength of the insulator strings and surge impedance of the tower itself. Proposed theoretical analysis is backed by the most recent propositions of parameters that define statistical distributions of lightning current parameters and then applied to the typical HV transmission line example. This analysis might be useful when scrutinizing the several first transmission line towers incoming to the HV substations. Hence, here presented analysis could be subsequently applied for the design of the HV substation overvoltage protection. Finally, a sensitivity analysis is provided, in regards to the parameters of the log-normal distributions defining lightning current amplitude and steepness, as well as in regards to the tower footing impulse resistance. Copyright © 2011 John Wiley & Sons, Ltd.

Electrokinetic Model Refinement Via a Perturbation Finite-Element Method—From 2-D to 3-D

A method for increasing the contact area of a grounding system with the earth is the installation of counterpoise wires. Counterpoise wires improve the reliability of overhead power transmission lines. They are conductors buried in the ground parallel to or at an angle to the line conductors. This paper presents an electrokinetic model refinement via a perturbation finite-element method to calculate the grounding resistance of counterpoise wires. The perturbation method is herein developed for refining the electric field distribution in soil starting from simplified models, based on electric field distributions from 2-D models, that evolve towards a 3-D accurate model. The analysis of the distribution of the electric field and of the electric potential around the tower footing allows accurately determining the tower footing resistance. The comparisons between the grounding resistance simulated and calculated analytically of a counterpoise wire are presented.

Comparison of metal-oxide surge arresters circuit models and implementation on high-voltage transmission lines of the Hellenic network

The transmission lines of high-voltage networks are protected against lightning strikes and internal overvoltages with the use of overhead ground wires and surge arresters. The appropriate modelling of a power system's components is a significant issue, in order to obtain reliable theoretical simulation results for the lightning performance of the line that will be used in the design of a new line or the improvement of an existing one. Metal-oxide surge arresters present difficulties in their modelling owing to their dynamic characteristics, which depends on the rise time of the injected impulse current. Several equivalent circuit models have been proposed in the technical literature for the adequate simulation of this frequency-dependent behaviour. The current paper presents the most important surge arresters models, comparing the residual voltage that each model extracts for different lighting currents. For each model, the arresters failure probability is computed implementing them on Hellenic high-voltage transmission lines, performing a sensitivity analysis for the tower footing resistance and the arresters interval installation.

A Practical Evaluation of Surge Arrester Placement for Transmission Line Lightning Protection

The use of metal-oxide varistor surge arresters (MOVs) in lightning protection of overhead transmission lines to improve reliability is of great interest to electric utilities. However due to economic reasons, it is not possible to completely equip an overhead transmission line with surge arresters at each transmission structure. In this paper, an evaluation of lightning protection design on a 115 kV transmission line using surge arresters, utilizing a model based on field data, is presented. The model developed is used for computer simulation using the Alternative Transients Program. Various design procedures aimed at maximizing the reliability of service on the transmission line using a minimal number of surge arresters are analyzed. Different designs considered for transmission line lightning protection using MOV arresters include: the use of a different number of surge arresters per tower, distance between towers with surge arresters and the dependence of these configurations on tower footing resistance. The lightning protection designs are analyzed using `lightning flashover charts,' proposed in this paper. Also, an analytical model of two 115 kV transmission lines in Southwest U.S. has been developed and different surge arrester location strategies used on these transmission lines have been analyzed. Practical experiences and effectiveness of various lightning protection designs used on these transmission lines are discussed.

Centrifuge modelling of the pushover failure of an electricity transmission tower

Centrifuge model tests were conducted to examine foundation failure mechanisms during rapid horizontal pushover of an electricity transmission line support tower, simulating a broken transmission line response or wind gust loading. A model transmission tower supported on four pad foundations in clay and backfilled with sand was loaded horizontally and the loads at each foundation were measured during fast and slow pushover. The tests examined the influence of tensile resistance mobilized at the underside of the footings, which is difficult to reliably incorporate within design practice due to a lack of accepted quantitative design methods. The measured performance of the tower footings was compared with results from a series of tests where a single footing is subjected to purely vertical loading in compression and tension and was found to be in good agreement. The tower response was back-analysed as a simple push–pull model and the calculated uplift capacity of the footing backfill provided a close match to the observed response of the tower footings subjected to slow pushover. During fast pushover, the additional capacity mobilized due to tensile resistance (suction) created by the reverse bearing capacity beneath the base of the footings subjected to uplift was quantified using a suction capacity factor.

Assessment of surge arrester failure rate and application studies in Hellenic high voltage transmission lines

Abstract The use of transmission line surge arresters to improve the lightning performance of transmission lines is becoming more common. Especially in areas with high soil resistivity and ground flash density, surge arresters constitute the most effective protection mean. In this paper a methodology for assessing the surge arrester failure rate based on the electrogeometrical model is presented. Critical currents that exceed arresters rated energy stress were estimated by the use of a simulation tool. The methodology is applied on operating Hellenic transmission lines of 150 kV. Several case studies are analyzed by installing surge arresters on different intervals, in relation to the region's tower footing resistance and the ground flash density. The obtained results are compared with real records of outage rate showing the effectiveness of the surge arresters in the reduction of the recorded failure rate. The presented methodology can be proved valuable to the studies of electric power systems designers intending in a more effective lightning protection, reducing the operational costs and providing continuity of service.

Heat conduction with seasonal freezing and thawing in an active layer near a tower foundation

In this paper, seasonal freezing and thawing of an active layer near a power transmission tower foundation are investigated experimentally with a metal rod buried in an enclosure filled with soil. The measured soil temperatures are used to examine thermal effects of the tower footing and snow cover on seasonal freezing and thawing cycles. It was found that the metal tower has significant thermal effects on the local region around the tower footing. This increases the thaw depth of the active layer and lengthens the freezing/thawing cycle, which adversely affects the foundation stability and safety. These results provide useful new insight of practical utility in the design and maintenance of power transmission line foundations and other infrastructure in northern regions.

Study on Transmission-Line Arresters for Tower With High Footing Resistance

The countermeasure against lightning faults is one of the most decisive factors in designing a transmission line in a high grounding resistance area. The higher the footing resistance is, the higher the resulting lightning surge voltage borne by the insulator string is. But it is practically not so easy to decrease the tower footing resistance. Lightning arresters for the transmission line have been developed and are already in successful service. However, the application effect of line arresters to the tower of high footing resistance has not been studied yet. This paper describes the results of computer calculation regarding the application effect of transmission-line arresters assuming typical parameters of the 77-kV line in Japan. As a result, the line arresters can effectively prevent the lightning faults in an area of high footing resistance.

Ground Heat Transfer From a Varying Line Source With Seasonal Temperature Fluctuations

In this paper, transient heat conduction between a line heat source and a semi-infinite medium (representing a foundation of a power transmission tower and the surrounding ground) is analyzed numerically and experimentally. The tower foundation is represented by a metal rod buried in a semi-infinite medium. Using an experimental test cell with a data acquisition system, heat transfer and temperature measurements within the domain are collected. The experimental studies are first applied to unidirectional heat conduction, wherein the analytical solutions are compared against measured temperature responses. Then two transient heat transfer cases are studied: one case with a steady heat input provided by an electrical heater and another with sinusoidal temperature variations achieved by temperature-controlled fluid in a heat exchanger. The analysis shows that a metal tower footing has significant thermal effects on the temperature response of the local half-space around the footing in the foundation. This thermal effect varies with time, as well as spatially at different positions around the tower footing. In particular, measured results from the case of sinusoidal temperature variations show that the tower footing introduces additional temperature increases in the “summer” periods and temperature decreases in the “winter” periods.

An experimental study of lightning overvoltages in wind turbine generation systems using a reduced‐size model

AbstractWind turbine generation systems are built at locations where few tall structures are found nearby so as to obtain good wind conditions, and thus, they are often struck by lightning. To promote wind power generation, lightning‐protection methodologies for such wind turbine generation systems have to be established. This paper presents the result of an experimental study of lightning overvoltages in wind turbine generation systems using a reduced‐size wind turbine model. Overvoltages observed at wavefronts of lightning surges are focused on in this study. In the experiments, lightning strokes to one of the blades and to the nacelle were considered, and voltages and currents at various positions of the wind turbine model were measured. The following points have been deduced from the results: (i) The voltage rise due to the tower footing resistance can cause a significant voltage difference between the tower foot and an incoming conductor led from a distant point. Also, a voltage difference between the bottom of down conductors installed inside the tower and an incoming conductor can be of significance. (ii) The lightning current flowing through the tower body induces voltages in main and control circuits which form loops, and the induced voltages can cause overvoltages and malfunctions. (iii) Traveling‐wave phenomena in a wind turbine generation system for a lightning strike to the tip of a blade and to the nacelle have been clarified from the measured waveforms. This information can be used for developing an EMTP simulation model of wind turbine generation systems. © 2007 Wiley Periodicals, Inc. Electr Eng Jpn, 158(4): 22– 30, 2007; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/eej.20466

An Experimental Study of Lightning Overvoltages in Wind Turbine Generation Systems Using a Reduced-Size Model

Wind turbine generation systems are built at locations where few tall structures are found nearby so as to obtain good wind conditions, and thus, they are often struck by lightning. To promote wind power generation, lightning-protection methodologies for such wind turbine generation systems have to be established. This paper presents the result of an experimental study of lightning overvoltages in wind turbine generation systems using a reduced-size wind turbine model. Overvoltages observed at wavefronts of lightning surges are focused on in this study. In the experiments, lightning strokes to one of the blades and to the nacelle were considered, and voltages and currents at various positions of the wind turbine model were measured. The following points have been deduced from the results: (i) The voltage rise due to the tower footing resistance can cause a significant voltage difference between the tower foot and an incoming conductor led from a distant point. Also, a voltage difference between the bottom of down conductors installed inside the tower and an incoming conductor can be of significance. (ii) The lightning current flowing through the tower body induces voltages in main and control circuits which form loops, and the induced voltages can cause overvoltages and malfunctions. (iii) Traveling-wave phenomena in a wind turbine generation system for a lightning strike to the tip of a blade and to the nacelle have been made clear from the measured waveforms. This information can be used for developing an EMTP simulation model of wind turbine generation systems.