Underground Transmission Lines Research Articles

Thermal Properties of Backfill Material for Underground Heat Exchange Applications

We are investigating the effect of the thermal properties of backfill material on the efficiency of underground thermal energy systems. We have used aluminum to increase the thermal properties of clay-bentonite and to investigate its potential as a backfill material in underground heat exchange applications. The measurements of thermal properties were made at room temperature using the Transient Plane Source (TPS) technique. The measurements were based on an infinite homogeneous medium using different composites of aluminum and clays in amounts of aluminum concentration ranging from 6 to 25 percent of the sample’s weight. The results confirmed an overall relative increase of 170% and 135% in the thermal conductivity and diffusivity, respectively as the Al content increased by 25%. The estimated average of the thermal transmittance (U-value ) of these samples was in the range from 0.40 to 1.1 W/m2K, which was directly proportional to the relative increase in the measured properties. Our findings can be applied to underground thermal energy systems, ground source heat pumps, and solid dielectric underground transmission lines.

Analysis of Increased Induced Voltages on the Sheath of Double-Circuit Underground Transmission Lines Guaranteeing Ampacity

This paper quantifies and discusses the increase in induced voltage on a sheath due to changes in duct banks in terms of type and dimensions along an underground transmission line, guaranteeing the ampacity required for a project. The four most common duct banks in double-circuit underground transmission lines with phase transposition were considered in this study, along with two special cross-bonding techniques: continuous cross-bonding (CCB) and sectionalized cross-bonding (SCB). These techniques aim to reduce sheath currents and enhance the distribution of the induced voltage on the sheath. The analysis considers two distinct scenarios in which the profile of the induced voltage is calculated: the first one accounts for underground obstructions, intersections with important traffic avenues, and ground with high excavation costs that force changes in the duct bank dimensions and configuration, which is the most exact and realistic case. The second one solely considers one typical configuration of a duct bank along the route. This last scenario is normally applied to calculate the induced voltage when an underground transmission design is required. The results show that when installing cables at a greater depth, it is imperative to increase the distance between them to guarantee the ampacity. The induced voltage on the sheath will rise as the distance increases. Furthermore, the results reveal that instead of calculating the induced voltage by considering the scenario that is exact and most like a real case, it is enough to calculate following the second scenario and then add a scaling factor according to each duct bank configuration.

Optimization of Ampacity in High-Voltage Underground Cables with Thermal Backfill Using Dynamic PSO and Adaptive Strategies

This article addresses challenges in the design of underground high-voltage transmission lines, focusing on thermal management and cable ampacity determination. It introduces an innovative proposal that adjusts the dimensions of the backfill to enhance ampacity, contrasting with the conventional approach of increasing the core cable’s cross-sectional area. The methodology employs a particle swarm optimization (PSO) technique with adaptive penalization and restart strategies, implemented in MATLAB for parameter autoadaptation. The article emphasizes more efficient solutions than traditional PSO, showcasing improved convergence and precise results (success probability of 66.1%). While traditional PSO is 81% faster, the proposed PSO stands out for its accuracy. The inclusion of thermal backfill results in an 18.45% increase in cable ampacity, considering variations in soil thermal resistivity, backfill properties, and ambient temperature. Additionally, a sensitivity analysis was conducted, revealing conservative values that support the proposal’s robustness. This approach emerges as a crucial tool for underground installation, contributing to continuous ampacity improvement and highlighting its impact on decision making in energy systems.

Retracted: Design of an Underground Transmission Line Condition Fault Monitoring System for Power Grids Based on Data Analysis Algorithms

Retracted: Design of an Underground Transmission Line Condition Fault Monitoring System for Power Grids Based on Data Analysis Algorithms

Thermal Diffusivity in the Subsoil: A Case Study in the Asturias (Northern Spain)

This study presents a novel methodology for determining the apparent thermal diffusivity of subsoil in situ, employing two heat transfer models within the subsurface: one method is based on heat conduction caused by air temperature oscillations, while the other considers heat transmission via both conduction and convection due to groundwater flow. Differential equations were solved, and non-linear regression analysis was employed. This method has direct applications in various engineering and environmental domains, such as underground transmission lines, oil and gas pipelines, radioactive waste management, and geothermal systems, especially in the context of implementing horizontal geothermal collectors (HGC). The apparent thermal diffusivity value of 1.514 × 10−6 m2 s−1, within a 95% confidence interval spanning 1.512 × 10−6 m2 s−1 and 1.516 × 10−6 m2 s−1, was obtained from the section between 1.67 and 3.86 m depth in a research borehole located in Asturias, Northern Spain, using twenty-one temperature sensors. The method allowed for the calculation of the subsoil’s apparent thermal diffusivity up to a depth of 14.55 m.

Life Prediction of Cable Steel Bracket Considering Carbon Steel Corrosion

Replacing the underground cable steel bracket within the service life will increase the maintenance cost, so the life prediction of the cable steel bracket is aroused. The mechanical properties and atmospheric corrosion of carbon steel are the key factors affecting the service life of the bracket. In this paper, a life prediction method of cable steel brackets considering carbon steel corrosion is proposed. First, the mechanical properties of the bracket are analyzed by the finite element model to determine the thickness of the damage. Based on electrochemical corrosion experiments, a life prediction method based on Tafel curve extrapolation is constructed to obtain the corrosion rate; At last, taking the 110 KV underground transmission line of a power grid as an example, the service life of the angle steel bracket of the cable is calculated, which provides a theoretical basis for the selection of steel bracket.

Real-time probabilistic backfill thermal property estimation method enabling estimation convergence judgment

This study proposes a real-time probabilistic estimation method for determining the thermal properties of backfill used in underground power transmission lines and ground heat exchangers. The thermal properties of backfill, serving as a heat conductor between the heat source and surrounding soil, are crucial design parameters for underground transmission lines and ground heat exchangers because temperature limits their maximum operational output. However, an unpredictable environment and human fieldwork during on-site backfill construction lead to considerable performance uncertainty, requiring in situ thermal property estimation. Furthermore, when transmission lines are installed over long distances, on-site assessments are necessary at multiple locations, making it essential to reduce the experimental time. The proposed estimation method enables real-time monitoring of temporal changes in the estimated probability distribution of the thermal property and quantifies the estimation uncertainty. The proposed method also allows immediate decision-making regarding experiment termination by evaluating real-time convergence by calculating the Jensen-Shannon divergence of sequentially estimated probability distributions. The effectiveness of the proposed method was tested on two backfill materials, demonstrating its ability to determine the estimation convergence by capturing contextual information affecting the estimation uncertainty.

Insights into the Thermal Performance of Underground High Voltage Electricity Transmission Lines through Thermo-Hydraulic Modelling

In this paper, a flexible numerical framework to provide thermal performance assessment for the underground buried cables, considering different geological and meteorological conditions, has been presented. Underground cables tend to retain the heat produced in the conductor, so complex coupled thermo-hydraulic response of the porous medium surrounding the cables needs to be assessed to prevent cable overheating and the associated reduction in cable capacity for carrying current. Applying a coupled thermo-hydraulic model within the developed numerical framework to conduct a health assessment on a subset of National Grid Electricity Transmission’s underground cables, this study provides novel insights into the thermal behaviour of buried circuits. The results indicate that backfill and surrounding native soil have the dominant effect on the thermal behaviour of cables, while the amount of precipitation and ambient temperature were found to have less impact on cable’s thermal behaviour. The findings strongly infer that the nature of the overloading which is undertaken in practice would have no ongoing negative impact, suggesting that more frequent or longer duration overloading regimes could be tolerated. Overall, this study demonstrates how the developed numerical framework could be harnessed to allow safe rating adjustments of buried transmission circuits.

Design of an Underground Transmission Line Condition Fault Monitoring System for Power Grids Based on Data Analysis Algorithms

With the continuous expansion of the scale, capacity, and coverage of the modern transmission network, the role of the power system in the national economy is increasingly prominent, and the disconnection of the power system will have a huge impact on society and people’s lives. Due to the long transport distance and wide coverage of transmission lines, natural conditions and human factors have caused great difficulties in line operation and maintenance. How to effectively improve the operation and maintenance of transmission lines to ensure the stability and safety of the power grid has become a common problem for the power industry and scientific researchers to discuss. In recent years, the information society has stepped into the era of big data, and big data has developed rapidly, becoming a hot area favored by academia and industry, and is widely used. Through big data analysis, potential operation rules can be discovered from a large amount of grid information, providing maintenance personnel with corresponding maintenance decision support. Using big data technology for transmission line fault analysis can effectively reduce accident processing time and avoid accident expansion. Therefore, this paper combines the underground transmission line fault of power grid with the fuzzy KNN algorithm model to apply the underground transmission line state intelligent monitoring system and conducts the study of real-time data collection and fault diagnosis analysis of the underground transmission line fault of power grid, and this paper conducts the transmission line fault analysis experiment, which fully confirms the feasibility and effectiveness of the algorithm model proposed in the paper and concludes that the data analysis model proposed in this paper. The proposed data analysis model has good innovation and practical feasibility.

Modelling and Stress Analysis of Shaft Work Subsidence of Underground Power Transmission Line Tunnel

This research presents an analysis of soil subsidence around shaft work under its construction process and drilling tunnel by finite element method (FEM). The study is focused on the underground power transmission tunnel in the Chao Phraya River construction project (Southern Bangkok Station to Suksawat Road, Thailand) of the Metropolitan Electricity Authority. This research aims to provide information for designing shaft work and preventing drilling tunnel problems. The research analyzed the soil subsidence around shaft work with 12 m of inside diameter and 1 m of thickness that located 33 m underground during constructing a tunnel chimney by sinking method. It is assumed that the analyzed horizontal reaction is the net resistance of the lateral earth pressure around the shaft work. This study found that the subsidence of soil around shaft work during the construction and drilling process had the value between 0.80 - 53.30 mm. In the part of the base slab, the soil subsidence value is between 3.40 - 16.00 mm. It can be concluded that the top of shaft work during the construction and drilling process had a soil subsidence value of 53.30 mm with dmax/H ratio of 0.00162.
 HIGHLIGHTS
 
 The Metropolitan Electricity Authority has an underground power transmission tunnel in the Chao Phraya River Construction (Southern Bangkok Station to Suksawat Road, Thailand) project which used to distribute electricity from the source station to the other side of the Chao Phraya River
 This research studied the behavior of the shaft work in sinking reinforced concrete shaft work and drilling tunnel process by finite element method (FEM) with the Midas Gen 2021 program
 The analysis results of the maximum soil subsidence around the shaft work wall occurred in the top area (very soft to medium stiff clay layer)
 
 GRAPHICAL ABSTRACT

Technical Comparison of Modern HVAC and HVDC Transmission System Along with Cost Analysis

This research aims to provide comprehensive information on current state-of-the-art technology in the field of high voltage transmission lines. Also, this study compares the performance of present high voltage AC/DC power systems and classifies potential future opportunities. It presents a contextually updated cost analysis for researchers’ working on modern HVAC and HVDC transmission systems. Besides this paper explained the most important aspects of the modern features of transmission systems. It also describes the modern technology, conductors, and accessories equipment for the transmission system, which greatly reduces the loss of power transmission. This paper further describes the cost breakdown of HVAC and HVDC overhead and underground transmission lines, cost estimation, explanation of comparative HVAC and HVDC transmission costs, and price variation. As the variation between the HVAC and HVDC transmission lines increases, a certain amount of voltage drops and losses occurs which is explained in detail.

Development of Manhole Monitoring System using the Existing Power Cables as Transmission Line

The realization of condition based maintenance for underground transmission lines is of particular interest to advance the maintenance utilizing the digital transformation. As a problem, it was difficult to develop an inexpensive, secure, and reliable communication method for information transmission from the manhole in a conduit. We have developed a manhole monitoring system by applying power line communication technology, in which a metal shielding layer of power cables is used as a transmission line. This paper describes the development of a system to collect sensor information and camera images using a 154kV and a 66kV XLPE cable installed in a conduit manhole.

SPECIFICS OF DIAGNOSTICS OF TECHNICAL CONDITION OF TRANSPORT AND TECHNOLOGICAL SYSTEM “MINING GAS PIPELINE – MINE WORKING”

The purpose of the paper is to analyze a deformation mechanism of the mine degassing pipelines to forecast their spatial changes in terms of intensification of underground mining of coal-gas seams.
 Methodology. The paper deals with expert assessment of the available approaches to diagnostics of technical condition of mine degassing pipelines, which are constructed within the in-seam underground mine workings with the floor rocks prone to heaving. The results of scheduled surveying measurements of technical condition of in-seam development workings have helped identify the potentially hazardous zones of rock mass deformation and indices of changes in spatial location of section degassing pipelines mounted in those mine workings. To determine the operating modes of a degassing pipeline under such operating conditions, a computer model of interaction of the elements of transport-technological system “mine gas pipeline – mine working” has been developed
 Findings. Diagnostics of technical conditions of the mine gas transmission lines and examination of their dismantled components have helped understand that deflections, mainly resulting in water accumulation zones, intensive corrosion of internal pipe walls, and mechanical depositions of coal and rock dust take place right within the flange connection areas. Formation of such zones is argued by health of the degassing pipeline as well as mine air inflow. Availability of internal corrosion, water accumulations, and mine air inflow decreases substantially capacity of the underground gas transmission line inclusive of qualitative characteristics of the captured methane-air mixture and efficiency of MDS on the whole.
 Originality. New approaches to diagnostics of technical condition of mine degassing gas pipeline in difficult mining and geological conditions of development of gas-bearing coal seams are substantiated and it is offered to consider indicators of their functioning as interacting in space and time transport-technological system "mine gas pipeline - mining".
 Practical implications. The operational parameters of mine degassing systems notes that the equipment performance with the least underpressure losses created by vacuum pipes requires that the degassing pipeline should have minimum hydraulic resistance of the gas transmission network. Pipeline aeration from the mine workings and water accumulations should be prevented by means of qualitative hermetic sealing of its flange connections as well as the pipeline straightness with the corresponding pitches. Consequently, the basic requirements for operating mine degassing pipelines involve their design profile, tightness of flange connections of pipes as well as operative control of the facility health.

Identification of Fault in Invisible Underground Broken Transmission Lines

ABSTRACT The underground electricity distribution cables are subjected to stress and pressure as they are laid underneath the earth. Due to many factors, these cables create different types of faults that include short circuits. If these faults are not addressed, the power supply is disrupted and resulting in permanent damage. To deal with faults in underground cables, a device has been devised that includes detection of underground broken wires using an android application. The suggested approach measures the electric field around the underground wire. This approach differentiates the short circuit fault from other kinds of faults in underground cables. Only methods for detecting short circuit errors are considered in standard fault detection techniques. The proposed system makes use of low-cost electronic devices that are simple to construct. As a result, the proposed underground fault detection product provides a low-cost optimization solution to locate short-circuit defects.

On the Power Lines—Electromagnetic Shielding Using Magnetic Steel Laminates

Protection against the electromagnetic fields around high-voltage transmission lines is an issue of great importance, especially in the case of buildings near power lines. Indeed, the developed fields can be harmful for the habitants and electrical/electronic devices, so the implementation of appropriate measures to address the above electromagnetic interference issue is necessary in order to ensure the safety of both human beings and equipment. Several practices have been proposed to reduce the electric and the magnetic fields around overhead and underground transmission lines (minimum distance, shielded cables, anechoic chamber etc.). In this context, the scope of the current paper is the use of highly permeable magnetic sheets for shielding purposes, along with the development of an appropriate procedure, based on finite element analysis (FEA) for the efficient design of passive shielding. The simulation results are compared with laboratory measurements in order to confirm the adequacy of the proposed methodology. The good agreement between the FEA outcomes and the experimental results confirms that the developed FEA tool can be trustfully used for the design of the shielding means in the case of overhead or underground power lines.

Electromagnetic Field Exposure in Kindergarten Children: Responsive Health Risk Concern.

Long-term exposure to physical agents can be detrimental to children due to their vulnerability. This study aimed to assess and compare the electromagnetic field (EMF) exposure level around the kindergartens from the underground transmission line (UGTL). We investigated randomly selected 24 kindergartens based on the location of the UGTL. The EMF emission levels were measured using an EMDEX II (Electric and Magnetic Digital Exposure Meter). The maximum mean value of the EMF emission level was 13.5 mG around the kindergartens and 17.7 mG from the point of UGTL to kindergartens. EMF emission level around the kindergartens was significantly associated with the location of the UGTL (t = −7.35, P < 0.001). These estimates are not trivial, as long-term exposure to EMF among kindergarten children can lead to different health problems. Routine monitoring of EMF emission levels is recommended including the awareness of EMF exposure to public citizens.

A Study of Fast Front Transients of an HVDC Mixed Transmission Line Exposed to Bipolar Lightning Strokes

Bipolar lightning strokes are associated with multiple polarity electrical discharge with no current intervals in between, making their behavior quite peculiar. This work presents a fast front analysis of a mixed high voltage direct current (HVDC) transmission link, evaluating the factors that influence the line transients due to shielding failures and backflashovers (BFOs), considering both overvoltage and repeated polarity reversal at the cable sending terminal. The research process includes a detailed modeling of a bipolar lightning stroke, frequency-dependent HVDC overhead, and underground transmission line sections. Noticeable findings include the occurrence of only a positive polarity insulator BFO for the adjacent and subsequent tower, despite the dual polarity of the lightning stroke with relatively small values for the lightning parameters. The influence of traveling waves on the insulator flashover performance of the line with varying parameters (such as the riser section length, the tower grounding impedance, and the location of the lightning stroke) is recorded and explained.

Dosimetry analysis of the magnetic field of underground power cables and magnetic field mitigation using an electromagnetic shielding technique

Non-ionizing dosimetry investigations of extremely low frequency (ELF) magnetic fields that are generated by underground power cables as well as the minimization of their health effects are significant topics handled by numerous researchers. In this study, ELF magnetic fields and current densities caused by three-phased underground transmission lines induced in the human model were examined utilizing both analytical and numerical methods. Analyses were carried out using a two-dimensional problem scenario for the comprehensive head and body model. The results of the finite element method (FEM)-based simulation studies and the analytical calculations are consistent with each other. Moreover, a magnetic field shielding method utilizing conductive material was presented in the study. The shielding technique performed with copper material was carried out to mitigate the magnetic field and possible dosimetry hazards in the ELF region. The proposed shield was a 4-mm reverse U-shaped copper material.

The effect of soil thermal conductivity and cable ampacity on the thermal performance and material costs of underground transmission line

This paper presents an optimisation of material costs of a 400 kV cable line arranged in a flat formation for different values of soil thermal conductivity and cable ampacity. The Finite Element Method is adopted to calculate the temperature field within the cable system. The Particle Swarm Optimisation method is used to minimise the material costs of the cable system. To ensure safe and reliable operation of the cable line, the temperature of the cable core should be lower than 90 °C. Under this assumption, the design optimisation is performed to calculate the optimum size of cable bedding cross-sectional area and select the nominal cable conductor area correctly. An application of two commonly used cable backfill materials is studied: Fluidized Thermal Backfill and Sand-Cement Mix. The calculations are performed for current rating varied from 1,000 A to 1,400 A, and thermal conductivity of soil varied from 0.6 W/(m × K) to 1 W/(m × K). The results showed that for a low soil thermal conductivity, i.e. 0.7 W/(m K), it is possible to reduce the overall material cost of the system (cables cost and thermal backfill material cost) when applying Fluidized Thermal Backfill (FTB) instead of commonly used Sand-Cement Mix. The reason is that FTB application allows reducing the cross-sectional area of power cables, which cost has a major effect on the overall material cost of the underground transmission line.

Life cycle cost assessment method considering multiple factors for economic evaluation of cable line steel brackets

In China, the bracket of underground high voltage cable transmission line is mainly made of various carbon steels. However, no unified standard exists for type selection of cable brackets in practice. Here, a selection method of the bracket based on life cycle cost assessment is proposed from the viewpoint of technical economy, in which the size of brackets, steel grade, carbon steel corrosion and eddy current loss are considered as influencing factors to the costs. Stress performance analysis under identical pole arm stress constraints is employed to determine the size and grade of carbon steel which influences the initial cost. And 3D coupled electromagnetic fluid-dynamical and thermal finite-element analysis is applied to calculate the eddy current loss of cable brackets which influences the operation cost. Meanwhile, multi-physics coupling analysis and experiment are conducted to prove the correctness of loss simulation calculation. Tafel curve measured by electrochemical corrosion experiment is used to obtain the corrosion rate of carbon steel which influences the maintenance cost by affecting service life of brackets. A case study is demonstrated, and the results show that the channel steel type is the most economical for the commendatory and the proposed method is effective.