Underground Power Transmission Research Articles

Enhancing resilience in urban utility tunnels power transmission systems: Analysing temperature distribution in near-wall cable fires for risk mitigation

Urban utility tunnels are integral to the underground power transmission systems of cities. Enhancing the understanding of cable fire hazards within these tunnels is pivotal for fostering risk-resilient underground environments. This study delves into the temperature distribution characteristics of near-wall cable fires in urban utility tunnels. Employing a reduced-scale utility tunnel model, a series of fire experiments were conducted, focusing on the safety monitoring of horizontally arranged cables. Cone calorimeter tests were initially performed to ascertain the heat release rate of cable fires. The study meticulously considered cable spatial position parameters, such as cable height and near-wall distance, as dependent variables, and monitored real-time temperature changes within the utility tunnel under various fire scenarios.The findings reveal that the cable’s proximity to the wall significantly influences the fire temperature field. For instance, when a fire near the sidewall (at 1 cm) is compared to one in the middle of the tunnel (at 20 cm), the ceiling maximum temperature is reduced by 38.9 %. The sidewall’s limiting effect diminishes fire energy release, moderates vertical temperature decay, and enhances longitudinal temperature attenuation. The study introduces dimensionless mathematical models that account for the sidewall effect, quantifying the characteristics of fire-induced smoke temperature distribution in utility tunnels, such as ceiling maximum temperature rise, vertical smoke temperature profile, and longitudinal smoke temperature decay. The models’ predicted values closely align with the experimental results.These insights are pivotal for integrating cable numbers and spatial positioning in scenario-based fire prevention and mitigation strategies. The study’s implications extend to the design and operation of urban utility tunnels, emphasizing the need for strategic cable placement and the potential for mathematical models to inform risk mitigation measures. The research contributes to the advancement of sustainable urban development by providing a framework for enhancing the safety and resilience of critical infrastructure against fire hazards.

Modelling of a High-Temperature Superconductor HVDC Cable Under Transient Conditions

The carbon neutrality goal of achieving net-zero by 2050 has sparked significant interest in offshore wind farms. With the offshore wind farms installed > 100 km away from the seashore, underground power transmission infrastructure is a necessity. Considering the power cable length and low net effective losses, a high-voltage direct-current (HVDC) system is chosen for this study. Also being lossless in DC operation, the HVDC cables are considered to be made out of high-temperature superconductor (HTS) material. But, unlike copper/aluminium, HTS material has a sharp transient behaviour as a function of the operating current, temperature and field. Thus, under transient conditions, as the fault current ramps up, this can lead to an increase in the HTS operating temperature, causing either degradation or permanent damage to the HVDC cable. In this paper, an HTS HVDC cable model has been developed in MATLAB/Simscape coupling both electrical and thermal models. For this study, a 100 km long coaxial 100 kV/1 GW DC HTS power cable is considered and modelled both as a lumped element and distributed element model with 100 elements to compare and evaluate the cable parameters along the length. The parameters include temperature distribution, resistance, critical current, and losses at different spots throughout the length of the cable. To simulate the transient condition, a line-to-ground (LG) fault is considered and the current distribution between the copper former and HTS tapes is studied. Using this cable model, the maximum temperature of the HTS and coolant both in the superconducting state and transient state are evaluated and presented. In comparison to the distributed model, the lumped model displayed different thermal and electrical values.

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.

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

Hydrogen evolution and electromigration in the corrosion of aluminium metal sheath inside high‐voltage cables

The urgent need for power transmission is the reason for leading research on the safer operation of high-voltage cables. These high-voltage cables are emerging as an efficient technology for underground power transmission. However, the electrochemical corrosion of aluminium (Al) metal sheaths in these cables is a common and challenging degradation process. Herein, the corrosion mechanisms and electrochemical analysis are investigated by using scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FTIR), electrochemical impedance spectroscopy (EIS) and laboratory-designed electrochemical characterisation techniques. In the corrosion experiments, the authors evaluated the corrosion rate by measuring the release rate of hydrogen gas and explored the different roles of sodium polyacrylate (NaPA) during the corrosion process. It is found that the complexation reactions between NaPA and Al inhibited corrosion while increased the resistance of the buffer layer. The proposed mechanisms of corrosion in this study can improve the lifespan and sustainability of high-voltage power transmission.

A Bayesian Inference Reliability Evaluation on the Corrosion-Affected Underground High-Voltage Power Grid

The underground high-voltage power transmission cables are high value engineering assets that suffer from multiple deteriorations through-out life cycles. Recent studies identified a new failure mode – the pitting corrosion deterioration on the layer of phosphor bronze reinforcing tape, which protects the oil-filled power transmission cables from oil leakage due to deterioration of the leads heath. Two models estimating the phosphor bronze tape life were established separately in this study. The first model, based on mathematical fitting, is generated using a replacement priority model from the power supply industry. This is considered as an empirical-based model. The second model, based on the corrosion fatigue mechanism, utilizes the information of the pit depth distribution and the concept of pit-to-crack transfer probability. The Bayesian inference approach is the conjunction algorithm to update the existing probability of failure (PoF) model with the newly identified failure modes. Through this algorithm, the integrated PoF model contains a more comprehensive background information while maintaining the empirical knowledge on the engineering assets’ performance.

Deep learning with small datasets: using autoencoders to address limited datasets in construction management

Large datasets are necessary for deep learning as the performance of the algorithms used increases as the size of the dataset increases. Poor data management practices and the low level of digitisation of the construction industry represent a big hurdle to compiling big datasets; which in many cases can be prohibitively expensive. In other fields, such as computer vision, data augmentation techniques and synthetic data have been used successfully to address issues with limited datasets. In this study, undercomplete, sparse, deep and variational autoencoders are investigated as methods for data augmentation and generation of synthetic data. Two financial datasets of underground and overhead power transmission projects are used as case studies. The datasets were augmented using the autoencoders, and the project cost was predicted using a deep neural network regressor. All the augmented datasets yielded better results than the original dataset. On average the autoencoders provide a model score improvement of 7.2% and 11.5% for the underground and overhead datasets, respectively. MAE and RMSE are lower for all autoencoders as well. The average error improvement for the underground and overhead datasets is 22.9% and 56.5%, respectively. Variational autoencoders provided more robust results and represented better the non-linear correlations among the attributes in both datasets. The novelty of this study is that presents an approach to improve existing datasets and thus improve the generalisation of deep learning models when other approaches are not feasible. Moreover, this study provides practitioners with methods to address the limited access to big datasets, a visualisation method to extract insights from non-linear correlations in data, and a way to improve data privacy and to enable sharing sensitive data using analogous synthetic data. The main contribution to knowledge of this study is that it presents a data augmentation technique for transformation variant data. Many techniques have been developed for transformation invariant data that contributed to improving the performance of deep learning models. This study showed that autoencoders are a good option for data augmentation for transformation variant data.

A Monte Carlo Model for Pitting Corrosion in Phosphor Bronze Tape Used in Underground Power Transmission Cables

Life of underground oil-filled power transmission cables used with phosphor bronze tapes is greatly reduced by pitting corrosion and hence accurate prediction of the pit growth in these tapes becomes essential. In the present work, the probability distribution of corrosion pit depth on phosphor bronze tapes is calculated using probabilistic Monte Carlo simulations and compared with the measured pit depth distribution on samples of broken tapes which have been in service for about 50 y. This Monte Carlo simulation is performed on every stable pit that nucleates, propagates, and repassivates on the metal surface. Due to the random nature of pitting corrosion, the probability of failure of this class of cables can be simulated based on the Monte Carlo model. This paper shows that the simulated pit depth distribution is very similar to the experimental data. The results demonstrate that the Monte Carlo model by Engelhardt and Macdonald can be effectively applied to long-term field data of phosphor bronze tapes, even over 50 y. In addition, the probability of failure due to pitting corrosion can be evaluated analytically, without need of conducting expensive and time-consuming experimental campaigns. Therefore, this probabilistic pit depth distribution model will be a powerful tool in the decision-making strategy for the replacement of underground power transmission cables near their end of life.

Settlement and damage analysis of working shaft for underground high-voltage electricity cables

Working shafts or manholes, usually made of concrete or reinforced concrete, are essential parts of the underground electrical power transmission system. However, the working shaft frequently encountered differential settlement, tilt and other damage like cracking when construction activity was carried out near the high-voltage electricity cables. This paper reported a severe damaged working shaft in South China including its settlement and cracks. Numerical simulation was also performed to reveal the evolution of structural damage with an increased differential settlement. The results show a good agreement with field observation in respect to the initiation of concrete tension damage. It is proposed that if a differential settlement higher than 5mm is observed, preventative measures are needed and the risk is conditional acceptable.

Fault Detection in Power Transmission Line

Now a days, there are many transmission line fault occurs due to environmental disaster like heavy rain, cyclone, lightening etc.. In this paper small GSM based fault detection and alert system was used to accurately indicate the exact line where the fault had occurred and sends the message to the control station. This paper seeks to design an automatic and efficient fault detection and alert system for both over head and underground power transmission network using fault indicator technology. In addition to that, if when the transmission line fault occurs it automatically trip the line to avoid the current leakages. In conclusion the time required to locate a fault is reduced as a system automatically and provide the accurate line fault information.

Corrosion fatigue of phosphor bronze reinforcing tapes on underground power transmission cables - Failure analysis

Abstract This paper is an investigation on the failure mechanism involved in underground power transmission cables with their life limited by corrosion of phosphor bronze reinforcing tapes. In the present work, a detailed analysis of failed bronze tapes in an ammonium free environment has been undertaken and corrosion fatigue failure mechanism has been identified. A detailed examination of the tape samples is carried out using 2D and 3D optical microscopy and SEM. It follows a mechanical approach that confirms corrosion fatigue as the failure mechanism. SEM images reveal that the pits present on the surface could be the starting point for the crack that eventually leads to failure. Stress calculation shows that the tape could fail only if corrosion pits are present on the tape surface. Presence of corrosion pits, multi cracks and striations on the fractured surface demonstrates corrosion fatigue cracking as the failure mechanism across the tape samples.

物理探査法による地中送電線周辺土壌の固有熱抵抗の評価（その2）―統合物理探査データを用いた未固結砂の熱抵抗プロファイリング―

物理探査法による地中送電線周辺土壌の固有熱抵抗の評価（その2）―統合物理探査データを用いた未固結砂の熱抵抗プロファイリング―

Life Prediction of Phosphor Bronze Reinforcing Tape Used in Underground Power Cables

Phosphor bronze tape is a key protection component of underground power transmission cables. During the service life, it is affected by both corrosion and fatigue effects, leading to the failure of material and eventually the failure of cables. In the present work, the combined effect on the cable failure is studied in five stages: pit initiation, pitting corrosion control, pit-crack transfer, crack propagation control, and material failure. The cable time to failure is defined as the time needed for a corrosion pit on the tape surface to transfer into a crack resulting from a cyclic fatigue load condition. The pit-crack transfer is described statistically by a pitting corrosion to crack propagation transfer probability function. The result shows that a life prediction model can convey the long-term data of cables and can predict the years of failure. Subsequently, the life prediction model is extended and tested to include crack transfer probability function.

Cable joint installation time optimization

Emphasis on renewable energy demands power transmission from scattered sources. Underground power transmission and distribution is the key way due to right of way (ROW) and smart city initiative. This is increasing the demand of subterranean cable networks, which is leading to the proportional increase in the installation of cable joints. Cable joint installation is a highly time consuming process requiring very high level of human skill in the form of preparing cable ends by cutting, shaping and polishing them with a very low tolerance, without which the joint becomes the weakest link in the cable network. In view of this rapid growth of joint installations, there is now a two pronged requirement of lowering of installation time and lowering of human skill requirement for cable joint installation. In the present study all the major time consuming steps in the joint installation have been identified. A critical re-look of this decades old process was carried out in terms of materials properties and their influence on the steps involved in the process. Subsequently, based on experiments we propose a novel modification of the cable joint installation process which will lead to a 30% reduction in the installation time with a concomitant reduction in the human skill requirement.

Insulation materials for HVDC polymeric cables

Polymeric insulation materials have been very successfully used in the extruded high voltage alternating current (HVAC) cables underground power transmission for a long time. Although the development history of extruded polymeric HVDC cables is not very long, it has shown great advantages in power transmission over HVAC cables and other kinds of HVDC cables such as OF and MIND cables. So the demand on polymeric HVDC cables is increasing and its application range is being expanded. On one hand, offshore wind power and photovoltaic power transmission call for long distance and large capacity HVDC cable power transmission. On the other hand, underground HVDC cable power transmission can solve the contraction of power transmission and cityscape during urbanization. Therefore, polymeric HVDC cables will be rapidly developed in the near future but the most important issue is the insulation material for polymeric HVDC cables.

Measurement and Modeling of Pitting Depth Distribution for Phosphor Bronze Tapes Used in Underground Power Transmission Cables

Phosphor bronze tapes used in underground power transmission cables are affected by pitting corrosion that can lead to cable failures. In the present work, the probability distribution of corrosion...

지중송전계통에서 절연접속함 위치 및 접지방식 변경의 효용성 평가

Dielectric breakdown accidents have been mainly occurred in the vicinity of service entrance section in underground power transmission systems. One cause among them is due to the excessive component value of sheath located around service entrance of cable. In this paper, as one of the alternative to prevent these accidents, the change of cross bond grounding system and the location change of IJ(Insulation Joint) are suggested. Also, to evaluate effectiveness of this changing system, circulating current and induced voltage of sheath were analyzed in steady and transient state. By comparison of the analytical results for the several possible changing systems, a grounding system and location of IJ which has the smallest sheath component values is proposed. In this paper, analysis to evaluate the proposed system is carried out by EMTP/ATPDraw. It can be used as a valuable operational material to prevent accident of the service entrance section in underground power transmission system.

物理探査法による地中送電線周辺土壌の固有熱抵抗の評価 －地中送電線埋設地点への適用－

物理探査法による地中送電線周辺土壌の固有熱抵抗の評価 －地中送電線埋設地点への適用－

物理探査法による地中送電線周辺土壌の固有熱抵抗の評価 －土壌試料の固有熱抵抗と電気比抵抗との関係－

地中送電線路の設計において，ケーブルサイズ選定のための送電容量の計算に重要な土壌の固有熱抵抗をより簡易に低コストで調査できる手法が必要とされている。土壌の固有熱抵抗及び比抵抗は，水飽和度や間隙率といった同じパラメータに応じて変動することから，両者の間に相関性を持つことが示唆され，地表からの物理探査により得られる物性値から固有熱抵抗を間接的に導出できる可能性がある。そこで，固有熱抵抗と比抵抗を測定する室内試験を実施し，両者の相関性を評価することで，比抵抗から固有熱抵抗を求める可能性について検討した。その結果，固有熱抵抗は水飽和度や間隙率とともに，土壌を構成する土粒子の固有熱抵抗に応じて変化しており，既往の実験式であるヨハンセンの式を用いた計算値が室内試験とほぼ同じ値を得られることが明らかとなった。また，比抵抗は水飽和度や間隙率，間隙水の比抵抗などといったパラメータに応じて変化し，アーチーの式と整合した傾向であった。そして，両式を組み合わせることで比抵抗から固有熱抵抗を求める手法を提案した。室内試験結果を用いた検証の結果，比抵抗の測定値から求めた固有熱抵抗は室内試験結果とほぼ一致した値が得られた。以上より，電気探査により得られる比抵抗値を用いて固有熱抵抗値を推定できる可能性が示された。

The Development of Underground Power Transmission Line Inspection Robot

The small underground power transmission Line inspection track robot is developed. The robot is composed of left and right track walking mechanisms driven independently and camera tripod head with better environmental adaptability and flexibility of movement. The robot is controlled by wireless remote control mode. The portable handheld controller is also developed with communication, control, display and storage function. The inspection work experiment at the cable room of 110kV underground substation has been done using this robot. The result indicates that the inspection robot has better flexibility of movement, better reliability of communication and better convenience of operation, which is the better method for underground power transmission Line inspection.