Underground Power Cable Systems Research Articles

Finite Element Method Based Design of a Computer Application Interface for Thermal Analysis of Underground Power Cable System

Finite Element Method Based Design of a Computer Application Interface for Thermal Analysis of Underground Power Cable System

Consistency in thermal conductivity measured via lab-, field-scale test, and numerical simulation for newly developed backfill materials for underground power cable system

This study investigates the thermal properties of newly developed backfill materials (normal and prepacked concrete) for an underground power cable system (UPCS) through different methods (laboratory test, in-situ horizontal thermal response test (TRT), and numerical analysis). First, the thermal conductivities of small-scale backfill materials were measured in a laboratory using a hot disk sensor. Afterward, a 5 × 3 × 2 m full-scale backfill was prepared for the TRT. The infinite line source (ILS) model was employed to estimate the thermal conductivity of the backfill materials using data from the TRT. Finally, a numerical analysis was conducted to simulate the heat transfer process during the TRT and back-calculate the thermal conductivity of the proposed backfill materials. All tests were also conducted using conventional backfill materials, namely natural sand, for comparison with the proposed backfill materials. The results indicated that the prepacked concrete type has the highest thermal conductivity among the three backfill materials regardless of the testing method employed. In addition to a high thermal conductivity, this newly developed material also has good workability (382 mm for the grout), low bleeding, and satisfactory excavation ability for the backfill material (1.56 MPa). Furthermore, the ILS model and numerical model slightly overestimated the thermal conductivity (less than 8 %) compared to the measured values from the laboratory test involving normal and prepacked concretes; however, natural sand exhibited a significant difference (1.365 W/(mK) and 1.8 W/(mK), 32 %) owing to the influence of the water content on its thermal conductivity. In addition, thermal conductivities calculated by the ILS model and numerical simulation were in very good agreement (with the difference being lower than 3 %), demonstrating that the proposed ILS model is appropriate to use to estimate the thermal conductivity of large-scale backfill material using horizontal TRT data.

Thermophysical Properties of Bentonite–Sand/Fly Ash-Based Backfill Materials for Underground Power Cable

The surrounding (backfill) materials around the underground power cable systems are essential for dissipating the heat away from it, during the exertion phases. The heat dissipation restrains the thermal instability and risk of progressive drying of the backfill materials, thus reducing the thermal stress on the power cable. Thermal instability indicates the reduction in thermal properties (conductivity or diffusivity) due to the migration of moisture because of heat accumulation. Thus, the backfill materials should have adequate thermal properties and water retention capacity to transfer the heat from the heat source to the surrounding area with minimal moisture migration. The bentonite has high water retention capacity, but low thermal conductivity, whereas sand/fly ash exhibits low water retention and has higher thermal conductivity than bentonite. The addition of bentonite promotes the water holding capacity and thermophysical properties of sand and fly ash. Therefore, this study presents the thermal properties of backfill materials, bentonite–fly ash (B–F) and bentonite–sand (B–S), at varying weight-percent of sand and fly ash with bentonite. Various compositions of the mixtures were compacted to varying dry densities, and water contents and thermal properties variation of backfill materials were measured using a dual thermal needle probe ‘KD2 Pro’ at room temperature. The study deals with the systematic evaluation of the volumetric specific heat capacity, thermal conductivity, and diffusivity of backfill materials against varying dry density and water content. The threshold water content (TWC) has been determined from the thermal diffusivity–water content variation curve, and it has correlated with plastic limit (PL) and optimum moisture content (OMC). Thereafter, the efficacies of two thermal conductivity prediction models have also been evaluated statistically with respect to experimental results.

Evolution of Temperature Field around Underground Power Cable for Static and Cyclic Heating

Power transmission covering long-distances has shifted from overhead high voltage cables to underground power cable systems due to numerous failures under severe weather conditions and electromagnetic pollution. The underground power cable systems are limited by the melting point of the insulator around the conductor, which depends on the surrounding soils’ heat transfer capacity or the thermal conductivity. In the past, numerical and theoretical studies have been conducted based on the mechanistic heat and mass transfer model. However, limited experimental evidence has been provided. Therefore, in this study, we performed a series of experiments for static and cyclic thermal loads with a cylindrical heater embedded in the sand. The results suggest thermal charging of the surrounding dry sand and natural convection within the wet sand. A comparison of heat transfer for dry, unsaturated and fully saturated sand is presented with graphs and colour maps which provide valuable information and insight of heat and mass transfer around an underground power cable. Furthermore, the measurements of thermal conductivity against density, moisture and temperature are presented showing positive nonlinear dependence.

Heat Storage of Paraffin-Based Composite Phase Change Materials and Their Temperature Regulation of Underground Power Cable Systems.

Excessive heat accumulation in backfill materials causes thermal fatigue damage in underground power cable systems that significantly affects the cable carrying capacity. To improve the thermal conditions of the system, two types of composite phase change materials (CPCMs) were prepared by incorporating paraffin into porous ceramsite (CS)/expanded graphite (EG) in this study. EG and CS can carry 90 and 40 wt.% paraffin, respectively. The phase change temperature of paraffin/CS and paraffin/EG CPCMs was approximately 65 °C, and the corresponding latent heats were 63.38 J/g and 156.4 J/g, respectively. Furthermore, the temperature regulation by CPCMs was evaluated experimentally by designing a setup to simulate the underground power cable system. The reduction in the maximum temperature of the backfill materials with paraffin/CS CPCM and paraffin/EG CPCM was approximately 7.1 °C and 17.1 °C, respectively, compared to reference samples. A similar conclusion was drawn from the heat flux curves. Therefore, the prepared CPCMs could significantly alleviate temperature fluctuations, where the paraffin/EG CPCM provided better temperature regulation than paraffin/CS CPCM. Both materials have potential applications for use in backfill materials for underground power cable systems.

Multiobjective optimization of underground power cable systems

This paper presents a modified Jaya algorithm (MJaya) for optimizing the material costs and electric-thermal performance of an Underground Power Cable System (UPCS). Three power cables arranged in flat formation are considered. Three XLPE high voltage cables are situated in the thermal backfill layer for ensuring the optimal thermal performance of the cable system. The cable backfill dimensions, cable backfill material, and cable conductor area are selected as design variables in the optimization problem. In the study, the Finite Element Method model is validated experimentally.The Particle Swarm Optimization (PSO), Jaya, and MJaya algorithms are used for multiobjective optimization in order to design a cable system in such a way to minimize the cable backfill costs and maximize the allowable electric current flowing through the cables. For the case study, calculations performed using the Jaya algorithm indicated 1.7 mln Euro cable system costs while cable ampacity is equal to I = 1460 A. The calculations are performed for the objective function values equal to w1 = 0.5 and w2 = 0.5. Such an optimization parameters set allow obtaining low costs of UPCS alongside with reasonable cable line ampacity.What is more, the results of the optimization obtained by Jaya, MJaya, and PSO algorithms are compared. Therefore, Coverage and Hypervolume metrics are incorporated. It is concluded that both the Jaya and MJaya algorithms performed better when compared to the PSO algorithm.

Analysis of an application possibility of geopolymer materials as thermal backfill for underground power cable system

The circular economy is a closed cycle that allows one to reuse the industrial waste, as well as minimize the energy and resources losses during the production process. This paper presents an innovative idea of the application of a geopolymer cable backfill for underground power cable system installation. The closed cycle, in this case, is formulated as follows: the primary resource is the waste from the combustion of fossil fuels, i.e., fly ash that is utilized to form the geopolymer matrix. The geopolymer then is used as thermal backfill in underground power cable systems. Utilization of combustion by-products in the form of a geopolymer is a highly profitable solution since landfill waste disposal, in this case, generates considerable costs for the electrical energy producers. In typical applications, geopolymers are used as insulators. By adding individual components, the thermal conductivity of 2.0 W/(m K), higher than of typical thermal backfills (Fluidized Thermal Backfill), which value is close to 1.5 W/(m K), is reached. What is very important, geopolymers can absorb water better than typical sand–cement mixtures. As a result, a high thermal conductivity with the temperature increase is maintained. The application of geopolymers as thermal backfills has the potential to improve the flexibility of underground power cable systems, as well as to minimize the material costs of installation. The case study is presented to show the economic benefits of using the combustion by-products as a geopolymer thermal backfill. The finite element method model of an underground power cable system is developed, and optimization of backfill dimensions is provided to minimize the material costs using the geopolymer thermal backfill and to maximize the underground power cable system performance. The main result of this paper is that the application of geopolymers leads to a decrease in underground power cable system costs, compared to traditional backfill (sand–cement mixture). The reason is the higher value of thermal conductivity, which allows selecting a cable with a smaller cross-sectional area. Also, the environmental benefits of geopolymer application for cable bedding are discussed.Graphic abstract

Experimental Validation of a Heat Transfer Model in Underground Power Cable Systems

This paper presents the laboratory test stand that is used for experimental validation of underground power cable system models. Determination of temperature distribution in the vicinity of the cable is the main goal of the study. The paper considers a system of three power cables, situated in the in-line arrangement, and buried in sand. Three electrical heaters of special construction are used in order to simulate the heat flux that is generated in the power cables during their operation. The test stand is designed to be placed in a thermoclimatic chamber, which allows testing the system in various thermal conditions when the ambient temperature changes by 20 °C to 30 °C. Numerical computations of the steady-state temperature fields are performed using the finite element method.

Numerical study of heat transfer in underground power cable system

Underground power cable system has been wildly used as an effective transmission line. This paper presents the thermal analysis of the two formations (Flat and Trefoil) of buried system. Temperature-dependent thermal conductivity of soil was considered in the research and the Fluidized Thermal Backfill (FTB) was applied to increase the ability of heat dissipation of power cable system. Further, the influences of heat transfer performance and ampacity on power cable systems were studied when FTB was applied in two formations. Firstly, it is found that the geometric parameters have significantly effects on the heat transfer performance of underground power cable system. Secondly, considering the temperature-dependent thermal conductivity, the maximum temperature of cables will rise rapidly, the limited temperature is easy to reach. The application of FTB can enhance the heat transfer of the whole system, especially for trefoil formation. Finally, the analysis of ampacity shows that the FTB will ensure the higher loading current in a security condition with the temperature-dependent thermal conductivity.

Thermal performance optimization of the underground power cable system by using a modified Jaya algorithm

This paper presents a modified Jaya algorithm for optimizing the material costs and electric-thermal performance of an Underground Power Cable System (UPCS). A High Voltage (HV) underground cable line with three 400 kV AC cables arranged in flat formation in an exemplary case study is considered. When buried underground, three XLPE high voltage cables are situated in thermal backfill layer for ensuring the optimal thermal performance of the cable system. The study discusses the effect of thermal conductivities of soil and cable backfill material on the UPCS total investment costs. The soil thermal conductivity is assumed constant and equal to 0.8 W/(m K). The cable backfills considered in the study are as follows: sand and cement mix, Fluidized Thermal Backfill™ (FTB) and Powercrete™ a product of Heidelberg Cement Group. Constant thermal conductivities of the backfills in the dry state are assumed, respectively, 1.0 W/(m K), 1.54 W/(m K) and 3.0 W/(m K). The cable backfill dimensions and cable conductor area are selected as design variables in the optimization problem. The modified JAYA algorithm is applied to minimize material costs of UPCS under the constraint that the cable conductor temperature shall not exceed its optimum value of 65 °C. The cable temperature is determined from the two-dimensional steady state heat conduction equation discretized using the Finite Element Method (FEM). The performance of the modified Jaya algorithm was compared with classical Jaya and PSO algorithms. The modified Jaya algorithm, for the presented case study, allows one to obtain lower values of the cost function.

Thermal analysis of underground power cable system

The paper presents the application of Finite Element Method in thermal analysis of underground power cable system. The computations were performed for power cables buried in-line in the ground at a depth of 2 meters. The developed mathematical model allows determining the two-dimensional temperature distribution in the soil, thermal backfill and power cables. The simulations studied the effect of soil and cable backfill thermal conductivity on the maximum temperature of the cable conductor. Also, the effect of cable diameter on the temperature of cable core was studied. Numerical analyses were performed based on a program written in MATLAB.

Estimation of Thermal Instabilities in Soils around Underground Electrical Power Cables

Core Ideas We studied coupled water, vapor, and heat flow around underground cables. There is a water–vapor flow cycle in the soil around these cables. This water–vapor cycle breaks down at the critical heat dissipation rate. The critical heat dissipation rate depends on soil hydraulic properties. The decentralized production of electrical energy from wind requires the installation of more and more underground power cables. From an economic standpoint it is very important to estimate the cables' lifetime, which strongly depends on the temperature. A correct estimation of cable temperature requires an accurate description of the heat balance equation around the cable. Most of the models used to assess heat dissipation from cables, however, do not consider that the soil thermal conductivity strongly depends on soil water content and hydraulic dynamics in the vicinity of the cable. The high temperature around the cable induces a water–vapor cycle in the soil: liquid water evaporates near the cable, and this vapor condensates in the more distant and colder parts of the soil. Above a critical heat dissipation rate, the soil around the cable dries out and the water–vapor cycle breaks down. In this study, we analytically estimated the critical heat dissipation rate as a function of soil type, soil temperature, and water content. We validated the analytical estimation by comparison to a numerical solution of the coupled heat–water–vapor transport for various soil types and conditions. The numerical code was validated by simulating a soil drying experiment. The proposed estimation of thermal instabilities in soils could become a powerful tool in the design of underground electrical power cable systems.

Trend adjusted lifetime monitoring of underground power cable

Insulated power cables are the most critical devices used in electrical network for urban areas. Also, lifespan knowledge of these devices is the important information for decision-makers to make their investment planning. This study has presented a practical method for estimating the remaining lifetime of a power cable. The validation of the method has been tested under two different cases for 154kV XLPE insulated underground power cable system. Power cable thermal network has been modeled for a short-duration transient according to electrical analogy. Lifetime monitoring has been carried out by considering loss of life trend depending on the load raising an annual basis. The results show that the accuracy of the remaining service life estimation is related to receiving data with high sample-rate and determining loss of life trend curve belonging to the power cable. The realistic estimations are carried out using trend-adjusted exponential smoothing.

The performance analysis of a new thermal backfill material for underground power cable system

Lafarge Gruntar™ material is proposed in this paper as a new thermal backfill component, and different mass fractions such as 5%, 10%, and 15% are examined. The thermal properties of the developed backfill are determined by experiments with respect to water content and time. Based on the experimental data it is found that the 15% mass fraction Lafarge Gruntar™ material provides the most favorable thermal conductivity i.e. over 1.00W/(mK) in the dry state. This paper also presents a thermal performance optimization procedure for the high voltage underground power cable system. The new thermal backfill material is considered to be installed within the cable system to improve its thermal performance. The analyzed system consists of three underground power cables situated in a flat formation (in−line arrangement) in thermal backfill, buried in the native soil. To avoid extensive mechanical loads caused by vibrations, when locating beneath a paved ground (e.g. under road crossings), the cables are situated in High Density Polyethylene (HDPE) casing pipes filled with sand−bentonite mixture. The pipes are then placed into a thermal backfill and buried in the native soil. The Installation of thermal backfill material is relatively expensive since in many cases the buried line is installed over many kilometers. Therefore, it is important to determine the optimal dimensions of the cable bedding layer to minimize the material and installation costs while keeping efficient heat dissipation from the cables. The momentum-type Particle Swarm Optimization (PSO) solver, with a dynamic penalty function, is used to minimize the cable backfill cross-sectional area while not exceeding the allowable temperature of the cable operation. The performed optimization procedure obtains the converged solution. The temperature distribution in soil, cables, and the cable backfill layer is determined using the Finite Element Method. The Campbell − de Vries thermal conductivity model is employed for the soil surrounding the underground power cable system. An MATLAB code is written for solving the heat conduction equation and to determine the temperature distribution within the underground power cable system. By using the momentum-type Particle Swarm Optimization algorithm, it is possible to design the best-found dimensions of the cable bedding layer (width and height). Moreover, the dynamic penalty function employed in the optimization procedure has assured the determination of the maximum temperature of cable conductor close to its best-found value.

지중송전케이블 종단접속함 종류에 따른 외기 및 운전온도에 의한 열특성 분석

This paper describes the thermal characteristics of underground power cable system terminations according to the change of ambient temperature as well as operating temperature. Recently, the failure has been gradually increasing in outdoor termination during winter season because the power demand was increased by electricity heating system. The power demand and outdoor temperature is difference between day time and night time. The temperature difference has an influence on conductor extension and shrinkage due to conductor force as well as thermal mechanical characteristics. These phenomena have daily repeated during heating and cooling period of conductors. In these cases, the insulation of outdoor terminations might be degraded by the reduced interface pressure surrounding stress relief cone. Therefore, in this paper, the thermal characteristics are variously analysed by simulation considering power demand and ambient temperature condition during winter season at epoxy type termination as well as slip-on type termination.

Optimizing of the underground power cable bedding using momentum-type particle swarm optimization method

Thermal performance optimization of underground power cable system is presented in this paper. The analyzed system consists of three underground power cables situated in an in-line arrangement. The HDPE (High-Density Polyethylene) casing pipes, filled with SBM (Sand-Bentonite Mixture), covers the cables to protect them from heavy mechanical loads (e.g. vibrations). The FTB (Fluidized Thermal Backfill) layer is applied to prevent the cables from overheating. Due to the substantial costs of FTB backfill material (in relation to the native soil or dry sand), the cross-sectional area of FTB bedding layer has to be minimized. Furthermore, the maximum cable conductor temperature is expected not to exceed the optimum operating temperature. Therefore, the optimization procedure i.e. momentum-type PSO (Particle Swarm Optimization) is applied. The FEM (Finite Element Method) is used to solve the two-dimensional steady-state heat conduction problem. As a result, temperature distribution is determined for the native soil, FTB bedding, and cables. The performed computations considered the temperature dependent current rating and volumetric heat generation rate from cable conductor. The applied optimization procedure resulted in determination of the optimum cable spacing and cross-sectional area of the rectangular-shaped FTB bedding layer. Moreover, the obtained maximum temperature for the cable core do not exceed the allowable value.

Numerical simulation of heat dissipation processes in underground power cable system situated in thermal backfill and buried in a multilayered soil

This paper presents the thermal analysis of the underground transmission line, planned to be installed in one of the Polish power plants. The computations are performed by using the Finite Element Method (FEM) code, developed by the authors. The paper considers a system of three power cables arranged in flat (in-line) formation. The cable line is buried in the multilayered soil. The soil layers characteristic and thermal properties are determined from geological measurements. Different conditions of cable bedding are analyzed including power cables placement in the FTB or direct burial in a mother ground. The cable line burial depth, measured from the ground level, varies from 1m to 2.5m. Additionally, to include the effect of dry zones formation on the temperature distribution in cable line and surroundings, soil and FTB thermal conductivities are considered as a temperature-dependent. The proposed approach for determining the temperature-dependent thermal conductivity of soil layers is discussed in detail. The FEM simulation results are also compared with the results of the simulation that consider soil layers as homogeneous materials. Therefore, thermal conductivity is assumed to be constant for each layer. The results obtained by using the FEM code, developed by the authors, are compared with the results of ANSYS simulations, and a good agreement was found.

DC 혼합배전시스템에서 지중계통의 뇌과전압 해석

This paper describes the overvoltage through lightning surge analysis on underground system in DC combined distribution systems. It is considered that operating micro grid including distributed generation with smart grid can make possibility of composing new distribution system different from existing one. However, there are many papers about low voltage DC distribution in grids or buildings but not many about replacement or distributing 22.9kV AC distribution system to DC system. Among many research need for DC system development, overvoltage is studied in this paper. Overvoltage is simulated on DC cable when lightning strikes to overhead grounding wire which is installed at the nearest location from power cable section. Analysis as well as modeling is performed in EMTP/ATPDraw. It is evaluated that analysis results can be used to design of DC underground distribution power cable system.

Generated heat within power cable sheaths per unit time and volume

An assessment of the generated heat per unit time and volume within cable sheaths is required to determine the corresponding temperature distribution through the domains with underground power cable systems. If the sheaths of a cable line, laid in a flat formation, are bonded and earthed at both ends of an electrical section, and if conductor currents form a balanced three-phase system, the generated heat per unit time and volume within solid sheaths can be accurately evaluated using: i) the filament method in the case of the cables laid in either touching or close arrangements, ii) both the filament method and the simple analytical IEC (International Electrotechnical Commission) relations associated with sheath losses in the case of spaced formations. By using the filament method, the modeling of the ground return path for sheath currents is not a mandatory task, regardless of the cable spacing.

Technical Review on Parallel Ground Continuity Conductor of Underground Cable systems

The Induced sheath voltage is significantly increased at single point bonding section when the ground fault occurs on power cable system because there is no return path of fault current. For solving this problem, therefore, many researchers recommend the PGCC (Parallel Ground Continuity Conductor). In this paper, the characteristics of PGCC are extensively analyzed for reducing the level of induced sheath voltage at the single point bonded section for Korea underground power cable system