Soil Thermal Resistivity Research Articles

The Heat and Buried Cable Conundrum: A Method to Help Determine Underground Cable Ampacity

One of the most important factors affecting underground cable ampacity is the thermal resistivity of the soil. A soil's thermal resistivity varies with moisture content, and the heat generated by cables can cause the soil to dry, increasing its resistivity. The ability of the soil to maintain its thermal resistivity in the presence of a heat source is known as thermal stability. Since no soil has perfect stability, accounting for the fluctuating thermal resistivity of soil makes the cable ampacity calculations difficult. This article will examine the information available from standard soil tests and the information these tests may provide relating to the migration of moisture in the soil and the resulting changes in soil resistivity. Furthermore, we suggest a method for including this information in underground cable ampacity calculations.

Determination of Electrical Resistivity of Soil Based on Thermal Resistivity Using RVM and MPMR

This article adopts Relevance Vector Machine (RVM) and Minimax Probability Machine Regression (MPMR) for prediction Soil Electrical Resistivity(RE) of soil. RVM uses an improper hierarchical prior. It optimizes over hyperparameters. MPMR is a probabilistic model. Two models (MODEL I and MODEL II) have been adopted. Percentage sum of the gravel and sand size fractions (F) and Soil Thermal Resistivity(RT) has been takes as inputs in MODEL I. MODEL II uses F,RT and saturation of soils(S) as input variables. The results of RVM and MPMR have been compared with the Artificial Neural Network (ANN). The developed RVM and MPMR proves his ability for prediction of RE of soil.

지중배전케이블의 허용전류용량 증대를 위한 적정 회선배치 및 관로 이격거리 분석

Power demand has continuously increased with technological and economical development. The load density is also growing in the center of downtown area. In particular, underground facilities have been increased on the purpose of the prevention of urban disasters and pedestrian environment improvement. Based on this situation, the underground space in urban surroundings has gradually decreased because of the limited space. The ampacity of buried cables is affected by various factors such as cable size, soil thermal resistance, burial depth and filling material. The thermal capacity of the facilities is determined by the absorb heat surrounding the cable and the soil. The maximum operating temperature of cable is the highest temperature when the insulator of cable is not damaged in the case of high enough temperature. In this paper, the most effective cabling configuration is suggested using the duct array adjustment. It was also considered to increase the number of cable line. This underground distribution system was simulated by using ETAP(Electrical Transient Analysis Program).

Cable Ampacity Calculations: A Comparison of Methods

When designing electrical power systems, it is often necessary to determine underground cable ampacity. Various methods are in use today, including computer simulation, ampacity tables, and a method that has recently been suggested that includes the effects of moisture migration through soil. Each of these methods can yield substantially different ampacity results for the same installation. Regardless of the method, using the correct value of soil thermal resistivity is critical and using the wrong value can result in cables that are incorrectly sized. This paper examines several commonly used methods and their underlying assumptions. Examples are provided to illustrate the differences in the results obtained from various methods and the consequences of using incorrect assumptions. It is hoped that these examples will provide guidance on the implementation of each method.

Experimental Analysis of Thermal Fields Surrounding Horizontal Cylindrical Geothermal Exchangers

In the engineering field there are many applications where it is important to evaluate the heat exchange between cylindrical surfaces and the surrounding ground (horizontal geothermal exchangers for heat pumps, underground pipelines, underground electric cables, etc...). For these reasons, there is an increased interest in the correct analysis of the soil thermal resistance. In this study different types of soils with linear heat sources were considered and, through a physical model reproduced in laboratory, the thermal field was evaluated. The model reproduced an “undisturbed” portion of soil and the influence of different thermal conditions of the linear heat sources and of the environmental parameters were evaluated. This paper will be extended through the implementation of the experimental device into a numerical model by applying a calculation software to the finite volumes; the result is a predictive model of the soil resistance able to furnish accurate information of the real mean radial thermal resistivity surrounding the geothermal exchanger.

Thermal characterization and prediction model of typical soils in Nanjing area of China

The thermal resistivity of soil is a basic parameter to describe its heat transfer characteristic and also is a key parameter in analysis of soil heat transfer in geotechnical engineering. The thermal probe testing technology and its theory were introduced. Four different typical soils in Nanjing area were measured by non-steady-state probe. The relationship between the soil thermal resistivity and moisture content was analyzed at different dry densities. It could be concluded that the non-steady-state probe can effectively measure the thermal resistivity of soils. With an increase in both moisture content and dry density, the magnitude of thermal resistivity decreased. There is a dramatical decrease of thermal resistivity in the initial stages of adding water to the dry soil. The thermal resistivity decreases rapidly within moisture content in a relatively small range and then approach equilibrium with further increase in moisture content. The critical moisture content is a great importance index which depends on the natural properties of soils. Thermal resistivity decreases with the increase of dry density and particle size. This effect is also related to mineral composition, particle shape and other factors. Based on the existing prediction model and testing results, a new prediction model was proposed to predict the thermal resistivity of soils. The results indicate that the predicted values generally agreed with the measured values within an absolute difference of less than 15%.

Underground electric cables a correct evaluation of the soil thermal resistance

Nowadays companies supplying medium/high voltage electric energy, near residential areas, tend to use underground cables laying. Hence the design engineer is required to estimate the thermal resistance around the underground cable to perform a right dimensioning of the cables; as a matter of fact what it should be avoided is the overheating determined by a bad heat dissipation due to the Joule effect.IEC rules provides a formula for the evaluation of the soil thermal resistance which is easy to apply. But from an experimental point of view, as the bibliography shows, it was discovered it tends to underestimate the problem when dealing with very dry soils in particular.Thanks to an experimental system, some useful data were collected for the validation of a 2D FEM model of an excavated area with a linear heat source reproducing the underground conduit. The numerical model presents a variation of both the geometrical parameters of the excavated area and the distance characterizing other cables (hence heat sources) located in the same site of installation examined. In this way two dimensionless coefficients, useful to correct the values of the thermal resistance furnished by the current regulations, were determined.

Evaluation of bentonite-based thermal backfill materials

The knowledge of soil thermal resistivity is required for projects such as thermal backfills for underground power cables, high-level nuclear waste disposal, pipelines carrying hot fluid and so on. The role of the backfill is to dissipate heat easily with minimal loss of moisture and low shrinkage. Clayey soils such as bentonite have high water retention capacity but low strength, high shrinkage and high thermal resistivity. Cohesionless geomaterials such as sand and fly ash exhibit low shrinkage and water retention. These materials have lower thermal resistivity than bentonite. The mix of sandfly ash with bentonite contributes to higher strength. Therefore, it is possible to mix bentonite with sand or fly ash to obtain an optimal thermal backfill mix with requisite thermal and engineering properties. There are few studies that deal with the thermal resistivity of bentonite-based backfill mixes. The present study deals with the development of the thermal resistivity variation curve (TRVC) for such mixes, which is the variation of thermal resistivity with water content. The critical moisture content has been determined from the TRVC and has been correlated with geotechnical properties such as plastic limit and optimum moisture content.

Utilization of Gaussian Process Regression for Determination of Soil Electrical Resistivity

Soil electrical resistivity (RE) is an important parameter for geotechnical engineering projects. This article employs Gaussian process regression (GPR) for prediction of RE of soil based on soil thermal resistivity (RT), percentage sum of the gravel and sand size fractions (F), and degree of saturation (Sr). GPR is derived from the perspective of Bayesian nonparametric regression. Two models (Model I and Model II) have been developed. The developed GPR has been compared with the artificial neural network. It gives the variance of the predicted RE. The results show the developed GPR is an efficient tool for prediction of RE of soil.

An in situ thermal response test for borehole heat exchangers of the ground-coupled heat pump system

Thermal response tests (TRTs) are crucial for the estimation of the ground thermal properties and thermal performance of the borehole heat exchanger (BHE) of the ground-coupled heat pump (GCHP) system. In this article, a TRT apparatus was designed and built to measure the temperature response of inlet and outlet sections of BHE in the test borehole, the apparatus can effectively operate under both constant heating flux modes and heat injection and extraction modes with a constant inlet temperature. A TRT for a project of GCHP located in the Jiangsu province of China was carried out by the experimental apparatus. Based on the experimental data, the heat transfer performances of BHE under heating and cooling modes were evaluated, and the ground thermal properties, which include the ground thermal conductivity, ground volumetric specific heat, borehole thermal resistance and effective soil thermal resistance, were determined by the line source model. The results indicate that the experimental device and analysis model proposed in this article can be effectively applied to estimate the ground thermal properties and thermal performance of BHE. During the process of thermal response of ground, the fluid temperatures vary acutely at the start-stage of 8 h, and then tend to be a steady state after 40 h. The test data during the start-stage should be discarded for improving the estimation accuracy of ground thermal properties. At the same time, the effective soil thermal resistance increases continuously with time and a steady-state value would be reached after the start-time, and this steady-state thermal resistance can be used to evaluate the required length of BHE. In addition, the heat transfer rate of the BHE under different operating conditions can be used for the further evaluation on long-term operation performance of GCHPs.

Water Migration Impacts on Thermal Resistivity Testing Procedures

This study investigated water redistribution during measurement of the thermal dry-out curve (i.e., soil thermal resistivity ρT versus volumetric water content θ). Although there is not a standard method for measuring the ρT–θ relationship, curves are typically generated with incremental ρT measurements as a specimen is dried. However, drying within a specimen may not be uniform, even in a controlled, low-temperature drying environment. In this study, ρT and θ were measured at the top, middle, and bottom of remolded cylindrical soil specimens of a range of soil types after staged drying in a low-temperature oven. Water distribution was highly varied; for example, tests on silty sand revealed θ values up to nine times greater at the bottom than at the top of the specimen, and spatial variations in θ erroneously affect measured values of ρT. Numerical models validated water migration within soil specimens during staged drying. The results indicate a need for modification of the standard thermal resistivity testing procedure to account for variations in water content across a specimen and along the sensor length during drying.

Factors influencing ampacity and temperature of underground power cables

This paper presents the factors that influence ampacity and temperature rise of three-phase, single-core 33- and 500-kV XLPE underground cables (UGC) using CYMCAP software. These factors are conductor cross-sectional area, soil thermal resistivity, cable burial depth, cable separation, sheath bonding, bedding and backfill heights and thermal conductivities, nearby parallel heat source, formation of dry zone, loss tangent and segmented conductors. Results reveal that increasing the separation distance between phases gives higher ampacity, contrary to the burial depth. The rate of conductor temperature reduction due to the increase in the bedding thermal conductivity is more pronounced than that achieved by increasing backfill thermal conductivity. Furthermore, increasing the native thermal conductivity and/or the maximum conductor temperature increases the UGC ampacity and consequently increases the induced sheath voltage. Sheath losses are significant in transmission UGC where the load currents are always high. High conductor temperature and hence degradation rate is expected for UGC carrying currents of highly fluctuating loads. UGC must be derated as they age (increasing loss tangent), or when dry zones are formed around them, or when a nearby parallel heat source. Finally, it is found that the increase in the number of conductor segments nonlinearly increases the UGC ampacity.

현지 측정에 의한 남한지역의 지중유효열전도도, 보어홀 전열저항 및 초기온도 분석

Investigation of the effective soil thermal conductivity() is the first step in designing the ground loop heat exchanger(borehole) of a geothermal heat pump system. Another important factor is the borehole thermal resistance(). Thermal response tests offer a good method to determine the ground thermal properties for the total heat transport in the ground. The first step is measured for initial soil temperature. This is done by supplying a only pump power into a borehole heat exchanger. They need to supply into water unload heat power more than 30 minutes. In this study, the initial soil temperature was found to analysis ,the ratio was 68.7% represented. In this case of , was 2.1~3.0 , was 0.11~0.20 . In this work, it is also shown that the distribution of a soil thermal conductivity and borehole thermal resistance were on the influence of initial soil temperature. And soil thermal conductivity was related with factors of equation by linear least square method, borehole thermal resistance was on the influence of composite factors.

저압 지중케이블의 토양 열저항률 적용에 관한 연구

The current-carrying capacities in IEC standards for underground refers to a soil thermal resistivity of 2.5[K?m/W] where no measured data are available. But this value is considered too conservative and may not justifiable economically as to need precaution in using the value. In this paper, the standard practices on the application of soil thermal resistivity of some countries(UK, USA etc) are surveyed and proposed a reasonable representative value 1.0[K?m/W] of soil thermal resistivity considering the domestic soil thermal properties with regard to the application of IEC standard.

Artificial neural network models for predicting electrical resistivity of soils from their thermal resistivity

The knowledge of soil electrical and thermal resistivities is essential for several engineering projects such as laying of high voltage buried power cables, nuclear waste disposal, design of fluidized thermal beds, ground modification techniques etc. This necessitates precise determination of these resistivities, and relationship between them, which mainly depend on the soil type, its origin, compaction density and saturation. Such a relationship would also be helpful for determining one of these resistivities, if the other one is known. With this in view, efforts were made to develop artificial neural network (ANN) models that can be employed for estimating the soil electrical resistivity based on its soil thermal resistivity and the degree of saturation. To achieve this, measurements of electrical and thermal resistivities were carried out on different types soils compacted at different densities and moisture contents. These models were validated by comparing the predicted results vis-à-vis those obtained from experiments. The efficiency of these ANN models in predicting the soil electrical resistivity has been demonstrated, if its thermal resistivity is known. These ANN models are found to yield better results as compared to the generalized relationships proposed by the earlier researchers.

Effects of modelling assumptions on the rating calculation for externally forced cooled high-voltage cables

To increase the rating of a high-voltage cable circuit the cable group can be externally forced cooled, using additional coolant pipes in proximity to the buried cable group. This complicates modelling of the heat transfer problem to obtain ratings as coolant temperature and therefore heat transfer coefficient varies along the cable route. The most common approach for obtaining the circuit rating is the finite difference (FD) method outlined in Electra 66. This method is computationally efficient and quick to solve. To investigate the assumptions underlying this approach and provide confidence over a range of model parameters, this paper presents the development of an extended 2-D heat-transfer finite element method (FEM) model. The ratings of two cable circuits have been modelled using this approach and are compared with results from Electra 66. Cable ratings from the two methods are consistent in trend but offset favourably by 2.6% using the FEM model for all burial depths tested. With the FEM model verified for standard assumptions the model provides a useful tool for rapid investigation of sensitivity to model assumptions. A sensitivity analysis to changes in ac resistance, burial depth, dielectric loss, soil thermal resistivity and surface boundary condition is presented.

Thermal and magnetic analyses of gas-insulated lines

This paper presents the factors affecting current rating “ampacity” of buried three-phase, naturally cooled isolated-phase gas-insulated lines (GIL) with flat formation and finite-element simulation of double-circuit, three-phase GIL inside a tunnel to calculate the magnetic-flux and current densities. In the former analysis, the depth of GIL burial, interspacing of phases and soil thermal resistivity are investigated to show their impacts on the GIL ampacity. In the latter analysis, the effects of electrical and physical arrangements of GIL phases on the magnetic-flux density inside and outside the tunnel, and the current density in few installed liquefied natural gas (LNG) pipelines, which run in parallel to the GIL inside the tunnel, are also studied. In any GIL configuration, the magnetic field impact is always extremely low. Localized hot spots along the periphery of the GIL enclosure are found and corrosion can be accelerated due to temperature gradient. Moreover, there is a possibility of galvanic corrosion of the enclosures when they are bonded together and with the gallery steel reinforcement at very short length intervals. The simulation results also give the best location of the LNG pipelines or any cables to have the lowest electromagnetic induction. The configuration that has the best “magnetic behavior” is distant-placed arrangement (DPA) with (abc–cba) phase arrangement. The induced eddy-current density in the LNG pipelines is very low and hence its thermal effects can be neglected.

Artificial neural network models for predicting soil thermal resistivity

Thermal properties of soils are of great importance in view of the modern trends of utilizing the subsurface for transmission of either heated fluids or high power currents. For these situations, it is essential to estimate the resistance offered by the soil mass in dissipating the heat generated through it. Several investigators have tried to develop mathematical and theoretical models to estimate soil thermal resistivity. However, it is evident that these models are not efficient enough to predict accurate thermal resistivity of soils. This is mainly due to the fact that thermal resistivity of soils is a complex phenomenon that depends upon various parameters viz., type of the soil, particle size distribution and its compaction characteristics (i.e., dry density and moisture content). To overcome this, Artificial Neural Network (ANN) models, which are based on experimentally obtained thermal resistivity values for clay, silt, silty-sand, fine- and coarse-sands, have been developed. Incidentally, these soils are the most commonly encountered soils in nature and exhibit entirely different characteristics. The thermal resistivity of these soils, corresponding to their different compaction states, was obtained with the help of a laboratory thermal probe and compared vis-à-vis those obtained from the ANN model. The thermal resistivity of these soils obtained from ANN models and experimental investigations are found to match extremely well. The performance indices such as coefficient of determination, root mean square error, mean absolute error, and variance account for were used to control the performance of the prediction capacity of the models developed in this study. In addition to this, thermal resistivity of these soils obtained from ANN models were compared with those computed from the empirical relationships reported in the literature and were found to be superior. The study demonstrates the utility and efficiency of the ANN model for estimating thermal resistivity of soils.

Ampacity Derating Factors for Cables Buried in Short Segments of Conduit

Buried cables are often routed through short segments of conduit, and when this situation occurs, the ampacity must be reduced or the cable will overheat as a result of the high thermal resistance created at the location of the conduit. This problem is examined for extruded cables by using a finite element heat transfer software program to determine the derating in ampacity that cables in conduits must experience in order to remain below a maximum conductor temperature. The derating factors are provided as a function of conduit length, soil resistivity, burial depth and number of cables in the conduit. The results show that once the length of conduit exceeds about 20 times its outer diameter, then the ampacity of the circuit must be reduced to the value that it would have if the entire length were buried in the conduit. Factors that result in lower cable ampacities, such as high soil thermal resistivity and deeper burial depths lead to larger derating factors.

Generalized relationship for determining soil electrical resistivity from its thermal resistivity

The knowledge of soil electrical and thermal resistivity finds its application in many of the real life engineering projects like laying of high voltage buried power cables, ground modification techniques etc. This necessitates determination of soil electrical resistivity and thermal resistivity and development of a relationship between them. However, as these resistivities mainly depend on the type of the soil (i.e. its physical composition) and its saturation, efforts have been made in this paper, to develop a generalized relationship to relate them. Validation of the relationship has been conducted vis-à-vis the results obtained from the laboratory experiments and those reported in literature.