Soil Thermal Resistivity Research Articles

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.

Thermal analysis of the influence of harmonics on the current capacity of medium‐voltage underground power cables

AbstractIn this article, an algorithm is proposed and used to study the influence of harmonics on the behavior of medium‐voltage underground cables in flat formation. The proposed algorithm is a thermal model based on the heat equilibrium of the thermal circuit nodes of the medium‐voltage cable system. The impact of harmonics on the temperature rise of the cable elements and the cable capacity is evaluated in this article. Also, the impact of harmonics on the derating factors of cable for different soil types is presented. Finally, the measurement of temperatures of cable cores is carried out experimentally and compared with the calculated results to validate the proposed algorithm. One of the algorithm merits is that several harmonic percentages can be taken into account for each cable phase individually, and the heat exchange between the cable phases and their sheath is also taken into consideration. From the obtained results, it is illustrated that the presence of harmonics has a remarkable influence on the cable core temperature; mainly, harmonics of the third and fifth orders may lead to dry zone formation around the cable. It is also observed that the presence of harmonics has an important influence on the cable current, especially when it is buried in soil that has high thermal resistivity during the summer season (suction tension = ∞). In summer, the cable core temperature reached 152.162°C, 139.053°C, and 133.375°C when lime, sand, and silty sand, respectively, are used as backfill materials, rather than 90°C in the normal operating condition of the 11 kV three‐phase single‐core cable. It is observed also that with the increase of the soil thermal resistivity, the ratio of /) reached about 1.2 times at 2.5 K m/W soil thermal resistivity. In addition, it is also observed that the impact of harmonics leads to a percentage reduction in the derating factor of the cable center phase by 11.88%–12.37% depending on the composition of the backfill materials.

Extended Thermal Probe Method for Measuring Thermal Stability of Soils Surrounding Buried Power Cables

The single most influential factor in determining the rating of underground cables is the thermal resistivity of the soil that surrounds the cables. An accurate measurement of soil thermal resistivity is therefore critical to the determination of cable ratings. Ratings are also strongly influenced by the thermal stability of the soil, because heat generated by the cables can often drive soil moisture away from the cables resulting in an unexpected steep rise in the soil resistivity and a corresponding sharp reduction in cable ratings. Soil thermal stability can be determined by a simple, short-duration test that involves a thermal probe that has traditionally been used to measure thermal resistivity if the duration of the test is extended. Therefore a single test with a thermal probe can be employed to provide a measure of the soil's thermal stability as well as its thermal resistivity. Armed with knowledge of both resistivity and stability of the soil, a utility engineer can confidently calculate cable ratings that will result in safe cable operating temperatures even under the most adverse unstable soil conditions. Measurements of soil stability are shown to be influenced by the soil's properties, the heat generated in the cables, the diameter of the cable/soil interface and the duration that the soil experiences heating by the cables. Experimental measurements of soil stability using the extended probe method are demonstrated for four different soils, and measurements of stability are shown to influence the rating of several different cable installations.

Thermal design of Earth-to-Air Heat Exchanger: Performance analysis of new transient semi-analytical model for short period of continuous operation

This paper concerns the thermal design of Earth-to-Air Heat Exchanger (EAHE). The paper presents new transient semi-analytical models to estimate i) the disturbed soil thermal resistance of EAHE pipe as a function of duration of operation and ii) thermal performance of EAHE operating under transient conditions. The main aim of this study is to evaluate the parameter choices of this new model for short period of continuous operation of the system (less than 70 h). The results show that time discretization steps of 15 min and 1 h present and lead to appreciable results compared to the experiments. Therefore, the variation of the layer length step of the buried pipe does not alter simulation results. In terms of precision of the soil temperature profile, refined solutions were obtained by the decreasing of the disturbed soil radius and the increasing the number of roots of Bessel's functions. Furthermore, the effect of the variation of surrounding soil radius is discussed for short period of continuous operation of the system. The time step of 1 h and the interval of the surrounding soil radius of 0.4 m–1.1 m are found accurate for good results with a reasonable computational time.

Effects of biochar particle size, biochar application rate, and moisture content on thermal properties of an unsaturated sandy loam soil

Although biochar is considered as a potential soil amendment, the effect of its rates and particle sizes on soil properties such as soil thermal properties, especially in a light-textured soil has not been widely discussed. Due to the importance of soil thermal properties in many engineering, climatological and agricultural applications, this study aims to investigate and model the effects of biochar particle size, biochar rate, and soil moisture content on the thermal properties of a sandy loam soil through column experiments. The experiments were conducted using three biochar application rates (0 (as control), 1%, and 5% w/w), three biochar particle diameters (<0.15, 0.15–1, and 1–2 mm), and three water saturation (Sw) levels (0.07, 0.2, and 0.78) in three replicates. Given the experimental data, soil thermal properties were modeled using the Campbell, Zhao et al., and de Vries models. According to the results, a higher biochar application rate (i.e. 5%) significantly (P < 0.05) altered soil physical and chemical properties, i.e. bulk density, porosity, EC and pH, and soil thermal properties. Biochar particle sizes had a remarkable effect on soil thermal properties. Adding 5% biochar with particle sizes of < 0.15, 0.15–1 and 1–2 mm decreased soil thermal conductivity to 21%, 26%, and 32% at Sw= 0.07, respectively indicating a lower reduction of thermal conductivity when biochar with a smaller particle size (<0.15 mm) is applied. Applying 5% biochar of the smallest size decreased soil thermal conductivity and diffusivity, and heat capacity by 21%, 17%, and 9%, respectively, and increased thermal resistivity by 27% at Sw= 0.07. The effect of soil moisture on thermal properties (especially thermal conductivity and resistivity) was more robust than biochar amendment. Experimental modeling showed a linear relationship between soil moisture and thermal conductivity, diffusivity and heat capacity, and a power law relationship between soil moisture and thermal resistivity. Mathematical modeling showed the Campbell, de Vries, and Campbell/de Vries models perform the best in estimating thermal conductivity, heat capacity, and thermal diffusivity, respectively. The findings of this research could be used for controlling and predicting soil temperature, improving irrigating scheduling, and optimizing the quantity of biochar amendment required in agricultural fields.

Life-Based Geometric Design of HVDC Cables—Part I: Parametric Analysis

This study, in two parts, investigates the cable design of high-voltage direct-current (HVDC) cables depending on life modeling of the insulating material. This article is the first part, in which a parametric analysis is carried out, illustrating the way in which different parameters affect the life-based feasible designs of an HVDC cable with known nominal voltage and ampacity. An <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">ad hoc</i> MATLAB code has been developed for this study. Results show great sensitivity of the lifetime to insulation geometry, as a slight variation in inner and outer insulation radii leads to a significant variation of cable life due mainly to the related electric field variation. The effect of temperature can be hardly observed in the life map due to the predominant effect of electric field. Increasing the maximum conductor temperature may not necessarily extend the feasible design area, which is also limited by the maximum temperature drop across insulation thickness. The higher the applied voltage, the greater the cable dimensions. This study also emphasizes the need to keep a low enough value of soil thermal resistivity, which considerably reduces the cable dimensions. Temperature coefficient of conductivity shows a nonlinear (mainly drastic) effect on the life. On the contrary, field coefficient of conductivity shows a slight positive effect on the life. Part II of this study will investigate the effect of electrical and thermal transients on the life-based geometric design of HVDC cables.

Research on the Method of Improving the Current-carrying Capacity of Power Cable

The inadvertent current carrying capacity of power cable is bound to be affected by many factors, such as air temperature, ground temperature, backfill sand, soil covering depth, grounding mode, soil thermal resistance coefficient, number of circuits, ventilation design, laying mode, groundwater, metal sheath circulation, layout mode, solar radiation, surrounding heat source . This paper analyzes the common factors in the work, and expounds the mode of flow promotion through the analysis and numerical calculation of power cable ampacity. It also compares the effects of the two methods, hoping to provide new ideas for the follow-up research on related issues.

Estimation of soil and grout thermal properties for energy piles: A new parameter estimation method

It is critical to determine the thermal parameters of the soil and pile for designing energy piles. Based on the infinite different-thermal properties cylindrical heat source model, this paper proposes a method that can use short-term thermal response test (TRT) data to estimate the soil thermal conductivity, thermal diffusivity and thermal resistance in the pile. A three-dimensional numerical model was established and verified by a large sandbox test. The numerical model was used to generate virtual TRT data, and the pattern search algorithm was adopted to estimate the parameters. Then the accuracy of the parameter identification results was discussed. It is found that, compared with the previous methods, the proposed method can greatly shorten the time required for TRT. At the same time, this method can estimate the thermal parameters more accurately, especially the soil thermal conductivity that has the greatest impact on heat transfer performance, with an estimated error of 1.23%.

Using Neural Network Feedback Analysis Technology to Predict Soil and Carbonaceous Rock Thermal Resistivity

Using Neural Network Feedback Analysis Technology to Predict Soil and Carbonaceous Rock Thermal Resistivity

A New Approach for Characterizing Pile Heat Exchangers Using Thermal Response Tests

Pile heat exchangers offer a cost effective route to implementation of ground-source heat pump systems for many large commercial buildings compared with traditional boreholes. Such projects typically use thermal response tests to determine the key input parameters for system design, namely soil thermal conductivity and heat exchanger thermal resistance. However, this brings challenges for pile heat exchanger based systems, where in situ thermal response tests are known to be less reliable due to the large thermal capacity of the pile. This paper presents a new “black box” resistance capacitive model for applications to pile thermal response tests. The approach is tested against case study data and shown to perform well. Additional test duration savings are shown to be possible if a novel combination of borehole and pile thermal response tests is applied together to determine design parameters.

Analysis of Current Carrying Capacity of Power Cable Based on DTS

This article uses a distributed optical fiber temperature measurement system (DTS, Distributed Temperature Sensing) to fix the temperature measurement optical fiber on the surface of the power cable, collect the real-time operating temperature of the power cable, conduct the ampacity test, and analyze the cable laying method and depth, soil temperature, Soil thermal resistance coefficient, cable spacing and irradiation strength and other test conditions, the impact on the current carrying capacity of power cables. According to the real-time monitoring temperature of the cable, the alarm can be made when the cable temperature is too high or the temperature rise speed is too fast. And on the basis of determining the working temperature and load operating conditions of the cable conductor core, dynamically calculate the current carrying capacity of the cable, which is conducive to improving the performance of the power cable and increasing the load capacity of the power cable.

Hong Point Estimate Method to analyze uncertainty in the underground cables temperature

Due to the uncertainty in parameters such as ambient temperature, soil thermal resistivity, backfill thermal resistivity, current being carried by the cable, the conductor temperature has an inherent uncertainty that can be described by a probability distribution function. For the first time, Hong Point Estimate Method (HPEM) has been used to analyze the uncertainty in the temperature of underground cables in concrete duct banks and pdf of cable temperature is found. In this paper, the simple and fast HPEM has been used to calculate the j’th moment of conductor temperature based on the moments of the probabilistic input variables (j = 1…4). Simulation results from underground cables temperature have been compared to that of Monte Carlo simulations and the First Order Second Moment (FOSM) method and show superior performance with negligible error. To show an implementation of HPEM, for the first time, statistical cable configuration optimization has been studied and simulated by taking into account the uncertainty problem. Depending on the confidence level chosen, the total ampacity of the underground cable in the concrete duct bank is changed. The results show that when the confidence level increases the total ampacity decreases.

Experimental Investigation of Green Roof Impact on Buildings Energy in Hot Climate

Green roofs are a layer that effectively working on blocks of solar radiation from entering the building's structure below partially. Its work as a passive cooling technique, and have the potential to reduce the high surface temperature of conventional roofs because of the soil thermal resistance, evapotranspiration, and several effects for foliage shading. This affects the heat flux flow of the roof that in turn influences the indoor thermal conditions and the building energy demand. The research goals are to test the influence of the green roof on reducing heat transfer to the interior of heavy structural buildings. The experimental part was done to examine the effect of the green roof and compare it with a standard roof under influence of ambient air temperature, solar radiation, and wind speed and test the effect of the green roof on reducing heat transfer inside the building. The model site was at the Diwanyah city (Latitude: 31.9868 and Longitude: 44.9215), the engineering college campus, Qadisiyah University. The experimental setup includes two cubicles, with equal internal volume values (163cm * 163cm * 105cm). The experimental results showed for the period 5-8 September 2019. The maximum zone temperature was 46.4 °C for traditional roof and 37.06 °C for the green roof at 3:24 p.m. The results showed that the maximum internal temperature was 46.97 °C and 36.42 °C for the traditional roof and green roof respectively at 3 p.m. Also, the results for period 18-21 July 2019 noted that the zone temperature decreased by 7.5 °C and the maximum temperature of the internal traditional standard roof’s surface is 45.66 °C and 37.41 °C for the green roof.

CHARACTERIZATION OF SOIL THERMAL PROPERTIES FOR DESIGN OF UNDERGROUND CABLE ROUTES AT THE WIND FARM POWER PLANT AREA, PANYIPATAN AND PELAIHARI SUB-DISTRICT, SOUTH KALIMANTAN

The estimation of soil thermal resistivity is essential for many types of big engineering projects because of the required knowledge about subsurface transmission of either heated fluids or high power currents. Thermal conductivity measurements were carried out at 12 locations, eight measurement point locations in South Kalimantan, the Sapu Angin hill region, sub-district Penyipatan and four points in the area of Kampung Baru, sub-district Pelaihari made at 2-3 variations in depth of 50 to 100 cm, which adjusted to the ability of the soil drilling support equipment to make holes in the soil. Samples were also collected from the locations for physical parameters description that influences thermal resistivity, subjected to grain size distribution and compaction analysis. The calculation of thermal conductivity is done by using CT-Lab software ver. 1.0.2 with a sensor resistance value of 82.69 ohms / m, heater voltage 4.0 V, and heater power 4.3857 W/m. The results show that the thermal conductivity values range from 0.593 to 3.239 W/mK. For the Sapu Angin hill region, sub-district Penyipatan, soil layers are generally in the form of sandy-clay (λ&gt; 1.2 W/mK), but at some points, the value of thermal conductivity in these layers have decreased (λ &lt;1.2 W/mK) due to rainwater infiltration (BH-02, BH-03, BH-04, BH-05, and BH-07). The infiltration causes weakening or loosening of bonds between the constituent particles of the soil. While in the area of Kampung Baru sub-district Pelaihari, the soil layer is generally in the form of sandy-clay, which has not well compacted (BH-08, BH-10, BH-12) with high water content (BH-09).

The relationship between thermal conductivity and electrical resistivity of silty clay soil in the temperature range − 20 C to 10 C

With increasing engineering projects carried out in cold regions, the analysis and evaluation of the thermal conductivity and electrical resistivity of frozen soil have become important considerations in engineering construction and theoretical research. Through experiments performed on silty clay, the effects of the initial water content and soil temperature on the thermal conductivity and electrical resistivity of silty clay soil were analyzed. Theoretical models were also formulated for the soil thermal conductivity and electrical resistivity. The results showed that the variations in the thermal conductivity and electrical resistivity can be discussed as three stages: the freezing prophase, freezing metaphase and freezing anaphase. The electrical resistivity increases rapidly in the temperature range − 2 °C to −6 °C. The thermal conductivity increases rapidly from −2 °C to −4 °C and then decreases slightly from −4 °C to −6 °C due to the development of frost heave cracks in samples with higher water contents (15%, 18% and 20%). However, the thermal conductivity of samples with a water content of 10% continues to increase under these conditions. At the same temperature, the thermal conductivity increases linearly with water content in general. The electrical resistivity decreases approximate linearly with water content from 10 °C to −4 °C and increases with increasing water content from −4 °C to −20 °C. The thermal conductivity is inversely proportional to electrical resistivity from 10 °C to −4 °C and is proportional to electrical resistivity from −10 °C to −20 °C. The results of the experiments performed on silty clay soil verify the soundness of the proposed model for the thermal conductivity and electrical resistivity of unfrozen and frozen soil.

Effect of soil moisture on current-carrying capacity of low-voltage power cables

One of the factors affecting current-carrying capacity of underground power cables is the thermal resistivity of soil. Its value in the close proximity of the cable is the most important, and for this reason, in some cases, the local soil is replaced with an another soil type or with a cement-sand mixture. The thermal resistivity of the soil is strongly affected by moisture, and in the case of a cement-sand mixture – as tested by the authors – also by this mixture initial water content. The paper presents results of investigation of soil moisture influence on the soil thermal resistivity, and an analysis of the current-carrying capacity of a low-voltage power cable for various soil parameters, in particular its part directly surrounding the cable.

Two-Zone Geological Soil Moisture Migration Model for Cable Thermal Rating

An optimized two-zone model based on the critical degree of saturation is proposed in this paper for ampacity calculations of underground power cables. Moisture migration is fully considered based on geological soil models. Comprehensive finite-element method (FEM) simulations are performed to study the effect of the moisture content and its effects on soil thermal resistivity and critical temperature rise. The critical degree of saturation ( S cr ) is used in the improved two-zone model instead of the critical temperature rise ( Δθx ) used in the IEC Standard 60287. The comparative analysis presented between the new method and the IEC Standard shows that IEC is more susceptible to changes in the ambient conditions. The use of a fixed critical temperature rise may yield significant errors. Shortcomings of numerical solutions are discussed highlighting the advantages of the new (optimized) two-zone model. The optimized method is useful for the design of new installations as it accounts for changes in soil thermal resistivity and critical temperature rise. The new method has been validated with available test results.

Cyclic Loading of Underground Cables Including the Variations of Backfill Soil Thermal Resistivity and Specific Heat With Temperature Variation

The aim of this paper is to calculate the temperature distribution of a cable conductor and the soil surrounding the cable under cyclic loading conditions, taking into account the formation of a drying zone in the backfill material. The thermal model of an underground cable is considered by using the IEC 60853-2 method, including the effects of the variations in temperature on the thermal resistivity and specific heat of the soil. The results are compared with IEC 60853-2 and thermoelectric equivalent method calculations. This study is done on eight sandy soil types as backfill materials, and calculations are carried out by using a 33-kVx cable. The results indicate that the modifications done on the IEC 60853-2 method are required to improve its capability to simulate the field conditions during load cycling of power cables.

Load cycling of underground distribution cables including thermal soil resistivity variation with soil temperature and moisture content

As it is known the load of distribution cables is not constant with respect to the daytime, increasing the current in some hours may lead to an increase of soil surrounding the cable resistivity due to the migration of soil moisture content. Formation of dry zones may cause failure of cable insulation. Also because the soil thermal properties change with time according to the weather conditions of different seasons, the current capacity changes significantly from time to time. This paper studies the effect of dry zone formation during load cycling of underground cables on their temperature rise and the temperature distribution in the surrounding which is not considered by IEC 60853-2. The thermal model of the cables using the thermoelectric equivalent method is modified by including the thermal soil resistivity variation with soil temperature and moisture content. The finite element method is used also in this study to obtain heat map of the cable and surrounding soil. Additionally, experimental work of three soil types was investigated to study the effect of temperature and moisture content variation on the soil thermal resistivity and the dry zone formation of each soil type. Field measurements of temperature distribution surrounding the cables are done.

Thermal design of Earth-to-air heat exchanger. Part II a new transient semi-analytical model and experimental validation for estimating air temperature

Abstract This paper is the second in a series of thermal design of Earth-to-Air Heat Exchanger (EAHE). The first part explains a transient semi-analytical model ‘named RBM model’ to estimate the disturbed soil thermal resistance of EAHE pipe as a function of duration of operation. In this part, a new semi-analytical model, called Generalized RBM model (GRBM model), is developed to predict the thermal performance of EAHE operating under transient conditions for cooling mode. Therefore, an experimental investigation was carried out in the region of Biskra (Algeria) to validate the developed model. The GRBM model development is based on the subdivision of the soil and the pipe into many layers and the use of RBM model, presented in the first part, to estimate the temperature of disturbed soil surrounding the inspected layer. Furthermore, the experimental horizontal EAHE setup, installed in the University of Biskra, is described. The experimental tests were performed in May 2013 for 6 h of continuous operation mode of the EAHE setup. To validate the GRBM model, a comparison between the calculation and the experimental results at various sections along the pipe is conducted. The results of the GRBM model are in a good agreement with the measured temperatures with an average relative error of 2.8%. Furthermore, both analytical aspect and the simplicity of the GRBM model will make the thermal design of EAHE more accurate with significantly reduced calculation time.