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Backfill materials for underground power cables, Phase I. Interim report. Thermal resistivity measurement methods, backfill treatments, heat and moisture flow analysis. [HEAT]
Abstract
Because the allowable current loading of buried electrical transmission cables is frequently limited by the maximum permissible temperature of the cable or of the surrounding ground, there is need for cable backfill materials that can maintain a low thermal resistivity (less than 50°C-cm/watt) even while subjected to high temperatures for prolonged periods. The results of studies aimed at development of improved methods for placing backfill around underground power cable systems and special treatments to reduce the thermal resistivity and increase the thermal stability of the backfill materials are reported. The thermal needle method used for measuring the thermal resistivity of backfill materials in the laboratory is discussed. Samples compacted wet and then dried backfill materials were tested. Three water-absorbing polymers for the prevention of water migration were tested and found to retard, but not prevent water migration. Once dried, samples treated with these materials had poorer thermal properties than did the untreated soil. No additives were found that can produce a material with a thermal resistivity significantly less than that of the untreated wet compacted material. A finite element computer program HEAT was used to study transient and steady-state heat flows and temperature distributions for typical buried cable systems. The dominating influence of the thermal resistivity of the trench backfill on the temperature distributions and allowable thermal loading is readily apparent from the analyses. Field tests should be conducted to evaluate both the performance of the most promising backfill treatments and the accuracy of the developed heat and moisture flow predictive methods.
... The thermal properties of the biochar, soils, and BAS were measured by compacting in an acrylic mold with an inner diameter of 80 mm and a height of 60 mm. The selected dimension is sufficient to prohibit the boundary effect due to the needle probe's heating (Mitchell et al. 1977;Brandon and Mitchell 1989;Cai et al. 2015). The distance between the probe and the outer boundary (i.e., acrylic wall) was 5.86 times the probeto-probe spacing to avoid any boundary effect, which is more than the recommended value (i.e., 2.37 times) provided by Campbell et al. (1991). ...
Thermal backfill is essential for projects such as underground crude oil pipelines and crude oil storage tanks to control the heat migration from the source. The properties of locally available soil may not be adequate as thermal backfill and hence need suitable amendment. Biochar is a low thermal conductive material and may contribute to an increase in soil strength. There are no such studies that deal with the thermal properties of biochar-based backfill. Further, the influence of biochar particle size fractions on soil thermal properties has not been reported until now. Therefore, the possibility of biochar as a soil amendment for modifying thermal backfill characteristics is explored in this study. Two types of soils (highly plastic silt and clayey sand) are amended with three biochar content amounts (5%, 10%, and 15%), and three different particle size fractions [coarse (4.7-2 mm), medium (2-0.425 mm), and fine (0.425-0.075 mm)], and their compaction characteristics as well as thermal and mechanical properties are investigated. It was observed that the amendment of biochar in soil reduced the thermal and mechanical properties of the soil. Further, the reduction in soil thermal conductivity and volumetric heat capacity with biochar amendment was more in coarser biochar fraction than finer and medium fractions. In comparison, reduction in unconfined compressive strength (UCS) was more in finer biochar fraction. Additionally, the thermal properties reduction was higher in clayey sand than highly plastic silt. An inverse linear correlation of thermal conductivity with pH and electrical conductivity were observed for both soil biochar mixes. The relationship between the thermal properties and UCS indicates that the medium fraction biochar provides the optimized value for reducing thermal properties and UCS of biochar amended soil (BAS). This proves the efficacy of BAS as thermal backfill.
A technique is presented for calculating the temperature rise and load capabilities of power cables with provision for statistical variations of various soil, boundary and loading conditions. The novel. technique exploits the ampacity sensitivity algorithm described in a previous paper [1] in conjunction with statistical data gathered for a variety of soil materials, ambient parameters and load cycle characteristics. The technique provides useful information concerning the expected. values and confidence levels of. ampacity and cable temperature rise as well as the probabilities associated with excessive temperatures and thermal instability. The paper describes the probabilistic approach and illustrates a variety of its applications.
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