The Effect of Drying around Power Cables on the Vadose Zone Temperature

Abstract

The effects of moisture migration away from power cables depend strongly in most soils on whether the soil has reached the so-called critical conditions (e.g., critical temperature) for a dry-out. However, research on how to estimate this critical temperature has been limited. We solved numerically a differential equation for the planar case of steady-state coupled heat and mass transfer in porous media, using the van Genuchten-Mualem model for the hydraulic properties of the medium. The critical isotherm is assessed by the temperature at which the steepest change occurs in the moisture content with temperature. We considered the impact of the properties of the surrounding medium and of the ambient temperature and moisture content on the dry-out temperature. We found that seasonal variations in ambient conditions in a field in Italy have a greater impact on the critical temperature than the variations in soil properties in that field (under the assumption of a nearly uniform moisture content with depth). However, our analysis shows that, under certain conditions, a change in the van Genuchten parameter n and in a specific combination of parameters that we call a dynamic parameter can have as large an effect on the critical temperature. This suggests a direction in the optimization of backfill materials.Simultaneous transfer of heat and moisture is of interest for a wide variety of agricultural, industrial, and energy production processes. Drying of porous solids and soils, geothermal energy production, thermally induced oil recovery, underground high-level waste disposal, and heat transfer from buried pipelines and electrical cables are just some of the examples (Udell, 1985).The release of Joule heat from buried electrical cables increases the temperature of the ground in their vicinity and leads under certain conditions to the formation of a zone with a very low moisture content. Besides heat conduction through grain-to-grain contacts and the water bridges between the grains, heat is also transferred as a result of a "heat pipe cycle" (Groeneveld et al., 1984). Water evaporates in high-temperature areas and condenses in lower temperature regions, causing a gradient in moisture content. This results in liquid water flow in the opposite direction. At high heat fluxes, large gradients in temperature and moisture content develop. When certain critical conditions are exceeded, this results in almost complete drying out of the soil near the heat source (Groeneveld et al., 1984).Among the critical conditions, those involving the critical temperature are the most important for using standardized steady-state cable ratings. To prevent overheating in the cable insulation or the surrounding environment due to a low thermal conductivity caused by drying, the International Electrotechnical Commission recommended solution procedures for the steady-state rating and thermal analysis of power cables (CIGRE Study Committee 21, 1992; International Electrotechnical Commission, 2015). That analysis adopts a two-zone model-a steady...

Lifetime Assessment of PILC Cables with Regard to Thermal Aging Based on a Medium Voltage Distribution Network Benchmark and Representative Load Scenarios in the Course of the Expansion of Distributed Energy Resources

“…In addition, drying-out of the soil at higher temperatures sets in (cf. [13,14,22,23]). According to IEC 60287 (2005), in stationary observations, drying-out takes place at a critical excess temperature of 15 • C. Above this temperature, a specific soil resistance of 2.5 km/W and below 1.0 km/W is assumed [7].…”

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

“…The effective dissipation of heat away from the underground power cable restrains the thermal instability and reduces risk of progressive drying of the back ll material and thus, maintain surrounding temperature within the permissible limit [8]. Thermal stability is the ability of back ll materials to maintain relatively constant thermal properties during heating effect and hence, enhancing the amapacity (current carrying capacity) of these underground power cable system [9][10][11].…”

Experiment for validation of numerical models of coupled heat and mass transfer around energy cables

“…The fundamental approach of Philip and de Vries (1957) represents the basis of most contributions, reducing the complex microscopic transfer phenomena within soils in general to four macroscopically described transport phenomena. Most recently, this approach was used to study the effect of dryingout mechanisms in the framework of a realistic operation of energy cables (Anders & Radhakrishna, 1988;Freitas et al, 1996;Groeneveld et al, 1984;Hruška et al, 2018;Kroener et al, 2014Kroener et al, , 2017Radhakrishna et al, 1984;Snijders et al, 1981). Most of the studies have a strong theoretical or numerical approach and lack of experimental data for proper validation purposes.…”