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Thermal sand is widely used throughout the power industry to improve underground electrical cable performance. An electrical cable primarily made up of two elements the conductor and the insulator, the limiting factor on a power cable capacity is the temperature that the insulation can withstand and the more power or current that is flowing through the conductor the more heat is generated by the cable.
Different insulation materials are available PVC is rated to 60 deg C and XLPE a cross linked poly ethylene is rated at 90 deg C meaning that for the same size conductor the current carry capacity of the cable can be increased by as much as
Other insulating materials based on ceramic materials are used in special applications but these types of insulations are very expensive and reach the point where there cost is the same as the conductor meaning it easier and cheaper to increase the size of the conductor.
It’s widely recognised that a cable will perform differently depending on where and how the cable is installed. Burying the cable in the ground increases the cable capacity, the ground act as a heat sink taking heat away from the insulation and reducing the cable temperature. Thermal sand is a way of increasing this thermal heat transfer.
Thermal sand is now widely used when installing underground cables. The thermal sand is rated or tested to provide a known heat transfer efficiency. The cable and the thermal sand are installed so that the thermal sand is in direct contact with the cable. The thermal sands high heat transfer efficiency ensures the heat generated by the cable is transferred from the cable through the thermal sand to the surrounding earth.
Thermal sand is specified to a thermal resistivity which is measured in dig K m/W or °C-cm/W and typically the thermal sand may need to meet a required standard for example “ thermal resistivity shall be less than or equal to 0.9 °K m/W”
The thermal sand becomes the connection between the cable and the general mass of the earth. All the heat generated by an underground power cable must be dissipated through the soil. This is quantified by the soil thermal resistivity (or thermal rho, °C-cm/W), which can vary from 30 to 500°C-cm/W. Electrical engineers understand the performance of the cable quite well, but to most, the soil behaviour is a mystery, usually handled by using a thermal backfill with a supposedly “safe” thermal rho.
The ability of the surrounding soil to transfer the heat determines whether an operating cable remains cool or overheats. Improving the external thermal environment and accurately defining the soil and backfill thermal rho commonly results in a 10% to 15% increase in cable amp capacity, with 30% improvements noted in some cases.
Compacted granular backfills can have good thermal properties. Since most of the heat conduction is through the soil mineral particles and their contacts, one must ensure a high-density soil mixture to maximize these contacts. A well-graded sand to fine gravel can be a good thermal backfill when compacted to its maximum density as determined by a laboratory standard Proctor test (ASTM D698). The total cost of a compacted backfill must include material and transportation costs, as well as installation labour and quality-assurance costs. Yuleba Minerals makes a thermal sand which has been used for projects in and around Roma, Chinchilla and the Surat basin.
The one often-neglected factor about compacted backfills is the need for quality assurance during installation. If the gradation of the backfill is not correct (sieve analysis ASTM D422), or it is not at the optimum moisture content (ASTM D698), or not enough compaction effort is applied, or the backfill lifts are too thick, then the maximum density will not be achieved and the thermal capability degraded.
Gas Power projects in the Surat basin have been the catalyst for Yuleba Minerals developing and testing a thermal sand for use in underground cable installation.