Effect of Dry-Zone Formation around Underground Power Cables on Their Ratings

Effect of Dry-Zone Formation around Underground Power Cables on Their Ratings

Copyright: © 2016 |Pages: 23
DOI: 10.4018/978-1-4666-6509-5.ch009
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Abstract

Current ratings of buried cables are determined by the characteristics of surrounding soils and cable properties as given in IEC 60287-1-3 (1982). In this standard the soil thermal resistivity of the surrounding soil is supposed to be varies from 0.5 oC m/w to 1.2 oC m/w but under loading the heat dissipated from underground power cables increases the soil thermal resistivity and this may leads to cable thermal failure and thermal instability of the soil around the underground cables. For this reason de-rating factors for cable loading taking the dry zone formation into consideration has to be considered during distribution cable network design. Several approaches have been adopted to establish current ratings of buried cables based on constant values of soil thermal conductivities. Mathematical models are suggested by many researches to study the drying out phenomenon around underground power cables. In this chapter de-rating factor for underground power cables taking dry zone formation into account is calculated depending on IEC 60287-1-3 (1982). This chapter also contains an experimental work carried out on different types of soils to investigate the formation of dry zone phenomena under loading by heat source simulates the underground cables.
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9.1 Experimental Study

9.1.1 Soil Samples Used in Testing

Several experiments are carried out on different types of natural soils to study the dry out zone formation under different loadings conditions. Six types of natural soils are investigated for studying the drying out phenomena and the thermal behaviour of the soil around the power cables. These types of soil can be classified in composition as given in table 1.

Table 1.
Classification for investigated soil types
Soil typeWeight percentage (%)Classification
GravelSandSiltClay
Sand11.588.510-Very coarse sand, poor in gravel, moderately poor in silt
Sand2288.59.5-Moderately fine sand, poor in gravel, moderately poor in silt
Sand313843-Medium to coarse sand, some gravel and traces of silt
Sand4892--Medium to coarse sand, some gravel
Silty sand86030-Medium to coarse sand, some gravel
Clayey
Silty sand
3373030Medium to coarse sand, some gravel

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