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. |
