Thermal behavior of distribution MV underground cables

The present paper reports an experimental investigation on the thermal behavior of medium voltage underground cables laid in different types of soils and under different conditions of the ambient temperatures. The same paper shows as thermal degradation of the insulating system of the cables and their joints can become more consistent and faster due to the effect of the continuously overheating due to the ambient temperatures and to the thermal resistivity of the soil of higher values. An other source of thermal degradation of the cables and their joints may be indicated in the fault currents affecting the metallic shields in systems operating with a compensated neutral connection to ground. In particular, in these systems the fault current is lasted for a time of some tens of seconds to allow a faster localization of the failure, through the automatic sectioning switches. During this time, currents flow through the metallic shields of the cables, overheating the same shields especially in correspondence of pour connections which may be present inside the joints. In case that the single-fault-to-ground evolves in a double-fault-to-ground the same shield will be interested by much higher current (short-circuit) which will create a deeper degradation of the semiconductive compounds and the insulation located nearby the metallic shields. The paper also presents the results of visual inspections of failed cable joints, due to thermal causes. Based on these considerations, important solutions may be indicated to reduce the failure rate of the MV electrical system allowing improvements in the overall power quality of the entire electrical systems.