Failure rates reduction in SmartGrid MV underground distribution cables: Influence of temperature

Temperature plays an important role on the failure rate of electrical cables and their joints as well as of any electrical components. In fact, Arrhenius's Law (1889) states that temperature accelerates any chemical reaction and, hence, it is evident that aging, or a fast degradation, of any insulating materials increases with temperature. For this reason the maximum operational temperature which can be accepted for the insulation of the extruded underground cables must be limited controlling the transmitted current by proper relays (integrated in the switchgear). Anyway, the temperature of the insulation of underground cables is function of both ambient temperature and thermal resistivity of the ground, which have to be carefully evaluated, case by case, at the time of the original installation. Anyway, during the hottest months of the year both these parameters (ambient temperature and thermal resistivity of the ground) present their highest values and in consequence it is possible that the cable insulation may reach higher temperatures of those admitted, with faster degradation of the insulating materials and, hence, an increase of failure rate. For this reason the thermal current capacity of the cables installed in the distribution networks of modern Smart Grid can be optimized having on-line information on both ambient temperatures and load currents. Also a more accurate control of the same parameters may lead at a reduction of the number of failure registered on these distribution cables. This paper reports a preliminary study which classifies failures of medium voltage underground cables and their joints in function of the estimated or measured ambient temperatures and other parameters. Also some remarks on the relation exiting among failures of cables, ambient and cable temperatures and thermal resistivity of the ground are reported.