MV Underground Cables: Effects of Soil Thermal Resistivity on Anomalous Working Temperatures

One of the parameters that has to be considered in designing underground cables is the thermal resistivity of the soil along their routes. In fact, the ampacity of the MV underground cables having different types of insulation is normally evaluated through the application of the IEC Standard 60287-x "Electric cables - Calculation of the current rating". This series of IEC Standard is formed of three parts as in the following: part 1: Formulae of ratings and power losses; part 2: Formulae for thermal resistance; part 3: Sections on operating conditions. In Italy, the current capacity of the same MV cables is established by the Italian Standard CEI-UNEL 35027 (2009) "Power cables with rated voltages from 1 kV to 30 kV - Steady state current ratings: cables laid in air and in ground", which are based on the above mentioned IEC Standard. These thermal ampacities of the underground cables are function of ambient temperature and the thermal resistivity of the soil. The soil thermal resistivity is a parameter not easy to evaluate and for this reason, several times, only typical values are assumed just in function of the kind of the interested terrain (organic, loam, quartz sand, etc.). This assumption ignores the fact that the thermal resistivity varies continuously day by day especially in function of humidity soil content, being strongly dependent from this parameter. This paper describes as important differences of thermal resistivity may be present in consequence of seasonal variation in moisture content of the soil. It is also shown as such variations may cause important anomalous increases of cable maximum working temperature, which may be assumed to be at the origin of unexpected failures especially in correspondence of MV cable joints. In fact, the joints are, still today, the weakest points of the underground cables where the breakdown easily occurs due to poor workmanship, moisture ingress and thermal degradation. This thermal degradation may be also due to circulation of fault current in the metallic sheaths.