On the Influence of Mechanical Stress on the Conductivity of DC Polypropylene Cables

HVDC cables may have bending and mechanical stress gradients when laid down on field or tested in a lab. This might affect locally intrinsic electrical conductivity and, therefore, electric field distribution. In this paper, the influence of mechanical stress on the conductivity of Poly-Propylene (PP) insulation is investigated. Measurements of charging-discharging current, at various mechanical stress, electric field and temperature values, were performed on specimens taken at various locations along insulation diameter of a full-size HVDC cable. It is shown that mechanical stress may have significant influence on conductivity, which varies with location of specimens within cable insulation thickness. Conductivity decreases with stress and its behavior can be fitted to an empirical model. This should be considered where bending radius of cables are tight (e.g. type and PQ testing), since this dependence can have a non-negligible effect on field distribution in the cable dielectric layer.