Charge front and negative differential mobility in HVDC model cables

Space charge measurements have been carried out by means of pulsed-electro-acoustic (PEA) method on mini-cables with 1.5 mm-thick cross-linked polyethylene (XLPE) as insulation. Measurements were realized at room temperature or with a temperature gradient of 10°C through the insulation (Tin = 70°C, Text = 60°C) under DC voltage of -30kV and -55kV. Space charge results highlight the enhancement of the field at the outer-electrode when combining thermal and electrical stresses. They also reveal systematic occurrence of a negative front of charges generated at the inner electrode that moves toward the outer electrode at the beginning of the polarization step. It is observed that the transit time of the front of negative charge increases, and therefore the mobility decreases, with the applied voltage. Further, the estimated mobility -in the range 10^-14 - 10^-13 m²V^-1s^-1 for the present results, increases when the temperature increases for the same condition of applied voltage. The possible origin of this charge front is discussed on the basis of transient charging current measurements realized on the same cables. The features substantiate the hypothesis of negative differential mobility used for modelling space charge packets.