Study of an initial transient relaxation in XLPE cable insulation by TSDC and PEA

Space charge profiles in electrets of cable XLPE insulation have been determined by pulsed electroacustic method (PEA). A single positive peak close to the cathode, which decreases with annealing time, is observed. Typically, thermally stimulated depolarization currents (TSDC) of those electrets show heteropolar peaks initially. A new transient homopolar peak emerges in TSDC profiles at 99°C with annealing at high temperatures. No direct correlation can be established between PEA and TSDC results, since the case of the largest value of the maximum of this TSDC peak corresponds to the smallest charge peak observed in PEA profile. The transient behavior is explained by means of charge trapped on crystals surfaces due to the differences between the conductivities of amorphous and crystalline phases. The homoplar peak is associated with dipolar contaminants located in the interlamellar amorphous phase, which are reoriented by the field produced by charge trapped on crystal surfaces, during the TSDC discharges. Detrapping of charge trapped on crystal surfaces occurs when lamellae melt, which contributes homopolarly to the current. Both contributions result in the peaks observed in the TSDC discharge.