Dielectric and physicochemical behavior of aged PVC insulated cables

Radial field, medium voltage cables operated at 6.6 kV are the object of this study. Dielectric properties of those aged PVC insulated cables have been measured as a function of temperature and voltage in a frequency range of 1 mHz to 1 kHz. The range of temperature tested was between 30°C to 110°. The dielectric material shows a resistive behavior at low frequency. Deduced conductivities are in good agreement with DC conductivities measured at the considered temperatures. The conductivity follows an Arrhenius law over the range studied with an activation energy of 0.96 eV. The conductivity of some sample is high enough to induce an increase of the losses at industrial frequency. The increase of the low frequency capacity is revealing an interfacial polarization. This increase of capacity is due to local accumulation of electric charges in the insulation, particularly at the electrodes. Physicochemical characterizations have been done to understand this decrease of resistivity. A migration of plasticizers did not occur as evidenced by IR micro-spectroscopy. Therefore, the decrease of resistance cannot be attributed to such mechanism. Physical aging is evidenced by DSC, which leads to the working temperature of the cables. TMA has shown modifications of thermo-mechanical properties along aging. These evolutions are attributed to the formation of double bonds resulting from dehydrochlorination.