The effect of temperature gradient on space charge accumulation at SR/XLPE interface under DC stress

In the compound insulation which is comprised of cable accessory (e.g. terminal or sheath of cable), interface between different dielectrics or electrode and dielectric is often claimed to be the weakest points due to space charge accumulation and then the increase of adjacent electrical field. Furthermore, the higher temperature of cable conductor caused by Joule heating leads to conductivity gradient along the insulation under DC stress, which will cause space charge accumulation and distort electrical field at the interface or in the bulk. In order to investigate the space charge profiles in the compound insulation of cable accessory, Space charge distributions in silicone rubber/cross-linked polyethylene (SR/XLPE) laminates with pre-stress or not under DC stresses and temperature gradient were measured by pulsed electro-acoustic (PEA) method. At the same time, the resistivities of silicone rubber and XLPE were measured from room temperature to high temperature. The pre-stressed test results show that SR possesses lots of deep trapping sites. A strange phenomenon is found that SR easily injects charges not at high stress but at low stress, although it has a high conductivity at a temperature range of 20-80°C. Furthermore, the higher the temperature gradient the more negative charges injection from cathode, which leads to the increase of negative charges at the interface.