Space charge and electric field in thermally aged multilayer joints model

This paper is concerned with space charge accumulation and electric field distribution at XLPE/EPDM interfaces after thermal aging. XLPE, EPDM and multilayer XLPE/EPDM/XLPE flat samples have been manufactured and aged thermally during 240 and 480 h at 105 °C. Material degradation by oxidation during aging has been assessed by FTIR spectroscopy. The conductivities and permittivities of the samples have been measured before and during ageing. Using the values of conductivity and permittivity values obtained experimentally, the charge density values at the XLPE/EPDM interfaces and the electric field distribution across a multilayer XLPE/EPDM HVDC cable joint model has been calculated for different thermal aging times. The results show that the surface charge density values vary with the ageing time (decrease at aging beginning and then increase) and determine important enhancements of the electric field, in the vicinity of the inner but also of the outer semiconductor. The simultaneous presence of the volume and interfacial charges determines a less increase of the electric field for homocharges and more important for heterocharges, both for unaged and aged samples.