Ion generation and transport in low density polyethylene under electric stress

Synthetic materials such as cross-linked polyethylene (XLPE) are increasingly used as electrical insulation in HVDC power cables, compared to oil impregnated insulation. However, because of their dielectric properties, these materials can accumulate charges in the insulation, generated by dissociation of species or injected at the electrodes. This accumulation phenomenon can cause an increase of the electric field which can lead to the dielectric breakdown of the material, meaning thereby stopping the distribution of electricity. It is therefore important to understand these charge accumulation mechanisms for HVDC application, and particularly the role of ions, generated by dissociation of by-products of cross-linking agents, that can transport and take part to space charge accumulation. In this goal, we have considered a simple model material consisting of low-density polyethylene (LDPE) films soaked in cumylalcohol, a by-product often encountered in peroxide-crosslinked XLPE and thought to dissociate under field to provide ionic carriers. The impact of the soaking time, electric field and temperature on space charge density profiles recorded using the Pulsed Electro Acoustic method (PEA) and on charging current are presented and compared to those obtained on un-soaked LDPE samples.