Impact of conditioning on space charge formation in XLPE under dc electrical stress

Whereas synthetic materials have been used for long in the insulation of HVAC cables, their use in the case for HVDC applications is still pending due to a lack of knowledge on the behaviour of space charge and on their impact on the reliability of cables. As volatile cross-linking by-products are known to have an impact on space charge accumulation, it is important to control the optimum outgassing conditions in respect to such application. In this work, the accumulation of space charge in cross-linked polyethylene (XLPE) samples is investigated under dc electrical fields at room temperature using the pulsed electroacoustic (PEA) method. Organic semiconducting screens were attached on both sides of the dielectric during the cross-linking procedure and were used as electrodes. Five different degassing conditions were studied. The samples degassed at room temperature developed heterocharges. Conditioning by holding at 50degC for 2 days decreased the accumulation of heterocharges. Surprisingly, no heterocharges were observed in some fresh samples and heterocharges only changed sign (the density remained almost the same) when the applied voltage was reversed. These results appear consistent with space charge build up resulting from a spatially inhomogeneous distribution of permanent dipole molecules, like crosslinking by-products, present in the material. A model, based on diffusion of volatile cross-linking by-products and polarization orientation features, was developed to describe the heterocharges formation in XLPE. Simplified assumptions (one dimensional infinite sample, desorption phenomenon) were adopted in order to obtain a simple expression of the polarisation charge density. Theoretical results predict, basically, that the heterocharges move in time from the electrodes into the bulk due to the permanent dipole gradient. The magnitude of the polarization charge is also affected by the temperature and the initial dipoles concentration.