Space charge accumulation in two power cable grades of XLPE

The accumulation of space charge in XLPE (cross-linked polyethylene), cross-linked using DCP (dicumyl peroxide) or a silane-based grafting process, was studied via the LIPP (laser-induced-pressure-pulse) technique. Planar samples 0.5 mm thick were obtained from the XLPE insulation of power distribution cables. dc fields to 10 kV/mm were applied at temperatures in the range 20 to 90 degrees C. Usually the Dcp samples developed heterocharge, and the silane samples homocharge. However, the observed space charge densities were not very different. Nearly all of the charge accumulated within 100 mu m of each electrode, with very little in the remainder of the volume. Both sample types showed a near-perfect inversion of the equilibrium space charge profiles on reversing the applied voltage polarity. This observation is explained in terms of charge injection at the electrodes, electron transfer between electrode and XLPE in either direction involving the same narrow 'window' of combined donor and acceptor states in the insulator, centered on the Fermi level.