Space charge accumulation in power cable XLPE insulation

The accumulation of space charge in the crosslinked polyethylene (XLPE) insulation of a power cable sample under de stress was investigated. The sample was held at 82 degrees C and atmospheric pressure, and at 82 degrees C and 0.1 Pa, for periods up to several days. Such conditioning removed pre-existing space charge. When an external de field was applied after conditioning at atmospheric pressure and 82 degrees C, space charge accumulated at a rate considerably faster than that observed before conditioning. However the rate of accumulation following conditioning at 82 degrees C under reduced pressure was much slower than that observed before conditioning. The faster rate could be recovered by reconditioning the sample at atmospheric pressure after conditioning under reduced pressure. Inversion of the equilibrium space charge profile, following reversal of the applied field, was observed. This observation is consistent with a model incorporating a spatially-inhomogeneous polarization in the amorphous volume of the XLPE, and electron injection/extraction at the electrodes. The electron transfer between electrode and XLPE in either direction involves the same narrow window of combined donor and acceptor states in the insulator, centered on the Fermi level. The spatially-inhomogeneous polarization in the XLPE originates in a spatially-inhomogeneous distribution of dipole complexes. It is suggested that the dipoles are formed by a process equivalent to non-permanent oxidation of the XLPE, requiring water as a catalyst.