Space charge accumulation in power cable XLPE insulation

The accumulation of space charge in the crosslinked polyethylene (XLPE) ins ulation of a power cable sample under de stress was investigated. The sampl e was held at 82 degrees C and atmospheric pressure, and at 82 degrees C an d 0.1 Pa, for periods up to several days. Such conditioning removed pre-exi sting space charge. When an external de field was applied after conditionin g at atmospheric pressure and 82 degrees C, space charge accumulated at a r ate 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 pressu re 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-inh omogeneous polarization in the amorphous volume of the XLPE, and electron i njection/extraction at the electrodes. The electron transfer between electr ode and XLPE in either direction involves the same narrow window of combine d donor and acceptor states in the insulator, centered on the Fermi level. The spatially-inhomogeneous polarization in the XLPE originates in a spatia lly-inhomogeneous distribution of dipole complexes. It is suggested that th e dipoles are formed by a process equivalent to non-permanent oxidation of the XLPE, requiring water as a catalyst.