The results of space charge evolution in cross-linked polyethylene power cables under dc electrical field at a uniform temperature and during external voltage polarity reversal are presented in the paper. A mirror image charge distribution was observed in the steady state, but the pre-existing field altered the way in which the steady state charge distribution was formed from that obtaining when the cable was first polarized. Polarity reversing charge was generated in the middle of the insulation and moved towards the appropriate electrodes under the influence of a field in excess of the maximum applied field. Our results show that the mirror effect is a steady state effect that is due to crossinterface currents that depend only on the interface field and not its polarity. Measurements on cable sections with an elevated mean temperature and temperature gradient show that the interface currents are temperature dependent, and that differences between the activation energies of the interface and bulk currents can eliminate and possibly even invert the polarity of the space charge distribution.Index Terms -Space charge, PEA, XLPE insulated power cables, voltage polarity reversal, temperature gradient, "mirror image effect".

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Repetition of partial discharge (PD) activities within a dielectric insulation of high voltage equipment may lead to dielectric breakdown, eventually resulting in failure of the whole equipment. Thus, PD measurement is essential in high voltage insulation system. Modeling of PD activity may increase an understanding of PD phenomenon. One of the parameters which can be determined from PD modeling is the statistical time lag. In this work, a physical model of PD using finite element analysis (FEA) method has been developed to determine the relationship of statistical time lag with different applied stresses; these include different applied voltage, frequency and temperature. The statistical time lag as a function of different applied stresses was determined through comparison between measurement and simulation results. The proposed experimental-modeling approach may increase an understanding on the physical explanation about the statistical time lag.

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