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Space Charge Accumulation in Models of Inhomogeneous Insulations of DC Cable Joints

This paper presents a study on the accumulation of the space charge in the volume and on the surface of models of bi-layer insulations of DC cable joints, unaged and thermally aged. The bi-layer samples were manufactured from XLPE and EPR or EPDM and were thermally aged up to 60 days at 120 oC. It is shown that the space charge accumulates in the bulk of the sample, more in the layer connected to the HV electrode, and that its values depend on the ageing time. Space charge also accumulates at the interface between the two dielectric layers, with a negative polarity under applied voltage and becoming positive after de-energization, more as the sample is thermally aged. The variation of the average charge density value was also analysed, and it was found that it increases with the ageing time and is up to 3 times higher in XLPE / EPDM than in XLPE / EPR samples. The space charge changes the distribution of the electric field under applied voltage, the field becoming locally up to 5 – 7 times more intense than its average value. Finally, it is shown that the increase of the ageing time leads to the increase of the values of the average density of the remaining space charge (and, respectively, of the remaining electric field) in all the samples, which remains at significant levels even after 600 s.