Calculation and Measurement of Space Charge in MV-size Extruded Cables Systems under Load Conditions

A load current in dc high voltage cables results in a temperature drop across the insulation and hence a radial distribution of the insulation conductivity is found. Direct consequence is an accumulation of space charge in the bulk of the insulation, that may significantly affect its reliability. This phenomenon was modeled in terms of the macroscopic properties of the cable insulation and a numerical procedure was developed for the calculation of the time-dependent space charge and electric field in extruded-type cables. Results of calculations were compared to those of space charge measurements performed on MV-size XLPE cables at different values for the temperature drop. The analysis of the results indicates that the space charge induced by the temperature drop is measurable and that its magnitude and location can be reasonably well estimated by means of the proposed numerical procedure. Moreover, different space charge accumulation mechanisms could be distinguished taking into account the knowledge obtained of the space charge distribution induced by the temperature drop.