The Role of Right Interpretation of Space Charge Distribution for Optimized Design of HVDC Cables

In the field of high-voltage transmission systems, different degradation phenomena affect the reliability of the employed components. In particular, under dc stress, the space charge accumulation phenomenon is believed to be the most responsible of the dielectrics lifetime reduction. To measure the accumulated space charges in flat specimens, the pulsed electro-acoustic (PEA) method is one of the most used techniques. The working principle of the PEA cell is based on the acoustic waves propagation and detection. As is well known, the acoustic waves propagating in different means are partially transmitted and partially reflected. Therefore, the piezoelectric sensor of the PEA cell is subjected to both direct and reflected waves. The latter, which are also present in the PEA cell output signal, if are not taken into account, could be easily confused as a charge accumulation. For this reason, in this paper, the acoustic waves' behavior within the PEA cell has been simulated in order to avoid incorrect interpretation of the PEA cell output charge profiles. In this paper, compared to other works present in the literature in which only the surface charges were considered, the space charges accumulated in the insulation bulk are also considered for a complete analysis of the waves behavior during the PEA measures. The results found in this paper are very useful for cable manufacturing industries in which the choice of the dielectric materials is strongly influenced by the space charge accumulation phenomenon.