Interaction of Coupled Thermal Effect and Space Charge in HVDC Cables

Currently, zero-emission targets require future global energy concepts to be based on renewable energy sources; therefore, huge investments are being made in bulky offshore wind parks worldwide. In this context, there is ongoing and enormous development and a need for HVDC submarine cables (both static and dynamic) to connect offshore wind farms. One of the basic problems when analyzing the operating conditions of HVDC cables is assessing the effects of the load current, which generates thermal and electric fields on the insulation systems in these cables. This article considers the problem of the influence of the thermal effect and space charges—the field effect—on the electrical conductivity of polymeric insulating materials and, thus, on the distribution of the electric field intensity in the cable insulation. An analytical methodology for joint analysis of the thermal-effect- and space-charge-related influence is presented. The critical value of the electric field intensity at which the electrical conductivity is significantly modified under coupled thermal–electric exposure is determined. Special focus is placed on the analysis of the coefficient representing the dependence of the electrical conductivity on the temperature in a much broader range than typically assumed. Hence, the intention of this paper is to highlight the limit values of the electric field strength under the simultaneous action of the space charge and temperature gradient. Recognizing the changes in the electric field intensity value in the insulation is of fundamental importance from the point of view of HVDC cable technology and construction.