Polarity reversal in HVDC joints - the effect of the axial thermal conduction

It has been shown that the establishment of a thermal gradient over the radius of HVDC cables involves the accumulation of space charge within the dielectric layer. High thermal gradients over the insulation thickness of loaded cables can lead to the inversion of the radial electric field pattern. In this scenarios, transient overvoltages and polarity reversal can lead to local and transitory peaks of electric field. Since the temperature distribution plays an important role in reaching critical values of the electric field, it has been considered interesting to have a more in-depth view of the thermal behavior of HVDC systems close the discontinuities of the geometry along the cable axis. The main goal of this research is to investigate the effect of the axial thermal conduction on space charge phenomena occurring on joints between two HVDC cable segments. In particular, the values of the electric field have been calculated in several probe points in a joint area during a polarity reversal event. The calculations are carried out by means of Comsol Multiphysics® on a 2D axial symmetric geometric model. The results demonstrate that the role of the axial heat transfer in the behavior of the electric field during a voltage transient strongly depends on the thermal conductivity of the joint insulation.