Effect of Heat Exchange Transient Conditions with Moving Water-Air Interface on Space Charge Accumulation in Undersea HVdc Cables

The accumulation of space charge inside the dielectric layer of an HVdc cable is one of the most important issue to be considered in the design stage and during the operating conditions. The separation of space charge is due to several factors including the dependence on temperature of the electrical conductivity of the insulation and the establishment of a thermal gradient under load conditions. This article is focused on the investigation of the effect of an axial heat transmission on the space charge and electric field distributions in an HVdc cable. In order to assess the impact of this phenomenon, a case study involving a cable immersed partly in water and partly in air has been simulated by means of a 2-D thermal and electrical model in time domain. The electric field calculation is based on the quasi-static Maxwell model and the current continuity equation. To simulate the effect of waves, a moving water-air interface is considered, with time varying sinusoidal law. The results show the establishment of an electrical field peak near the water-air interface due to an increased temperature drop over the insulation in this area. The approach and the findings described in this article attempt to improve the knowledge in the relationship between thermal and electrical phenomena in HVdc cables. Moreover, this article focuses on the advisability of appropriately considering the impact of the axial heat transfer in the standards on the thermal rating of HVdc cables