Load cycling and radial flow in mass impregnated HVDC subsea cables

For transmission of large amounts of electric energy across long sea crossings so called mass impregnated HVDC subsea cables remain the state-of-the-art technology. The cooling period after a power reduction or turn-off is the most critical part of the op eration of such a cable. It turns out that differences in thermal expansion and contraction may cause cavities to form within the insulation during cool-down, leading to a dielectric breakdown. Thus, the power rating of such cables, both with regard to sh ort term overloads and on a continuous basis, is largely set by considering this risk. However, the cavity formation process is far from well understood, and it is reasonable to assume that the true capacity and operational flexibility this cable technolo gy can offer, are not fully exploited. The cavities are essentially a result of an insufficient radial flow of the mass impregnation. Thus, the main R&D challenge of the project is to identify and quantitatively assess the importance of the factors govern ing these processes. The approach will be largely experimental. 10 - 15 m lengths of the cables used in the NorNed link will taken into the NTNU/SINTEF high voltage laboratories and subjected to various load cycles under different environmental conditions . By using appropriate instruments and experimental techniques, in particular partical discharge measurements at very low frequency AC, the radial mass flow and cavity formation will be studied. In parallel the mass flow will be modelled by using MathLab or other software tools.