Modelling of Internal Pressure Dynamics in Mass-Impregnated Non-Draining HVDC Cables

Abstract

Changing the current loading of mass-impregnated non-draining power cables causes the pressure in the insulation to vary over a wide range. It is generally recognized that under certain conditions—typically after a large load reduction—the pressure becomes so low that shrinkage cavities (voids) form in the insulation, and that partial discharges that may damage the insulation ignite. A finite element model for temperature, electric field and pressure dynamics throughout the insulation layer has been developed. Comparisons with previously obtained measurements on full-scale HVDC subsea cable samples subjected to a variety of loading patterns, external pressures (sea depths) and ambient temperatures, show that the model replicates the pressure behavior of the insulation reasonably well. Most importantly, the complicated viscoelastic properties of the lead sheath, which have a profound effect on the internal pressure dynamics and thereby on the prospects of forming potentially harmful cavities, appear to be sufficiently accurately modelled.