Abstract
During temperature cycling the radial flow of oil, which is caused by differences in thermal expansion between the oil and paper, may lead to cavity formation in mass impregnated HVDC cables. Partial discharge activity in these voids is an important failure mechanism in this type of cable, but the governing processes are not fully established.This paper contributes to flow modeling through a literature survey and a comparison between two alternative flow patterns in the insulation. Only one detailed model of radial flow in this kind of insulation has been found in the literature. The model, which is based on porous media flow in a homogeneous insulation with constant porosity, and its suitability as a starting point for further modeling is reviewed.Two possible flow paths, radially outwards through the paper or between paper tapes from one layer to the next, have been considered. Based on previously unpublished experimental results, the flow resistance R of the two alternatives has been estimated. This quantity is found to be of the same order of magnitude in both cases, meaning that there probably will be flow both through and between the paper tapes.
Highlights
During temperature cycling the radial flow of oil, which is caused by differences in thermal expansion between the oil and paper, may lead to cavity formation in mass impregnated HVDC cables
Radial flow may result from load variations which, due to ohmic losses in the conductor, create temperature gradients in the insulation with corresponding thermal expansion or contraction
Thermal expansion of the oil will fill these before any radial flow can occur [5]
Summary
During temperature cycling the radial flow of oil, which is caused by differences in thermal expansion between the oil and paper, may lead to cavity formation in mass impregnated HVDC cables. The published models, such as [6] and [5], have treated the insulation as a homogeneous material, disregarding the structure with layered papers and butt gaps. The paper itself is porous, allowing radial flow of oil, in addition to around 10% of the insulation volume being comprised by the helical butt gap channels.
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