Structure of Paper-Oil Insulation for Mass-Impregnated HVDC Cables

The electrical insulation in mass-impregnated high voltage direct current (HVDC) cables consists of many layers of paper tapes that are wound helically around the conductor and subsequently impregnated in oil. The oil fills the interstices between the fibers in the paper, as well as the larger gaps ("butt gaps") between the revolutions of the helices. In order to assess the electric field in such insulation, the insulation needs to be modeled either as a homogeneous material, or as a composite of several materials. One approach is to model the butt gaps as oil and the rest as a homogeneous material, without paying special attention to the interfaces between the butt gaps and the rest of the insulation. In this work, scanning electron microscopy (SEM) has been used to assess whether or not such an approach is appropriate. The results show that the internal structures in the impregnated paper tapes are much smaller than the paper-free butt gaps. Moreover, the internal structures contain paper-oil interfaces that are comparable to the interfaces between the butt gaps and the rest of the insulation. This justifies the mentioned approach.