Lightning Impulse and Switching Surge Breakdown of Liquid Nitrogen-Impregnated Wrapped Dielectrics for Cryogenic Cables

In this paper experimental results are presented which indicate that flexible resistive cryogenic EHV cables employing a liquid nitrogen-impregnated dielectric wrap will withstand values of electric stress under lightning impulse and switching surge conditions comparable to those achievable in EHV oil/paper cables. To simulate the geometry of a cable, each test specimen consisted of a tape-wrapped insulation layer between concentric cylindrical electrodes (with stress cones added at the ends of the sample). The samples included fifteen wrapped with spunbonded olefin fiber tapes, three wrapped with high-density cellulose EHV cable paper, and for comparison purposes, a liquid cell with concentric cylindrical electrodes. The data are consistent with observations on similar insulation systems at 60 Hz since the wrapped dielectrics impregnated with liquid nitrogen withstood a substantially higher electric stress under these conditions than the liquid nitrogen alone.