A study of electric stress enhancement. Part 2. Stress enhancement on semiconductive shields

For pt.I see ibid., vol.11 p.976-82, (2004).This paper evaluates the electric stress enhancement due to surface protrusions on the tape of semiconductive shield compounds used in power cables. The results of a conventional grade and a supersmooth grade shield compounds are reported. Using a laser-scanning instrument, the shape of protrusions on the tape surface of these compounds can be quantitatively determined. An algorithm is developed to portray the geometry of a protrusion so as to allow the calculation of its stress enhancement. First the supersmooth grade shield compound is illustrated as much smoother than the conventional shield compound. Besides exhibiting a lower number of protrusions, the surface protrusions on a supersmooth shield compound also are much "flatter" than those of the conventional shield compounds. Insulation thickness is confirmed to influence the stress enhancement and, consequently, different distributions of stress enhancement factor are observed with low voltage, say, 15 kV, and high voltage, say, 500 kV, applications. The conventional shield compound exhibits higher stress enhancement than the supersmooth grade compound in higher voltage cables, though the difference is much less with lower voltage settings. Since the total number of protrusions on the conventional shield surface is more than one order of magnitude higher, the supersmooth shield therefore can offer much better protection to a cable by minimizing the electric stress degradation in its insulation material.