Characterization of smoothness via laser profiling of a surface

Abstract

A number of instruments are in use for the inspection of polyethylene-based insulation products for power cable. The trend towards higher voltages and long-term XLPE cable system reliability emphasizes the importance of not only the insulation materials, but also the semiconductive shields and the perfection of the shield/insulation interface. Protrusions from the semiconductive shields penetrating into the insulation have been observed to initiate tree growth and thereby give premature failures. Consequently, the need to quantitatively evaluate the smoothness of the carbon filled shield material has been recognized. Currently, extruded semicon tapes are manually examined under a strong light or at low magnification for undispersed black or other protrusions on their surface. This technique to evaluate extruded tape smoothness is qualitative and subject to the errors associated with human judgement. To quantify the smoothness characteristics of a semicon extruded tape surface, a laser-light profiling device has been developed to count and size the number of surface imperfections. This instrument permits rapid quantitative analysis of large surface areas of semiconductive tape directly from an extruder with immediate data analysis and presentation. The electrical stress enhancement resulting from these protrusions from the shield into the insulation may then be evaluated by numerical methods.