Void Detection In Semiconductor Shielded Power Cable Insulation By Measurements Of Submillimeter Radiation Scattering

Abstract

Void Detection in Semiconductor Shielded Power Cable Insulation byMeasurements of Submillimeter Radiation ScatteringP. R. Cunningham, P. K. Cheo, and J. D. FarinaUnited Technologies Research Center, Silver Lane, East Hartford, Connecticut 06108AbstractThe feasibility of detecting voids in triple extruded power cable insulation by measure-ment of Mie scatter power of incident far -infrared (FIR) laser radiation is explored. Mea-surements of scatter power of a 100 -250 on range of void sizes in polyethylene insulationshow that void scatter power and experimental SNR decrease as wavelength increases from 119to 447 um. Extrapolations of experimental data to larger void sizes show that detection ofvoids larger than 250 on in polyethylene insulation at 447 on wavelength would have accept-able SNR using direct detection methods with no noise reduction techniques. Triple extrudedinsulation was modeled by taking into account the attenuation of an insulation shield at 447on with 14 percent transmission. Modeling results show simulated detection of voids intriple extruded insulation would provide acceptable SNR for detection of voids larger than250 on using higher laser irradiance levels than those required for nonshielded, or tandemextruded, insulation.IntroductionThe Association of Edison Illuminating Companies (AEIC) have established various specifi-cations which are intended to maintain a reasonable level of quality standards for all manu-factured power cables. One specification requires that the insulation of the complete cableshall be free from any void larger than 50 um. Until recently, the power utility industrydid not have a reliable instrument to inspect cables. For cables manufactured by the tandemextrusion process, where the insulation is extruded over a semiconducting shield whichcovers the central conductors in one process, a new FIR laser scanner performs on -line in-spection of cable insulation with a nearly complete coverage by measuring Mie scatter of 119on radiation.1,2 In recent years, cable manufacturers introduced the triple- extrusion pro-cess which involves simultaneous extrusion of both inner and outer semiconducting shieldlayers and the insulation layer at one time. These cables cannot be inspected by the far -infrared laser scanner because the semiconducting compounds presently used contain a highconcentration of carbon black, which scatters the FIR radiation and severely reduces trans-mission through the outer layer.An experimental investigation was conducted to determine if voids could be detected byscatter power measurements using radiation longer than 119 um. FIR transmission of avail-able commercial and experimental semiconducting materials was determined by FTIR spec-troscopy and are reported elsewhere.3 All commercial semiconducting materials were found tobe opaque at 625 on thicknesses out to 500 on wavelengths. However, several experimentalmaterials had transmissions of several percent to 14% at 500 um wavelengths. These resultsare listed in Table 1. Using these measurements as a baseline reference, laser scatteringexperiments were designed at appropriate laser wavelengths to measure scattered power fromvoids in cable insulation. With these results, a modeling effort determined the minimumdetectable void size in triple extruded cable insulation by using an optimum laser wave-length which can be obtained from the available FIR laser prototype cable inspection instru-ment.Previous calculations based on a Mie- scattering model indicated that the detectivity ofthe laser inspection system decreases significantly with increasing laser wavelengths.4Longer wavelength radiation is necessary to compensate for scattering losses through pre-sently available carbon black semiconductor layers and previous calculations indicated thatminimum detectable void size was expected to increase as wavelength increases. Experimentalscatter power measurements were required in order to (1) verify predicted void scatter powerbased on previous calculated void scatter cross -sections, (2) determine minimum detectablevoid size at each wavelength from measured SNR, and (3) determine experimental scatter powerfor basis of modeling effort for void detection in triple extruded insulation.