Accelerated water tree tests on three layer miniature cables

Abstract

AB 8 TRACTMiniature cables were extruded on a true triple extrusion line using conventional conductor shield, insulation shield and different insulation materials. As they were extruded the cables underwent continuous vulcarisation using steam at pressures 2 4 kg/cm . Suitable termination procedures were developed and used to subject these miniature cables to accelerated water tree growth tests. All tests were carried out at room temperature by immersing the cables in 0.1M NaCl solution and aging them at an average electrical stress of 6 kV/mm, at both 60 Hz and 1 kHz. Samples of the cables aged at 60 Hz test frequency were removed after a preset time and characterised by measuring the dielectric loss and ac breakdown strength. Aged cables were also sectioned after the breakdown test and examined for water trees.The 1 kHz test cables were allowed to age till breakdown.The ac breakdown strength of conventional crosslinked polyethylene (XLPE) insulated 15 kV cables were measured after accelerated water tree tests and compared to the results on miniature cables. For engineering purposes miniature cable tests at room temperature can predict the results for full-size cables under AEIC test methodology at a significantly lower cost. The miniature cables have been shown to be useful in comparing the dielectric performance of different insulation and semiconductive compounds for power cables.Accelerated aging tests on any insulating material used for power cables yield often conflicting results because of unrealistic test conditions and test sample.Test conditions closer to real service aging should, in theory, allow a more reliable approximation of the accelerated test results. Test samples closer to the geometry of the real size cables are also more desirable and expected to add to the validity of different accelerated test methodologies. This paper describes a true triple extrusion technique developed to produce miniature cables as a more desirable test sample for accelerated aging. The obvious benefits of a three layer extrusion system were retained along with an objective of following a system approach to develop materials for power cables.Miniature cables with thin overlays of the insulation alone have been shown to be useful 0-7803-0649-X/92 $3.00 0 1992 IEEE for research purposes [l] as well as for screening effectively the performance of new materials [ 2 -4 1 .A correlation between miniature and full-size cable testing based on extensive accelerated aging tests has also been established [ 3 ] .