THERMOLUMINESCENCE IN XLPE CABLE INSULATION

Crosslinked polyethylene (XLPE) has been employed in underground trans mission and distribution cables because of its excellent electrical an d mechanical properties, such as low permittivity and dielectric loss, high degree of toughness, and good flexibility. An underground power cable operates at temperatures above ambient and the polymeric insulat ion is usually crosslinked to provide mechanical strength to withstand the high temperatures. Chemical crosslinking is commonly employed; ho wever, chemical crosslinking creates byproducts such as acetophenone, alpha-methylene styrene, and cumyl alcohol. The general practice is to decrease the concentration of the volatile crosslinking byproducts fr om the newly manufactured transmission class cables before they are co mmissioned into service. The concentration of the byproducts is decrea sed by treating the cables at a high temperature in a vacuum oven. At present, to determine the residual concentration of the byproducts, th e treatment has to be stopped, a sample of the polymer has to be cut f rom the treated cable and the byproducts have to be extracted for seve ral hours from the polymer before they can be analyzed. This paper des cribes a novel, non-destructive optical method for determining the con centration of the byproducts in XLPE prior to cable installation. The method involves in situ detection and measurement of thermoluminescenc e emitted by the crosslinking byproducts during the pretreatment of th e cable. It is shown that the measurement of the intensity of thermolu minescence provides a direct indication of the concentration of the by products and that it is more sensitive than mass spectrometry.