Investigation of the acoustical material properties of xlpe dependent on the degree of crosslinking
The typical material for the insulation layer of modern power cables is crosslinked polyethylene (XLPE), which has a high electrical strength and good thermal properties. Due to the crosslinking of the polyethylene, XLPE does not melt at elevated temperatures in comparison to thermoplastic materials, which allows an increase of the operation temperature and therefore an increase in transmission power. Insufficient crosslinking results in a high risk for the power cable, since a non-crosslinked insulation system can melt during high load or short circuit conditions. A possible deformation of the coaxial design of the cable can lead to increased field stress in the insulation, which can cause an electrical breakdown of the insulation system. Hence, the crosslinking process is checked using destructive hot-set-tests at the beginning and end of each production length. Since a hot-set-test is a destructive measurement method, continuous monitoring of the crosslinking process is not possible. So, local but critical variations in the degree of crosslinking during production cannot be detected. It is known, that crosslinking changes the thermomechanical properties of XLPE. Furthermore, ultrasonic measurements can be used to detect changes in the mechanical material properties. It can therefore be assumed, that non-destructive ultrasound could be used to assess the degree of crosslinking of XLPE. The basis for the development of an ultrasonic monitoring system is the precise characterization of the acoustical material properties. In this paper the dependency of the acoustical material properties of XLPE on the degree of crosslinking is presented. At first, samples of XLPE with different (defined) degrees of crosslinking are produced at a laboratory scale. The range varies from fully-crosslinked to non-crosslinked samples with different sample thicknesses to represent different voltage levels. Afterwards ultrasonic measurements are performed on the samples, while varying temperature (25-75 °C) and ultrasonic testing frequency (2-5 MHz). The temperature variation is used to take varying process conditions in the vulcanization line into account. The results are evaluated in the frequency and time domain to characterize the differently crosslinked samples. Besides ultrasonic sound velocity and sound impedance, the attenuation and frequency behavior of the received signals are evaluated. It can be seen, that the evaluated measurement parameters show a dependency on the degree of crosslinking. The discriminability of different degrees of crosslinking is reduced with increasing temperature. The results indicate, that in the range of typical production temperatures a distinction between crosslinked and non-crosslinked samples is possible based on the evaluation of the acoustical material properties of XLPE.
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